Energy Response Characteristics of Several Neutron Measuring Devices Determined By Using the Scattered Neutron Calibration Fields of KAERI

Transcription

1 Energy Response Characteristics of Several Neutron Measuring Devices Determined By Using the Scattered Neutron Calibration s of KAERI B.H. Kim 1, J.L. Kim 1, S.Y. Chang 1, J.K. Chang 1, G. Cho 2 1 Korea Atomic Energy Research Institute, P.O.Box105, Taejon, , Korea 2 Department of Nuclear Engineering, Korea Advanced Institute of Science and Technology, Taejon, , Korea INTRODUCTION Neutron measuring devices used for radiation monitoring in the field of radiation protection have very different response characteristics with energy. Mismatches of measured quantities with the variation of energy spectra come from monitoring instruments that are not calibrated using neutron fields similar to the workplaces. This is the reason why the Realistic Neutron Calibration s (RNCF) (1) should be used in the calibration of neutron remmeters and dosimeters if possible. Normal calibrations with the radioactive neutron sources recommended by the International Organization for Standardization (ISO) are performed using the direct component of incident neutron, mainly fast neutrons, to the devices to be calibrated at the scatter-free or low scatter facility. The effects of scattered neutron in the reference point were corrected or subtracted in determining the response of the neutron measuring devices and generally the dose contribution of these scattered neutrons to the total dose is not significant. Most neutron working fields to be monitored have a considerable component of scattered neutrons with relatively low energy including thermal neutron and multi-directional or not unidirectional. Typical neutron remmeters overrespond in the intermediate energy region and the albedo type personnel dosimeters do so in the low energy region. Therefore, when the calibration is performed, it is necessary to use the representative neutron field encountered in the workplace for minimizing an error due to the spectral difference at the time of monitoring. At recently, the construction and use of RNCF were suggested by ISO to calibrate the neutron measuring devices, because the RNCF would be assumed as closer to the real working field and more appropriate than the neutron reference fields of ISO-8529 (2). Ten kinds of the scattered neutron fields for calibration have been constructed using a means similar to the new draft of the ISO (PTB s method) at the Korea Atomic Energy Research Institute (KAERI) and used to determine the energy response characteristics of several neutron measurement devices used popularly for radiation protection purposes. MATERIALS AND METHOD Irradiation Facility and Neutron s of KAERI The neutron irradiation room of KAERI is a bunker room whose dimensions are 8 m long, 6 m wide and 6 m high, and it is enclosed with 60 cm thick concrete walls and ceiling. Two kinds of neutron sources, 252 Cf and 241 AmBe can be used for calibration. 252 Cf is placed at the center of the room and moved from storage to an irradiation position of a height of 2.9 m at the time of irradiation and this source is mainly used for routine calibration. In using 252 Cf, bare and D 2 O moderated neutron spectra are available. D 2 O moderator assembly is a 0.53 mm thick Cd-covered sphere with diameter of 32.3 cm to cut-off thermal neutrons, so there are few thermal components in the reference calibration spectrum. Descriptions of the scattered neutron fields of KAERI constructed in the irradiation room are shown in Table 1. Two kinds of ordinary phantoms, a Poly-Methyl Meta-Acrylate (PMMA) and the ISO water filled phantom, used for irradiation of personal dosemeter and the shadow cone were placed to produce the scattered neutron fields between the source and the calibration point. The shadow cone was made of iron with a length of 50 cm. The front end was 20 cm iron with a diameter of 30 cm and the back end 30 cm air with a diameter of 35 cm. All reference positions were fixed at a center-to-center distance of 100 cm between the source and the detectors except for place E, where it is behind the wall in the irradiation room. The thickness of the concrete wall is 21 cm. The reference calibration point in this study was usually 100 cm from the effective center of the source. The dosimetric quantities including the neutron fluence rate at the test point were determined by means of the Bonner Multi-sphere Spectrometry System (BMSS) (3). The calibration of the BMSS was performed in the KAERI neutron irradiation room by using the 252 Cf source with the same technique as done by Liu et al. (4). Integral properties such as the spectral mean neutron energy, Eave., the neutron flux and dose rate on the reference date of Jan. 1, 2000, the fractional contribution of thermal neutrons to the total fluence, ϕ th /ϕ, and the fluence to ambient/personal dose equivalent conversion factors averaged over the neutron energy spectra, h*(10) and h p (10), with its averaged mean neutron energy, E*ave. and Ep,ave. are summarized in Table 2 for ten scattered neutron fields (5). The mean neutron energies and the conversion factors for the ambient and personal 1

2 dose equivalents were calculated using the values obtained from the interpolation method of cubic spline for the conversion factors for mono-energetic neutrons given by ICRP 74 (6). Table 1. Description of KAERI scattered neutron fields. Notation Description 252 Cf (bare) 252 Cf (D 2 O) [A] [B] [C] [D] [E] [F] [G] [H] [I] Direct and scattered; unmoderated source only Scattered; using the PMMA phantom (40 x 40 x 15 cm 3, contacted to the source guide holder) Scattered; using the shadow cone (distance from the source to the front end : 32 cm) Scattered; using the PMMA phantom(same as B) and polyethylene sheet with 5% boron (61x 61 x 5 cm 3, distance from the source to the surface : 40 cm) Behind the concrete wall in the irradiation room (distance from the source : 385 cm) Direct and scattered; moderated source only Scattered; using the PMMA phantom(same as B, distance from the source : 32 cm) Scattered; using the shadow cone (distance from the source to the front end : 32 cm) Scattered; using the water filled phantom (30 x 30 x 15 cm 3, 32 cm from the source) 241 AmBe [J] Direct and scattered; unmoderated source only Table 2. Integral properties of several scattered neutron fields in the neutron irradiation room of KAERI. Eave. a) (MeV) Flux (n.cm -2 sec -1 ) ϕ th /ϕ (%) Dose rate b) (msvhr -1 ) h*(10)/ E*ave c) (psvcm 2 /MeV) h p (10)/Ep,ave c) (psvcm 2 /MeV) [A] / /1.64 [B] / /1.42 [C] / /1.03 [D] / /1.78 [E] / /1.22 [F] / /1.47 [G] / /1.19 [H] / /0.70 [I] / /0.98 [J] / /3.30 a) spectral mean energy. b) ambient dose equivalent rate, H*(10). c) fluence to ambient (and personal) dose equivalent conversion factor and dose equivalent averaged mean energy reffered to the conversion factors given by ICRP-74 (6). Detector Response Response characteristics of several neutron measurement devices were determined by getting the quotients of the detector indication by the dose equivalents given in table 2, which were determined by using the BMSS of KAERI. All neutron detectors were calibrated in the field of D 2 O moderated 252 Cf source and the indication values of the detector multiplied by the calibration factor. At the time of calibration, the effect of scattered neutrons for each neutron detector was corrected by the semi-empirical fitting method of ISO (7). The six kinds of neutron detectors and their calibration factors are listed in Table 3. 2

3 Table 3. Neutron detectors and calibration factors for D 2 O moderated 252 Cf. Detector model Detection method Calibration facor 1) NG-2 (NRC, USA) Cylindrical moderator and cylindrical BF 3 proportional counter ESP2/NRD (Eberline, USA) Spherical moderator and cylindrical BF 3 proportional counter Ludlum 12-4 (Ludlum, USA) Spherical moderator and spherical BF 3 proportional counter REM-500 (HPI, USA) Tissue equivalent proportional counter Dineutron (Nardeux, France) Two spherical 3 H proportional counter (2.5 & 4.2 inch) A300 (Teledyne, USA) Thermoluminescence dosimeter ) 1) Calibration factor for the D 2 O moderated 252 Cf. 2) Dosimeters for calibration were irradiated on the ISO water-filled phantom at a distance of 50 cm using the same neutron source as 1) (8). RESULTS AND DISCUSSION Indications of all remmeters were lower than the quantities determined by BMSS, as a reference. This means that BMSS overestimates the dose equivalents for most neutron fields to be monitored and this trend of BMSS is still reasonable in view of conservative radiation protection. When the calibration was performed in the field of D 2 O moderated 252 Cf, dose equivalents were over estimated from 20 % to 80 % roughly for the moderator type remmmeters in these measurements. These values can be reduced by the adoption of a calibration factor for a 252 Cf source. This is not common because the discrepancies between the reading values and the conventional true dose equivalents basically result from the big change in detector response with energy. Two ways to solve this problem were suggested by Naismith et al. (9): one is to use a similar neutron field to the workplace in the calibration of neutron detectors for use and the other is to apply the correction factors which are categorized for the specific neutron fields using the relations between the detector responses and some databased neutron fields. The responses and calibration factors were obtained for the ten kinds of neutron fields of KAERI and are given in Tables 4 and 5, and Figures 1 and 2. Even though these fields are not representative for the workplaces, the figures given in Tables 4 and 5 show that it is necessary to correct the response according to the neutron fields. In the case of TLD used in the field of the 252 Cf source only, [A], corrections more than four times should be applied to the readings if TLDs are conventionally calibrated in the field of D 2 O moderated 252 Cf. All moderator type remmeters have the similar response shape with energy and the calibration factors ranged from 1.22 ~ 1.92, whereas REM-500 and Dineutron show different responses in the case of more scattered neutron fields or at low energy region. REM-500 has relatively low responses at low energies as shown in the paper by Thomas (10). In the case of Dineutron, special care is necessary to correct the low sensitivity when it is used in more scattered neutron fields for monitoring. Although it is not possible to determine the energy response by the definition without the use of mono-energetic neutron sources, this paper shows what type of instruments are necessary to a specific neutron field and how important it is to correct the response of neutron detectors used in workplace monitoring. Table 4. Responses of neutron detectors in several scattered neutron fields. [A] [B] [C] [D] [E] [F] [G] [H] [I] [J] Detector NG ESP2/NRD Ludlunm REM Dineutron A300(TLD)

4 Table 5. Calibration factors of neutron detectors in the several scattered neutron fields. Detector [A] [B] [C] [D] [E] [F] [G] [H] [I] [J] NG ESP2/NRD Ludlum REM Dineutron A300(TLD) Fig. 1. Responses (up) and calibration factors (down) of remmeters for the scattered neutron fields. 4

5 Fig. 2. Responses (left) and calibration factors (right) of TLD for the scattered neutron fields. ACKNOWLEDGEMENTS This study was the partial product of the national projects for long term nuclear energy development supported by the Ministry of Science and Technology. REFERENCES 1. International Organization for Standardization (ISO), Reference Neutron Radiations: Characteristics and Methods of Production of Simulated Workplace Neutron s, International Standard ISO-12789, Draft (1997) 2. International Organization for Standardization (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) 3. R.L.Bramlett et al., A New Type of Neutron Spectrometer, Nuc. Inst. Meth., (9), 1-12 (1960) 4. J.C. Liu, F. Hajnal, C.S. Sims and J. Kuiper, Neutron Spectral Measurements at ORNL, Radiat. Prot. Dosim.30(3), (1990) 5. B.H. Kim, J.L. Kim, S.Y. Chang and G. Cho, Scattered Neutron Calibration s of KAERI, J. Nucl. Sci.Technol. (in press) 6. ICRP and ICRU, Conversion Coefficients for Use in Radiological Protection Against External Radiation, Report of the Joint Task Group, ICRP 74 (1997), ICRU 57 (1998) 7. International Organization for Standardization (ISO), Reference Neutron Radiations : Dosimetry Fundamentala Related to the Basic Quantities Characterizing the Radiarion, International Standard ISO , Draft (1995) 8. S.Y. Chang, B.H. Kim and J.L. Kim, Intercomparison of Neutron Personnel Dosimeters in Korea, Radiat. Prot. Dosim. (in press) 9. Naismith, B.R.L. Siebert and D.J. Thomas, Response of Neutron Dosemeters in Radiation Protection Environments: An Investigation of Techniques to improve Estimates of Dose Equivalent, Radiat. Prot. Dosim. 70(1-4), (1997) 10. D.J. Thomas and G.C. Taylor, Response Function Measurements for the REM 500: a Microdosimetric Counter Based Area Survey Instrument, Proceedings of Int l Conf. on Radiat. Dosim. and Safety, Mar. 31- Apr. 2, Taipei, Taiwan, (1997) 5