|Year : 2011 | Volume
| Issue : 3 | Page : 153-156
Neutron and gamma dose measurement in the working area of a research reactor using passive dosimeters
Deepa Sathian1, AK Bakshi1, Sajin Prasad2, N Kalyanasundaram2, MP Chougaonkar1
1 Radiological Physics & Advisory Division, Bhabha Atomic Research Centre, Mumbai, India
2 Radiation Safety and System Division, Bhabha Atomic Research Centre, Mumbai, India
|Date of Web Publication||27-Sep-2012|
A K Bakshi
Radiological Physics & Advisory Division, Bhabha Atomic Research Centre, Mumbai
Source of Support: None, Conflict of Interest: None
Radiation dose measurement at selected locations around the experimental facilities of a research reactor was carried out. Passive dosimeters based on CR-39 and CaSO 4 :Dy Teflon disc and 6 LiF:Mg,Ti and 7 LiF:Mg,Ti based neutron and gamma dosimeters were used in the study for the measurement of ambient dose equivalent (H*(10)). Measured neutron and gamma dose equivalent rates were found to be within the stipulated limit as per the guidelines of Atomic Energy Regulatory Board (the regulatory authority of India) for the radiation workers.
Keywords: Research reactor, CR-39, CaSO 4 :Dy disc, dosimeter
|How to cite this article:|
Sathian D, Bakshi A K, Prasad S, Kalyanasundaram N, Chougaonkar M P. Neutron and gamma dose measurement in the working area of a research reactor using passive dosimeters. Radiat Prot Environ 2011;34:153-6
|How to cite this URL:|
Sathian D, Bakshi A K, Prasad S, Kalyanasundaram N, Chougaonkar M P. Neutron and gamma dose measurement in the working area of a research reactor using passive dosimeters. Radiat Prot Environ [serial online] 2011 [cited 2020 Mar 31];34:153-6. Available from: http://www.rpe.org.in/text.asp?2011/34/3/153/101681
| 1. Introduction|| |
A national facility (DHRUVA research reactor) for neutron beam research is operated for use of neutron in condensed matter physics research. It is a 100 MW (Th) natural uranium fuelled vertical tank-type research reactor with maximum thermal neutron flux of 1.8 × 10 14 n/cm 2 /s and is the national facility for neutron beam research. Neutron scattering experiments and condensed matter research are done using neutron beams available through radial and tangential beam holes, through tube, and guide tube laboratory. [Figure 1] shows the schematic diagram of the experimental facilities used for neutron scattering / condensed matter research in DHRUVA research reactor. The researcher has to spend long duration around the experimental facilities to record and analyze the experimental data. In the present study, radiation dose equivalent H*(10) measurement of several such locations around the experimental facilities of this research reactor was carried out where there is a likelihood of occupancy.
|Figure 1: Experimental facilities for neutron scattering experiments and condensed matter research inside DHRUVA reactor hall|
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| 2. Materials and Methods|| |
The locations for measurements were chosen based on the measured dose equivalent rate by 3 He-based REM counter (Neutron Monitor model 2222A manufactured by Wedholm Medical, Sweden). Two types of passive neutron detectors were used in the experiment, namely, CR-39 based fast neutron detector and thermoluminescent dosimeter (TLD) based on CaSO 4 :Dy (Bakshi et al., 2009) and TLD 600 to cover the entire energy range of neutrons prevalent in the reactor environment. The CR-39 dosimeters are prepared using 3 cm × 3 cm × 0.625 mm foil packed along with 1 mm polyethylene radiator in a triple laminated aluminized pouch. CaSO 4 :Dy based neutron/gamma TL disc used in the study is of 6 mm diameter and 0.8 mm thickness and were packed in polyethylene pouch for irradiation. TLD 600/700 chips used in the study are of size 3 × 3 mm 2 with 0.89 mm thickness and procured earlier from M/s Harshaw Chemical, USA. Set of selected TLDs of all types having sensitivity within ±5% with respect to the average was chosen for the experiment. Time of irradiation was decided in such a way that signal-to-noise ratio of the dosimeter was always more than by a factor of 10. The irradiation time was about 15 days during December 15-31, 2011 during which the reactor was operated at 40 MW power. TLDs were readout on Harshaw 3500 research reader about a week after the irradiation. The temperature range used for the readout of TLDs was 30 °C-300 °C for CaSO 4 : Dy disc and 30 °C-270 °C for TLD 600/700 chips with heating rate of 5 °C/s.
| 3. Calibration of Tld and Cr-39 Detectors|| |
The CR-39 based dosimeter was calibrated against D 2 O moderated 252 Cf source, whereas TLD-based dosimeter was calibrated against thermal neutron and D 2 O moderated 252 Cf. For the purpose of calibration, six number each of CaSO 4 :Dy based neutron and gamma sensitive discs and TLD-600, TLD 700 were irradiated to 1 mSv of thermal neutrons from a thermalized 241 Am-Be source (Bakshi et al., 2009) and 10 mSv of D 2 O moderated 252 Cf covered with 1 mm Cd. The dose equivalent delivered to TLDs for the purpose of calibration is H*(10). The neutron fluence rate for thermalized 241 Am-Be source was measured with gold foil at the position of the irradiation and is then converted to H*(10) using a conversion coefficient given in International Organisation for Standardisation (ISO) 8529-3 (ISO 8529, 1998). Measurement of neutron fluence rate was carried out by gold foil with and without Cd cover. The ratio of bare to Cd covered gold foil activity was 5 for thermalized 241 Am-Be source. This result ensured that at the point of irradiation, contribution of thermal neutrons (average energy 0.4 eV) to total fluence is 80%. The fluence used for the purpose of calculating dose equivalent was always due to the thermal part of the total fluence. Two calibration factors, one from the irradiation of thermal neutron and another from D 2 O moderated 252 Cf stated above in terms of mSv/TL count were determined from the average of six readouts and were used appropriately depending on the locations. Calibration factor for neutron was always calculated based on the net TL count of TLDs (TL of TLD 600-TL of TLD 700 and TL of neutron sensitive CaSO 4 :Dy disc - TL of only gamma sensitive CaSO 4 :Dy disc) after subtracting the gamma part. Separately calibration of TLD-700 and CaSO 4 :Dy based gamma discs were carried out by irradiating the dosimeters to 1 mGy of 137 Cs gamma rays in air. Gamma and neutron sensitive TLDs (both neutron and gamma sensitive CaSO 4 :Dy discs and TLD 600 and TLD 700) were always irradiated together to account for gamma part.
The passive detectors used for fast neutron measurements in the reactor were CR-39 solid state nuclear track detectors which are in use for the country wide neutron personnel monitoring program (Pal et al., 2011). These detectors were initially calibrated with ISO recommended (ISO Report 8529-I, 2001) neutron source 252 Cf+D 2 O, since the neutron spectrum outside the reactor is generally moderated fission spectrum. For this purpose, five number of CR-39 foils along with 1 mm thick polyethylene proton radiator packed in a triple laminated pouch (CR-39 detector) were irradiated in air at 75 cm distance from a D 2 O moderated 252 Cf source for a neutron dose equivalent of 1 mSv (ambient dose equivalent, H*(10)). It may be noted that 252 Cf+D 2 O moderator is covered with 1 mm Cd. After irradiation, these foils were processed along with five number of unirradiated control foils with optimized etching conditions of 5 h low frequency (100 Hz), 40 min high frequency (3.5 kHz) at 1300 V in 7N KOH solution maintained at a constant temperature of 60 °C. The foils after etching were washed, dried, and evaluated with the help of an image analyzer system with 2× magnification and tracks having diameter in the range of 60-250 μ m were counted for dose evaluation. A response factor of 135 ± 8 tracks cm−2 mSv−1 was obtained after subtraction of background tracks and this factor was used for the evaluation of neutron dose at various locations of the reactor. All the passive dosimeters used in the present study were meant for the measurement of ambient dose equivalent. ,,,
| 4. Results and Discussion|| |
[Table 1] shows the ambient dose equivalent rate measured by TLDs and CR-39 at the selected locations inside the reactor hall. Dose rate measured by TLDs represent the lower energy neutrons from thermal to 200 keV, whereas CR-39 represents that of higher energy neutrons from 100 keV to 6 MeV. Dose equivalent rates measured by TLDs and CR-39 dosimeters are added to get the total dose rate. It may be noted that at every location both neutron and gamma sensitive TLDs were deployed. Neutron dose equivalent (H*(10)) was always determined based on the net TL count [TL (Gamma+Neutron) -TL (Gamma)]. Based on the measurement of passive dosimeters, the maximum dose equivalent rate was found to be about 51 μ Sv/h near the beam hole R 3001, whereas the minimum was of the order of 1 μ Sv/h near the neutron monitor and GT lab. Upon confirmation from the health physicists of the reactor about the presence of thermal neutron, thermal neutron calibration factor was applied for two locations, namely, GT Lab and near neutron monitor opposite HS-1019, For all other locations, calibration factor of intermediate energy neutrons (based on D 2 O moderated 252 Cf) was applied considering the fact that the neutrons those which are leaking out of the reactor are all moderated. In the present experiment, REM counter was used to measure the dose rate at different locations to get an idea of the dose rate before deploying the passive dosimeters. Hence, the dose rate values presented in the last column of [Table 2] are the indication of dose rate at the respective locations. For the purpose of dosimetry, the dose rate values based on passive dosimeters are considered to be more accurate as REM counter values can vary up to ±20%. However, for the purpose of comparison on the trend of dose rate with respect to locations as indicated by the REM counter, data on the dose rate measured by REM counter are also presented in the last column of [Table 2]. Reasonable agreement between the trend in the measured dose rate by REM counter and that of passive dosimeters for all locations was observed except HS-1019 left side. The disagreement in the dose rate indicated by REM counter and that of measured by passive dosimeters at the location mentioned above could be attributed to the fact that (i) REM counter measure instantaneous dose rate, whereas passive detectors measure cumulative dose rate and (ii) REM counter, due to its bigger size (9 in. diameter moderating sphere with BF 3 counter at the centre) could not be positioned at the same point as that of TLDs, and (iii) difference in the neutron spectrum at the location of measurement with that of used for the calibration of TLDs and CR-39 detectors.
|Table 1: Measured Dose Rate of Lower Energy Neutrons (Thermal to 200 keV) by CaSO4:Dy based TLD and 6LiF:Mg,Ti Chips (TLD 600) and Fast Neutron (100 keV-6 MeV) by CR-39 based Neutron dosimeter at Different Locations of DHRUVA Reactor|
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|Table 2: Measured Gamma Dose Rate by CaSO4:Dy Teflon Discs and 7LiF:Mg, Ti Chips (TLD-700) at Different Locations of DHRUVA Reactor|
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It can also be seen from [Table 2] that there is a difference in the measured dose rate by neutron sensitive CaSO 4 :Dy Teflon disc and TLD-600. This could be attributed to the difference in the neutron response of the detectors to neutron spectrum due to the difference in their composition and energy deposition mechanism although the neutron sensitive compound ( 6 LiF) is same in both the TLDs. [Table 2] gives the details of gamma dose rates measured by TLD 700 and CaSO 4 :Dy based Teflon disc for all the locations except one selected for neutron measurement. A good correlation in the trend of gamma dose rate measured by TLDs could be observed with that of neutron dose rate with respect to the locations. The highest gamma dose rate is found to be near R 3001 beam hole, whereas the minimum is near HS-1019 opposite to area neutron monitor. The difference in the dose rate measured at several locations by the two types of TLDs could be attributed to the difference in their energy response. Except one location R 3001, the combined dose rate of neutron and gamma is well within the stipulated limits laid down by the Regulatory Authority, AERB. The overall error in the evaluation of ambient dose equivalent using TLDs and CR-39 is about ±10% including the sensitivity variation among the TLDs/CR-39 detectors.
| 5. Conclusions|| |
Dose rate due to neutron and gamma are measured using passive dosimeters and compared with that of REM counter values inside the experimental hall of a research reactor. Except few locations, neutron dose rates measured by two methods (instantaneous and cumulative) are in good agreement. Considering the occupancy of the locations, which is either rare or occasional, the measured dose equivalent rates are considered to be well within the stipulated limit as per the guidelines of Atomic Energy Regulatory Board (the regulatory authority of India) for radiation workers.
| References|| |
|1.||Bakshi AK, Pradhan AS, Kher RK, Srivastava K, Varadharajan G, Chatterjee S, et al. Study on the Response of Indigenously Developed CaSO 4 : Dy Phosphor-based Neutron Dosemeter, Radiation Protection Dosimetry. 2009;133:73-80. |
|2.||International Organisation for Standardisation Reference Neutron Radiations, Part-1 Characteristics and method of production, ISO - 8529-1 (2001). |
|3.||International Organisation for Standardisation. Calibration of area and personal dosimeters and determination of their response as a function of neutron energy and angle of incidence. ISO 8529-3 (1998). |
|4.||Pal R, Sathian D, Jayalakshmi V, Bakshi AK, Chougaonkar MP and Mayya YS, Present Status of Fast Neutron Personnel Dosimetry System based on CR-39 Solid State Nuclear Track Detectors, BARC Report BARC/2011/E/015 (2011). |
[Table 1], [Table 2]