Available online at www.sciencedirect.com
Radiation Measurements 36 (2003) 499 – 502 www.elsevier.com/locate/radmeas
Angular response characteristic of pyramid shape fast neutron dosimeter using CR-39 &lms D. Gopalani∗ , S. Saravanan, A.S. Jodha, S. Kumar Defence Laboratory, Ratanada Palace, Jodhpur-342 011, India Received 21 October 2002; received in revised form 18 March 2003; accepted 5 May 2003
Abstract CR-39 is a highly sensitive etched track detector for neutron monitoring and dosimetry applications but its dose equivalent response is strongly direction dependent with respect to the incident neutrons. This is considered to be a major drawback for their use. In the present study, an attempt has been made to develop a pyramid shaped dosimeter, which consists of polyethylene material of thickness 1 mm with the provision to hold three CR-39 &lms at an angle of 35◦ to each other. The response of CR-39 in this con&guration under optimum electrochemical etching at elevated temperature have been found nearly angular independent and therefore the dosimeter can be used for neutron monitoring, i.e. personnel as well as area monitoring. c 2003 Elsevier Ltd. All rights reserved. Keywords: SSNTD; Fast neutron dosimeter; Electrochemical etching of CR-39; Pyramid shaped neutron dosimeter
1. Introduction CR-39 material found potential application in fast neutron monitoring (Cartwright et al., 1978), as it is insensitive to beta and gamma radiations and recorded signals are stable for a long period of time. Recoil protons, generated in a converter of high hydrogen content or in the detector material itself, can be detected for neutron energies ranging from few tens of keV to above 20 MeV with the use of electrochemical etching procedure. To use CR-39 &lms as fast neutron dosimeter, its dose equivalent response should be reasonably independent of both neutron energy and angle of incidence. For the energy independence behaviour of CR-39 &lm the ratio of track density to dose equivalent remains constant for di>erent neutron energies. The angular independence behaviour of CR-39 &lm indicates that its dose equivalent response for various incident angles of neutrons remains identical. In real situations, for fast neutrons, the dose equivalent response of electrochemically etched CR-39 &lm is strongly energy and direction dependent. The reasons for energy dependence are ∗ Corresponding author. Tel.: +91-0291-510554; fax: +910291-511191. E-mail address:
[email protected] (D. Gopalani).
thickness and material of the radiator, amount of etching, the strength of electric &eld, the polymerization conditions of CR-39 and the selection of the tracks that are counted. While angular dependence is mainly attributed to two causes: (i) small value of VT =VB ratio of the proton tracks (where VT is the track etching rate and VB is the bulk etching rate). Consequently, the cone angle and thus the critical angle of the etched tracks will be very wide. This will therefore limit the registration especially for those neutrons scattered with an angle ¿45◦ . The critical angle is energy dependent and with increasing proton energy the critical angle increases and this indicates the angular dependence. (ii) Protons are usually forward scattered and this tendency increases with the neutron energy. Improvement of the etching conditions could increase the VT =VB ratio only to a certain extent. Various workers have studied these aspects in detail (Al-Najjar and Piesch, 1987; Cross et al., 1986; Gopalani et al., 1997; Mattullah and Durrani, 1987; Pitt et al., 1985; Tommasino et al., 1984). The etching at an elevated temperature appears to o>er the best neutron energy response (Gopalani et al., 1997; Tommasino et al.,1984) in the range 0.4 –4:4 MeV and these conditions can be used for energy independence behaviour of CR-39 &lms. The Bat dose equivalent response was achieved by various suggested geometries but
c 2003 Elsevier Ltd. All rights reserved. 1350-4487/03/$ - see front matter doi:10.1016/S1350-4487(03)00180-X
500
D. Gopalani et al. / Radiation Measurements 36 (2003) 499 – 502
side of the detector was kept in contact with a very weak electrolyte (∼ 0:1 N NaOH). The electrochemical etching spot density was measured under 400× magni&cation using a Zenval optical microscope. To intercompare pyramid dosimeter with Bat shaped CR-39 &lm dosimeter, 2 cm × 2 cm CR-39 &lms along with 1 mm polyethylene radiator in a black plastic badge were also exposed with neutrons. After exposure, these &lms were etched with the same electrochemical conditions. The intercomparison of pyramid shape dosimeter and Bat shape dosimeter was also carried out with simple chemical etching in 6 N NaOH at 70◦ C for 5 h. 3. Results and discussion
Fig. 1. Pyramid dosimeter.
development of such a dosimeter based on these geometries has not yet been achieved due to its complexity in the &eld use. In the present study, a simple and an alternative approach has been worked out by developing a pyramid-shaped dosimeter. This dosimeter consists of polyethylene material of thickness 1 mm with the provision to hold three CR-39 &lms of size 2 cm × 2 cm. These &lms were kept at an angle of 35◦ to the axis of dosimeter. The &tting details of three &lms in dosimeter and then their exposure view with relative to incident neutrons are shown in the Fig. 1 (a and b), respectively. 2. Experimental procedure CR-39 &lms (M/S Pershore Moulding Ltd.) were kept in the pyramid shape holder and exposed with neutrons (241 Am–Be of activity 37 GBq) at di>erent angles (0◦ , 30◦ , 45◦ , 60◦ and 90◦ ) of incidence. After exposure these foils were etched electrochemically using two step technique (Gopalani et al., 1997) i.e. at 1500Vrms with a frequency of 100 Hz for 5 h followed immediately by an etching at 1500 Vrms with a frequency of 2 kHz for 1 h. The other
The angular response of pyramid dosimeter and Bat dosimeter of CR-39 &lms etched electrochemically is shown in Fig. 2. From the &gure it is clear that for Bat shape dosimeter the relative track density at larger angles, i.e. 60◦ –90◦ is less by 43% as compared with normal incidence of neutrons whereas for pyramid dosimeter this track density is less only by 22%, which o>ers a nearly angular independent response within an acceptable variation. Fig. 3 shows the angular response of pyramid dosimeter and Bat dosimeter of CR-39 &lms etched chemically. The relative track density for pyramid dosimeter is 45% less at larger angles (¿60◦ ) than at normal incidence (0◦ ), while in the Bat dosimeter this track density is 60% less. There is reduction in track density for the chemical etching in pyramid and Bat shape dosimeter as compared to electrochemical etching due to the di>erence in the registration of recoil proton tracks in the two techniques. The sensitivity (tr/n) of pyramid dosimeter was also measured by exposing with di>erent neutron Buence (i.e. from 1:57×107 to 2:86×107 n=cm2 ) of 241 Am–Be source. The irradiated &lms were etched electrochemically and track density was measured. The sensitivity obtained was 0:48 × 10−3 tr/n. After determining the sensitivity of the pyramid dosimeter, it was used for measurement of neutron Buence in multi-neutron irradiation facility of Defence Lab, Jodhpur. In this facility 252 Cf source of strength ∼13 g is housed. The pyramid shape dosimeter was exposed at a distance of 9 cm and the Buence was calculated by, Neutron Buence = Track Density/Sensitivity. The theoretical value of neutron Buence obtained for this facility with these exposure conditions is 10 × 107 n=cm2 whereas experimentally it was found as 9 × 107 n=cm2 . The di>erence between these values is within the error limit and hence this dosimeter is suitable for neutron mapping also. The consistent behaviour of pyramid dosimeter was also studied by exposing four dosimeters with doses of 1 m Sv. It is found that the variation in the track density results was within 5%.
D. Gopalani et al. / Radiation Measurements 36 (2003) 499 – 502
501
Electrochemical Etching
Pyramid Flat
Relative Track Density (%)
100
80
60
40
20
0 0
20
40
60
80
100
Neutron Incident Angle (in Degree) Fig. 2. Relative track density as a function of
241 Am–Be
neutron incidence angle for pyramid and Bat dosimeter etched electrochemically. Chemical Etching
Relative Track Density (%)
100
Pyramid Flat
80
60
40
20
0 0
20
40
60
80
100
Neutron Incident Angle (in Degree) Fig. 3. Relative track density as a function of
241 Am–Be
neutron incidence angle for pyramid and Bat dosimeter etched chemically.
The calibration curve at normal incidence was also plotted between the track density (tr/cm2 ) and neutron dose (mSv) for a pyramid dosimeter. The results are shown in Fig. 4. It is apparent from the &gure that the track density increases with the neutron dose, so a linear relationship was established by a least-squares &t, Tr = aDn + b; where Tr is the total recoil proton track density of three &lms (tr/cm2 ) and Dn is the fast neutron dose (mSv). The values of a and b are 1725 (tr/cm2 /mSv) and 955 (tr/cm2 ), respectively.
4. Conclusion A practical fast neutron dosimeter in the shape of pyramid has been successfully designed and developed which has angular independent behaviour and is useful for neutron mapping and personnel monitoring applications. Acknowledgements The authors are greatful to Shri R.K.Syal, Director and Dr. M.P.Chacharkar, Assoc. Director, Defence Laboratory, Jodhpur for their keen interest, useful comments
502
D. Gopalani et al. / Radiation Measurements 36 (2003) 499 – 502 14000
Track density (Tr/cm2 )
12000
10000
8000
6000
4000
2000 0
2
4
6
8
10
12
Neutron Dose (mSv) Fig. 4. Calibration curve between fast neutron dose and recoil proton tracks recorded on CR-39 &lms kept in pyramid dosimeter.
and valuable guidance during the entire course of this work. References Al-Najjar, S.A.R., Piesch, E., 1987. Angular response characteristics of a CR-39 detector of cylindrical geometry. Radiat. Protect. Dosim. 20 (1–2), 67–70. Cartwright, B.G., Shirk, E.K., Price, P.B., 1978. A nuclear track recording polymer of unique sensitivity and resolution. Nucl. Instrum. Methods 153, 457–460. Cross, W.G., Arneja, A., Ing, H., 1986. The response of electrochemically etched CR-39 to protons of 10 keV to 3 MeV. Nucl. Tracks 12, 649–652.
Gopalani, D., Baheti, G.L., Kumar, S., Ramaseshu, P., 1997. Angular characterization of water based multidirectional fast neutron dosimetry. Radiat. Protect. Dosim. 74 (3), 145–150. Mattullah, Durrani, S.A., 1987. A cubical fast neutron dosimeter based on electrochemically etched CR-39 detector with polymetric front radiators. Radiat. Protect. Dosim. 20(1–2), 77–80. Pitt, E., Scharmann, A., Werner, B., 1985. Electrochemical etching and angular dependence of CR-39 neutron detection. In: Proceedings of the Fifth Symposium on Neutron Dosimetry, EUR 9762, Munich, Neuherberg, pp. 501–510. Tommasino, L., Zapparoli, G., Spiezia, P., GriNth, R.V., Espinosa, G.G., 1984. Di>erent etching processes of damage track detectors for personnel neutron dosimetry. In: Proceedings of the Twelveth International Conference on SSNTD, Mexico, Nucl. Tracks 8, 335 –339.
