comparison of imaging plates with track detectors for fast-neutron ...

Radiation Protection Dosimetry (2004), Vol. 110, Nos 1-4, pp. 333±336 doi:10.1093/rpd/nch170

COMPARISON OF IMAGING PLATES WITH TRACK DETECTORS FOR FAST-NEUTRON DOSIMETRY

A. Belafrites1, A. Nourreddine1, D. Mouhssine1, A. Nachab1, A. Pape1, A. Boucenna2 and F. FernaÂndez3 1 Institut de Recherches Subatomiques, 23 Rue du Lúss, BP 28, F-67037 Strasbourg Cedex 2, France 2 DeÂpartement de Physique, Universite de SeÂtif, Setif 19000, Algeria 3 Grup de FõÂsica de les Radiacions, Departament de FõÂsica, Universitat Aut onoma de Barcelona, E-08193 Barcelona, Spain

Imaging plate (IP) radiation detectors are widely used in industrial radiography, medical imagery and autoradiography. When an IP is exposed to ionising radiation, some of the energy is absorbed to form a latent image. The energy stored, which is proportional to the dose received, can be liberated by a selective optical stimulation and collected to reconstitute the distribution of the ionising radiation on the IP. In this work, IPs for use in fast-neutron measurements are characterised. The response of our IP dosemeters in conjunction with their reading system was found to be linear in dose between 75 mSv and 10 mSv. This performance is compared with those of dosemeters based on the plastic track detectors PN3 and CR-39.

A schematic diagram of the IP dosemeters used in this work is presented in Figure 1. These dosemeters are constituted of two 1 mm Pb plates to reduce gamma rays that accompany the neutron ®eld, two 1 mm Al plates to absorb beta rays after neutron capture by the Pb and two IPs (31 mm  41 mm  0.36 mm) separated by a polyethylene (CH2)n radiator(3). IP2 (see Figure 1) is assumed to receive the same radiations and in the same amount as IP1 except for the additional protons ejected from the proton radiator(4). Thus by taking the signal in IP2 less that in IP1, one obtains the signal due to the recoil protons. The 1 mm thickness of (CH2)n

PN3 detectors (NE Technology, Beenham, England), 20 mm  25 mm  1.5 mm(7) were placed in a pouch (Plastiplast, France, composed from inside to outside of Al (40 mm), (CH2)n (20 mm), cellulose (40 mm) (see Figure 2). This arrangement served for the PN3-based dosemeters. After exposition the PN3

neutrons Pb Al

IP2

IMAGING PLATE DOSEMETER

TRACK DOSEMETERS

Radiator

In the framework of a study of passive dosimetry and its applications, the response of imaging plates (IPs) incorporating BaFBr:Eu2‡ and of the solid state nuclear track detectors (SSNTDs) PN3 and CR-39 to fast-neutrons from an Am±Be source delivering a dose of neutrons equal to 0.022 mSv hÿ1 at 1 m(1) at normal incidence was measured. The IPs offer an interesting alternative to SSNTDs due to the fact that they can be erased and reused many times and do not require etching. This paper presents a description of IPs used in fast-neutron dosimetry and a comparison of their response with that of two SSNTDs. Experimental results for PN3 are compared to a Monte Carlo simulation using the MCNP (Monte Carlo n-particles) code(2).

ensures a proton yield independent of radiator thickness for Am±Be neutrons(5). The dosemeters were exposed to up to 10 mSv neutron doses in order to determine the response and estimate the detection threshold. Immediately after exposition, the IPs were scanned by a DenOptix(6) laser reader that visualises a (symbolic) darkening produced by the exposure in units of `optical density'. The scanning cycle takes 2.25 min for the 1±29 IPs mounted on the reader drum.

IP1

INTRODUCTION

Al Pb

Plastic case

Corresponding author: abdelmjid.nourreddine@ IReS.in2p3.fr

Figure 1. Con®guration of the imaging plate dosemeter.

333 Radiation Protection Dosimetry Vol. 110, Nos 1-4 ã Oxford University Press 2004; all rights reserved

A. BELAFRITES ET AL.

The CR-39 detectors were electrochemically etched using 6 N KOH at 60 C according to the following steps(8):

Pouch

PN3

(i) 20 kV cmÿ1 (rms) at 50 Hz for 5 h, (ii) 20 kV cmÿ1 (rms) at 2 kHz for 1 h, (iii) 15 min post-etching at 0 V.

Al (CH2)n Cellulose

neutrons

Cellulose (CH2)n Al

Pouch

The track density of CR-39 was measured over an area of 0.53 cm2, using an imageJ(9) software system coupled to a SONY PAV-A7E camera in turn connected to a PC with a WinTV card(10), without the use of a microscope. RESULTS

Climafol

Methacrylate

CR-39 39 CR-

(CH2)n

neutrons

Climafol

Figure 2. Con®guration of the PN3 dosemeter.

Figure 3. Con®guration of the CR-39 dosemeter.

detectors were pre-etched for 1 h in a mixture of 60% (by volume) methanol and 40% 6.25 N NaOH at 70 C. This pre-etch removes about 60 mm from each side, polishes the surfaces and eliminates most super®cial alpha-particle background tracks and scratches. The background track density after the pre-etch was 21  3 cmÿ2. Next, the PN3 detectors were etched for 6 h in 6.25 N NaOH at 70 C. Finally the detectors were neutralised in a weak HCl solution and washed with warm then cold tap water. The PN3 track counting was performed using an automated system composed of a movable XYZ stage and a CCD camera mounted on an optical microscope at magni®cation of 10x. The whole is connected to a PC for the piloting of the stage and the acquisition and treatment of images coming from the CCD camera via a numerical interface card. Analysis of the track images was performed by Visilog software (Noesis S. A., Courtabúuf, France). The CR-39 dosemeters (Figure 3) consisted of 3 mm (CH2)n as radiator, CR-39 (Pershore Mouldings Ltd., England) of dimensions 20 mm  20 mm  0.5 mm and 5 mm methacrylate as mechanical backing support. The dosemeter was put in a Climafol pouch made of (CH2)n covered with aluminum.

The response of the IP and SSNTD (PN3 and CR-39) dosemeters is linear as a function of dose up to at least 10 mSv. For the IP dosemeters, the dose response was found to be linear up to 10 mSv (Figure 4) with a detection threshold with our DenOptix reading system of 75 mSv, or possibly less. These results and those for the SSNTD are given as a function of the dose D (dose rate  time). A conversion must be applied to transform the absorbed dose into dose equivalent H. The sensitivity of the PN3 dosemeter was found to be 380  54 tracks cmÿ2 mSvÿ1 for neutrons from the Am±Be source (Figure 5), comparable to a result 429  90 tracks per cm2 per mSv found in the literature(11). An MCNP simulation gave a linear response to dose with an average of 384 tracks per cm2 per mSv. The ¯uence-to-dose conversion factor was taken from ISO 8529(12). The method of estimating the detection threshold for PN3 was done according to Curie(13). The response of the CR-39 dosemeter was 274  72 tracks per cm2 per mSv for Am±Be neutrons, this value being somewhat less than the 401  5 tracks per cm2 per mSv obtained by other authors(14). The difference might be explained by their different electrochemical etching conditions (1st electrochemical etching 30% KOH, 60 C, 5 h, 22 kVpeak cmÿ1, 2nd electrochemical etching 30% KOH, 60 C, 30 min, 25 kVpeak cmÿ1, 10 kHz). In Figure 6 is given the dose response of the CR-39 dosemeter. The response is linear to 10 mSv. No Monte Carlo simulation was made here for CR-39 since this has already been reported(8). Table 1 groups together a comparison among the dosemeters of the present study. CONCLUSION The response of the IP dosemeter is linear in dose with a threshold of 75 mSv or less. In spite of a slight a signal loss due to fading, the IP presents several advantages, being more sensitive, reusable and faster to read in comparison with PN3 or CR-39. These

334

1 .6 x 1 0

5

1 .2 x 1 0

5

8 .0 x 1 0

4 .0 x 1 0

4 O p tic a l d e n s ity

O p tic a l d e n s ity

IMAGING PLATE AND TRACK DETECTOR NEUTRON DOSIMETRY

4

3 .0 × 1 0

4

2 .5 × 1 0

4

2 .0 × 1 0

4

1 .5 × 1 0

4

1 .0 × 1 0

4

5 .0 × 1 0

3

0 .0 0 .0

0 .2

0 .4

0 .6

0 .8

1 .0

H (m S v )

0 .0 0

2

4

6

8

10

H (m S v )

-2

Track density (cm )

Figure 4. Response of the IP dosemeter to Am±Be neutrons.

4x10

3

3x10

3

2x10

3

1x10

3

0 0

2

4

6

8

10

H (mSv) Figure 5. Response of the PN3 dosemeter to Am±Be neutrons.

®rst results show the interest of using this IP dosemeter for fast-neutron dosimetry. The study shows that IPs can be adapted to the passive dosimetry of low to high doses and can replace SSNTD

dosemeters in fast-neutron dosimetry. Some other advantages of the IP dosemeter are given in Table 1. In the future, work will be carried out to study fading in the IP dosemeter.

335

2x10

3

1x10

3

-2

Track density (cm )

A. BELAFRITES ET AL.

0 0

2

4

6

8

10

H (mSv) Figure 6. Response of the CR-39 dosemeter to Am±Be neutrons. Table 1. Certain comparisons among dosemeters. Dosemeters based on SSNTDs (PN3, CR-39) Insensitive to gamma rays Necessitate etching (several hours) Visual reading (automatic) Reading time on the order of 15 min per detector Not reusable No fading Without radiator for PN3 Threshold 70 mSv for PN3

5.

Dosemeter based on IPs (BaFBr:Eu2‡)

6.

Sensitive to all ionizing radiations No etching

7.

Reading (automatic) by scanning laser Reading time 2.25 min for 1±29 IPs Reusable Fading (4% one hour after the irradiation, less afterwards) With a (CH2)n radiator Threshold 75 mSv (or less)

8.

9. 10.

11.

REFERENCES 1. Lorch, E. A. Neutron spectra of 241Am/B, 241Am/Be, 241 Am/F, 242Cm/Be, 238Pu/13C and 252Cf isotopic neutron sources. Int. J. Radiat. Isot. 24, 585±591 (1973). 2. Briesmeister, J. F. MCNPÐA General Monte Carlo N-Particle Transport Code, LA-13079-M, Los Alamos National Laboratory, Los Alamos, NM (2000). 3. Mouhssine, D., Nourreddine, A., Heilmann, C., Fernandez, F. and Pape, A. CaracteÂrisation des deÂtecteurs photostimulables (BaFBr:Eu2‡): Applications aÁ la dosimeÂtrie passive. JourneÂes de Lard, 18±19 octobre 2001, Barcelone (2001). 4. Boukhair, A., Heilmann, C., Nourreddine, A., Pape, A. and Portal G. Fast-neutron and gamma ray dosimetry

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with imaging plates. Radiat. Meas. 34, 513±516 (2001). Fernandez, F., Domingo, C., Baixeras, C., Luguera, E., Zamani, M. and Debeauvais, M. Fast-neutron dosimetry with CR-39 using electrochemical etching. Nucl. Tracks Radiat. Meas. 19(1±4), 467±470 (1991). DenOptix, Digital Imaging System, Gendex Dental XRay Doc. 10/97 Rev 1 (1997). www.gendexkray.com/ deroptix.htm. Fiechtner, A., Gm ur, K. and Wernli, C. A personal neutron dosimetry system based on etched track and automatic readout by Autoscan 60. Radiat. Prot. Dosim. 70(1±4), 157±160 (1997). Fernandez, F., Bouassoule, T., Domingo, C., Luguera, E. and Baixeras, C. Response of a CR-39 fast-neutron dosemeter with a polyethylene converter improved with Makrofol. Radiat. Prot. Dosim. 66(1±4), 343±347 (1996). Freely available at: http://rsb.info.nih.gov/ij Amgharou, K., Font, Ll., Allbaracin, D., Domingo, C., Fernandez, F. and Baixeras C. Semi-automatic evaluation system for nuclear track detectors applied to radon measurement. Radiat. Meas. 33, 203±209 (2001). Harvey, J. R., French, A. P., Jackson, M., Renouf, M. C. and Weeks, A. R. An automated neutron dosimetry system based on chemical etch of CR-39. Radiat. Prot. Dosim. 70(1±4), 127±132 (1997). ISO Neutron reference radiations for calibrating neutron measuring devices used for radiation protection purposes and for determining their response as a function of neutron energy. International Standard ISO 8529 (1989). Curie, L. A., Limits for qualitative detection and quantitative determination: application to radiochemistry. Anal. Chem. 40, 586±593 (1968). Turek, K. and Dajk o, G. Comparison of experimental and calculated responses of CR-39 to neutron spectra of 252 Cf sources. Radiat. Meas. 34, 625±628 Am±Be and (2001).