Radiation Protection and Environment

ARTICLE
Year
: 2011  |  Volume : 34  |  Issue : 1  |  Page : 44--46

Response of CR-39 based personnel neutron dosemeter in terms of directional dose equivalent, in free air and on phantom


Rupali R Pal, Deepa Sathian, V Jayalakshmi, MP Chougaonkar 
 Radiological Physics and Advisory Division, BARC, Mumbai, India

Correspondence Address:
Rupali R Pal
Radiological Physics and Advisory Division, BARC, Mumbai
India

Abstract

CR-39 is the most sensitive of nuclear track detectors for protons and is recommended as an effective neutron dosimeter because of itSQs low threshold energy of 100 keV neutrons. The fraction of protons that gives detectable tracks in CR-39 depends on the energy of the proton angle of incidence and etching conditions. As a consequence the registration efficiency of neutrons in the CR-39 plastics used for neutron personnel monitoring is strongly influenced by the direction of radiation incidence. This paper presents the relative response of CR-39 at varying neutron incident angles, for 241 Am-Be neutron source spectra in free air and on ISO phantom, in terms of operational quantities. It is observed that the angular dependence of CR-39 for irradiations in air and on phantom is essentially the same indicating that the phantom does not affect the directional response of CR-39.



How to cite this article:
Pal RR, Sathian D, Jayalakshmi V, Chougaonkar M P. Response of CR-39 based personnel neutron dosemeter in terms of directional dose equivalent, in free air and on phantom.Radiat Prot Environ 2011;34:44-46


How to cite this URL:
Pal RR, Sathian D, Jayalakshmi V, Chougaonkar M P. Response of CR-39 based personnel neutron dosemeter in terms of directional dose equivalent, in free air and on phantom. Radiat Prot Environ [serial online] 2011 [cited 2020 Aug 4 ];34:44-46
Available from: http://www.rpe.org.in/text.asp?2011/34/1/44/93949


Full Text

 1. Introduction



CR-39, a thermosetting plastic is the most sensitive of nuclear track detectors and is recommended as an effective neutron dosimeter. It has a wide energy response from 100KeV to 14 MeV, a high sensitivity to protons for track formation (Zapparoli et al., 1986). These properties of CR-39 make it suitable for neutron personnel monitoring for radiation workers in neutron environment such as reactor and accelerators. However, CR-39 track etched detectors show a strong angular dependence due to the fact that charged particles are recorded in the detector surface only if the angle of radiation incidence with respect to normal is more than the critical angle of registration θc (Jang-Lyul Kim et al, 1988). The critical angle depends on the detector material, the etching conditions, the type of charged particle, and varies significantly with particle energy (Piesch et al, 1989). As a consequence the registration efficiency of neutrons in the CR-39 plastics used for neutron personnel monitoring is strongly influenced by the direction of radiation incidence (Tanner et al, 2005).

ICRU in the publication 39, has introduced a dose quantity namely 10mm directional Dose Equivalent H'(10) for individual monitoring that depends on the direction of the incident radiation. This quantity is suggested by the ICRU as a calibration analog to the individual monitoring, the individual dose equivalent Hp (10) (ICRU 39, 1985). It is stated that the directional dose equivalent H'(10), ambient dose equivalent H*(10) in free air and individual dose equivalent Hp (10) on phantom, are operational quantities designed to provide reasonable estimate of the appropriate protection quantity under normal working conditions. It must be emphasized that the radiation fields to which people are exposed are usually associated with broad energy spectra and various irradiation geometries (ICRP 74, 1996). In this paper, the response of CR-39 exposed to 241 Am-Be neutron source spectra in free air and on ISO phantom for varying angle of incidence have been presented in terms of operational quantities.

 2. Materials and Methods



2.1 Description of dosimeters and irradiation

A typical neutron dosimeter used for personnel monitoring at BARC, comprises of CR-39 dosimeters of dimensions 3 cm by 3 cm with 1mm polyethylene radiator in front of it, sealed in an air tight triple laminated aluminized pouch. The CR-39 sheets, 625 micron thick (dosimetric grade) and curing time of 32 hrs, are procured from Pershore Moulding, U.K.

The reference conditions used for irradiation were (i) neutron energy spectra of 241 Am-Be, average energy: 4.4MeV and yield: 2.5 x 10 6 n/sec (iii) radiation background: 1 ΅Sv /hr and (iv) distance between detector and source: 75 cm. (v) different conditions of exposure used were, dosimeter in free air and on ISO phantom.

For obtaining the angular response, sets of 10 dosimeters each along with 1mm radiator in front were irradiated in free air and on phantom separately, to a neutron fluence (Φ) of 2.44E+06 n/cm 2 , corresponding to 1 mSv dose equivalent at 75 cm. The neutron dose delivered was calculated by applying the neutron fluence to dose conversion coefficients H *Φ(10) (391 pSv cm -2 ) for air and H pΦ(10) (411 pSv cm -2) for phantom in Am-Be spectra (ISO 8529-3,1998). The dosemeters were irradiated at varying angles in multiples of 15° from 0°-90° with respect to the normal to the plane of detector. Irradiation of CR-39 dosemeter was carried out in a low scatter laboratory. Measurements on phantom were made using an ISO water slab phantom having outer dimensions of 30 cmΧ30 cmΧ15 cm and the walls made of PMMA. The front wall is of thickness 2.5 mm, all other walls of thickness 10 mm, and the phantom is filled with water. The personnel dosimeters were fixed on the front face of the phantom such that the reference point of the dosimeter coincided with normal to phantom front face. The variation of angle of incidence was effected by rotating the phantom around the vertical axis which passes through the point of test. When the ISO water slab phantom was used as described above, no corrections were applied to the reading of the dosimeter under test due to differences in backscatter between this and the ICRU tissue slab phantom (International Standard ISO 8529-3,1998).

2.2 Processing technique

The ECE technique is a two step Electro Chemical Etching technique (Massand et al, BARC Report,1990), carried out in specially designed Electro Chemical Etching Cell (ECE cell) with 7 N KOH as etchant, maintained at temperature of 60°C. For the first step - etching at low frequency, the power supply operates at 1250 volts constant potential and low frequency of 100 Hz for 4 hours, followed by the second step - etching at high frequency of 3.5 kHz for 40 mins duration, using field strength of 25kVcm -1 . The processed dosimeters were evaluated by counting tracks in area of 1 cm 2 (track cm -2 ) using a PC based image analysis system.

 3. Results and Discussion



The response (tracks cm -2 ) of CR-39, irradiated to 241 Am-Be spectra in free air and on ISO phantom, for various angles of incidence and the corresponding fluence to personal dose conversion coefficient H p,slabΦ(10,α) for 241 Am-Be spectra (ISO 8529-3,1998) is presented in [Table 1]. The neutron dose in terms of operational quantities has been evaluated from the response at various angle of incidence is also shown in [Table 1]. The dose is derived by multiplying the calibration factor and the response of CR-39. The calibration factor N for the dosimeter under calibration is obtained by relation: N = Φ hΦ / M in mSv-cm 2 tracks -1 where Mis the measured net tracks cm -2 mSv -1 at 0°, Φ is fluence and h Φ is the fluence to dose conversion factor. The calibration factors are 0.00386 mSv-cm 2 track -1 for air and 0.00374 mSv-cm 2 track -1 , for phantom measurements.{Table 1}

It is observed that as the angle of incidence of the neutron normal to dosimeter, approaches 90°, the response drops rapidly due to the fact that the recoil protons enter almost parallel to the surface of the dosimeter having incident angles within the critical angle and are not registered as these low depth (angular) tracks are removed by the etching process. The angular dependence of the dosimeter, in air and phantom is nearly the same indicating that phantom does not influence the angular response of CR-39. These observations are in good agreement with the data available in literature (Hankins et al, 1987).

The relative response of CR-39 dosimeter in terms of H'(10, α)/H*(10) in free air and in terms of Hp(10,α)/Hp(10,0°) on ISO phantom at varying angle of incidence is shown in [Figure 1]. It can be seen from the [Figure 1], that the ratio Hp(10,α)/Hp(10,0°) calculated using the fluence and corresponding dose conversion coefficient for 241 Am-Be spectra, falls with increasing angle of incidence but fall is more steep in case of the ratios H'(10, α)/H*(10) measured in free air and Hp(10,α)/Hp(10,0°) measured on ISO phantom, with increasing incident neutron angles. This is due to the influence of the detector.{Figure 1}

 4. Conclusions



The response of CR-39 neutron dosimeters decreases as the angle of incidence increases, and the angular dependence is the same for both irradiations in air and phantom. CR-39 based neutron dosimeters worn by persons in workplace are irradiated to neutrons from various incident angles. Hence, it is important to evaluate it's performance with respect to variation in angles. As per International Standards Organization (ISO) standards on performance and test requirements of passive neutron dosimeters, the arithmetic mean of the response of a dosimeter at angles of incidence of 0°, 15°, 30°, 45° and 60° should not differ by more than 40% from the response at normal incidence. In our neutron badges, the average angular dependence of CR-39 detector differs by 20% with respect to normal incidence, which is within the limits recommended by ISO.

 5. Acknowledgements



The authors wish to acknowledge the keen interest shown by Shri H.S. Kushwaha, Director HS&E Group, BARC and the constant encouragement given by Dr.Y. S. Mayya, Head, RP&AD in the activities related to Fast Neutron Personnel Monitoring.

 6. References





Hankins D.E, Steven H.G. and Joane W. (1987), Personnel neutron dosimetry using electrochemically etched CR-39 Foils, Lawrence Livermore National Laboratory Report No. UCRL-53833.ICRP Publication 74 (1996), Conversion coefficients for use in external radiation, Vol. 26(3/4). ISO 8529-3: (1998(E)), Reference neutron radiations-Part 3: Calibration of area and personal dosimeters and determination of their response as a function of neutron energy and angle of incidence.Jang-Lyul Kim, et al. (1988), Energy and Angular Response of CR-39 Neutron track Detector, J of the Korean Nuclear Society, Vol 20, (2). Massand O.P, Kundu H.K., Marathe P.K. and Supe S.J. (1990), BARC Report, 1528.Piesch E, Al-Najjar S.A.R and Ninomiya K, (1989), Neutron dosimetry with CR-39 Track detectors using ECE: Recent improvements Dosimetric characteristics and aspects of routine application. Radiation Protection Dosimetry, Vol.27 (4), 215-230.Tanner R.J, Bartlett D.T, and Hager L.G. (2005), Operational and Dosimetric Characteristics of Etched-track Neutron Detectors in routine Neutron Radiation Protection Dosimetry, Radiation Measurements, 40, 549-559.Zapparoli G., Tommasino L., Djeffal S. and Maiorana A. (1986), Additional results with electrochemically etched CR-39 Neutron dosemeters. Int. J of Radiation Application and instruments Nucl. Tracks, Vol.12, (1-6), 675-678.