radiation protection and environment

RADIATION PROTECTION AND ENVIRONMENT (A quarterly journal Published by the Indian Association for Radiation Protection)

Vol. 30, No.1- 4

Editorial Committee Dr. B.S. Rao Convener, ex BARC Shri R.C. Rastogi Co-convener, ex BARC and ex IAEA Dr. R.K. Kher RP &AD, BARC Dr. A.G. Hegde HPD, BARC Dr. Pushparaja RSSD, BARC Dr. M.S. Kulkarni RSSD, BARC Dr. D.Dutta HPD,BARC Smt. B.K. Sapra EAD, BARC Dr. P.R. Sangurdekar RSSD, BARC Miss Pramila D. Sawant IDD, BARC Shri R. Sarangapani IGCAR, Kalpakkam Shri. D.S. Katoch ex BARC Shri Pradip Bhargava HPD, BARC Shri Munish Kumar RP &AD, BARC

All correspondence may be addressed to Dr.B.S. Rao, Convener, Publication Committee, Radiation Protection and Environment, IARP office, c/o RPAD,Room No. 212, CTCRS Bldg., Anushaktinagar, Mumbai400094. Opinion expressed by the authors do not necessarily reflect the official view of the IARP.

January- December 2007

EDITORIAL

2

Radiation Protection and Environment, Vol. 30 , No. 1- 4, 2007

RADIATION PROTECTION AND ENVIRONMENT (A quarterly journal Published by the Indian Association for Radiation Protection)

Vol. 29, No.1- 4

January- December 2007

Contents Page No. EDITORIAL Synthesis of BaSO4: Eu Nanoparticles for Dosimetry of Ionizing Radiation

5

Numan Salah, S P Lochab, Ranju Ranjan, D Kanjilal, V E Aleynikovand A A Rupasov

Thermoluminescence properties of Cu and P doped LiNaSO4 phosphor

9

Ba0.97Ca0.03SO4:Eu Nanoparticles for ion beams dosimetry

12

Preparation And Characterization Of CaF2: Mn Phosphor For High Dose Dosimetry

16

a

,

-Jyoti Mehra, P.D.Sahare, Ranju Ranjan , Numan Salah S.P. Lochab, Amitansu Patnaika \,Ajay Kumar a.

-S P Lochab1, Numan Salah1, Ranju Ranjan2, D Kanjilal1, V E Aleynikov3 and A A Rupasov4

-

Vijay Kumar and A.K. Bakshi

Safety Provisions In Electron Accelerators At Rrcat, Indore

20

-Dimple Verma, M.K.Nayak, Vipin Dev, G.Haridas, K.K. Thakkar, P.K. Sarkar and D.N. Sharma

Beam Loss Detection Technique Using Radiation Detectors in Electron Accelerators at RRCAT, Indore

25

-M. K. Nayak, G. Haridas, Vijay Kumar, P. K. Sahani, K.K. Thakkar, P.K. Sarkar and D.N .Sharma

Fast neutron spectrometry from 20 MeV 1H+NAT.Cu using activation foils

29

-S.P. Tripathy1, C. Sunil, M. Nandy, A.A. Shanbhag, P.S. Sharma, D.S. Joshi, P.K. Sarkar and D.N. Sharma

BaMgAl10O17:Eu PDP Phosphor for TLD Applcations

33

Shield Design for the Material Science Beam Line of Medical Proton Cyclotron Facility

36

-S.J. Dhoble, A.D. Deshmukh, G.V. Bramhe* N. Sinha, R.G. Sonkawadeand D.R. Peshwe

-R. Sarangapani, V. Meenakshisundaram, K.V. Subbaiah and R. Indira

Radiation environment in positive ion accelerators: Experimental and theoretical investigations

40

-Maitreyee Nandy

Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

3

Bremsstrahlung Contribution around X-Ray Diffraction Beam line At Indus-2 Synchrotron Radiation Source

47

-M.K.Nayak, Vipin Dev, G.Haridas, Dimple Verma, K.K.Thakkar, P.K.Sarkar,D.N.Sharma

Safety from RF and Microwave Radiation at RRCAT, Indore

50

-M.K.Nayak, Vipin Dev, G. Haridas, K.K.Thakkar *PK Sarkar, DN Sharma

Radiation Safety Aspects of High Energy Particle Accelerators

54

-K.V. Subbaiah

Physics Study of the Criticality Safety of a Precipitator for using in a Fuel Reprocessing Facility

55

- D. Datta and L.G. Guneshwor

Background Radiation, People And The Environment: A Review

61

-T.V. Ramachandran

Thermoluminescence Dosimetric Characteristics of Europium Doped BAM Phosphor

72

-A.S. Sai Prasad, A.P.Zambare and K.V.R. Murthy

High Energy Photon Dosimetry: Present Status and Future Challenges

75

-Munish Kumar R. K. Kher, G.Sahani, Kanta Chopra and S.P.Agarwal

Performance evaluation of low level radiation monitoring instruments

78

-Liji Daniel, R.A. Satam, S.M. Tripathi and Suresh Rao

4


SYNTHESIS OF BaSO4: EU NANOPARTICLES FOR DOSIMETRY OF IONIZING RADIATION Numan Salah1, S P Lochab1, Ranju Ranjan2, D Kanjilal1, V E Aleynikov3 and A A Rupasov4 1 Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110067, India 2 Department of Physics and Astrophysics, University of Delhi, Delhi-110007, India 3 Joint Institute for Nuclear Research, Dubna 141980, Russia. 4 P.N. Lebedev Physical Institute, Russian Academy of Sciences, Leninsky Prosp. 53, Moscow 117924, Russia. Abstract : Barium sulfate activated with europium (BaSO4:Eu) is a highly sensitive phosphor commonly used for personal and environmental radiation dosimetry using the thermoluminescence (TL) technique. The nanocrystalline powder of this material with average grain size ~ 20 nm has been prepared by the chemical co-precipitation method. They were characterized by UV–Vis spectrometry and X-ray diffraction (XRD). TL response of these nanoparticles have been studied and compared with the corresponding microcrystalline powder. It has been observed that the TL glow peak at 497 K, seen prominently in microcrystalline sample, appeared as a small peak in nanocrystalline powder, while that observed as a shoulder in the former at 462 K, dominantly appeared in the latter. The observed TL sensitivity of the prepared nanocrystalline powder is less than that of the microcrystalline sample at low doses, while it is more at higher doses. The TL response of the nanomaterial phosphor shows a very linear response with exposures increasing up to very high values (as high as 7 kGy), where the same for microcrystalline phosphors show saturation. This linear response over a large span of exposures (0.1 Gy–7 kGy) makes the nanocrystalline phosphor useful for its application to estimate low as well as high exposures of ãrays.

Key words : Naoparticles, BaSO4:Eu, Thermoluminescence, high dose, Band gap.

INTRODUCTION Thermoluminescence (TL) is a common technique used for the dosimetry of ionizing radiations. There are several thermoluminescent materials are used for this propose (Fox et.al., 1988; Noh et. al.,2001). However, the saturation effects at high exposures as well as the less sensitivity for low doses of ionizing radiations are the major problems in these materials. Recently, there are several studies on different nanoscrystalline materials have been suggested for the use at higher exposures of ionizing radiations (Salah et. al., 2006; 2007; Sahare et. al., 2007; Lochab et. al., 2007a; 2007b). These nanomaterials have showed a very interesting feature, such as good sensitivity for high doses and linear in a very wide range of exposures. But, these nanometerials, however, have a very less sensitivity at detecting low doses of ionizing radiations. The observed excellent performance of the microcrystalline form of BaSO4:Eu for measuring the ãrays exposures (Madhusoodanan, et. al.,1999), encouraged us to prepare its nanocrystalline material and to study its TL response to gamma rays. There are several studies on this material for its TL (Madhusoodanan, et. al.,1999; Dixon and Ekstrand, 1974; Gupta et, al.,1974; Holzapfel and Krystel,1976; Gundu Rao et al.,1995, Bhatt et. al.,1997, Azorin ,1998). However, the TL response of BaSO4:Eu nanoparticles Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

has not been reported. In the present work, the nanomaterial of BaSO4:Eu is prepared by the chemical co-precipitation method. Formation of the compound was done by the XRD. Furthermore, the band gaps of the micro- and naocrystalline materials were obtained, using the UV-Vis spectroscopy. TL response to gamma rays is investigated in more details. EXPERIMENTAL Barium chloride of analar grade was dissolved in triply distilled water. Europium chloride of AR grade (0.2 mol %) was then added to the solution. To control the size of particles to be produced on precipitation, ethanol was added to the solution. Further ammonium sulfate was added drop wise to the solution until the precipitation was complete. The precipitate was filtered out and washed several times with distilled water. The nanophosphor was finally obtained by drying the precipitate at 363 K for 4 h. The nanocrystalline powder thus obtained was further annealed 673 K for 1 h in a quartz boat and quenched by taking the boat out of the furnace and placing it on a metal block for better sensitivity. To confirm the formation of the compound, XRD was studied at room temperature for the nanomaterial sample by using Cu-target (Cu-Kα1 line, λ = 1.5045 Å) on Philips-X’Pert Model-98 XRD machine and matched with the standard data available (JCPDS card No. 245

Synthesis of BaSO4: Eu Nanoparticles for Dosimetry of Ionizing Radiation - Numan Salah et.al.

1035). The band gaps of the micro- and nanocrystalline materials were also obtained using UV-visible absorption spectra. The measurements were done using U-3300 spectrophotometers (Hitachi). The band gap values obtained are 3.36 eV and 3.45 eV for the micro- and nanocrystalline phosphors, respectively. The obtained nanoparticles were irradiated with different doses of γ-rays from a Co60 source ranging from 0.1 Gy to 14.5 kGy at room temperature. The TL glow curves were recorded at a heating rate of 5 Ks”1 on a Harshaw TLD Reader (Model 3500) having a neutral density filter, taking 5mg of sample each time. For comparison of the sensitivity and the structure of the glow curve, the glow curves of both the microcrystalline BaSO4:Eu phosphor and the standard one CaSO4:Dy were also recorded under identical conditions.

α ( h ν ) ~ ( h ν − E g )1 / 2 where h í is the photon energy and α is the optical absorption coefficient near the fundamental absorption edge. The absorption coefficients α were calculated from these optical absorption spectra. Figure 3 shows the values of (áhí)2 for nano- and microcrystalline BaSO4:Eu plotted as a function of incident photon energy. The energy band gap was obtained by extrapolating the linear portion of the graph and making (á h í)2 = 0. The optical energy band gap for the nano- and microcrystalline BaSO4:Eu are found to be 3.36 and 3.45 eV, respectively. The widening of the band gap while going from micro- to nanosize might be due to the absence of the crystal field effects.

RESULTS AND DISCUSSION 4

Absorption (arb. units)

Particle size Broadening in the XRD lines in nanocrystalline powder sample (Fig. 1) was utilized to determine the size of these nanparticles by using Scherrer’s formula:

0.9λ D= β cos(θ )

a) b)

2

Microcrystalline powder Nanocrystalline powder

a

b

0

-2

Where D is the average grain size of the crystallites; λ the incident wavelength; θ the Bragg angle and β is the diffracted full-width at half-maximum (in radian) caused by the crystallites. The average grain size of these nanoparticles is estimated to be approximately 20 nm.

300

400

450

500

550

W avelength (nm)

Fig. 2: Uv- visible absorption spectra for the BaSO4:Eu micro-and nanocrystalline samples.

120

a) b)

Microcrystalline powder Nanocrystalline powder

100

(αhν) (a. u.)

30

20

80

2

In t e n s it y ( a . u )

350

60

10

40 a

20

0 20

25

30

35

40

45

50

2 θ (d e g re e s )

Eg for microcrystalline powder

b

Eg for nanocrystalline powder

0 3.5

Fig. 1: X-Ray diffraction pattern of BaSO4:Eu nanocrystalline powder. Energy Band gap Figure 2 shows the optical absorption spectra of the nano- and microcrystalline BaSO4:Eu samples near the fundamental absorption edge. The optical energy gap Eg of the nanophosphor as well microphosphor were calculated by the following relation (Pankove, 1971; Sze, 2004): 6

4.0

Photon Energy (eV)

Fig. 3: Plot for (ahn )2 as a function of the incident photon energy (hn) for the BaSO4:Eu micro-and nanocrystalline samples TL glow curves and response Fig. 4 shows a typical TL glow curve for BaSO4:Eu nanocrystalline powder exposed to 10 Gy of ã-rays from Radiation Protection and Environment, Vol. 30 , No. 1- 4, 2007


6x10

8

5x10

8

4x10

8

a) b) c)

BaSO 4:Eu nanocrystalline powder BaSO 4:Eu microcrystalline powder CaSO 4 :Dy standard phosphor

a 3x10

8

2x10

8

b c x0.05

1x10

8

0 350

400

450

500

550

600

650

700

Temperature (K)

Figure 4: Typical TL glow curves of BaSO4:Eu micro-and nanocrystalline samples exposed to 10 Gy of γ-rays from a Co60 source. TL glow curve of CaSO4:Dy phosphors is also shown for comparison. The ordinate is to be multiplied by the numbers at the curves to get the relative intensities. On exposing the materials to a wide range of gamma ray doses it is found that the TL response of the nanomaterial phosphor shows a very linear response with exposures increasing up to very high values (as high as 7 kGy), where the same for microcrystalline phosphors show saturation (Fig. 6). This strange behavior of the TL responses of the nanophosphors has been also observed in other materials (Salah et. al., 2006; 2007; Sahare et. al.,2007; Lochab et. al., 2007a;2007b). This could be explained by track interaction model (TIM) given in (Mahajna and Horowtz, 1997; Horowitz et. al., 2001). Accordingly, the number of TC/LC generated by highenergy radiation in a track depends not only on the crosssection of the tracks but also on the length of the tracks inside the matrix. In the case of a microcrystalline powder sample, high-energy radiation could generate a track of approximate length equal to dimensions of the crystal/ crystallites while passing through it. This could be of the order of several mm. But in the case of nanocrystallites the length of such tracks will be only of the order of a few tens of nanometres (dimensions of the nanoparticles). If Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

Fig. 5: TL glow curves of BaSO4:Eu nanocrystalline powder exposed to various doses of γ-rays from a Co60 source. The ordinate is to be multiplied by the numbers at the curves to get the relative intensities. we, therefore, compare the number of TC/ LC generated in the nanoparticle, it would be much less than the number of such centers generated in a single crystal or a microcrystalline powder sample for low doses. However, as the dose increases more overlapping tracks occur that may not give extra TL in the case of the single crystal and as a result of which saturation occurs in these materials. In the case of the nanoparticles, there still exist some particles that would have been missed while being targeted by the high-energy radiation, due to their very tiny size. Thus, on increasing the dose, these nanoparticles, which had earlier been left out from the radiation interaction, now generate TC/LC in them. Thus, we do not get saturation for the nanophosphor at a higher dose, whereas saturation is obtained in the case of the microphosphor. However, at much higher doses, i.e. well beyond 7 kGy, saturation is obtained even in the case of the nanophosphor due to the same reason of overlapping of tracks as was obtained in the case of the microphosphor. Never less, this property beside its good sensitivity for low doses, makes BaSO 4 :Eu nanophosphor a good candidate for detecting low as well as high doses of ionizing radiation.

10

9

10

8

10

7

10

6

a

TL intensity (a. u.)

TL Intensity (a.u)

a Co 60 source (curve a). TL glow curves of the microcrystalline BaSO4:Eu and CaSO4:Dy phosphors are also shown for comparison (curves b and c). As can be seen in this figure that the TL glow peak at 497 K, seen prominently in microcrystalline sample, appeared as a small peak in nanocrystalline powder, while that observed as a shoulder in the former at 462 K, dominantly appeared in the latter. This variation in behavior of the TL glow curves of the micro- and the nanophosphors is possibly due to the widening of the band gap in the nanophosphor from 3.36 to 3.45 eV. This increase in the band gap has probably resulted in changing the population of the trapping/luminescent centers (TC/LC).

b

a) b)

10

-1

10

0

10

1

n an o c ry sta llin e p o w d e r m ic ro c r ys tallin e p ow de r

10

2

10

3

10

4

E x p o s u re (G y )

Figure 6: TL response curve of BaSO4:Eu nanocrystalline powder to γ-rays of Co60. 7


Its comparison with the corresponding microcrystalline phosphor is also shown. CONCLUSIONS BaSO4:Eu nanoparticles of average grain size ~20 nm have been prepared by the chemical co-precipitation method. XRD results confirmed the formation of the compound and utilized for determining the particle size. UV-Vis studies have showed a widening in the band gap while going from micro-to nanocrystalline material. The nanophosphor exhibits a linear TL response to gamma radiation over a very wide range 0.1 Gy–7 KGy, whereas its corresponding microphosphor has a linear range only from 0.1 to 10 Gy. This has been explained in the framework of track interaction model. The nanomaterial of BaSO4:Eu is found to be sensitive for low as well as high doses of gamma radiation, thus, might be used as a TL dosimeter. ACKNOWLEDGMENTS We are thankful to the Department of Science and Technology (DST), New Delhi, India and the Russian Science Academy (RAS), Moscow, Russia for providing financial assistance under Project No A.2.53. REFERENCES Azorin Nito Juan (1998), Thermoluminescence and Optical Characteristics of Europium-Doped Barium Sulphate, Radiat. Phys. Chem 51, 471-472. Bhatt, B. C. et. al. (1997), A comparative study of the dosimetric characteristics of BaSO4:Eu and CaSO4:Dy Teflon TLD Discs, Radiat. Protect. Dosim. 105, 105-110. Dixon, R. L. and Ekstrand, K. E., (1974), Thermoluminescence of rare earth activated CdSO4, SrSO4 and BaSO4, J. Lumin. 8, 383-390)

materials, Nucl. Instr. and Meth. B 184, 85-112. Lochab, S.P. et. al.(2007a), Thermoluminescence and Photoluminescence study of nanocrystalline Ba0.97Ca0.03SO4:Eu, J. Phys. D: Appl. Phys. 40, 1343– 1350 Lochab, S. P. et. al. (2007b) Nanocrystalline MgB4O7:Dy for High Dose Measurement of Gamma Radiation, Physica Status Solidi (a) (at press). Madhusoodanan, U. et. al.(1999), Development of BaSO4:Eu thermoluminescence phosphor, Radiat. Meas. 30, 65-72. Mahajna S and Horowtz Y S (1997), The unified interaction model applied to the gamma ray induced supralinearity and sensitization of peak 5 in LiF:Mg,Ti (TLD-100) J. Phys. D: Appl. Phys.30 2603-2619 Noh, A.M. et. al.(2001), Investigation of some commercial TLD chips/discs as UV dosimeters, Radiat. Phys.Chem.61 497-499. Pankove, J. J. (1971) Optical Process in Semiconductors, (New York: Dover) p 39 Salah Numan, et. al. (2006), TL and PL studies on CaSO4: Dy nanoparticles, Radiation Measurements, 41, 40–47 Salah Numan, et. al. (2007), Theromluminescence of nanocrystalline LiF:Mg,Cu,P, J. Luminescence, 124, 357-364 Sahare, P.D, et. al. (2007), K3Na(SO4)2:Eu nanoparticles for high dose of ionizing radiation, J. Phys. D: Appl. Phys. 40, 759–764 Sze, S. M. (2004), Physics of Semiconductor Devices 2nd edn (New York: Wiley) p 39

Fox, P. J et.al. (1988), Spectral characterization of six phosphors used in Thermoluminescence dosimetry, J. Phys.D:Appl. Phys. 21,189-197 Gundu Rao, T. K. et al. (1995), Electron spin resonance, thermoluminescence anf fluorescence studies on BaSO 4:Eu and BaSO 4:Eu,P thermolumienescent phosphors, J. Phys.: Condens. Matter 7, 6569-6581. Gupta, N M. et, al.(1974), Trapping and emission centres in thermoluminescent Barium Sulfate, Radiation Effects 21, 151-156 Holzapfel, G. and Krystel, M. (1976), Thermally Stimulated Luminescence and Exoelectron Emission from Barium and Strontium Sulpate Doped withEuropium, Phys. Stat. Sol. (a) 37, 303-312. Horowitz, Y. S., et. Al.(2001), Theoty of heavy charged particle response (efficiency and supralinearity) in TL

8


THERMOLUMINESCENCE PROPERTIES OF Cu AND P DOPED LiNaSO4 PHOSPHOR Jyoti Mehraa, P.D.Saharea,c, Ranju Ranjana, Numan Salahb, S.P. Lochabb, Amitansu Patnaika ,Ajay Kumar a. a Department of Physics and Astrophysics,University of Delhi,Delhi-11000,India;b Inter bUniversity Acclerator Center,Aruna Asaf Ali Marg,New Delhi-110067,India. cDepartment of Physics,University of Pune,Pune. Abstract : Thermoluminescence (TL) is a good tool to investigate materials and has many applications in diverse fields, but it is more popular for dosimetry of ionizing radiations. The present paper reports the TL of ã-rays irradiated LiNaSO4:Cu and LiNaSO4:P phosphors (with Zeff »15). TL glow curve of LiNaSO4:Cu phosphor has a simple structure with a prominent peak at 137 0C along with smaller one at 205 0C. Similarly, LiNaSO4:P sample, it has a major peak at 152 0C , beside a minor one at 210 0C. The TL intensities of these phosphors are found to be 3.3 and 4.25 times of the standard CaSO4:Dy phosphor. These intensities have been optimized with appropriate concentration of activators. The position of the main peaks of these phosphors along with the growth of the higher temperature peaks has been observed to be dependent of the impurities concentrations. With increasing the dopants concentration, the 137 and 152 0C peaks of Cu and P doped LiNaSO4 samples are observed to shift to the lower temperature side and the corresponding higher peaks i.e., 205 and 210 0C are vanishing. These results, however, might be a useful tool to understand the TL mechanism and the role of these activators for generating trapping/ luminescent centers inside the host of such materials.

Key words : Thermoluminescence, LiNaSO4, activators,ppm.

INTRODUCTION Thermoluminescence (TL) is a phenomenon that has been used as a technique for dosimetry of ionizing radiations. Since till date the mechanism of TL is not fully understood. The role of the impurities normally used as activators are also doubtful. For understanding such important phenomenon, there are several factors to be investigated in more details and their affects need to be carefully detected. The first factor is being the host; nature of its cation elements, band gap and radicals formation. Second one is the impurity; its valence state, ionic radii, electronic transitions, and so on. As part of an effort to understand the TL mechanism, the valence state and the ionic radii of some selected well-known activators are subjected to this kind of investigations. P5+ and Cu+ are selected to be the activators and LiNaSO4 is chosen to be the host material. The mixed sulfate, LiNaSO4 doped with rare earth impurities have been reported earlier by several workers [1–6]. This phosphor when doped with proper activator (i. e. Eu); has a higher sensitivity than even the commercially available standard TLD phosphor, CaSO4:Dy (3 times of CaSO4:Dy). In the present study we have prepared LiNaSO4 by the same method adopted earlier [1-4, 6] and doped it with different concentrations of Cu and P. A typical value for gamma rays irradiation from Co60 source was given to the prepared powder and the TL glow curves were recorded and studied. EXPERIMENTAL PROCEDURES For preparing the LiNSO4:Cu, Li2SO4 and Na2SO4 of analar grade were stoichiometrically dissolved in Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

double-distilled deionized water. The solution was filtered and then boiled till a paste was formed. The paste was dried in an oven. The dried material was again dissolved and chloride salt of Copper was added to the solution. Copper concentration was varied from 0.005 to 0.1 mol. %. The solution was then slowly evaporated at 345 K and a powder was obtained. The resulting powder was heated in a quartz crucible till it melted. The melt was slowly cooled to room temperature and then crushed and sieved to yield particles of size 100-125 µm. The powder was annealed in a quartz boat at 700 K for 2 hours and quenched by taking the boat out of the furnace and placing it on a metal block. P doped samples were prepared by the same method with varying the dopant concentrations from 0.05-0.5 mol.%. Samples were exposed to gamma rays from a CO60 source for 1 Gy at room temperature. TL glow curves were recorded for 5 mg of sample each time at a heating rate of 5 K/s. TL glow curves were recorded on a Harshaw TLD Reader (Model 3500). For comparison CaSO4:Dy glow curves were also recorded under identical conditions. RESULTS AND DISCUSSION Fig.1 shows a typical TL glow curve for Cu and P doped LiNaSO4 microcrystalline powder exposed to 1 Gy of ã-rays from a Co60 source (curves a and b, respectively). TL glow curves of the commercially available CaSO 4:Dy phosphor is also shown for comparison (curve c). As can be seen in this figure that the TL glow curve of Cu doped sample has a simple structure with a prominent peak at 137 0C along with smaller one at 205 0C. Similarly, LiNaSO4:P sample, it has a major peak at 152 0C , besides a minor one at 210 9

Thermoluminescence properties of Cu and P doped LiNaSO4 phosphor - Jyoti Mehraa

0

However, other studies are needed for more understanding, like PL and ESR.


4x10

8

3x10

8

2x10

8

a) b) c)

a

LiNaSO 4:C u LiNaSO 4:P CaSO 4:Dy

b

1x10

8

c 0 149

249

347

360

0

T em perature ( C )

Fig. 1: Typical TL glow curves of Cu and P doped LiNaSO4 microcrystalline samples exposed to 1 Gy of γ−rays from a Co60 source. TL glow curve of CaSO4:Dy phosphors is also shown for comparison. 2.5x10

7

2.0x10

7

1.5x10

7

1.0x10

7

5.0x10

6

a)LiN aSO 4:C u b)LiN aSO 4:C u c)LiN aSO 4:C u d)LiN aSO 4:C u e)LiN aSO 4:C u

Intensity (a.u.)

a

100ppm 200ppm 500ppm 1000ppm 50ppm

e b

c d

0.0

50

100

150

200

250

.

Tem perature ( C)

Fig. 2: TL glow curves of LiNaSO4 doped with different concentrations of Cu.

Intensity (a.u.)

C. The TL intensities of these phosphors are found to be 3.3 and 4.25 times of the standard CaSO4:Dy phosphor. These intensities have been optimized with appropriate concentration of activators. The optimum value for Cu is found to be 100 PPM(Parts per million), while that for P, its 1000 PPM. These have been showed in figures 2 and 3, respectively. The position of the main peaks of these phosphors along with the growth of the higher temperature peaks have been observed to be dependent on the impurities concentrations. With increasing the dopants concentration, the 137 and 152 0 C peaks of Cu and P doped LiNaSO4 samples are observed to shift to the lower temperature side and the corresponding higher peaks i.e., 205 and 210 0C are vanishing. Though these activators are much different in their ionic radii and their valence states, even for their optimized values, they show somewhat similarity in their glow curves structure and have the same trend with different dopants concentrations. It has been reported by Patil et al. (2003), that Cu is incorporating in LiNaSO4 matrix in its monovalent form. Its ionic radius is 0.73 Å, which is very close to that of Li+ and even close to the ionic radius of the other cation Na+ (0.97 Å). In other words Cu+ is occupying Li+ ions sites, giving the best fitting, without even charge compensations. Therefore, a 100-PPM activator is sufficient to generate maximum number of active trapping center/luminescent center (TC/ LC) and yields maximum TL intensity. This is in contradiction with P ions. It is incorporating as P5+ in LiNaSO4 and will suffer from un-complete charge compensation, beside its ionic radius(0.38 Å ), which is much smaller than the two cations of LiNaSO4 host ( i. e. Li+ and Na+). Therefore, the optimum value of this activator is found to be 1000 PPM, which gives TL intensity close to that given by 100-PPM Cu+. Moreover, positions of the TC/LC seem to be a property of the host material, while role of the impurities is just to fill these vacancies. The positions of the TL glow peaks at almost the same temperatures for both activators are a direct indication for this assumption.

1.4x10

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1.2x10

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1.0x10

7

8.0x10

6

6.0x10

a) b) c) d)

LiNaSO 4:P LiNaSO 4:P LiNaSO 4:P LiNaSO 4:P

(500 PPM) (1000 PPM) (2000 PPM) (5000 PPM)

100R D OSE

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a b

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c

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100

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200

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.

Tem perature ( C)

Fig. 3(: TL glow curves of LiNaSO4 doped with different concentrations of Cu.

10


Thermoluminescence properties of Cu and P doped LiNaSO4 phosphor - Jyoti Mehraa

CONCLUSION Doping of LiNaSO4 phosphor with two different activators has showed an interesting TL results. The TL sensitivity of Cu and P doped materials are around 3.3 and 4.25 times more than that of CaSO4:Dy. The PPM level of the activators to have a maximum TL intensity is strongly dependent on their ionic radii and valence states. From the other side, positions of the TC/LC seem to be a property of the host material, while role of the impurities is just to fill these vacancies. REFERENCES

1. P.D. Sahare, S.V. Moharil “Thermoluminescence in LiNaSO4” , Radiat. Eff. Defects Solids 114 (1990) 167. 2. P.D. Sahare, S.V. Moharil, “Thermoluminescence in mixed alkali sulphate phosphors”. Radiat. Eff. Defects Solids 116 (1991) 275.


3. S.V. Moharil, S.V. Bodade, P.D. Sahare, S.M. Dhopte, P.L. Muthal, V.K. Kondawar “Luminescence in LiNaSO4 :Eu phosphor”., Radiat. Eff. Defects Solids 127 (1993) 177. 4. A. Pandey and P. D. Sahare “Thermoluminescence characteristics of LiNaSO4 doped with Eu and Dy”. phys. stat. sol. (a) 199, No. 3, 533.540 (2003) 5. A Pandey, P D Sahare, J S Bakare, S P Lochab, F Singh and D Kanjilal “ Thermoluminescence and Photoluminescence Characteristics of nanocrystalline LiNaSO4:Eu “Phosphor. J. Phys. D: Appl. Phys. 36 (2003) 2400–2406 6. Numan Salah, P. D. Sahare and Awadhesh Prasad.Thermoluminescence and Photoluminescence of LiNaSO4:Eu irradiated with 24 and 48 MeV 7Li ion beam” J. Luminescence 121(2006) 497-506

11

Ba0.97Ca0.03SO4:Eu NANOPARTICLES FOR ION BEAMS DOSIMETRY S P Lochab1, Numan Salah1, Ranju Ranjan2, D Kanjilal1, V E Aleynikov3 and A A Rupasov4 1 Inter-University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi-110067, India 2 Department of Physics and Astrophysics, University of Delhi, Delhi-110007, India 3 Joint Institute for Nuclear Research, Dubna 141980, Russia. 4 P.N. Lebedev Physical Institute, Russian Academy of Sciences, Leninsky Prosp. 53, Moscow 117924, Russia. Abstract : Ba0.97Ca0.03SO4:Eu nanoparticles have been exposed to 48 MeV Li, 75 MeV C and 90 MeV O ion beams. These ions have been accelerated using the 16 MV Van de-Graff type Electrostatic pelletron Accelerator at Inter University Accelerator Centre, New Delhi. The fluence range is 1x109-1x1013 ion/cm2. Thermoluminescence (TL) of the ion beams irradiated nanomaterials is studied. The TL glow curves along with the response curves of these nanomaterials have been investigated and compared with the corresponding microcrystalline samples. Moreover, the TL glow curve of the nanomaterials exposed to ã-rays has been included in the studies with the aim of reporting some of the comparative measurements. The glow curves of the ion beams irradiated nanomaterilas are similar in their shapes to that induced by ã-rays, with a shift in the peak positions to the higher temperature side by around 30 K. The TL intensity of the ion beams irradiated materials is found to decease, while going from low to high atomic number (Z) ions. Though the ions with higher Z have higher energies, the ion species is found to be the major factor for the generation of densely ionized zones, which have a direct relation to the amount of luminescent/ trapping centres (LS/TS), then the yield luminescence. Similar trend was also observed with the linearity of the TL response curves. The response curve of Li ions irradiated materials is linear in a very wide range i. e. 1x109-1x1013 ion/cm2. More or less, C ion has a similar range, while that of O ion; it is linear in a shorter-range i. e. 1x109-1x1012 ion/cm2, then above this it saturate. The TL response curve of the microcrystalline form of Ba0.97Ca0.03SO4:Eu is as reported in our earlier study is only linear in the range 1x109-1x1011 ion/cm2. This superiority for the nanomaterials makes Ba0.97Ca0.03SO4:Eu a suitable candidate for detecting the doses of these ions, especially C ions, which being recommended for radiotherapy applications.

Keywords : Ba0.97Ca0.03SO4:Eu; Nanocrystalline; Thermoluminescence; Li, C and O ion beams; HCP.

INTRODUCTION Thermoluminescence (TL) is a very powerful technique used for estimations of doses of high-energy ionizing radiations as the energy absorbed during irradiation and the TL intensity on stimulation (heating) is proportional to the radiation flux (doses). There are a number of commercially available thermoluminesent dosimeters (TLD) for this purpose (Fox, et.al., 1988; Noh et. al., 2001). However, efforts are still being made to improve the TL characteristics of these phosphors by preparing them using different methods. Currently, nanotechnology and nanomaterials has attracted several researchers from different fields, especially from the field of luminescence. The nanomaterials exhibit enhanced optical, electronic and structural properties (Gong X et al., 2000; Hadjipanayis and Siegel, 1993; Gleiter,1989). TLD materials, having nanostructure forms have also been prepared and their TL response to ionizing radiations has been studied (Salah etal., 2004; 2006; 2007; Sahare et.al., 2007; Lochab etal., 2007a; 2007b). The TL results of those

12

nanomaterials, however, are very interesting. They have revealed very imperative characteristics such as high sensitivity, saturation at very high doses and low fading (Salah et al., 2006; 2007; Sahare et.al., 2007; Lochab et al., 2007a; 2007b). This has encouraged us to study their TL response to ion beams irradiation. The TL saturation effect is the major problem in case of ion beams dosimetry. That is due to the tracks overlapping of these ions. However, with the use of very tiny materials such as TLD nanoparticles, this might overcome. Recently, the mixed sulfate, Ba0.97 Ca0.03 SO4 doped with rare earth impurity, Eu has been prepared in nanostructure form and studied for its TL characterization (Lochab etal., 2007a). Its TL response to ã-rays was studied and found to show a linear behavior in a very wide range (i. e 1 Gy to 20 kGy). In the present study, this nanomaterial, prepared by the same method, has been irradiated with 48 MeV Li, 75 MeV C and 90 MeV O ion beams at different fluences, ranging from 1x109 to 1x1013 ion/cm2. The TL response curves from the exposed materials were investigated in more details.


Ba0.97Ca0.03SO4:Eu Nanoparticles for ion beams dosimetry - S P Lochab et.al.

Ba 0.97Ca 0.03SO 4 nanocrystalline sample was prepared by the same method adopted earlier by Lochab et al. (Lochab et. al., 2007a ). Pellets of the prepared nanopowder with 0.6 mm thickness and 1 cm diameter were prepared taking 100 mg of the sample and 2 mg of Teflon powder, mixing together, putting in a die and applying 100 kgm/cm2 pressures each time by a hydraulic press. The pellets were again annealed at 973 K for 1 hour in Argon atmosphere and quenched rapidly to anneal out the deformations, if any, due to applied stress. Small pieces (approx. 5 mg) of the pellets were exposed to ã-rays from a Co60 source for various doses to see that the glow curve structure and the TL sensitivity remained the same as in the case of the powder samples. The samples in the form of pellets were irradiated at room temperature by Li, C and O ion beams at energies of 48, 75 and 90 MeV, respectively for different ion fluences in the range 1x109 – 1x1013 ions/cm2, using a 16 MV Tandem Van de-Graff type Electrostatic Pelletron Accelerator at the Inter-University Accelerator Center (IUAC), New Delhi, India. The full details of this set up are given by Kanjilal et. al (1993). The samples were mounted on a copper target ladder with a silver paste giving good thermal and electrical conductivity between them. This prevents sample heating during HCP irradiation. The ion beams were magnetically scanned on a 10 mm x 10 mm area on samples surfaces for a uniform irradiation and their spot sizes were 2.5 mm2. Three pellets were exposed to the same fluence, every time. For taking TL the irradiated surface was kept facing upwards towards the detector (PMT) of the TLD reader every time. TL glow curves were recorded using a Harshow TLD reader (Model 3500) fitted with a 931B photo multiplier tube (PMT). The heating rate was 5 Ks-1. Three glow curves were recorded for each sample to confirm uniform irradiation.

other hand, the glow curve of the microcrystalline material exposed to γ-rays has different structure. It has its own permanent peak at 460 K along with a small shoulder at 435 K. However, this change in the glow curves of the nanomaterials could be attributed to the change in the population of the trapping/luminescent centers (TC/LC), this in turn might be due to the extension in the host band gap. This already has been observed in our earlier work (Salah et. al., 2006; 2007; Sahare et.al., 2007; Lochab et. al., 2007a; 2007b). Reorganization of LC/TC is another effect, observed in several ion beams irradiated material, which results in shifting the peaks positions. With a wider band gap in case of nanomaterials, it’s expected to show a higher shift in the peak positions, which is around 30 K in the present nanomaterial. The TL intensity of the ion beams irradiated materials is found to decrease, while going from low to high atomic number (Z) ions. Though the ions with higher Z have higher energies, the ion species is found to be the major factor for the generation of densely ionized zones, which have a direct relation to the amount of luminescent/trapping centres (LS/TS), then the yield luminescence. With higher Z ions, the penetration through the material is less. This reflects in less TL intensity in case of C and O ion beams irradiated materials.


EXPERIMENTAL

7x10

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nanoparticle Li ion nanoparticle C ion nanoparticle O ion nanoparticle 10 Gy of γ-rays microparticle 10 Gy of γ-rays

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Temperature (K)

RESULTS AND DISCUSSION

TL glow curves Figure 1 shows typical TL glow curves of Ba0.97Ca0.03SO4 nanocrystalline phosphor, irradiated with 1x1011 ions/cm2 of 48 MeV Li3+, 75 MeV C and 150 MeV O ion beams (curves a, b and c, respectively). TL glow curves of the micro- and nanomaterial exposed to ã-rays of Co60 for a typical dose of 10 Gy, are also shown (curves d and e). As could be seen in this figure, the TL glow curves of Ba0.97Ca0.03SO4:Eu nanomaterials, exposed to these ions, are almost similar to that of ã-irradiated sample, with only shift in the peak positions towards higher temperature side by around 30 K, in case of ion beams irradiated samples. The peaks position appears at 464 K in case of ions beam irradiated samples, while is around 434 K for γ-rays irradiated sample. From the Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

Fig. 1: Typical TL glow curves of Ba0.97Ca0.03SO4:Eu nanomaterials exposed to 1x1011 ions/cm2 of 48 MeV Li, 75 MeV C and 90 MeV O ion beams. TL glow curve of 10 Gy γ-rays irradiated micro-and nanocrystalline samples are also shown. Figures 2, 3 and 4 show the recorded TL glow curves of Ba0.97Ca0.03SO4:Eu nanomaterials exposed to 48 MeV Li, 75 MeV C and 90 MeV O ion beams at different fluences in the range 1x109-1x1013 ions/cm2. There is a shift in the peak position, while going from 1x109 ions/ cm2 to 1x1013 ions/cm2 in case of Li ions irradiated materials. This was in association with the growth in the higher temperature peak at 529 K. More or less, C and O ions irradiated samples have showed similar trend. 13



This also can be ascribed to the reorganization/change of the populations of LC/TC.

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Figure 2: Ba0.97Ca0.03SO4:Eu Nanocrystalline sample exposed to different fluences of 48 MeV Li ion beam.

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TL response The TL response curves of Ba0.97Ca0.03SO4:Eu nanomaterials samples irradiated by 48 MeV Li, 75 MeV C and 90 MeV O ion beams, are shown in figure 5 (curves a, b and c respectively) in which the peak heights were used for measuring the TL intensities. The response curve of Li ions irradiated materials is linear in a very wide range i. e. 1x109-1x1013 ion/cm2. More or less, C ion has a similar range, while that of O ion; it is linear in a shorter-range i. e. 1x109-1x1012 ion/cm2, then above this it saturate. The TL response curve of the microcrystalline form of Ba0.97Ca0.03SO4:Eu is as reported in our earlier study is only linear in the range 1x1091x1011 ion/cm2 (Lochab et. al.2007c). This can be explained by track interaction model (Horowitz et. al. 1996). Accordingly, at low fluences the recombination of TC/LC occurs entirely within the tracks. Electrons escaping the tracks are intercepted by the nonradiative competitive centers in the intermediate region. The TL signal, therefore, is simply proportional to the number of ion beam tracks (the fluence). At higher fluences, the distance between the neighboring tracks decreases and the electrons escaping the track can reach the neighboring track resulting in the increased recombinations of the luminescent centers resulting in greater TL intensity. The sublinearity/saturation occurs due to more overlapping of the tracks at much higher fluences. The overlapping regions do not contribute to additional TL, since they do not result in additional charge carriers due to the full occupancy of the available TC and/or LC.

T e m p e ra ture (K )

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8

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8

9

a) b) c) d) e)

d

1x10 10 1x10 11 1x10 12 1x10 13 1x10

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b

5.0x10

a) Li ions b) C ions c) O ions TL intensity (a. u.)


Figure 3: Ba0.97Ca0.03SO4:Eu Nanocrystalline sample exposed to different fluences of 75 MeV C ion beam.

From the application point of few, Ba0.97Ca0.03SO4:Eu nanocrystalline sample is found to effective at recording irradiation from different ion beams, therefore, might be used as a dosimeter for medical applications.

7

a e

1E9

a b c 1E8

0.0 350

400

450

500

550

600

650

Temperature (K) 1E9

1E10

1E11

1E12

1E13

2

Fluence (ion/cm )

Fig 4: Ba0.97Ca0.03SO4:Eu Nanocrystalline sample exposed to different fluences of 90 MeV O ion beam Fig. 5: TL response curve of Ba0.97Ca0.03SO4:Eu to 48 MeV Li,75 MeV C and 90 MeV O ion beams. 14



CONCLUSIONS In this experiment, Irradiation of Ba0.97Ca0.03SO4:Eu nanoparticles by different ions having different energies, aimed to understand the TL behavior of this detector at a very wide range of fluences. The result is very promising. It shows that the size of the materials is playing a major role in the range of TL responses. The nanomaterials are found to show a wider range of linearity to heavy charged particles. This has been attributed to the extension of the band gap of the nanomaterials. From the application point of few Ba0.97Ca0.03SO4:Eu nanoparticles is a good candidate for ion beams dosimetry. ACKNOWLEDGEMENT We are thankful to the Department of Science and Technology (DST), New Delhi, India and the Russian Science Academy (RAS), Moscow, Russia for providing financial assistance under Project No A.2.53. Authors are also thankful to the Director of the Inter-University Accelerator Centre (IUAC), New Delhi for providing beam time. REFERENCES Fox, P. J. et.al. (1988), Spectral characterization of six phosphors used in Thermoluminescence dosimetry, J. Phys.D:Appl. Phys. 21,189-197 Gleiter, H.(1989) Nanocrystalline materials, Prog. Mater. Sci. 33 223-315 Gong X et al. (2000), effect of ã-rays irradiation on structure and luminescent properties of nanocrystalline MSO4:xEu3+ (M=Ca,Sr,Ba; x=0.001-0.005), J. Phys. Chem. Solids 61 115-121. Hadjipanayis G C and Siegel R W (1993), Nanophase Materials: Synthesis, Properties, Applications (NATO ASI Series), (Dordrecht: Kluwer) p 260


Horowitz, Y. S. et. al.(1996), The track interaction model for alpha particles induced thermoluminescence supralinearity: dependence of the supralinearity on the vector properties of the alpha particle radiation field. J. Phys. D: Appl. Phys. 29, 205-217 Kanjilal, D. et. al. (1993), Testing and operation of the 15 UD Pelletron at NSC, .Nucl. Instr. and Meth. A , 328, 97100. Lochab, S.P. et. al.(2007a), Thermoluminescence and Photoluminescence study of nanocrystalline Ba0.97Ca0.03SO4:Eu, J. Phys. D: Appl. Phys. 40, 1343– 1350 Lochab, S. P. et. al. (2007b) Nanocrystalline MgB4O7:Dy for High Dose Measurement of Gamma Radiation, Physica Status Solidi (a) (at press). Lochab, S.P. et. al.(2007c), Thermoluminescence of Ba0.97Ca0.03SO4:Eu irradiated with 48 MeV 7Li ion beam, Nucl. Instr. and Meth. B 254, 231–235 Noh, A.M. et. al.(2001), Investigation of some commercial TLD chips/discs as UV dosimeters, Radiat.Phys.Chem.61 497-499. Sahare, P.D, et.al. (2007), K3Na(SO4)2:Eu nanoparticles for high dose of ionizing radiation, J. Phys. D: Appl. Phys. 40, 759–764 Salah Numan, et. al. (2004), Luminescence characteristics of K 2Ca 2 (SO 4 )3 :Eu,Tb micro- and nanocrystalline phosphor,Radiation Effects & Defects in Solids, 159, 321–334 Salah Numan, et. al. (2006), TL and PL studies on CaSO4: Dy nanoparticles, Radiation Measurements, 41, 40–47 Salah Numan, et. al. (2007), Theromluminescence of nanocrystalline LiF:Mg,Cu,P, J. Luminescence, 124, 357-364

15

PREPARATION AND CHARACTERIZATION OF CaF2: MN PHOSPHOR FOR HIGH DOSE DOSIMETRY 1

Vijay Kumar and 2A.K. Bakshi Health Physics Unit, RRCAT, Indore 2 Radiological Physics and Advisory Division Bhabha Atomic Research Centre, Mumbai-400 085 Email: [email protected] 1

Abstract: CaF2: Mn phosphor is known for its high thermoluminescent sensitivity and dose linearity up to few kGy. In the present study CaF2 phosphor with different concentration of Mn dopant was prepared and its further characterization was carried out through different techniques. The phosphor was prepared through chemical root using CaCO3, HF acid and MnCl2 as raw materials following precipitation method . TL sensitivity of this phosphor was compared with other well established phosphors used for radiation dosimetry. It was found that the TL sensitivity is about twice that of LiF: Mg, Ti, TLD-100 phosphor. However the sensitivity is half to that of imported CaF2:Mn. X -ray diffraction studies revealed that the crystal prepared is CaF2. TL emission spectrum showed a peak at around 500 nm which confirms the characteristic emission of Mn. ESR of the phosphor was also done and was found that it is dominated by peaks corresponding to Mn which are in a group of six and the “g” value calculated to be 2.006. The dose linearity in the high dose range from 50 Gy to 3 kGy showed that the TL response is linear up to 3 kGy within an uncertainty of 10-15 %. In view of its linearity in the high dose range it will be very useful for the dosimetry of commercial irradiator used for food and grains irradiation and also for other application using industrial electron accelerator where routine dosimetry can be done using this system and can be established

INTRODUCTION A significant advantage of CaF2 is that it is available in naturally and can thus be obtained easily. However the disadvantage of using natural fluorite is its complex glow curve structure. The natural mineral contains many activators which appear to be predominantly the rare earths. Synthetic Mn doped calcium fluorite detectors were first introduced Ginther and Kirk (Ginther & Kirk,1957). The advantage of this synthesized phosphor over other phosphors used in dosimetry is its long range of dose linearity. In view of the increasing use of Industrial electron accelerator and gamma ray irradiator for extending the shelf life of grains and foods, it has become essential to have TL phosphor based routine dosemeter which can be used from few Gy to few kGy. Due to embargo on the import of the CaF2:Mn phosphor from USA based supplier, an effort was made to develop this phosphor indigenously in order to make it available easily for the above purpose. MATERIALS AND METHODS Preparation of CaF2: Mn Phosphor: For the preparation of CaF2: Mn, procedures suggested by Ginther & Kirk (Ginther et.al. 1954, Ginther & Kirk 1957) was followed. In the present process CaF2: Mn was prepared through chemical root using MnCl2 and CaCO3 in diluted HF acid solution. The precipitated 16

CaF2: Mn was given washing treatment with de-ionized water and dried. High temperature annealing at 650°C for two hour was given to the samples in silica crucible in the presence of ultra pure Argon gas to avoid the formation of MnO2 instead of the suggested annealing treatment at 1100 ºC in platinum crucible mentioned in the literature (Ginther et.al. 1954). The samples with different concentration of Mn e.g. 1,2,3,4 and 5 mol% were prepared using the above method. For the comparison purpose LiF: Mg, Ti, TLD-100 and imported (earlier) CaF2:Mn phosphors were also taken and given the required annealing treatment. Subsequent to annealing, for testing the TL sensitivity, about 100 mg of each phosphor sample was irradiated to 1Gy of Co-60 gamma rays using a 60Co Teletherapy machine. The output of the machine is calibrated and the accuracy is ±3 %. For the dose linearity study phosphor samples were irradiated from 50 Gy to 3 kGy using a Co-60 gamma chamber with the dose rate of 43 Gy/min at the point of irradiation. The readout of the phosphor samples were carried out on a TL research reader manufactured by M/s Nucleonix System Pvt. Ltd., Hyderabad. For the characterization of the crystal properties of the phosphor, X ray diffraction spectrum was recorded using a Philips X ray Diffractometer (Model PW1710) with monochromatized Cu-Kα radiation. Further for the detection of defect centers in the Radiation Protection and Environment, Vol. 30 , No. 1- 4, 2007

phosphor ESR spectrum for irradiated (500 Gy 60Co) was recorded on a Bruker ESP-300 model spectrometer at X band frequency (9-10 GHz). DPPH was used for calibration of the spectrometer.

RESULTS AND DISCUSSION

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Fig.1 shows the XRD spectrum of the prepared CaF2:Mn phosphor. The positions of the peaks having intensity of 100%, 88% and 30% at “d” values of 3.1569,1.9288 and 1.6488 ´, respectively, with identifications of the planes having Millar indices (111), (220) and (311) confirms (PCPDF-350816) that the sample prepared in the present work is CaF2 powder. Also in comparison with the reported XRD pattern in the literature (PCPDF-350816) it was confirmed that the system is face centered cubic.

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TL emission spectrum was taken on a home made emission spectrometer in the wavelength range 200-600 nm for phosphor sample having Mn concentration of 4 mol% after irradiating it to 5 kGy dose of 60Co.

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Fig.1 XRD spectrum of CaF2 : Mn Fig. 2 : shows the ESR spectrum of the irradiated phosphor samples. The spectrum has well known 6 peak structure of 55Mn hyperfine lines corresponds to I=5/2. The peaks were found of same intensity and the distance between the peaks was also same which confirms that the peaks correspond to Mn. Further with the ESR spectrum of DPPH for which “g” value is known, the “g” value of Mn species was determined which is gMn = 2.006. As per the study of Richardson (Richardson et al, 1971) the g value of Mn is 2.001±0.005 which matches with the g value calculated from the ESR spectrum, further confirms the presence of Mn2+ in the phosphor sample prepared in the present work.


Fig.3 shows the TL emission spectrum of the prepared CaF2: Mn (4%) phosphor. It can be seen that the wavelength of the peak is at about 495 nm. As per the study of Ginther R.J (Ginther 1954) the emission spectra of CaF2: Mn peaks at 490 nm for 0.05-1 mol% Mn and it shifts to higher wavelength (500-510 nm) with increasing the concentration of Mn. Prepared sample also shows the emission peak at 495 nm and matches well with the observation of Ginther study. Hence it is confirmed that the thermoluminescence is coming from the Mn activator.

17

Fig. 4 & 5 show the thermoluminescence glow curve of CaF2: Mn with different concentration of Mn prepared along with that of imported CaF2:Mn (fig.6). It can be seen that the glow peaks are generally appeared at around 260 °C for prepared phosphor. It has been reported by Ginther & Kirk that with increasing Mn content the glow peak temperature shifts to higher temperature side. However for 2% and 4% the glow curve is single peak structure with glow peak temperature at 260°C implies that Mn concentration present in the crystal is close to the calculated concentration. The slight difference in peak temperature could be due to difference in heating rate and preparation process.

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To test the TL sensitivity of the prepared phosphor in comparison with the well established phosphor like LiF: Mg, TI, TLD-100 and CaSO4:Dy (0.05mol%) and imported CaF2:Mn (3%) phosphor, samples of all the phosphors were given required annealing treatment and subsequent exposure of 1Gy of 60Co simultaneously. The TL readouts were taken 1 day after the exposure and the results on TL sensitivities are given in Table-1. It can be seen that the TL sensitivity of prepared CaF2: Mn (2%) is twice that of TLD-100. However it is half to that of imported CaF2: Mn and 10 times less sensitive than CaSO4:Dy. The cause of lower sensitivity of the prepared phosphor as compared to the imported one are; 1- The grain size of the prepared phosphor is of the order of few micron as it is prepared through chemical root. 2- The dull appearance of the phosphor after annealing, 3- possible the lower concentration of Mn inside the crystal with respect to the expected due to washing away of Mn with water and 4- difference in annealing treatment after preparation with respect to the treatment quoted in the literature (650 ºC -2h instead of 1100 ºC for few h). The maximum sensitivity amongst the prepared samples was that of CaF2: Mn (2%).

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Table 1 Comparison of TL sensitivity of prepared CaF2: Mn doped with different mol% of Mn with respect to other phosphors Phosphor

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100

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Fig 6 Glowcurved CaF2Mn (Imported)

18


Table-2 Dose Linearity of CaF2: Mn (2%) in the dose range 50 Gy to 3 kGy Delivered Dose TL /mg (Gy) (glow curve area) (± %1σ)

Relative output

50

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1.08

fruitful discussion on synthesis part of the phosphor, Dr. A.K. Tyagi, and Dr. Sadika Patwe of Chemistry Division for helping us in getting XRD data and Dr. V. Natarajan and Mr. Manoj Kumar Mahapatra of Radiochemistry Division for their valuable support in getting ESR data. REFERENCES Ginther R.J., (1954), Sensitized Luminescence of CaF2: (Ce+Mn) Journal of Electrochemical Society, 101, 248256. Ginther R J. and Kirk R. D, (1957), The Thermoluminescence of CaF 2 :Mn, Journal of Electrochemical Society, 104, 365-369. Richardson R.J., Sook Lee, and Menne T.J., (1971), Electron Spin Resonance of Mn2+ in CaF2, Physical Review B, 4, 3837-3845.

To test the dose linearity of the prepared CaF2:Mn phosphor, it was decided to use CaF2:Mn (2%) sample. The dose range used was 50 Gy to 3 kGy. The normalized TL output with respect to dose and relative response with respect to 50 Gy response are given in table- 2. It can be seen that the response is almost linear in the dose range used. However it was found that in the range of 500-1000 Gy there is little over response. The glow curve of CaF2f:Mn samples irradiated to linear doses ( 50 Gy-3 kGy) did not show any change in the glow curve structure, however there is a small shift (5 °C) in the glow peak temperature at 500 Gy. The peak temperature at 3 kGy comes back to that of 50 Gy. CONCLUSIONS In the present study CaF2: Mn phosphors of different concentration were prepared. The Phosphor was characterized and investigated for its thermoluminescent properties like glow curve structure and sensitivity with change in Mn concentration. The dose linearity study of the prepared phosphor was also carried out. It was found that phosphor sample wit 2% concentration of Mn is most sensitive and having glow curve with single glow peak structure at 260o C. The dose linearity study shows that the phosphor has almost linear dose response in the range of 50 Gy to 3 kGy within an over response of about 20%. This shows that the phosphor has got potential application for high dose dosimetry and can be used for the dosimetry of food / grain irradiation. Further study on the phosphor are in progress. ACKNOWLEDGEMENTS We would like to put on records our sincere thanks to Shri S Kannan, Head, RPAD and Dr D.N.Sharma for the encouragement in this work. Authors are also grateful to Shri M.P. Bellary of Chemical Engineering Division for


19

SAFETY PROVISIONS IN ELECTRON ACCELERATORS AT RRCAT, INDORE Dimple Verma, M.K.Nayak, Vipin Dev, G.Haridas, K.K. Thakkar * P.K. Sarkar and *D.N. Sharma Health Physics Unit Raja Ramanna Centre For Advanced Technology , Indore 452013 * Radiation Safety Systems Division, BARC, Mumbai 400085

Abstract : Safety Provisions in Electron Accelerators are designed in such a way that the doses to the working personnel and concentration of toxic and radioactive gases are well within the limits prescribed by AERB in normally accessible areas. This is achieved by putting appropriate radiation shielding, ventilation and radiation monitoring system. Personnel Protection System ensures that no one remains trapped inside the machine during its operation. This paper briefly describes the various Electron Accelerator machines in operation at RRCAT, followed by the radiation hazards prevalent in them. Finally the design and operational safety provisions at Electron Accelerators at RRCAT are described in detail

Key Words: Radiation Environment, Safety Provisions, Shielding, Zoning, Radiation Monitors, Administrative Controls.

CLASSIFICATION OF ELECTRON ACCELERATORS

e)

In order to understand hazards and safety provisions in electron accelerators it is desirable to have some classification based on the electron beam energy. Various Electron Accelerators at CAT are given in Table-1. RADIATION HAZARDS IN ELECTRON ACCELERATORS

Activity Induced in Cooling Water: Photoactivation products from O16, of concern, are O15 (T1/2 2 minutes) and C11 (T1/2 20.34 minutes), which are positron emitters. The induced activity in water is negligibly small.

Non-Ionizing Radiation (RF & Microwave):

Ionizing Radiation Hazard

Klystrons,magnetrons, RF cavities and connecting wave-guides are sources of RF or Microwave leakages, which are harmful to working personnel.

a)

Noxious Gases

b)

Prompt Radiation: Bremsstrahlung X-rays and photo-neutrons are the prompt radiations of major concern and are emitted only when the accelerator is ‘ON’. No prompt radiations exist when the machine is shut down. Residual Radiation: Electron beam line components (septums, vacuum chambers, collimators, etc.), which may get activated due to electron beam interaction gives rise to induced activity, which exists even after shutdown. It may pose hazard to maintenance personnel. However the induced activity levels in electron accelerators are low compared with ion accelerators.

c)

Radiation from Klystrons, Magnetrons & RF Cavities: They produce low energy X-rays (depending upon the operating voltage and peak current, normally in kilo-volts and hundreds of amperes respectively).

d)

Activity Induced in Air: Radioactive gases N13 (T1/2 10 minutes) and O15 (T1/2 2 minutes) are produced in air due to the interaction of radiation in air. The induced activity in air is negligibly small.

20

a)

Noxious gases like ozone and nitrogen compounds in low concentration are produced inside the containment area due to interaction of ionising radiation with air. Ozone production yield is proportional to the number of electrons coming out in air and the path length traversed in air. The effects of ozone are given in Table-2.

b)

SF6 gas is used in LINACS and DC Accelerator for electrical insulation. SF6 gas is six times heavier than air hence it settles on the ground and thus replaces air. The leakage of this gas results in the depletion of oxygen concentration in the area and persons may feel suffocated.


Safety Provisions in Electron Accelerators at RRCAT, Iindore - Dimple Verma et.al.

Table 1 : Electron Accelerators at CAT S.No. Type Of Machine

Name of the Facility

Location Indus-2 Building

Operating Parameters (Design)

1

Low Energy

X-Ray Set (Phillips)

2

Accelerators

X-Ray Powder Diffraction Set Laser A Block

60 kV, 30 mA, 1.8 kW

3

(< 200 KV)

Magnetron Test Facility

ADL Building

2998 MHz, 50 kV, 3 ms, 200 Hz, 120A

4

Medium Energy FEL Linac

Electron Gun

Indus-1 Building

40 kV, 500 mA, 2 ms, 1 Hz

Klystron

(FEL Block)

70 kV, 355 A, 10 ms, 10 Hz, 2856MHz, 10 MW, 10 MeV, 500 mA

IMA Building

12 MeV, 50 mA, 250 Hz, 2-4 ms 300 W

5

Accelerators (200 KV- 20 MeV)

LINAC Radiotherapy Machine (Microtron)

60 kV, 50 mA, 4 kW

Photon Energy: 6 MV, 12 MV Electron Energy: 6, 9, 12 MeV

6

DC Accelerator

IMA Building

750 kV, 20 mA, 20 kW

7

Linac for food irradiation

IMA Building

10 MeV, 10 kW, 450 Hz, 15 ms

Indus-1 SRS

Indus-1 Building

Microtron 20 MeV, 20 mA, 0.4 W

8

High Energy Accelerators (> 20 MeV)

9

Indus-2 SRS

Indus-2 Building

Table-2: Effects of Ozone at various concentrations S.No.

1

Observed Effects

Threshold of odour (normal person)

Concentration (ppm)

Booster

450 MeV, 30 mA

SRS

450 MeV, 100 mA 2.7 J

2.5 GeV, 300 mA, 425 J

3) No one should remain inside the accelerator hall when it is ‘ON’. These objectives are achieved by putting appropriate safety system components in the accelerators.

0.01 – 0.05

SAFETY SYSTEM COMPONENTS

2

Irritates the nose and throat for most of the persons

0.1

3

Headache, respiratory irritation and possible edema

1 – 10

4

Lethal to small animals within 2 hours

15 – 20

5

Lethal in few minutes

> 1700

OBEJCTIVE OF SAFETY SYSTEM IN ELECTRON ACCELERATORS 1) Radiation dose levels in working areas should be within the limits prescribed by AERB. In normally accessible areas, it should be less than 0.1mRem/h. 2) Concentration of toxic and radioactive gases in working areas should be within the limits. Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

1) Shielding: Shielding reduces the dose rates to less than 0.1 mRem/h at normally occupied areas. Shielding material used in Electron Accelerators at CAT are concrete, stone chips, earth soil, lead and mild steel. 2 ) Ventilation: To reduce concentration of noxious and radioactive gases and maintain required temperature, enough ventilation is provided in accelerator halls. DC Accelerators and LINACS require much higher ventilation rate as electron beam comes out in air in these accelerators leading to production of ozone and noxious gases. 3) Zoning: Based on actual radiation survey, the accelerator facility is divided into three zones namely Zone 1- Normally Occupied Areas (< 0.1 mRem/h), Zone 2-Restricted Entry Areas (0.1 – 1.0 mRem/h) and Zone 3-Prohibited Areas (> 10 mRem/h). 4) Search and Scram Switches: Search switches, installed in prohibited areas ensure no occupancy during operation. Before the accelerator operation, 21


Table 3 Safety Provisions in Low Energy Accelerators S. N o.

Safety Provisions

Facility Name X-Ray Powder Diffraction X-Ray Set (Phillips) 60 kV Set Covered with glass partition, lead Shielded X-Ray tube enclosed shielding at beam exit in a lead impregnated glass enclosure 1 Shielding 2 Access Control Cordoned Area 3 Flashing 1 No. Nil Lamps 4 Other Safety Restricted entry, Visual signs, Restricted Entry, Provisions Personnel Monitoring Monitoring

Magnetron Test Facility 50 kV Lead shielding, shielding for Microwave Leakage Cordoned Area 1 No.

Personnel Restricted Entry, Warning signs, grounding, Caution Boards

Abbreviations : TL3-Transport Line 3 Tunnel, ACPH- Air Changes Per Hour, E-Exit door to central well, TL3R- TL3 Tunnel Roof, ARM-Area Radiation Monitor, BLM-Beam Loss Monitor, ERM-Environmental Radiation Monitor, CWCentral Well, EH- Experimental Hall, S&S- Search And Scram, BH- Booster Hall, SR- Storage Ring, D- Entry door to ring, G- Gates to four partitions of Indus-2 ring, MCR- Microtron Control Room, OC- Ordinary Concrete. Table 4 : Safety Provisions in Medium Energy Accelerators Facility Name S.

FEL Linac 15 MeV

No. Safety Provisions

Radiotherapy Machine -12 MeV

Linac – 10 MeV Food Processing

DC Accelerator (750 keV)

Lateral: Outer and Inner Wall of 0.6 m Concrete; 1 m Concrete Wall, 0.9 m Concrete Roof

1

Area between Outer & Inner wall of 3.3 m – 4.0 m Stone Chips and Earth Filling

Vertical: Roof :- 1 m Ordinary Concrete + 2.25 m Earth

Vertical: Irradiator Room: wall 0.6 m, roof 1.0 m O.C Accelerator Room:Side wall 0.6 m O.C + 2.15 m stone

Shielding

2

Ventilation

1 ACPH

5 ACPH

10 ACPH

10 ACPH

3

S&S Switches

3 Nos*

3 Nos*

5 Nos*

4 Nos*

4

Access Control

1 Glass Door

1 Glass Door

1 Glass Door

1 Glass Door

5

Door Interlocks

1 No.

1 No.

1 No.

1 No.

6

Flashing Lamps

2 Nos.

2 Nos.

2 Nos.

2 Nos.

7 8 9 10

Sirens ARM BLM CC TV Cameras

1 No. 2 Nos. 1 No.

1 No. 3 Nos. 1 No. 1 No.

1 No. 3 Nos. 1 No. 2 Nos.

1 No. 3 Nos. 1 No. 2 Nos.

11

Ozone Monitor

-

-

1 No.

1 No.

Features specific to DC Accelerator: It has a portable SF6 leak detector, oxygen deficiency meter, Self Containing Breathing Apparatus and an earthing stick, to ground any residual charge left. Also Irradiator Room is maintained at negative pressure to prevent ozone leakage to accessible areas and entry to the room is permitted 10 minutes after shutdown to bring down ozone levels below 0.1ppm.

22



Table 5 : Safety Provisions in High Energy Accelerators S. Facility Name Indus-1 SRS (450 MeV) Indus-2 SRS (2.5 GeV) No Safety Provisions 1 Shielding Microtron : (Wall: 1 m, Roof: 0.9m) SR Tunnel & TL3 Tunnel: Concrete Booster: (Wall: 1-2 m, Roof: 0.3 m) Outer Wall : 1.5 m Concrete C t 8 cm MS + 8 cm Pb Hybrid Inner Wall: 0.6 m Concrete SR Area: Shielding Indus-2 Side 1 m Concrete Wall Roof: 0.6 m Concrete 2 5

Ventilation Door Interlocks

BLM Photons

1 ACPH 7 Nos. ( Microtron 1No., BH 2 Nos. ,SR Area 5 Nos.) 6 Nos. (MCR 1 No., BH 2 Nos., EH 5 Nos.) 3 Nos. ( BH 1 No., EH 1 No., Buzzer 1 No.) 10 Nos. (MCR 1 No., BH 2 Nos., EH 2 Nos., SR Area 4 Nos., Corridor 1 No.) 2 Nos.

6

Flashing Lamps

7

Sirens

8

ARM Photons

9

20 Nos. in Storage Ring and TL3 Tunnel

10

ERM Photons

Nil

5 Nos.

11

Neutron Flux Rate Nil Meters

10 Nos. in Storage Ring and TL3 Tunnel

12

Neutron Monitor

10 Nos. in Experiment Hall

13

Rem Meter based Radiation survey 2 Nos. Meters (Photons)

Nil

1 ACPH 8 Nos. (D1, D2, E1, E2, G1, G2, G3, G4) TL3- 2 Nos., SR- 8 Nos., CW- 3 Nos., TL3R-2Nos., EH7 Nos 1 No. 25 Nos. in Experiment Hall

4 Nos.

14

Rem Survey 1 No. Meters (Neutron)

2 Nos.

15

CC TV Cameras

TL3- 2 Nos., SR- 8 Nos., CW- 1 No., TL3R- 2 Nos., EH6 Nos.

16

Prohibited During Injection

4 Nos.

Areas Microtron Block Beam Booster Hall

Indus-2 Storage Ring Tunnel, TL 3 Tunnel, Open Area to Sky (Central Well), TL 3 Tunnel Roof

Storage Ring Area

* Search is carried out before machine is started. these are activated in a predetermined sequence by operation crew to ensure no occupancy. Scram switches allow a person to put off the machine if he is trapped in Zone-3 area during its operation.

7) Door Inter-locks: Doors leading to zone-3 are interlocked with the machine operation. Hence if some one opens the door during machine operation, it will put off the beam.

5) Radiation Monitors: On-line area monitors for photons and neutrons are positioned at selective locations in Zone-2 and Zone-3 areas to monitor radiation doses. Their readings are available remotely in control room by 4-20 mA signal.

8) Doors, Access Control Gates and Administrative Control: These are necessary to demarcate different zones and restrict entry of persons.

6) Audio-Visual Warning Systems: Siren, Flashing Lamps, PA System, and Buzzer etc. are put at various locations to inform the working personnel about status of the machine and giving instructions.


9) Personnel dosimetry devices: Personnel working in radiation areas are provided with dosimeters like Thermo Luminescent Dosimeter (TLD), Direct Reading Dosimeter (DRD) and Fast Neutron Monitoring foils (CR-39 foils) for recording their doses. 10) Radiation surveys: The radiation surveys are routinely done and they ensure that dose rates are 23


within prescribed AERB limits in working areas. 11) Training in Radiation Protection to working personnel: This training is given routinely to ensure minimum knowledge of hazards and safety precautions to be taken for work in accelerators. SAFETY PROVISIONS IN ELECTRON ACCELERATORS The Electron Accelerators at CAT are categorized into Low, Medium and High Energy Accelerators based on the electron beam energy. Low Energy Accelerators are used for generating RF and MW power, as injectors in accelerators, research purpose, etc. Medium Energy Accelerators are generally used for irradiation experiments and High Energy Accelerators are used for research purpose. Tables 3, 4 and 5 describe in detail the safety provisions employed in Low Energy Accelerators, Medium Energy Accelerators and High Energy Accelerators. COMMISIONING EXPERIENCE OF INDUS-1 SRS a) Shielding evolution of Indus-1 Storage Ring: During initial commissioning of Storage Ring, the ring had a shielding of 0.5 m concrete with 3 m height. Entry to Experiment Hall during beam injection to SR was prohibited. b) Radiation fields in accessible areas were found to be high during beam injection. Hence total shielding around Indus-1 Storage Ring was made of 1 m concrete. c) After commissioning of Indus-1 Storage Ring with 100 mA design current in year 1999, 1 m concrete shielding was totally removed and replaced by hybrid shielding comprising of 8 cm lead (Pb) + 8 cm mild steel (MS) with 2 m height. It has provision of one central hole for insertion of front end of Synchrotron Radiation beam line. d) During commissioning of Synchrotron Radiation beam lines, streaming of radiation was observed from area near central hole of shield. Hence lead wool and MS shot bags were filled in the gaps. e) High radiation fields were observed in IUC beam line (BM1 BL2) and Photo Physics beam line (BM3 BL2). Hence shielding augmentation was done inside

24

shielded areas so that these beam lines were also made accessible during beam injection. Presently HRVUV beam line (BM1 BL1) is inaccessible during beam injection. CONCLUSIONS The Safety Provisions in Electron Accelerators are planned in such a way that ionizing radiations are stopped by proper and adequate shielding. Thus the radiation field in the normally accessible areas is within the permissible limits. Adequate ventilation reduces the concentration of radioactive gases and noxious gases like oxides of nitrogen and ozone to acceptable limits. The non-ionizing radiations are taken care by fencing the area, restricted entry, grounding, shielding the source, tightening the joints and flanges and by using the personnel protective equipments. By applying the proper administrative controls like access control gates, zoning, safety interlocks, search and scram switches, etc. it is ensured that no working personnel is exposed to any high radiation pulsed field comprising of high energy photons. Also it is ensured that no one remains trapped inside the accelerator hall when the accelerator is ‘ON’. With all these safety provisions in place, a good level of safety has been achieved in Electron Accelerators at CAT. The radiation doses received by the CAT Employees in accelerators and other facilities (< 100 mRem /year) are much below the permissible levels prescribed by AERB (2 Rem/year averaged over a period of five years) and also the radiation fields in normally accessible areas are brought down to permissible limits. ACKNOWLEDGEMENT We are thankful to Shri Gurnam Singh, In charge, Indus Accelerator Complex, Shri H. C. Soni, Head, IMA Section, Shri S. C. Bapna, Head, DC Accelerator and Shri P. R. Hannurkar, Head, RF & MW Section for kind support and co-operation. We are very much thankful to Dr.V.C.Sahni, Director, RRCAT and Shri HS Kushwaha, Director Health, Safety and environment Group, BARC for encouragement and support for this work. REFERENCES a) Safety Provisions in Indus-1, Haridas G., etal., 13th National Symposium on Radiation Physics, ISRP, Dec. 1999. b) Instrumentation for Radiation Safety Systems of Indus-1, Marathe R.G., etal., Bulletin of Radiation Protection, 2000.


BEAM LOSS DETECTION TECHNIQUE USING RADIATION DETECTORS IN ELECTRON ACCELERATORS AT RRCAT, INDORE M. K. Nayak, G. Haridas, Vijay Kumar, P. K. Sahani, K.K. Thakkar * P.K. Sarkar and *D.N .Sharma Health Physics Unit, Raja Ramanna Centre for Advanced Technology, PO CAT, Indore (MP) 452013 *Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai Abstract : Electron beam loss in an electron accelerator leads to production of intense bremmstrahlung photons. Some of the photons interact with nuclei of surrounding material to produce residual induced activity by photonuclear reactions. These facts were effectively used by Health Physics Unit to determine the locations and magnitude of electron beam loss in 20 MeV Microtron and 450 MeV Booster Synchrotron in Indus-1 Complex at RRCAT, Indore. Three methods were employed for beam loss detection viz. (i)on line radiation monitoring using area monitors (ii) integrated dose measurement using pocket dosimeters and chemical dosimeters (off line) and (iii) induced activity measurement using radiation survey meter (off line). These techniques were occasionally used by Beam Physicists to solve the problems related with beam loss detection and efficient operation of the accelerators when needed. The experimental study has proved the effectiveness of conventional radiation detectors in beam loss detection in electron accelerators. There is plan to use on line signal from PIN Diodes as beam loss detectors developed by Electronics Division, BARC From the results of the beam loss detection experiments, we were able to conclude that significant beam loss takes place in the direction of circulating electron beam with in the RF cavity in Injector Microtron. At booster synchrotron, high beam loss was observed near the injection septum during normal operation. Beam loss location, a bellow in Transfer Line of 12 MeV Radiotherapy Machine was determined using this technique. Replacing a bellow with bigger inner aperture improved exit current at treatment head from 10 mA to 20 mA.

INTRODUCTION Each accelerator, be it electron, proton or ion accelerator can not afford to loose particles from the beam as it leads to following serious problems. a)

Efforts put in to get the desired accelerated beam energy and current gets wasted.

b)

Hence over all machine performance reduces increasing the cost of operating the accelerator.

c)

d)

e)

f)

In view of above it is desirable that each accelerator should have a good Beam Loss Detection Program with sophisticated electronics and on line signals available in Control Room to machine operators. This paper describes various beam loss detection techniques developed using various types of radiation detectors and actually used effectively in Electron Accelerators at CAT, Indore. BEAM DIAGNOSTIC SYSTEM OF INDUS ACCELERATOR COMPLEX

Beam life time reduces due to chronic beam losses thus severely affecting user time to perform or complete the experiments.

(A) Beam Current Measuring Devices 1)

Beam loss leads to high radiation field thus increasing radiation hazard. Hence more money is spent in putting shield, ventilation and other precautions or countermeasures.

Fast Current Transformer (FCT) - Measurement of beam current pulse

2)

DC Current Transformer (DCCT) - Measurement of average beam current

3)

Wall Current Monitor - Observation of bunch beam current

4)

2-Pi Monitor - Observation of bunch beam current

5)

Internal Probe – Copper target for measurement of current *

High beam loss leads to more generation of induced or residual activity hence work in shutdown becomes difficult giving more exposure to working personnel. This is more true for Proton and Ion Accelerators rather than Electron Accelerators. Due to higher beam losses, radiation damage to accelerator components increases leading to their malfunction, maintenance, replacement etc.


(B) Beam Position and Beam Profile Measurement Devices 25

Beam loss detection technique using ... - M. K. Nayak, et.al

6)

Beam Profile Monitor (BPM) - Visual observation of beam profile and position *

7)

Beam Position Indicator (BPI) - Measurement of beam position

8)

Hole Monitor - Visual observation of the injected beam at the septum mouth

9)

Scraper - Measurement of beam intensity profile along horizontal & vertical axes

10) RFKO Beam Shaker - Tune measurement 11) Stripline- Tune measurement & Beam signal observation

(C) Synchrotron Light Monitors

Table 1 Radiation Levels around Microtron Body mR/hour/mA* Induced FBX DRD Monitor Activity A 24 1020 Nil B 401 3900 5000 Nil B` mR/h ** C 0 180 Nil D 76 48 4.7 Nil D` mR/h E 3 180 Nil F 14 0 Nil Tra nsport Line –1 Ra dia tion Monitor Beam Viewer Up 4103 Beam Viewer down 183

12) Synchrotron Light Monitor - Observation of synchrotron light

FBX – Chemical dosimeter, DRD-Pocket dosimeter (Ion Chamber type)

13) Diagnostics Beam Line - Beam parameter measurement with synchrotron light

* mR/hour/mA of extracted current from Microtron

14) Sighting Beam Line- Observation of synchrotron light during machine commissioning

**Activity on RF Cavity due to Copper 64 (Positron emitter)

* Beam Interceptive Devices

A F

It can be seen that none of above devices are Beam Loss Detection Devices. They indirectly give indication about beam loss but can not pin point exact location of beam loss in entire complex.

B

B` E

Experiments Using Beam Loss Detection Techniques 1) Microtron Body (20 MeV Accelerator) It is injector system for Booster Synchrotron. It accelerates electron up to 20 MeV energy through 2856 MHz RF frequency. It provides an electron beam with a current of 20 mA in 1 µsec long pulse duration at a repetition rate of 1 Hz. This beam is transported to the synchrotron through Transport Line-1 (about 14 meter length). When electron beam in Microtron revolves spirally in the orbit it gets acceleration (or energy) in each orbit. We have done mapping around the outer side of the Microtron Body, shown in Figure 1, using Direct Reading Dosimeter (ion chamber based). The aim of this experiment was to pin point the maximum beam loss location around Microtron body. The results are tabulated in Table-1. It can be concluded from the Table-1 that maximum beam loss takes place at Location C on Microtron body. After every circulation, electron beam passes through RF cavity. The huge loss taking place at RF cavity is due to electrons which are out of phase in the Cavity and get lost in direction tangential to their 26

D

C D`

Extraction Channel

Fig.1 Layout of the Microtron body orbit. The radiation levels arising due to this beam loss was consistently observed by three types of radiation detectors. This result can be confirmed from the induced activity levels at point B’ from Table 1. Since electrons are having up to 20 MeV energy, when interact with RF cavity produces bremsstrahlung having energies up to the electron energy. These radiations further interact with the material leading to photo-neutron production, leading to residual activity on the material.

Booster Synchrotron Injection Septum (450 MeV Accelerator) The beam from the microtron is transported to Radiation Protection and Environment, Vol. 30 , No. 1- 4, 2007


the synchrotron through Transfer line-1 (TL-1), which has a length of about 14 m. The circumference of the synchrotron is 28.44m. The electrons are injected into the synchrotron by adopting a multi turn injection scheme using 1 µsec long electron beam pulse from the microtron at a repetition rate of 1Hz. A compensated bump producing maximum amplitude near the injection septum is produced using three injection kickers. After injecting the beam, the electrons are accelerated to 450 MeV in nearly 200 msec following a linear ramp using the RF cavity operating operating at 31.619 MHz. We used radiation dosimeters (DRDs) to locate the beam loss point. Maximum beam loss was found to take place at septum. Complete mapping around the septum was done and results are shown in Figure 2. These results are very much useful in optimizing the efficiency of the injection septum. The results indicate that maximum beam loss takes place towards the inner side of the injection septum in line with Transport Line 1. The induced activity levels around injection septum (Figure 2) also confirmed this . The maximum induced activity measured at inner side of the septum is in the range of mR/hr. Other locations showed background level.

point. These experiments were performed at different conditions of operation. Results showed that maximum beam loss was taking place at point C & D on inner side of septum. Refer Table 2. This helped us to rectify the problem. Induced activity measurement were also carried out around it and shown in figure 3. It also proved that location C is having maximum beam loss.

Radiotherapy machine (Microtron based) IMA building (12 MeV Accelerator) It was observed that extracted beam from Microtron was 20mA and the beam current measured at the exit of gantry after transfer line was 10 mA. Hence it was suspected that severe beam loss was taking place somewhere in transfer line. In order to identify the exact location of beam loss, dose mapping of transfer line was done using Direct Reading Dosimeter (Ion Chamber Type). By dose mapping, it was established that major beam loss was taking place at a bellow. The bellow was removed and replaced by a bigger bellow to increase the extracted beam current. Dose mapping in Radiotherapy Machine (using Direct Reading Dosimeter)

2.7 R/hr

Outer Side of Booster Injection Septum 2.7 R/hr

Beam path 12 R/hr

Inner Side of Booster Injection Septum

Location

Dose rate

Operating parameters

On walls

5 – 476 mR/hr

10 MeV, 25 Hz

On Transfer line General On bellow

~ 0.1-1 R/hour 20 mA

0 R/hr

28 R/hr 123 R/hr

4727 R/hr (FBX)

Induced Actvity (Inner side) few mR/h due to Mn 54, Co 57 Current in TL-1 - 9 mA, Booster Current -1mA

Fig. 2 Dose mapping around Booster Injection Septum using DRDs. Dose mapping of Booster Synchrotron

Extraction Septum (450 MeV Accelerator) The 450 MeV accelerated beam from Booster is extracted by deflecting beam by a fast kicker magnet and transported through a Transport Line-2 to the storage ring. On one occasion, the beam extraction from booster extraction septum was not taking place hence a series of experiment was conducted with DRDs and area monitors around the septum, shown in figure 3. The aim of this experiment was to locate the exact beam loss Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

315 – 1190 R/hr

Table-2 Field measurements in R/hr/mA around Booster Extraction Septum (DRDs)

S .No. 1 2 3 4 5 6 7 8 9 10

Lo ca tio n Ex p -1 Ex p-2 Ex p-3 A 0 0 1 B 1 0.6 0 C 165 195 75 D 120 120 60 E 75 75 45 F 0 0 15 G 0 0 H 0 0 I 3 4 J 30 15 30

Exp-1 Extraction kicker OFF, Septum OFF: Exp-2 Extraction kicker ON, Septum OFF Exp-3 Extraction kicker ON, Septum ON. On line Radiation monitor at point E-> 10 R/h 27


Extracted Beam

Outer side of the

ring

A

B

G

H

F

00.02

Gate valve

Booster Extraction Septum

Inner side of the

ring

3

1 0.07

C

D

I

J E

Circulating beam

Figure 3 Schematic Layout of the Booster Extraction Septum CONCLUSION

4)

Radiation dosimeters, on line monitors and induced activity measurements have proved to be a reliable beam loss detection technique. Following conclusions can be drawn.

Using these detection techniques, we can improve the performance of the machine operation not only in electron accelerators but also in other type of accelerators.

5)

We have procured PIN diodes from Electronics Division, BARC as part of continuation of this work in future for beam loss detection. PIN diodes are also being used for beam loss detection in Elettra SRS, Italy and LHC Project, CERN.

1)

Maximum beam loss in Microtron occurs at the point B of microtron body which is tangential to direction of beam orbit in RF Cavity.

2)

Maximum beam loss in booster synchrotron occurs at the inner side of the Booster injection septum in line of Transport Line -1.

3)

28

The result of experiments conducted at Booster extraction septum revealed that the beam loss location can be identified with the help of dose mapping.

ACKNOWLEDGEMENT Thanks are to Shri Gurnam Singh, Head, IOAPDD and members of IOAPDD, RRCAT for support during the course of work. Special thanks are due to Dr.V.C.Sahni, Director, RRCAT & Shri H.S.Kushwaha, Director, HSE Group, BARC for encouragement.


FAST NEUTRON SPECTROMETRY FROM 20 MeV 1H+NAT.CU USING ACTIVATION FOILS S.P. Tripathy1,*, C. Sunil1, M. Nandy2, A.A. Shanbhag1, P.S. Sharma1, D.S. Joshi1, P.K. Sarkar1,3, D.N. Sharma1 1 Radiation Safety Systems Division, BARC, Mumbai, 400085, India 2 Saha Institute of Nuclear Physics, 1/AF Bidhan Nagar, Kolkata 700064, India 3 Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Kolkata 700064, India

ABSTRACT : We report experimental measurements using several activation foils as threshold detectors to obtain the spectrum of fast neutrons originating from the interaction of 20 MeV protons on Cu target, at the 14 UD Pelletron facility of TIFR. A total of 13 reactions were explored. The resulting counts were then corrected for decay, detector efficiency, counting geometry, etc. to calculate the actual activity (Bq) at the end of irradiation. With induced activities as input for an unfolding code (which uses iterative method), we have generated the neutron spectrum. The cross-section values from the latest report of ENDF/B-VII.0 were used to construct the response matrix for unfolding purpose. The final spectrum is compared with that measured previously for the same system using NE213 liquid scintillator and by unfolding the neutron induced proton pulse height distribution. The shapes of both the spectra fairly agree with each other. The slight discrepancy observed at some energy points is due to inherent uncertainties associated in the experiments and unfolding procedures.

Keywords: neutron spectrometry, activation foil, threshold detector, unfolding

INTRODUCTION With accelerators entering almost all spheres of our everyday life, a high premium is put on neutron dosimetry, because neutrons dominate the prompt radiation field in these facilities. The energy dependence of an ideal neutron dosimeter is expected to match the energy dependence of radiation protection quantities. However, such a dosimeter does not exist in practice. Therefore, a detailed knowledge of both the neutron energy spectra and the dependence of detector responses on neutron energy are needed in order to achieve an appropriate level of protection against irradiation with neutrons. One of the most widely exploited methods of neutron spectrometry is the use of activation foils of different elements as threshold detectors, where the neutron induced nuclear reactions such as (n,p), (n,g), (n,a), (n,f), etc. leave the reaction products (many of which are gamma emitters) in an excited state. Cross-section of each of these reactions has a characteristic energy dependence. Hence the total activity of a particular type gives a measure of the neutron fluence in a given energy range (Byerly, 1960). Since the neutron energy spreads over a wide range, no single detector can cover the entire domain. The spectrum can be measured with a set of several such activation foils (a multi-detector system) (Roulti and Sandberg, 1985) with distinct responses to different energy windows. The irradiated foils or wires are counted separately after the irradiation, usually with ∗

gamma spectrometric methods (Knoll, 2000). Highresolution HPGe detector could be used for the purpose, and the yield of several nuclides from even the same activation foil (Roulti and Sandberg, 1985) is measured. This method is often used for neutron flux distributions in reactors, accelerators and in space (at high altitudes) due to the reaction specificity, simplicity of measurement, insensitivity to other radiation components and the relative easiness in constructing the response matrix for the unfolding purposes (Hurst, 1956; Holt, 1964; Mijnheert, 1981; Mukherjee, 2004; Gualdrini, 2004; Zanini, 2005). The measured yield of a radionuclide is the convolution of the neutron energy distribution with the response function of the spectrometer summed over the interaction energy range. The spectral information needs to be unfolded from these detector responses by using any suitable computational codes such as BUNKI (Lowry, 1984), MAXED (Reginatto, 1998), etc., based on several methods like least square, iterative, monte carlo, neural network, etc. The cross section values of each reaction are processed to construct the response matrix, which summarises the detector response as a function of incident neutron energy and provides basis for the unfolding process (Brooks, 2002). In this work, the activities (Bq) induced by 13 distinct reactions in the threshold detectors, after irradiation to fast neutrons, were obtained applying necessary corrections to the measured counts from HPGe gamma-spectrometer. These values of activity (Bq) along with a 51-group response function were used as the input for an iterative computational technique similar

[email protected] , [email protected]


29

Fast neutron spectrometry from 20 MeV1h+nat.Cu using activation foils - S.P.. Tripathy et.al.

to the well-known code BUNKI to obtain a well-resolved spectrum of fast neutrons originating from the interaction of 20 MeV protons on Cu target, at the 14 UD BARCTIFR Pelletron facility, Mumbai. MATERIALS AND METHODS Thin foils of Mg, Al, Ti, Fe, Co, Ni, Cu, Zn, In of 1 inch diameter, and 1 mm thickness were chosen on the basis of threshold energy for reaction, sensitivity, abundance, half-lives and gamma energies of the product nuclides, etc. The dimension of the foils is optimised properly to obtain sufficient activities to be counted without much attenuation or perturbation of neutron flux. Details of the detectors used in this study, the reactions, the induced activities and other parameters are listed in Table 1. The fast neutrons were generated from 20 MeV protons hitting natCu target, at BARC-TIFR Pelletron, Mumbai. With an average neutron fluence rate of 3.7 x 106 cm-2 s-1, the irradiation time (42 h) was sufficient to induce measurable activities in the foils which were placed at 90o angle with respect to the incident proton beam. The induced activities of the product radionuclides were measured with a low-background HPGe spectrometer coupled with a data station. Measured count rates were corrected for the usual experimental errors, dead time, gamma-ray branching ratio, detector efficiency, etc. to obtain the actual activities at the end of the irradiation (see Table 1). At any given angle, the measured activitiy of a particular type is related to the neutron spectrum by:

∫

Aj= Nσ j (E ) Φj dE

(1)

where Aj is activity of the jth detector, N is the total number of atoms in the sample, σj(E) is the reaction cross-section of the jth detector (in cm2/atom) as a function of E, Φj(E) is the energy dependent neutron flux density (n cm-2 s-1). This represents a Fredholm’s integral equation of first kind without any unique solution, because a finite number of discrete measurements cannot define a continuous function. In the present work, the discrete form of Fredholm’s equation of first kind (eq.1) was used:

inputs into an unfolding method. The response matrix was formed by multiplying the total number of atoms (in the sample) with cross section data (taken from the cross section library of the ENDF-B/VII.0) and then arranging them into 51 energy groups over the entire energy range on an evenly spaced logarithmic scale. The number of detectors that could reasonably be used in this type of measurements is generally much lower than the number of the energy points, thereby creating an underdetermined system of equations. The number of energy intervals was chosen so as to produce response up to maximum expected energy since in this type of underdetermined problem a larger energy interval may give spurious results as it tries to conserve the total number of neutrons. In the case of underdetermined problems, the unfolding programs generally make use of an ‘a priori’ information that constitutes the best approximation to the output to be determined. The accuracy of the unfolding results may strongly be dependent on how close is the input solution to the true distribution on one hand and the efficiency of the adjustment procedure on the other hand. In the present work, ‘a priori’ information was supplied in the form of a Maxwellian and 1/E initial trial spectrum from which the solution spectrum is determined. The activity values along with the associated errors and the response matrix were then processed through an iterative unfolding method (similar to BUNKI) based on BON algorithm to determine spectral distribution of neutrons. The algorithm is described below. In the matrix form, equation (2) can be written as:

A=R.Φ

…(3)

where A is the vector of activities in all the detectors, Φ is the unknown neutron spectrum, and R is the response matrix. For iterative solution, the equation 3 is multiplied by the transposed matrix RT to yield

RT.A= RT.R.Φ or, Y=X.Φ , with Y=RT.A, X=RT.R A physically relevant solution can be reached with the iteration n

Φj

n

Aj=

∑ i =1

σj(Ei)Φj (Ei)

(2)

where i =1,…,n: the energy bins; j = 1,…,m: the detectors (reactions) The values of measured activities along with a response function of the multi-detectors were fed as 30

( k+1)

=(YjΦ j ) ( k)

∑ j =1

(XjΦ j (k)), where k is the

iteration step. The solution is not very sensitive to the initial guess Φ(0), which favours the situations where ‘a priori’ information is not enough. In this work, a total of 2000 iterative steps were to


Fast neutron spectrometry from 20 MeV 1h+nat.Cu using activation foils - S.P. Tripathy et.al.

Table 1. Details of activation foils, the induced reactions along with other important parameters needed for activity measurement at the TIFR pelletron accelerator. Foil

W t (g)

Mg

1.4131

Al

4.4994

27

Ti

7.1755

46

Fe

7.6523

Reaction 24

Mg(n,p)24Na

1.50E+01

2.81

9.99E+02

1.32E+02

1.7

0.83

99.98

3.52

7.30E+00

0.85

98.9

3.48

1.29

43.21

2.51

0.81

99.45

3.59

1.48

23.59

2.25

0.51

35.7

5

1.12

50.6

2.82

0.34

45.82

6.62

59

1.07E+03

58

Ni(n,p) Co

1.70E+03

Cu(n,p)65Ni

2.52E+00

Zn(n,p)64Cu

1.27E+01

Co(n,p) Fe

65

Zn(n,2n) Zn

115

99.99

3.11

2.58E+00

66

1.12

5.64E+02

100

Fe(n,p)56Mn

64

2.4

0.98

56

4.7574

100

5

7.49E+03

Zn

1.37

1.01E+01 3.27E+02

Fe(n,p) Mn

65

2.4

10.4

4.37E+01

8.9727

100

67.91

54

Cu

1.37

A 0 (Bq)

0.16

54

58

h (%)

3.2 0.9

Ti(n,p)48 Sc

7.5087

5.6

BI (%)

47

Ti(n,p) Sc Ti(n,p) Sc

Ni

6.1

E g MeV

2.01E+03

48

59

E th (Eff) MeV

46

8.03E+01

0.5263

1.8955

Al(n,a)24Na

1.50E+01

47

Co

In

T1/2 (h)

115

In(n,n’)

In

m

5 3.1 1.3 3.3 1.7

5.86E+03 4.49E+00

11.4 0.34

9.40E+02 1.05E+01 3.62E+02 1.04E+02 4.79E+03 1.56E+01 5.36E+04

Wt=Weight of the samples, T1/2 = Product half-life, Eth (Eff) = Effective threshold energy, Eg = γ - energy, BI = Branching Intensity, η = Efficiency, A0 = Activity at the end of irradiation. be followed to obtain the final neutron distribution. RESULTS AND DISCUSSION It can be seen from Table 1 that, a total of 13 reactions were explored from 9 activation foils. The multiple reactions induced in some detectors were useful in supplying additional data points, which enhanced the quality of the final spectrum. The product nuclides monitored were 24Na, 46,47,48Sc, 54,56Mn. 59Fe, 58Co, 65Ni, 64 Cu, 65Zn, 115In. The unfolded spectrum is plotted in fig.1, spanning the energy range of 1 to 13 MeV. It has been observed that the high energy contribution is small and above 10 MeV it falls off by a factor of more than 60. From the shape of the spectrum it appears that evaporation process dominates, and there is some non-equilibrium contribution. The present measurement is compared with the one measured at the same field using NE213 liquid scintillator and by unfolding the neutron induced proton pulse height distribution. The shapes of both the spectra fairly agree with each other. However, the slight discrepancies at some energy points could be due to the inherent errors associated in the experiments as well as the uncertainties involved in the unfolding procedures. Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

Fig.1. The unfolded spectra of fast neutrons at 90o obtained from the interaction of 20 MeV protons on Cu target, at the 14 UD Pelletron facility of TIFR. The spectrum obtained by the activation foils (threshold detectors) is compared with the one obtained previously in the same system using NE213 liquid scintilator. Neutron spectrometry by passive methods using activation foils was employed successfully to study the neutron spectrum around the TIFR pelletron accelerator. 31

Fast neutron spectrometry from 20 mev 1h+nat.cu using activation foils - S.Pp. Tripathy et.al.

The fact that the high-resolution HPGe detector could be used successfully to measure more than one nuclide from even the same activation foil, has made it possible to explore several reactions simultaneously. Thus, by exposing a few foils, a well-resolved spectrum can be obtained. The proper combination of the multi-foil activation method and a suitable unfolding code can characterise the neutron flux density per unit energy from the measurements. The spectrum obtained by this method agrees fairly well with that obtained previously in the same radiation field using NE213 liquid scintillator. It was found that the spectrum quality largely depends, in addition to the “a priori information” and the unfolding codes, on the accuracy of the response matrix, which in turn depends on the cross-section values for each type of reaction. It is important that the unfolding programs should never be used as ‘black boxes’, and the user should always carry out test runs, especially when the fine structure of the spectrum is of interest. The work is in progress to develop a simple ‘response matrix generator’ for the activation foil unfolding, and to unfold the results using different unfolding methods and algorithms as well as to develop a more user-friendly unfolding code. ACKNOWLEDGEMENT Authors are thankful to the staffs of TIFR pelletron for the help during irradiation, to Dr. Anil Kumar for his valuable help in measurements with HPGe. The encouragement from Dr. H.S. Kushwaha, Director, Health, Safety and Environment group is gratefully acknowledged. REFERENCES Brooks F.D., Klein H. (2002). Neutron spectrometry: historical review and present status. Nuclear Instruments and Methods in Physics Research A 476, 1–11. Byerly, P.R. (1960). In: J.B. Marion, J.F. Fowler (Eds.), Fast Neutron Physics, Part 1, Interscience, New York, p. 657.

Holt, P.D. (1985). Passive detectors for neutron fluence measurement. Radiation Protection Dosimetry 10(1-4), 251-264. Hurst, G.S., Harter, J.A., Hensley, P.N., Mills, W.A., Slater, M., Reinhardt, P.W. (1956) Techniques of measuring neutron spectra with threshold detectorsTissue dose determination. The Review of Scientific Instruments 27(3), 153-156. Knoll G.F. (2000), Radiation detection and measurement, 3rd edition, John Wiley & Sons, Inc. p.744-751. Lowry, K. A., Johnson, T. L. (1984). Modifications to iterative recursion unfolding algorithms and computer codes to find more appropriate neutron spectra. NRL Memorandum Report 5340 (Washington, DC: Naval Research Laboratory). Mijnheert, B. J., Haringat, H, Noltheniuss, H. J., Zijps, W. L. (1981). Neutron spectra and neutron kerma derived from activation and fission detector measurements in a d +T neutron therapy beam, Phys. Med. Biol., 26(4), 641-655. Mukherjee Bhaskar (2004). ANDI-03: a genetic algorithm tool for the analysis of activation detector data to unfold high- nergy Neutron spectra, Radiation Protection Dosimetry, 110, Nos 1-4, 249-254. Reginatto, M., Goldhagen, P. (1998). MAXED, a computer code for the deconvolution of multisphere neutron spectrometer data using the maximum entropy method. EML-595 (New York: Environmental Measurements Laboratory). Roulti, J.T., Sandberg J.V. (1985). Unfolding activation and multisphere detector data. Radiation Protection Dosimetry, 20, 103-110. Zanini, A., Storini, M., Visca, L., Durisi, E.A.M., Fasolo, F., Perosino, M., Borla, O., Saavedra, O. (2005). Neutron spectrometry at high mountain observatories. Journal of Atmospheric and Solar-Terrestrial Physics 67, 755–762.

Gualdrini, G., Bedogni, R., Fantuzzi, E., Mariotti, F. (2004). The ENEA criticality accident dosimetry system. Radiation Protection Dosimetry 110(1-4), 465–469.

32


BaMgAl10O17:Eu PDP PHOSPHOR FOR TLD APPLCATIONS S.J. Dhoble, A.D. Deshmukh, G.V. Bramhe*, N. Sinha, R.G. Sonkawade** and D.R. Peshwe*** Kamla Nehru College, Sakkardara Square, Nagpur-440009, India *Govt. Autonomous P.G.College,Chhindwara-480001, India ** Inter University Accelerator Centre, New Delhi-110067, India ***Department of Metallurgy, VNIT, Nagpur-440011, India Abstract : Ba0.95MgAl10O17:Eu0.05 and Ba0.99Mg2Al16O27:Eu0.01 PDP phosphors prepared by combustion synthesis is reported in this paper. The TL results of phosphors shows the prominent TL peak at 185 ºC and its intensity is higher as compared to CaSO4:Dy TLD phosphor. Therefore above phosphors may be used as TLD phosphors of ionization radiations.

INTRODUCTION Flat display panels (FDPs) have been developed prosperously to replace the cathode-ray tube displays (CRTs) which currently are applied in television sets and computers. As one of the candidates of FDP, plasma display panels (PDPs), because of its obvious merits, such as a fast response, a wide viewing angle, large screen, low energy consumption and high scalability [1,2], are gaining most considerable attention. But there are some serious problems to be solved, for example, stability, efficiency, lifetime, contrast and brightness. Presently PDP phosphors are not satisfactory for practical applications, i.e. unstable characteristic and low brightness for blue phosphors poor color purity for red phosphors and long decay time for green phosphors. The commercial PDP red phosphor (Y,Gd ) BO3 : Eu 3+ green phosphor Zn2SiO4 Mn2+ and blue phosphor BaAl10O17:Eu has more demerits to be resolved [3]. Although BAM-based phosphors are used in lamps and displays and degradation and afterglow are a matter of concern for all device manufacturers, the type and density of defects in BAM are less known. This is surprising since lattice defects, in particular oxygen vacancies, can act as electron traps and therefore force the photooxidation of the unstable activators Eu2+. It can be expected that the crystallinity or defect concentration of BAM will strongly influence the stability of BAM and the luminescence properties. BaMgAl10O17:Eu2+ shows serious luminescence degradation by heating process for binder burn off and the irradiation of VUV during operation i.e. brightness decreases in about 30-40% after 15,000 h VUV irradiation. In order to conquer the above demerits of this phosphor, many groups had investigated the structure and luminescence mechanism. However, the stabilized BaMgAl10O17: Eu has never been obtained so far. The aim of this work is to gain insight in the influence of defects on the luminescence properties and to study the nature of the defects created under UV and VUV radiation by performing thermoluminescence Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

measurements. The primary objective of dosimetry is to monitor the radiation dose delivered to persons and environment during radiation leak or nuclear explosion with sufficient sensitivity. Thermoluminescence (TL) is a very common technique used for dosimetry of ionizing radiations. The energy absorbed by a phosphor on being exposed to some ionizing radiation is released as light on subsequently heating it. The intensity of light emitted by the phosphor on being heated gives an idea of the irradiation dose given to it. Different preparation methods and properties of several thermoluminescent materials have been studied so far and it is found that sulfates constitute a class of TL phosphors with good performances, especially when doped with proper activators [4-8]. The mixed sulfate K2Ca2(SO4)3: Eu prepared by the conventional solid-state diffusion method is found to be a highly sensitive TLD phosphor [5]. Moreover, recently LiNaSO4: Eu in its nanocrystalline form has also been prepared and its thermoluminescence properties are studied [9].In recent years, sensitive TL phosphors have been developed, namely K3Na(SO4)2: Eu [10], CaSO4: P, Dy [11], MgB4O7: Dy [12], Sr2B5O9Cl : Eu [13], Sr2(PO4)3Cl: Eu [14], which could be used in the personal and environmental radiation dosimetry. In this investigation, synthesis, photoluminescence (PL) and TL characterization of blue commercial PDP phosphor i.e. BaMgAl 10O 17 :Eu phosphor is chosen. The BaMgAl10O17: Eu phosphor prepared by combustion synthesis using urea as a fuel. The PL emission spectrum shows the blue emission of phosphor is suitable for commonly use PMT and emission is away from red emission of kanthal plate use in TL reader. EXPERIMENTAL Ba0.95MgAl10O17:Eu0.05 and Ba0.99Mg2Al16O27:Eu0.01 PDP phosphors were prepared by combustion synthesis. The detailed description of the method can be found in the original works of Patil and co-workers [15,16]. Starting materials were respective metal nitrates (oxidizer), and 33

Bamgal10O17:EU PDP phosphor for TLD applcations - S.J. Dhoble et.al

TL of these prepared phosphors were recorded using a standard TL reader Harshaw-3500 at IUAC, NewDelhi. The gamma dose (137Cs ) employed for irradiation is 1.8 R.

3500000

B

3000000 2500000 T L -In te n s ity (A r b .U n i ts )

urea as a fuel. All constituents in stoichiometric proportions, along with fuel and oxidizer were mixed together. The mixture on thoroughly grinding was transferred to a pre-heated furnace at 500 ºC. On rapid heating the mixture evaporates and ignites to yield a white fluffy product. Entire process completes within few minutes. Hence the combustion synthesis offers an efficient and easy way for the phosphor preparation.

2000000 1500000 1000000

A

500000 0 0

100

200

300

400

500

0

T e m pe r a tu r e ( C )

RESULTS AND DISCUSSIONS Fig. 1 shows the X-ray diffraction (XRD) pattern of BaMgAl10O17: Eu phosphor material. The XRD pattern did not indicate presence of the constituents materials, which is an indirect evidence for the formation of the desired compound. The XRD patterns of prepared phosphor well match with the standard data of JCPDs file no 026-0163. These results shows the final product was formed in homogeneous form.

Fig 2. TL glow curve of (A) CaSO4:Dy and (B) PDP Ba0.95MgAl10O17:Eu0.052+ phosphor exposed to 137Cs gamma rays(1.8 R)

10000000

B

9000000 8000000

T L -Inte nsity (A r b.U nits)

7000000

C o unts B am E u

400

300

6000000 5000000 4000000 3000000 2000000

A

1000000 0 0

200

100

200

300

400

500

0

T e m pe r a tu r e ( C ) 100

0 20

30

40

50

60

70

80

90

100

11 0

Fig.1 XRD pattern of BaMgAl10O17 phosphor P o s itio n [°2The ta]

RE activated aluminate phosphors give sensible response for TL characteristics.Figure-2,shows the excellent and well defined TL glow curve(B) of Ba0.95MgAl10O17:Eu0.05 phosphor irradiated with 120.33 mR/hr (137Cs ), and curve (A) shows the glow-curve of standard TLD phosphor,CaSO4:Dy.The TL glow curve exhibits a very well defined TL peak at 187ºC [ In most cases the practical dosimetric glow-peaks of a TL dosimeter are between 150-250º] and another TL peak observed at lower temperature 320ºC .The well defined TL peak intensity is 5.42 times higher as compared to conventional CaSO4:Dy TLD phosphor. Similar results are observed for Ba0.99Mg2Al16O27:Eu0.01 phosphors, but the peak intensity is 14.95 times higher than the CaSO4:Dy phosphor(Fig 3).

34

Fig 3.TL - glow curves for (A) CaSO4:Dy and (B) Ba0.99Mg2Al16O27:Eu0.012 + exposed to 137Cs gamma rays (1.8R) The above results shows the TL intensity in Eu activated Ba 0.95 MgAl 10O 17:Eu 0.05 and Ba0.99Mg2Al16O 27:Eu0.01 PDP phosphors is higher as compared to conventional CaSO4:Dy TLD phosphor. In the literature aluminate based high sensitive TL dosimetry phosphor first time reported in this paper. Therefore the new route open for investigation of new TLD phosphors in aluminate based materials. The detail dosimetric characterization published in future as early as possible. The above Eu activated Ba 0.95MgAl10O 17:Eu0.05 and Ba0.99Mg2Al16O27:Eu0.01 phosphors it may be considered as TLD phosphors of ionization radiations. CONCLUSION The phosphors Ba 0.95 MgAl 10O 17:Eu 0.05 , Ba0.99Mg2Al16O27:Eu0.01 phosphors prepared by combustion Radiation Protection and Environment, Vol. 30 , No. 1- 4, 2007

Bamgal10O17:EU PDP phosphor for TLD applcations - S.J. Dhoble et.al

synthesis. The TL results shows prominent TL peak at 185 ºC and its intensity is higher as compared to CaSO4:Dy TLD phosphor. Therefore above phosphors may be used as TLD phosphors of ionization radiations. ACKNOWLEDGMENT One of us SJD is thankful to BRNS (Project sanction letter no. 2005/37/19/BRNS/1745), Dept. of Atomic Energy, Govt. of India for financial assistant during the work. REFERENCES 1. P.S. Friedman, Inf. Disp. 2/91 6 (1991). 2. C.H. Kim, H.S. Bae, C.H. Pyun, G.H. Hong, J. Korean Chem. Soc. 5/42 588(1998). 3. C.R.Ronda, Proc. Int.Display Workshop 1995; 69. 4

.P D Sahare, S V Moharil and B D Bhasin J. Phys. D: Appl. Phys. 22 971(1989)

5. P D Sahare and S V Moharil J. Phys. D: Appl. Phys.23 567(1990) 6. S M Dhopte, P L Muthal, V K Kondawar, S V Moharil and P D Sahare J. Phys. D: Appl. Phys. 24 1869 (1991)


7. B T Deshmukh, S V Bodade and S V Moharil Phys. Status Solidi a 98 239(1986) 8. A Pandey, V K Sharma, D Mohan, R K Kale and P D Sahare, J. Phys. D: Appl. Phys. 35 1330 (2002) 9. A Pandey, P D Sahare, J.S.Bakre, S.P.Lochab, F.Singh and D. Kanjilal, J. Phys. D: Appl. Phys. 36 2400 (2003) 10. S.J. Dhoble, S.V. Moharil, S.M. Dhopte, P.L. Muthal, V.K. Kondawar, Phys. Stat. Sol. (a) 135 289(1993). 11. M.S. Atone, S.J. Dhoble, S.V. Moharil, S.M. Dhopte, P.L. Mutal, V.K. Kondawar, Phys. Stat. Sol. A 135 299 (1993). 12. D.I. Shahare, S.J. Dhoble, S.V. Moharil, J. Mater. Sci.Lett. 12 1873 (1993). 13. S.J. Dhoble, S.V. Moharil, J. Nucl. Instr. and Meth. B 160 274 (2000). 14. S.J. Dhoble, J. Phys. D 33 158 (2000). 15 .J.J. Kingsley and K.C. Patil, Mater. Lett. 6, 427 ( 1988). 16. S. Ekambaram and K.C. Patil, J. Alloy. Comp. 217, 104 (1995).

35

SHIELD DESIGN FOR THE MATERIAL SCIENCE BEAM LINE OF MEDICAL PROTON CYCLOTRON FACILITY R. Sarangapani, V. Meenakshisundaram, K.V. Subbaiah+ and R. Indira Radiological Safety Division, IGCAR, Kalpakkam – 603 102 + AERB-Safety Research Institute, IGCAR CAMPUS, Kalpakkam – 603 102 Email: [email protected] Abstract : A medical proton cyclotron facility is being commissioned by DAE in Kolkata. The facility will have three dedicated beam lines for i) the production of radioisotopes, ii) Accelerator Driven Systems (ADS) experiments and iii) material science studies. It is proposed to irradiate SS (D9 alloy) samples by Materials Science Division, IGCAR in the material science beam line. The proton energy and current are 30 MeV and 200 µA respectively. It is proposed to irradiate the sample for seven days and retrieve it after a minimum cooling period of 24 h. Proton-induced radionuclides are produced on the target during irradiation. In addition to the induced activity, protons impinging on the sample results in the emission of neutrons. The emitted neutrons interact with the structural materials and produces induced activity. The access to the area is permitted only during shutdown of the beam line and that too after sufficient cooling time. The irradiated target is transferred from irradiation chamber to lead transport cask. When the beam line is in the off state, neutrons are absent and the dose rate at the accessible locations is only from the gamma rays emitted by the target material and induced activity on the structural materials. Calculations have been carried out to estimate the required shielding thickness around the target so that the dose rate at the accessible locations is within acceptable levels. The shield thickness for the lead cask is also estimated.

INTRODUCTION A medical cyclotron facility is being commissioned by DAE in Kolkata. The facility will have three dedicated beam lines for i) Production of radioisotopes, ii) ADS experiment and iii) Material science studies. It is proposed to irradiate SS (D9 alloy) samples by MSD, IGCAR in the material science beam line. The proton energy and current of the material science beam line are 30 MeV and 200 µA respectively. Proton-induced radionuclides are produced on the target during irradiation. In addition to the induced activity, protons impinging on the target results in the emission of neutrons. The neutrons interact with the structural materials and produces induced activity. The main construction material for the beam line will be Aluminium. The dose rate at accessible locations during the operation of the beam will be due to i) neutrons emitted from the target, ii) gammas from induced radio nuclides in the target, iii) prompt gammas due to neutron absorption in the structural materials and iv) gammas from neutron induced activity in the structural materials. Access to the sample retrieval area is permitted only when the beam line is in shutdown state and that too after sufficient cooling time for carrying out operations such as retrieval of sample and maintenance work. When the beam line is in shutdown state, neutrons and prompt gammas are absent and the dose rate at the accessible locations is only from the gammas emitted by the irradiated sample and induced activity on the structural

36

materials. In the present work, calculations have been carried out to estimate the shield thickness required around the target so that the dose rate at the accessible locations is within acceptable levels. It is also to be noted that the accessible locations have to be classified as ‘controlled area’ if the design dose rate exceeds 1 mSv/ h and access to the area is to be controlled by implementing appropriate radiation protection measures. It is proposed to transfer the irradiated sample in a lead cask for further studies in shielded cells. The estimated shield thickness of the transport lead cask and induced activity in the Aluminium irradiation chamber are also given.

Details of the source, target and proton induced radionuclides The general layout of the medical proton cyclotron is shown in Fig.1. The proton beam of energy 30 MeV with current 200 µA is the primary source. The density of the D9 alloy is 7.8 g/cc and the volume of the sample is 1.25 cc [Nair, 2005, a]. The major elements in the D9 alloy sample are Fe (63.783%), Cr (14.5%), Ni (15.5%), Mo (2.5), Mn (2.35%), Si (0.75%) and Ti (0.25%). The proposed time of irradiation of the target is 7 days. The prominent proton induced radionuclides of the D9 alloy sample estimated using ALICE code is given in Table 1 [Nair,2005,b]. The emitted neutrons due to proton interaction with D9 alloy will have energies up to 17 MeV [Nair, 2006].


Shield Design for the Material Science Beam Line of Medical Proton Cyclotron Facility - R. Sarangapani et.al.

Fig.1: General layout of the Medical Cyclotron and Material Science Beam line SHIELD DESIGN FOR IRRADIATED D9 ALLOY SAMPLE It is proposed to shield the D9 alloy sample with ordinary concrete. The schematic shield design is given in Fig.2. The irradiated D9 alloy sample is assumed as a point source and the dose rates have been computed for various thicknesses of concrete shield. The dose rate calculations indicate an ordinary concrete of thick 750 mm and a minimum distance between the source and the receiver point is 1750 mm. The height of the concrete is assumed upto 4000 mm. The dose rate at the receiver point (Fig. 3) has been calculated for the radionuclides strength for an irradiation period of seven days with current 200 mA and cooled for 24 h. The dose rate at 10 mm and at 1750 mm from the target is 5.09E+3 Gy/h and 128.6 mGy/h respectively


Fig.2: Proposed concrete shield design for the D9 alloy sample

37


Table 1. Prominent radionuclides of the D9 alloy sample and their source strength [Energy of proton beam: 30 MeV, Current: 200 µA, Duration of irradiation: 7 days] S. No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

R a d io n u c lid e C o -5 5 C o -5 6 C o -5 7 C o -5 8 C r-5 1 C u -5 9 C u -6 0 C u -6 1 C u -6 2 C u -6 4 F e -5 3 M n -5 1 M n -5 2 M n -5 4 N i-5 7 Tc -9 3 Tc -9 4 Tc -9 5 Tc -9 6 V -4 8

H a lf-life (d a y s ) 7 . 3 0 E -0 1 7.73 E + 01 2.72 E + 02 7.09 E + 01 2.77 E + 01 9 . 3 8 E -0 4 1 . 6 5 E -0 2 1 . 3 9 E -0 1 6 . 7 6 E -0 3 5 . 2 9 E -0 1 5 . 9 1 E -0 3 3 . 2 1 E -0 2 5.59 E + 00 3.12 E + 02 1.48 E + 00 1 . 1 5 E -0 1 2 . 0 3 E -0 1 8 . 3 3 E -0 1 4.28 E + 00 1.60 E + 01

A c t ivit y (B q ) 1.18E + 12 1.83E + 11 1.01E + 10 4.27E + 09 1.50E + 11 2.45E + 10 1.01E + 11 3.15E + 10 2.79E + 10 6.53E + 09 1.65E + 11 2.24E + 11 4.76E + 11 2.20E + 09 5.37E + 11 1.02E + 10 2.88E + 10 3.83E + 10 2.64E + 10 9.05E + 09

DOSE RATE DUE TO INDUCED ACTIVITY IN CONCRETE As the D9 alloy sample undergoes proton irradiation, neutrons are emitted due to (p, n), (p, an), (p, dn) reactions etc. The neutron flux at 1 metre from the Aluminium target for proton energy 30 MeV is 6 x 106 neutrons cm-2 s-1 mA-1 [NCRP, 1977]. The neutron flux at 1m from the target for a current of 200 mA is 1.2x 109 n.cm-2.s-1. The neutron source emission from the D9 alloy sample is assumed to be same as the Aluminium target. Hence it becomes pertinent to estimate the radiation level contributed by the induced activity of the concrete shielding material at the receiver point. The elemental composition of the ordinary concrete is assumed to be the Portland concrete. The major nuclear reactions viz., (n, p), (n, 2n) and (n,a) considered for the concrete composition and the important isotopes formed due to activation of the concrete are given in Table 2. The calculation of induced activity for the important radionuclides has been carried out using MCNP code [Briesmeister, 1997]. The geometry of the concrete structure as shown in Fig.2 has been modeled in the code. The concrete composition has been specified and the saturation activity has been calculated using reaction rates tally. The concrete block of 750 mm thick was

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Table 2: Induced activity in the concrete shield and dose rate at the receiver point

Induced Activity

S.No 1 2 3 4 5 6 7 8

One week irradiation Saturation and one day Radionuclide activity (Bq) cooling (Bq) Na-22 2.82E+06 1.44E+04 Na-24 2.65E+09 8.73E+08 K-42 1.41E+09 3.68E+08 K-43 2.95E+08 1.40E+08 Ca-47 4.66E+03 2.63E+03 Mn-54 2.16E+09 3.33E+07 Cr-51 3.31E+07 5.18E+06 Mn-56 6.11E+08 1.02E+06 Total dose rate (Gy/h)

Dose rate on contact with concrete Saturation One week doserate irradiation (Gy/h) and one day cooling (Gy/h) 5.49E-10 2.79E-12 2.84E-06 9.35E-07 7.79E-08 2.03E-08 1.86E-07 5.51E-08 5.12E-13 2.88E-13 2.41E-07 3.71E-09 9.06E-11 1.42E-11 6.57E-11 7.74E-09 4.00E-06 1.02E-06

divided into 8 layers (seven layers of 100 mm thick and one layer of 50 mm thick) and the induced activity in each layer calculated separately. It is seen that the activity decreases with increase in shield thickness and the variation of activity across the slab is about 3 to 4 orders of magnitude. By treating the entire concrete block as a volume source with calculated activity distribution, the dose rate at the receiver point (refer Fig.2) has been calculated using a point kernel shielding code, QADCGPIC [Subbaiah, 2001]. The total induced activity in concrete and the dose rate at the receiver point (refer Fig. 2) is summarised in Table 2. It is noted that the dose rate due to induced activity in concrete is 1.0 mGy/h. SHIELDING REQUIREMENTS FOR THE TRANSPORT CASK OF IRRADIATED D9 ALLOY SAMPLE. The irradiated D9 alloy sample has to be transported from irradiation location to a laboratory in a shielded container. It is proposed to use lead cask for transport. On the basis of the induced activity on D9 alloy sample (refer Table 1), the dose rates with varying thickness of lead (Density: 11.34 g/cc) have been computed. It is found that the required thickness of lead is about 18 cm and dose rate outside the lead cask (18 cm thick) is 0.70 mGy/h, which is well below the maximum permissible regulatory limit of 2 mGy/h. INDUCED ACTIVITY IN CONSTRUCTION MATERIAL Aluminium is proposed as the main construction material for the beam line. The induced activity in Al has been calculated for neutron reactions viz., (n, p) (n,a) and (n, 2n). By assuming the Aluminum is at 10 cm from the source, the calculated induced specific activity and dose rates are given in Table 3.



Table 3: Induced activity in Aluminum and dose rate

Induc ed A c tivity (B q/g) Radionuc l ide M g-27 Na-24 A l-26

S aturation 2.11E + 08 3.64E + 08 7.47E + 08

O ne week irradiation and one day c ooling 3.25E -38 1.20E + 08 1.38E + 01

Dos e rate at 1 c m from A l and outs ide the c onc rete s hield (G y /h)/g

S aturation 2.75E -01 2.19E + 00 1.99E + 00

CONCLUSIONS The major contributors for the gamma dose rate outside the concrete shield due to proton induced activity are Mn52, Co55, Co56 and Ni57. Similarly Na24 is the major contributor to the gamma dose rates due to neutron induced activity in the concrete shield. Since the dose rates at the accessible locations even after a cooling time of 24 h is above 1 mGy/h, the area has to be declared as ‘controlled area’ as per AERB design criteria. A lead cask of 18 cm thick is required for the transport of irradiated D9 alloy sample. Na24 is the important induced radionuclide in the construction material of the beam line. ACKNOWLEDGEMENTS The authors thank Dr. K.G.M Nair, MSD, IGCAR for suggesting the problem and for fruitful discussions.

O ne w eek irradiation and one day c ooling 4.23E -47 7.24E -01 3.70E -08

O uts ide the c onc rete for one w eek irradiation and one day c ooling N egligible 1.06E -07 N egligible

Nair, K.G.M., D9 alloy sample composition (2005), Personal Communication, Material Science Division, IGCAR. Nair, K.G.M., List of proton induced radionuclides, (2005) Personal Communication, Material Science Division, IGCAR. Nair, K.G.M, Neutron spectrum for 30 MeV proton beam, (2006), Personal Communication, Material Science Division, IGCAR. NCRP, NCRP Scientific committee report on ‘Radiation protection design guidelines for 0.1 to 100 MeV particle accelerator facilities’, (1977), NCRP Report No 51. Subbaiah, K.V. and Sarangapani, R., GUI2QAD-3D: A graphical user interface for QADCGPIC program, (2006), Annals of Nuclear Energy, 33, 22-29.

REFERENCES Briesmeister J.F., MCNP- Monte Carlo Neutron and Photon Transport Code, Version 4A, (1997), RSIC-CCC200, RSICC, ORNL.


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RADIATION ENVIRONMENT IN POSITIVE ION ACCELERATORS: EXPERIMENTAL AND THEORETICAL INVESTIGATIONS Maitreyee Nandy Saha Institute of Nuclear Physics 1/AF, Bidhannagar, Kolkata – 700064. ABSTRACT : We report results of several experimental and theoretical studies on energy-angule distribution of neutron yield and ambient dose equivalent carried out to gain an idea of the radiation environment in positive ion accelerators Measured neutron yield and/or dose distribution for 12C incident on 12C, Ag, Ti targets, 16O on Ta, 20Ne on 12C, 19F on 27Al, Cu at beam energies in the range ~5.25A to 12A MeV, for 20 MeV protons on Cu and Be are compared with calculations from different models. A simplified computational model developed based on nucleon-nucleon scattering kinematics to calculate the angular distribution of neutrons from heavy ion reactions at low energy gives a fairly good estimate of the neutron yield. Empirical expressions have been developed to calculate total yield, energy spectra and angular distribution of neutrons from proton, alpha and heavy ion induced reactions in low to intermediate energy range. The empirical formalisms reproduce the measure data within ±10% on an average. These expressions serve as simple reliable and fast tools for shielding, transport calculation in positive ion accelerators.

INTRODUCTION Accelerators, first designed and constructed as research instruments, have in the recent past entered the very fabric of our life with wide application in medicine, industry, agriculture, basic research like cosmology, particle physics, nuclear physics, and energy production. Energy production through nuclear fusion is one of the promising future prospects of accelerator application. Although the radiological aspect of accelerators encompasses nearly all the issues of health physics, in some respect accelerator radiation safety is unique and has emerged as a discipline quite different from conventional radiation safety. This is attributed to various causes: firstly vast span of energy, intensity and variety of the projectile beams combined with a large number of possible targets produce a wide range of radiation components (both particles and photons), spread over a wide energy range. Moreover, the radiation field is usually pulsed in nature, and normal measurement techniques suffer from the interference from radio frequency (RF) sources used to produce a synchronized beam. Undoubtedly, measured energy-angle distributions are the most reliable sources for any type of safety design to be made or precautions to be taken. However such measurements are pitted with certain drawbacks. First of all, it is self-contradictory. Because safety design needs to be carried out before installing any accelerator facility. Secondly, experimental measurements for all the possible situations are not feasible because of innumerable combinations of possible target, projectile,

40

beam energy. Also because expensive, sophisticated instruments required to carry out such measurements are not affordable to many, if not most, of the small accelerator laboratories. Finally, no instrument can appropriately respond to the different radiation types over the entire energy range, nor do they exclusively separate out each type or can shut out the RF interference. Thus measurements and computation become complimentary for estimating radiation environment around an accelerator. In order to gain a rather comprehensive knowledge of the radiation environment in positive ion accelerators, we have carried out several experimental and theoretical studies. EXPERIMENT

Heavy ion reactions: Computational models and techniques always need to be validated against measured data. Though heavy ion accelerators are coming up in large numbers, measured neutron data in these accelerator facilities are not so plentiful. In order to replenish this neutron data bank we have measured the double-differential neutron spectra from some heavy ion reactions at low projectile energies. The target thicknesses were so chosen that the projectile beam was completely stopped inside the target and at the same time scattering of the neutrons produced was minimum. The neutrons detected, comprised of those coming in that direction directly from the nuclear reaction at the target as well as those scattered from other areas. The contribution from roomscattered neutrons was determined by repeating measurement at a given angle with an interposed perspex shadow bar which cuts off the direct neutrons.


Fig. 1. Angle-integrated neutron energy spectra measured and calculated by HION, BME and EMPIRE codes for 144 MeV 12C induced reactions on (a) Ti and (b) Ag targets. Angular and energy distribution of neutron yield from 144 MeV 12C+5 induced reactions on thick targets of 12C, Ag, Ti were measured at 0o, 30o, 60o, 90o. The experiments were carried out at National Institute of Radiological Sciences, Inage, Japan. Projectile beam current was about 5-15 particle-nanoampere. Neutrons were detected with the help of proton recoil scintillation detectors NE-213 and BC-501, 5 inch long with 5 inch diameter. Standard pulse shape discrimination technique was used to differentiate the neutron pulse from the gamma pulse. The neutron spectra were calculated from the measured pulse-height distributions with the help of the revised FERDO unfolding code (Uwamino 1992) using the calculated (Monte Carlo) Response functions. The total error associated with the unfolded spectra consists of i) the statistical error associated with the measurement, ii) the error arising from discretizing the continuous spectra and the response function, and iii) the statistical error inherent in the Monte Carlo calculations. The measured neutron spectra from 12C and Ti targets showed a broad hump at the extreme forward angle at emission energy of around 25 MeV which becomes weaker at backward angles. This broad hump was not present in the case of Ag target. Figure 1(a) and (b) show the comparison of the Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

experimentally measured energy spectrum with calculated results from nuclear reaction model codes HION (Nandy, 1999; 2001) and BME (Blann, 1985) for Ti and Ag targets. We also use EMPIRE-2.18 code (Herman, 2002) to get the multistep compound (MSC) and HauserFeshbach (HF) evaporation results. The Weisskopf-Ewing (WE) approximation of the evaporation part of the spectra had also been obtained. It had been observed that in both the cases the measure spectra were best reproduced by HION+HF calculations though the higher energy part is underpredicted. Neutron angular distribution was also measured from 19F9++27Al at 110 MeV, 19F8++Cu at 100 MeV at the BARC-TIFR Pelletron Accelerator Facility. A cylindrical (5 cm diameter, 5 cm height) BC501 liquid scintillator detector had been used for detecting the neutrons at 0o, 30o, 60o, 90o and 120o, which were differentiated from gamma pulses using the time-of–flight (TOF) technique. Our analysis showed that for 19F9++27Al, neutron emission reached a peak around 5 MeV at 0o, but the peak shifted towards lower energy side at back angles. Beyond this peak the neutron emission fell off smoothly but slowly indicating possible presence of some PEQ effects, though two distinct slopes were not seen. Neutron ambient dose 41

Fig. 2. Comparison of measured neutron ambient dose equivalent H*(10) at different angles with those calculated using EMPIRE 2.18 code for (a) 110 MeV 19F + Al, absorbed dose D is also shown separately by dotted lines with axis on right, (b) 100 MeV 19F + Cu.

equivalent (H*(10)) was computed from these measured spectra and compared with those obtained from model calculations. Figure 2(a) shows the comparison between measured H*(10) and those calculated using the exciton model code EMPIRE 2.18 (Herman, 2002). At 0o, 30o, 60o the calculations reproduced the measured data well, but slightly underpredicted the data at 90o and 120o. The measured angular distribution of H*(10) agreed fairly well with calculations of some empirical formalisms at forward angles upto 60o, but the agreement was poor at backangles. We had also calculated absorbed dose (D) from the measured neutron spectrum. Comparison of D and H suggested an average value of 10 for the neutron quality factor in this energy range (Nandy, to be published). In the case of 19F8++Cu reaction, the angular distribution of H*(10) showed similar type of flat slope, but the EMPIRE calculations failed to reproduce the experimental data (Fig.2(b)) well. The measured spectra are grossly overpredicted beyond 30o and underpredicted to some extent at 0o angle. Double-differential neutron spectra from 7.2A MeV 16O+5 projectile incident on thick Ta and 7.25A MeV 42

20

Ne on thick 12C targets had been measured at the Variable Energy Cyclotron Centre, Kolkata. Heavy ion projectiles were obtained from the ECR ion source. In the first case measurements were done at 0o, 30o and 60o while for the latter case 15o and 45o were also included. A 2"X2" NE-213 detector was used for detection of the neutrons. The measured neutron angular distribution from 16 +5 O +Ta was compared with the calculations of HION code in fig.3(a) which shows that the measured distribution was fairly well reproduced by the calculations. Experimental data showed that there was significant preequilibrium (PEQ) contribution at forward angles (Nandy, 2001). The forward angle spectrum could not be explained fully in the framework of Maxwellian evaporation spectra. But at 60o emission angle non-equilibrium contribution was absent. Neutron spectra (Bandyopadhyay, 2003) and the consequent dose H*(10) from 20Ne+C reaction showed a broad hump at higher energy side (similar to those in the cases of 12C+C and 12C+Ti) at all angles from 0o to 60o though the peak intensity decreased as we go to backward angles. Figure 3(b) shows the comparison between measured and HION calculated H*(10) for this reaction. It is seen that the calculations Radiation Protection and Environment, Vol. 30 , No. 1- 4, 2007

Fig. 3. Comparison of experimental measurement with calculations using HION code for (a) neutron energy spectra at different angles from 116 MeV 16O + Ta, PEQ contribution are shown separately by dashed lines, (b) neutron ambient dose equivalent from H*(10)145 MeV 20Ne + C. underpredicted the measurement except at low energies and extreme back angle 120o PROTON INDUCED REACTIONS Energy-angle distribution of neutron yield from thick targets of Cu and Be by 20 MeV incident protons were measured at the BARC-TIFR Pelletron Facility using the standard pulse-height unfolding (PHU) technique. H*(10) had been computed from the measured distribution and were compared with those calculated using the hybrid model code ALICE-91 (Blann, 1982) with different options for the inverse reaction cross section and the level density formalism [Figs. 4(a) & (b)]. ALICE-91 is a very well validated code for nucleon and a–particle induced reactions upto around 200 MeV. In the case of Be target, the measured distribution showed a broad peak at the higher energy end for 0o – 60o emission angles. The intensity of this peak was almost equal to the intensity at low energies. The calculated distribution failed to reproduce this peak, but otherwise reproduced the measured yields at these angles. This peak was much weaker at 90o and 120o where the experimental data were overpredicted by the calculations. For p+Cu reaction, the agreement between the measurement and Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

computation was not very good. Apart from the fact that the doses at low energy were overpredicted by the calculations, the measured doses were fairly well reproduced by the results of ALICE 91 at all angles except at 120o where it was overpredicted throughout the energy range. THEORETICAL STUDIES Heavy ion reactions are complex in nature and are yet to be understood fully. The existing heavy ion reaction model and codes are too complicated and time consuming to be used for practical applications. A simplified computational model was developed to calculate the angular distribution of neutrons from heavy ion reactions in the energy range of 10A-30A MeV (Nandy, 1999). This model was based on the kinematics of twobody scattering between nucleons. Different stages of the reaction were characterized by the no. of interactions that had taken place between one representative particle and a nucleon. The probability of the representative particle being scattered in various energy-angle bins was computed. Consequent to this the emitted neutron distribution was obtained by taking into account the probability of the scattered particle entering into further collision or being emitted. Angular distributions of neutron 43

Fig. 4. Comparison of measured neutron ambient dose equivalent H*(10) at different angles with those calculated using ALICE-91 code for 20 MeV proton induced reaction on (a) Be and (b) Cu targets emission cross section from thin targets were fairly well reproduced by this formalism, but thick target data were not always reproduced. Significant upgradation and improvement of the model are required which are going on as more and more experimental data become available. Neutron angular distribution in proton and alpha induced reactions on a large number of thick targets had been investigated in the framework of the hybrid model and Weisskopf-Ewing formalism. Empirical expressions for the total yield, energy spectra and angular distribution of neutrons as functions of incident energy, target mass and emission energy had been developed. The results obtained using these empirical expressions had been compared with the experimentally measured distribution. It was observed that our formalisms reproduced the experimental data within an accuracy of ~ 10%. For proton induced reactions (Maiti, 2004) the total neutron yield Ytot(Ep) was given as a polynomial function of the incident energy Ep, the coefficients of which were again polynomials in the target mass no. A. The energy distribution was divided into two emission energy ranges: the low energy emission could be described by an exponential function of the emission energy

[

( T )]

y (ε ) = C ε − exp ε R

44

W

(1)

where the fractional yield yR (e) = y(e)/Ytot at emission energy e and T was a parameter. The coefficients CW and T were functions of the target mass no. A. For higher energy emissions the energy spectra were given by, Here yR(e) =[ y(e)/Ytot]A0.6. The coefficients CPi were polynomial functions of the projectile energy EP. The angular distribution was given by a hyperbolic function similar to that as in Kalbach systematics, but the parameter values were different from it. For alpha-particle induced reactions similar expressions were developed (Maiti, 2005). These formalisms both for proton and alphainduced reactions might be successfully employed as simple, fast and reliable tools in calculating neutron yield and distribution in various applications like radiation safety, medical therapy, material damage studies, etc. For large scale transport problems use of this method reduces the computation time by a factor of 50 – 60. Figure 5(a) and (b) show the comparison of the energyangle distribution of emitted neutrons from 52 MeV p +Pb and 60 MeV a+Ta reactions calculated using our empirical formalism with measured data. For a-particle induced reaction the higher energy part was often underpredicted by our calculations. Heavy ion reaction data had also been investigated but it was found that with the data available so far no comprehensive empirical expression could be developed for total yield and energy distribution of neutrons for Radiation Protection and Environment, Vol. 30 , No. 1- 4, 2007

Fig. 5. Comparison of the measured neutron spectra at different angles with those calculated by the empirical formalism developed for (a) 52 MeV p+Pb and (b) 60 MeV a+Ta. different combinations of target, projectile and incident energy over a reasonable range. An empirical expression was formulated for the neutron angular distribution for projectile energies upto around 10 MeV per nucleon. This expression is an exponential function of the emission angle q. The calculated results were compared with the available experimental data. The shape of the measured angular distribution curve is not always very well reproduced but the absolute values of the angular yield are fairly well reproduced over the entire energyangle range. Figure 6 shows the comparison of our measured neutron angular distribution from 16O+Ta reaction 116 MeV with those calculated using this exponential angular distribution formalism. The experimental data were well reproduced by the calculated distribution. CONCLUSION We have tried to gain a comprehensive idea about neutron distributions, which are the major components of the radiation field, around low to intermediate energy positive ion accelerators. Experimental measurement is the best choice to have a quantitative knowledge, but not feasible for all possible and warranted situations. Our measurements showed that even for 8A-10A MeV heavy ion induced reactions the neutron spectra could not be fully described by evaporation mechanism alone. Existing reaction model codes used for estimating the Radiation Protection and Environment, Vol. 30 , No. 1-4, 2007

Fig. 6. Measured (solid circles) and calculated (solid line) (using exponential expression) energy distribution of neutrons at different angles from 116 MeV 16O+Ta reaction. energy-angle distribution need to be validated against measured data. Our analyses revealed that no model could unambiguously predict the neutron emission over a vast range of target-projectile combinations and reasonable energy domain. Performance of the computational methods developed to compute total yield, energy spectra, angular distribution of neutrons in proton, 45

alpha and heavy ion induced reactions was reasonably satisfactory, but there existed quite a number of points of disagreement. These areas need to be investigated thoroughly to incorporate improvements. Both the measured and the computed data constitute a valuable set of nuclear data and need to be archived properly. REFERNCES Bandyopadhyay T., Maiti Moumita, Nandy Maitreyee, Bhattacharya S. and Sarkar P.K. (2003). Neutron emissions from 7.25A MeV 20Ne+12C reaction” Proc. DAE-BRNS Symposium on Nuclear Physics, 46B pp. 186-187 Blann M. (1982) Lawrence Livermore National Laboratory Report UCID 19614; Blann M. International Centre for Theoretical Physics Workshop on Applied Nuclear Theory and Nuclear Model Calculations for Nuclear Technology Applications, Trieste, Italy, SMR/284-1. Blann M. (1985) Physical Review C 31 1245. Herman M. (2002) EMPIRE-II statistical model code for nuclear reaction calculations (2.18 Mondovi), IAEA, Vienna, Austria,. Maity M., Nandy Maitreyee, Roy S.N. and Sarkar P.K. (2004). An empirical expression to calculate thick target neutron yield distributions from proton induced reactions. Nuclear Instruments and Methods in Physics Research B, 215 pp. 317-325.

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Maity M., Nandy Maitreyee, Roy S.N. and Sarkar P.K. (2005). Systematics and empirical expressions for neutron emission from thick targets in a-induced reactions. Physical Review C 71, 034601. Maiti Moumita, Nandy Maitreyee, Roy S.N. and Sarkar P.K. (2006). Angular distribution of neutrons from heavy ion induced reactions in thick targets. Nuclear Instruments and Methods in Physics Research A, 556 pp.577-588 Nandy Maitreyee, Ghosh Sudip and Sarkar P. K., (1999). Angular distribution of Preequilibrium Neutron Emissions from Heavy Ion Reactions. Physical Review C 60, 044607(1-10). Nandy Maitreyee, Bandyopadhyay Tapas and Sarkar P. K. (2001). Measurement and analysis of neutron spectra from a thick Ta target bombarded by 7.2A MeV 16O ions Phys. Rev. C, 63, 034610 Nandy Maitreyee, Sunil C., Maiti Moumita, Palit R. and Sarkar P. K. (to be published) Estimation of angular distribution of neutron dose using time of flight for 19F+Al system at 110 MeV. Nucl. Instrum. Methods Phys. Res A Uwamino Y., Shin K., Fujii M., and Nakamura T., (1982). Nucl. Instrum. Methods Phys. Res. 204, 179


BREMSSTRAHLUNG CONTRIBUTION AROUND X-RAY DIFFRACTION BEAM LINE AT INDUS-2 SYNCHROTRON RADIATION SOURCE M.K.Nayak, Vipin Dev, G.Haridas, Dimple Verma, K.K.Thakkar P.K.Sarkar,* D.N.Sharma* Health Physics Unit, Raja Ramanna Centre for Advanced Technology, Indore-452013 * RSSD, Bhabha Atomic Research Centre, Mumbai-400085 Abstract : Indus-2 Synchrotron Radiation Source is designed for 2.5 GeV, 300 mA stored current of electrons. It has 172-meter circumference. Indus-2 SRS with a critical wavelength of 2 A0 from bending magnets has been designed to cater to the needs of X-ray users. There are twenty-seven beam lines to be installed for different applications. These beam lines are planned to be taken out from the bending magnets. The beam lines shall be used for performing the experiment using synchrotron radiation. In order to assess radiation hazard to SR Users, experiments were performed by Health Physics Unit in front of X-Ray Diffraction Beam line for measurement of radiation using a thin window (Mylar window) ion chamber made by Electronics Division, BARC. Experimental study of the Synchrotron and Bremsstrahlung contribution indicates that along with the synchrotron radiation (4 keV – 30 keV), higher energy bremsstrahlung radiation also comes out in SR beam line. This paper describes the percentage of bremsstrahlung contribution observed with the synchrotron using the detector. The study has helped us in deciding the amount of radiation shielding to be put around SR beam lines in Indus-2 experimental hall to protect the working personnel

INTRODUCTION Indus Accelerator Complex consists of two storage rings, having 450 MeV & 2.5 GeV energy of electrons with injector microtron of 20 MeV, 20mA and booster synchrotron of 450/600 MeV, 30 mA. Indus-2 is a 2.5 GeV Synchrotron radiation source with critical wavelength of about 2 A0. It has eight super periods. Each having two dipole-bending magnets, four focusing and five defocusing quadruples and six sextupoles. Indus-2 is an electron storage ring designed to accelerate electron from 600 MeV to 2.5GeV and store them at the peak energy for several hours [Singh etal.,]. It has 172 m circumference. There are 27 beam lines to be installed for different applications. These beam lines are planned to be taken out from the bending magnets and insertion devices like wigglers and undulators. The energy of synchrotron radiation which comes out from 200 micron Be window is within the range 4 keV to 30 keV. Experiments will be performed for various studies like EXAFS, X-ray diffraction, X-ray tomography, XRF etc.

The sensitivity of the detector was initially measured with 8 keV X-rays and for Co-60 radiation. For 8 keV, the sensitivity obtained was 12 pA/R/hr and for Co-60 it was 40 pA/R/hr. The experimental set up is shown in Figure 1. The detector was kept in front of the Be-window. The current output of thin window ion chamber detector was measured using current meter (make Keithly). Then radiation field was measured by putting different absorbers (Aluminum & Copper) one by one in front of Mylar window of detector in the forward direction. The measurement was carried out in the normal beam orbit conditions and for the orbit in the bumped condition. Radiation field was also measured at all other angles surrounding the beam line. Table-1 Specifications of SR Detector S.No.

Detector

Specifications

1.

Overall Dimensions

126 mm x 126 mm

2.

Detector Material

Perspex

EXPERIMENTAL SET UP

3.

Outer Housing

Aluminium

The X-ray diffraction beam line installed in Indus-2 is in the commissioning phase. Along with synchrotron radiation there is some probability of bremsstrahlung radiation coming out in beam line, which is generated due to interaction of accelerated electrons with residual gas molecules, materials etc in the storage ring. Thin window ion chamber detector used for measurement has the specifications given in Table-1.

4.

End Window

Mylar (1 mg/cm2)

5.

Sensitive Volume

400 cc

6.

Gas

Air at 1 atm


Aluminum body of the detector was grounded. The two electrodes were Mylar window and a graphite plate. Voltage applied was 500 V.

47

Table-2 Radiation field measurement around Beam Line with beam orbit correction

Ion Chamber Detector

Shield Wall Be Window

Experimental Station

DCM

High Voltage Output Current

Beam Line hole

Figure 1 Schematic layout of SR beam line with the position of synchrotron radiation detector. RESULTS AND DISCUSSION When 1 mA beam current was stored in Indus-2 at 1.9 GeV energy, Radiation field was first measured with bare detector and then with Al & Cu absorber. The measured data is shown in Table-2 and Table-3. It can be clearly seen that when orbit correction was made (beam path was adjusted by changing the setting of vertical steering coil), high radiation field is obtained as compared to normal beam orbit condition. It is shown in Table-2 that 3 mm Cu or 10 mm Al was found to be sufficient to stop Synchrotron Radiation. The remaining field after 3 mm Cu or 10 mm Al was due to bremsstrahlung photons. Table-2 explains that the percentage of radiation field after 3 mm Cu with respect to no absorber is less then 1% and if we put more thickness, the reduction in radiation field is not much significant. It shows that energy of emerging radiation after 3 mm Cu was not due to Synchrotron Radiation. The transmitted radiation after the thickness was due to bremsstrahlung photons. Table-3 gives the field in normal mode of operation. The radiation field is less compared to Table 2 as the beam was not steered properly but percentage reduction is same as Table-2. We have measured radiation field at angles other than 0 degree. Results are shown in Table 4. It indicates that as we go away from the forward direction of the Be-window, field reduced in the experimental hall up to 0.1 mR/hr. In the experiment, detector was kept at 10 meters from Be window, Radiation field measurement was done using Ion chamber based survey meter (make Babyline81) with maximum range up to 2000 R/hour.

48

Condition

Output % reduction Fractional ionization with respect reduction current to no absorber With Al absorbers No absorber 39.85 nA 100 1

5 mm Al plate 10 mm Al plate 15 mm Al plate

0.15 nA

0.38

266

0.13 nA

0.33

307

0.11 nA

0.28

362

100 0.53

1 190

0.35

285

0.13

797

With Cu absorbers No absorber 39.85 nA 0.21 nA 3 mm Cu plate 6 mm Cu 0.14 nA plate 9 mm Cu 0.05 nA plate

Table-3 Radiation field measurement around Beam Line without beam orbit correction

Condition Output % ionization reduction current with respect to no absorber With Al absorbers No 199.95 nA 100 absorber 5 mm Al 0.48 nA 0.24 plate 10 mm Al 0.05 nA 0.025 plate 15 mm Al 0.06 nA 0.03 plate With Cu absorbers No 199.95 nA absorber 3 mm Cu 0.05 nA plate 6 mm Cu 0.048 nA plate 9 mm Cu 0.046 nA plate

Fractional reduction

1 417 2000 3333

100

1

0.025

3999

0.024

4166

0.023

4347


Table 4 Radiation field measurements in lateral direction with beam line (Operating parameters 1.9 GeV, 3.5 mA) Location

Dose rate

0 degree

>2000 R/hr

45 degree

0.5 mR/hr

60 degree

Nil

90 degree

0.1 mR/hr

100 degree

Nil

Victoreen 451P (Ion chamber)

(Ion chamber) In front of Be window Bare Detector With 5 mm Al plate With 10 mm Al plate With 15 mm Al plate

1) 3 mm Cu absorber will be sufficient to stop Synchrotron radiation almost 99%. 2) When we put more thickness of absorber, the reduction in radiation field is not significant. This due to the transmission of bremsstrahlung photons through absorber. 3) Table-2 & 3 shows that contribution of High energy (bremsstrahlung) comes within 1% of total radiation field measured at about 1 mA stored current at 1.9 GeV. 4) Radiation is having highly forward directional nature.

Table 5 Comparison of radiation field measurement using different detectors Conditions Area Radiation Monitor

CONCLUSION

Babyline 81(open window) (Ion chamber)

ACKNOWLEDGEMENT Authors wish to thank Dr.Mary Alex and Shri M.D.Ghodgaonkar, Head, Electronics Division for development of SR detector for us. Thanks are also due to Dr.A.K.Sinha, Dr. G.S.Lodha, Dr.R.V.Nadedkar and members of SU & MRD, RRCAT for support in the experiments. Thanks are also due to Shri Gurnam Singh, Head, IOAPDD and Shri S.Kotaiah, Project Manager, Indus-2 for encouragement. Thanks are due to Dr.V.C.Sahni, Director, RRCAT and Shri H.S.Kushwaha, Director, Health, Safety and Environ Group, BARC for constant encouragement and support.

9.2 mR/hr

> 7 R/hr

> 100 R/hr

1.4 mR/hr

280 mR/hr

1 R/hr

REFERENCES

0.3 mR/hr

21 mR/hr

0.1 R/hr

G.Singh et.al., InPAC 2007

0.1 mR/hr

7 mR/hr

0.02 R/hr

Table-5 gives the comparison of measured radiation field using 3 types of radiation monitors. The responses of these monitors are varying as thickness of chamber is varying. Among the monitors the response by Babyline81 is the highest. This is due to the reason that the low energy response of the monitor starts from 8 keV onwards which is suitable for Synchrotron Radiation measurements. Low energy response of the other monitors start from a higher value and hence the Synchrotron Contribution will be cut off in the chamber wall itself.


49

SAFETY FROM RF AND MICROWAVE RADIATION AT RRCAT, INDORE M.K.Nayak, Vipin Dev, G. Haridas, K.K.Thakkar * PK Sarkar, *DN Sharma Health Physics Unit Raja Ramanna Centre for Advanced Technology, Indore * Radiation Safety Systems Division, BARC, Mumbai Abstract : Accelerator and Laser Program at Raja Ramanna Centre for Advanced technology, Indore has many RF and Microwave generating devices. During their testing, commissioning and operation, there is possibility of leakage of RF and Microwave radiation, which is harmful to working personnel. This paper discusses various RF and MW devices, which exist at RRCAT, instruments used for measuring RF leakages, level of leakage observed, standard of protection used and corrective measures taken to reduce leakages to internationally acceptable safety levels.

1. RF & MICROWAVE GENERATING DEVICES AT CAT

2. RF & MW LEAKAGE MEASUREMENT DEVICES USED AT CAT

CAT has many electron accelerators for various applications. In order to accelerate electrons, we have to obtain electric field generated by RF or MW power provided by generating devices. In CO2 laser, RF discharge is used for population inversion. List of RF & MW devices that exist at CAT are given in Table-1.

We use RF-MW Survey Meter (Model 94 Probe), which has three orthogonally oriented thin-film dipole circuits with schottky detectors. It gives an isotropic response. The model 495 has a six-minute power density averaging function, which automatically computes, and displays the average exposure measurement over the six-minute period required by safety standards. Its

Table 1 - RF & MW Equipments at CAT Location Indus-1 Building Indus-1 SRS Microtron Block Booster Hall Storage Ring FEL Linac Indus-2 Building RF Equipment Hall Ring Tunnel ADL Building Magnetron Test Facility PL & VTL Bldg. ECR Proton Source Laser B Block CO2 Laser Lab IMA Building Radiotherapy Machine Linac

Equipment

Voltage

Pulse Rate

Frequency

Power

Klystron RF Amplifier RF Cavity RF Amplifier RF Cavity Klystron

131 KV DC 7 KV DC 10 KV 7 KV DC 30 KV 70 kV

1 Hz,1µsec Ramping

2856 MHz 31.613 MHz

5 MW* 0.4 KW

CW

31.613 MHz

2 KW

10 Hz,10µsec

2856 MHz

10 MW*

Klystron RF Amplifier RF Cavities

20 kV

CW

505.8 MHz

64 KW

650 kV

CW

505.8 MHz

290 KW

Magnetron

47 kV

250 Hz, 4µsec

2998 MHz

2 MW*

Magnetron

50 kV, 30 mA

CW

2.45 GHz

2 KW

RF Oscillator 6.5 KV DC

CW

13.56 MHz

-

Magnetron Klystron

1 Hz, 4 µsec 300Hz, 15µsec

2998 MHz 2856 MHz

1.5 MW* 6 MW

42 KV 50 KV

* Peak Power 50


Table 2 - RF & Microwave Survey Meters and Dosimeters used at CAT

1

2 3 4

RF&MW Radiation Survey Meter

Personal Dosimeter Personal Dosimeter Personal Dosimeter

Frequency response

Model no. Company

Frequency Range (MHz)

Power Density (mW/cm2)

0.01-3

100

3-30

900/f2

30-100

1

100-1000

f/100

1000-300000

10

200kHz- 40 Model 94 RAHAM, GHz

100 kHz220MHz 1 – 50 GHz 0.05 – 2.5 GHz

General Microwave Corporation, USA Nardalert, USA 8844D-0.5 Nardalert, USA 8841D-5 General Microwave H600A

response time around 3 seconds. The personnel dosimeter is a portable, battery operated instrument intended for personal use. It consists of a magnetic field sensor and shielded circuit. The magnetic field sensor detectors electromagnetic radiation from RF and microwave sources in the desired frequency ranges. The model H600A uses an isotropic array of tandom loops, each feeding a schottky diode detector. The outputs of these loops are summed to a common feed line. To enable accurate operation of the sensitive circuits within the RF Radiation Badge in the presence of RF fields, an array of shields and absorbers is used. The circuits are contained within a faraday-shielded area, which is then covered by a graded absorber to minimize field disturbances. Details about RF-MW survey Meter and Personnel dosimeters used are given below in Table 2. 3. STANDARD FOR PROTECTION USED AT CAT We have adopted the safe limits provided by American National Standards Institute (ANSI) for RF/MW, which are given below. Basic limits of exposure are expressed in specific absorption rate (SAR) which is the power absorbed per unit mass and its SI unit is watt per Kilogram (W/Kg). Threshold limit values are selected to limit the average whole body SAR to 0.4 W/Kg in any six-minute period (0.1 hour). TLVs at various frequency ranges are given below.


f- Frequency in MHz

X axis - Frequency in MHz

1000 Average power density (mW/ cm 2)

S.No. Types of detector

Table 3 – TLV for RF & Microwave at Work Place (ANSI Standard)

100 10 1 0.1 0.01

0.1

1

10

100

1000

10000

100000 1000000

Frequency (MHz)

Figure 1 Threshold limit values (TLV) for RF/MW radiation in the work place 4. RESULTS OF RF LEAKAGE MEASUREMENT AT CAT RF and Microwave leakage measurements are carried out routinely in the facilities where these devices are operated. If leakage values are found to be higher than permissible value as per ANSI standard, then corrective measures are taken to bring down the leakages within permissible limits. By appropriate control measures, it is ensured that RF or Microwave leakages are within prescribed limits at CAT. 5. CONTROL MEASURES USED AT CAT To ensure the negligible leakage from RF & MW devices, following measures were taken. a)

Shielding - Sources should be enclosed in metallic enclosures. Metallic grill, wire mesh or metal sheet like copper etc. should be put around the source. All the screws, gaskets, flanges should be tightened. Cables should also be shielded by copper, aluminum cover.

b)

Grounding - Effective grounding of metallic enclosure is required.

c)

Restricted entry - Area around equipments should be fenced with proper demarcation for restricted 51

Table 4 - RF Leakage Measurement Location

Indus-1 Building Indus-1 SRS Microtron Block

Booster Hall

Storage Ring

FEL Linac Indus-2 Building RF Equipment Hall

Equipment

Klystron Wave guide (Klystron to RF Cavity) RF Amplifier Coaxial Cable (RF Amplifier to RF Cavity) RF Cavity RF Amplifier Wave guide (RF Amplifier to RF Cavity) RF Cavity Klystron & Wave guide Klystron Transmission Line RF Cavities

Ring Tunnel ADL Building Magnetron Test Facility Magnetron body Near transmission line Near dummy load PL & VTL Bldg. ECR Proton Source Near Magnetron Power supply Near flanges of Transmission line Near Plasma Chamber Near Magnetron body Laser B Block CO2 Laser Lab Around RF Oscillator Near Transmission line Around Discharge tube IMA Building Radiotherapy machine Near Cathode of Magnetron Near Pulse Transformer Near magnetron Wave guide Industrial Linac Klystron body & Wave guide

d)

52

Power Density before corrective measures

Power Density after corrective measures

(mW/cm2)

(mW/cm2)

radiation protection and environment

radiation protection and environment

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