Dosimetry of External Photon Fields Using Unfolding of Scintillation Gamma Spectrometry Data

Transcription

1 Dosimetry of External Photon Fields Using Unfolding of Scintillation J. Kluson, Tomas Cechak, Prague ntroduction The mathematical processing (unfolding) of pulse height spectra from a scintillation detector enables to calculate the photon fluence rate energy distribution in a measured photon field. The data processing is based on the knowledge of the detection system response function [, 7] (usually approximated by the response matrix) anddirectional dependence respectively. The other dosimetric quantities - e.g. exposure or air kerma rate, superficial/specific activities of radionuclides, dispersed in the soil surface layer (using the suitable source distribution and experimental arrangement models), etc. - can be calculated from the photon fluence rate. Main advantages of this approach are: 1) no experimental calibration and corrections of the energy dependence are necessary (both included in the mathematical model and resulting response matrix), 2) high sensitivity (depending on detector volume), 3) spectrometric information, 4) calculation of responses can be provided for any experimental arrangement, including special cases (e.g. accidental contamination of large territory), when experimental calibration is impossible. Method and applications The basic principle of designed method is apparent from the flow diagram in Fig. 1. Full procedure of the detector response matrix calculations is described by second flow diagram in Fig. 2. The individual experimental arrangements models and Monte Carlo partic1e transport simulation are used for detector response calculations. Examples of the sets of calculated 3"x3" Na(T) detector response functions for broad parallel beam (incident perpendicularly to the detector endface) and energy intervals up to 3 and 10 Me V are in Fig. 3, 4. The response matrixes with energy bins 15, 30, 60, 90 kev and different energy intervals (according to the application) were prepared from calculated responses. The iterative (Scofield-Gold) method [8, 2] (to solve corresponding matrix equation) is usually used for unfolding of experimental spectra. Using of neural networks for data processing and analysis was also tested. Developed method of the processing and interpretation of the experimental spectrometric data was used to a wide scope of applications. Dosimetric properties of the method were tested and examples are shown in Fig. 5 (calibration and test of linearity for very low doses region) and Fig. 6 (high sensitivity demonstration for the extremely low background field, both measurements at UDO lab in Asse salt mine). Next tests of discussed method were performed during the EU intercomparison experiments at Ris0 and PTB Braunschweig, 1994, where errors of the air kerma rate calculated values were determined. Errors for the free-field calibration experiments (for 137Cs, 60CO, 226Ra and natural background) have not exceeded ±3 % and errors for the high energy calibration (PTB 6 MeV facility) have been in the 5 % band. Figs. 7 and 8 show examples of results of the individual measurements at free field calibration (for 226Ra source) and high energy calibration respectively. The experimental spectrum and calculated air kerma 369

2 rate energy distribution respectively are weapon test site radiological assessment shown on the top and bottom part of the [3], etc. both figures respectively. The spectra Calibration factors for determination of the codes, mean count rates and total air kerma superficial/specific activities from scintilrates (exposure rates) are marked on the fi- lation or semiconductor detectors spectrogures. metry data were calculated (for defined ex- Example of developed method application peri mental arrangements). Calculations in the high energy field is shown on Fig. 9 were done for natural radionuclides (4 K (field in VVER 440 nuclear power plant re- and D, Th - series) as well as for 37CSand actor hall, 1 meter above centre of the set of another expected significant mansteam generator Jid).The older, 075 x 75 -made contaminants. mm detector and response matrix with wide The calibration (response functions calcuenergy bins (200 key) were used in this lations) and data processing method for aircase. borne gamma ray scintillation spectrometry Example of environmental in-situ gamma (system ENMOS, produced by PCODAS ray spectrometry result (air kerma rate Group. nc., Canada) were developed on energy distribution in the usual reference the similar basis [4]. Method was successpoint meter above ground) from partly fully introduced and can be used for fast restored depository of wastes from uranium and effective operational or accidental raore pre-processing is shown in the Fig. 10. dioactive contamination monitoring/mapp- Method was applied for monitoring of the ing over the large areas [5]. Method makes gamma fields in the spent fuel temporary it possible to calculate concentrations of the storage and study of its influence on the natural radionuclides and man-made conradiation fields in the storage close neigh- taminants (with given depth distributions bourhood. Measured differential air kerma models) from airborne spectrometry data. rate distributions in one of the selected re- The individual scans (i.e. usually 1 s expeference points near to storage building are rimental spectra), measured over the monishown in the Figs. 11 and 12. First measu- tored area, are processed and corresponding rement was done for empty storage, second maps of individual radionuclides disperafter storing first 6 containers (Castor type). sions can be constructed. The exposure (or Comparison of those results gives the first air kerma) rate distribution over the same evidence of negligible impact of the storage area can be calculated from determined inon radiation in neighbourhood. dividual radionuclides concentrations. Conclusions Described method was successfully used for environmental gamma ray fields study, monitoring and analysis, natural background components and variations studies [6], study of environmental impact of uranium ore mining and processing, assessment of doses in dwellings, workplaces monitoring (including mentioned nuclear power plant and spent fuel temporary storage), processing of the environmental spectral data from Chernobyl region, works within the project of Semipalatinsk nuclear References 1. Berger, MJ., Seltzer, S.M.: Response Functions for Sodium odine Scintillation Detectors. NM 104 (1972) Clements, P..: A Discussion of Variations on the Scofield-Gold terative Deconvolution Technique. AERE - R 7222 (1972). 3. Hill, P., Hille, R., Bouisset, P., Calmet, D., Kluson, J., Smogulov, S., Seysebaev, A.: Radiological Assessment of Long Term Effects at the Semipalatinsk Testsite. NATO-Semipalatinsk Project 1995/1996, Berichte des Forschungs- 370

3 zentrums Ji.ilich, JUL-, 3325 December ] 996, SSN , K]uson, J.: Calculation of the Fie]d Spectrometer Response and Calibration for the Operational and Accidenta] Monitoring of Nuclear Power Plant Neighbourhood. Progress in Nucl. Energy 24 (1990) K]uson, J., Ma]usek, A, Cechak, T., Jurza, P.: Airborne Gamma-Ray Spectrometry in Environmental and Accidenta] Monitoring. n Proceedings of RP A9 nternationa] Congress on Radiation Protection, April ] 4-] 9 ] 996, Vienna, Austria. 6, Pernicka, F., Kluson, J.: Variations of Natural Radiation Background and their Effect on Detector Response. Environ. nternal. 22, Suppl. ] (1996) , Rogers, D.W.O.: More Realistic Monte Carlo Calculations of Photon Detector Response Functions. NM ]99 (1982) 53]-548, 8. Scofield, N.E.: terative Unfolding. NAS - NS - 3]07 (1963) 108. Figure 1: Flow diagram of the method for the full data processing and analysis pulse height spectrum incident photon spectrum responses, response matrix, unfolding energy distribution of photon fluence rate detector directional dependence dosimetric quantities definition (CRU 33) 371

4 Figure 2: Calculation of the detector response and response matrix - flow diagram model of the source and geometry (photon field or contamined area, etc.) calculated angular-energy of photon fluence rate model of the detector distribution in the given photon field energy deposition spectrum detector response response matrix 372 Monte Carlo calculation Monte Carlo calculation convolution with Gaussfunction set of detector responses for required energy interval

5 ....un.1\ Dosimetry of External Photon Fields Using Unfolding of Scintillation Figure 3 Calculated 3"x3" Nal(T) Detector Response Functions \ f\ :::::::::::: x: ::::: :::::::::D... ::::: ::::: :p::... 7-' c ;:2...- P;;;/.... ;! " - V c ='".----/ f\ r\ L:\"?. ""' j<. ::.\.....\ i' =..../.. \ :::::::f.... \J....ḷ :r '. /:... f.7\. T\ i L, \/::: :::: V \J" ::.v l o Figure Calculated 3"x3" Nal(T) Detector Response Functions o

6 Figure 5: 160 uno lab in the Asse salt mine, test of linearity Nal(T) 3"x3' (S&D); correl. coell. r= for low doses <!J E- o; 80 >.; lu Ql : Ref. val. [ngy/h) Figure 6: uno lab in the Asse salt mine, background with shield 2.5 Differential Air Kerma Rate Distribution -1 dk,dt = ngy h,..., '7> 2.0 Q) '..0 > o S " [.lj "'Cl -- '" '" 0.5 "'Cl

7 Figure 7: Free field calibration, 226Rit source, distance 10 metres Measured Pulse Height Energy Distribution dn/dt = 573. S'. szu-ralo.gsc... : \..J\. t\,..'? ;;..,,.., ;.'. i".;'-::... :.'.., :.',:.' Calculated Air Kerma Rate Energy Distribution dkidt = ngy h- (dx/dt = 13.2 t-tr h'l) '> 400!.l _'$. 350 '..s::: >. 300 o s:: '" 100 N

8 Figure 8: PTB - 6 MeV facility, calibration measurement, distance 3 meters from the target Measured Pulse Height Energy Distribution dn/dt = 4848 S 1.. szu6m3.gsp - -\: -.- j \ \ : Energy [Me V]. :!'!.'T.....'-:..."' Calculated Air Kerma Rate Energy Distribution dkidt = 4436 ngy h- t,.--, ]6000 ' '"7.c ] 2000 >-. ] 0000 '--' 'u '" 4000 N o, a Energy [Me V] 7 8 9

9 n Dosimetry of External Photon Fields Using Unfolding of Scintillation Figure 9: VVER 440 nuclear power plant reactor hall, steam generator lid Differential Pulse Height Distribution dn/dt = 3218 S- pg75a.spe.- ' , -. -.: n' - - -i-.._ _ 500 ':>Q) ;:;s '...r::: >, ḍ s W 200. "'''0 100 Differential Air Kerma Rate Distribution dkjdt = 1306 ngy h- t o o c: 0.9 g ;:l.c 0.8 B en C<:l 0.5 aq) 0.4.:= <e: 0.3 Q). 0.2 ;S ;:, S 0.1 ;:l U

10 ---.- H 0.2 >.n_ Dosimetry of External Photon Fields Using Unfolding of Scintillation Figure 10: '\ Depository of wastes from uranium ore pre-processing, 1 meter above ground Differential Pulse Height Distribution dn/dt = 4334 s-1 s&d-bg2a.gsc _::--:J\. \ n_ ---'-- -- m_ u_" r\ "'1.:.. :. \..';r 'J..:!C.iL _._ n_ ".=:'.: DO _ '> Q) "7 800,..c: >. 0!::: 600 '--' 4:5 -- '" 400 N 200 Differential Air Kerma Rate Distribution dk,dt = 546 ngy h- t 0 D.a L' \.0!::: ::l.0 u n 0.8 E Q 0.60::: ell Q) '\0; rjl Q) ;;. S 0.\ ::l U

11 n '0 n 0 Dosimetry of External Photon Fields Using Unfolding of Scintillation Figure 11: Background in reference point near to spent fuel temporary storage (empty storage) ' 300 :.:;E '..c: 250 >. o J.l N Differential Air Kerma Rate Distribution dk,dt = 68.9 ngy h !:: 00c.2ke 0.9 g ::l,d 0.8 '0 +-' '" 0.7 is<ll 0.60::: <:<:l <ll :> 0.2 ::l n.l U n.o Figure 12: ';> <ll :.:;E 250 '..c: ' --0!f3 --- "io SO 50 Background in reference point near to spent fuel temporary storage (with 6 Castor containers) Differential Air Kerma Rate Distribution dk,dt = 68.0 ngy h !:: - l03c.2kel 0.9.g ::l.0 +-' '" 0.70" <ll +-' 0.6 <:<:l !::l < 0.3 <ll. 2,. 'E ::l U