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THERMAL NEUTRON DETECTION by ACTIVATION of Caso4dy + Kbr THERMOLUMINESCENT PHOSPHORS

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THERMAL NEUTRON DETECTION by ACTIVATION of Caso4dy + Kbr THERMOLUMINESCENT PHOSPHORS THERMAL NEUTRON DETECTION BY ACTIVATION OF CaSO4Dy + KBr THERMOLUMINESCENT PHOSPHORS Ana Maria Pinho Leite Goidon and R. Muccillo PUBLICAÇÃO IEA 527 MAIO/1979 IEA-Pub-527 CONSELHO DELIBERATIVO MEMBROS Klaus Riinach - Pmidann Roberto D'Utra Vaz Helcio Modatto da Ceuta Ivano Humbart Marchati Admar Ctrvellini PARTICIPANTES Ragina Elisabet* Azevedo Btratta FléVio Gori SUPERINTENDENTE Hômulo Ribeiro Piaroni PUBLICAÇÃO IEA 627 MAIO/1078 IEA-Pub-627 THERMAL NEUTRON DETECTION BY ACTIVATION OF CtóO40y + KBr THERMOLUMMESCENT PHOSPHORS Am Mtrla Pbtho Ltttt Gordon md R. MuecHIo CENTRO DE PROTEÇÃO RAOIOLÓGICA E OOSIMETRIA CPRDAMD 041 oi INMOIA ATÔMICA PAULO - MAML S*i* PUBLICAÇÃO IEA INIS Ctttgoriw and Dticripmri E4t B26 Thtrinolumfntfctnt dotimvtry Nfutfon dctictlon CUchim wHam Tiwrmoluml NOM: A **m. ***** • HMMttM THERMAL NEUTRON DETECTION BY ACTIVATION OF CaSO4£ry THERMOLUMINESCF.NT PHOSPHORS* Ana Maria Pinho Leite Gofdon and R. Muceillo ABSTRACT Thermolumineicenoe (TLI itudiei to detect thermal neutron» were performed in cold pressed CsS04:0,1%Dy • KBr eemplei. The detection it bewd on the Mil irradietion of the CeSO^Dy TL phosphor by the Br iiotopet activeted by exposure to a mixed neutrongemme field. In 1970 a new method for the detection of thermal neutrons by .the Thermoluminescence |TL) auto-activation technique was proposed . It consists in the measurement of the TL output of a phosphor previously exposed to a neutron-gamma field, thermally annealed to bleach the radiation effects, ang stored to undergo self-irradiation due to the decay of the thremal neutron activation products. The TL output can then be related to the activation of the phosphor and consequently to the thermal neutron fluence. The main advantage of the method is based on the ability to detect thermal neutrons without gamma contributions. In ref (3) the self-irradiation was provided by 45Ca. which is • beta-emitter, resulting from the activation of **Ca in natural CaF; phosphor. The low 44Ca natural abundance (2.08%) as well as its relative low thermal neutron cross section (0.7 b) lead to a minimum fluence detection of the order of 10* neutrons/cm1. Later on CaFj.Dy (TLD-200) commercial phosphors were used allowing an increase of the sensitivity of the method by more than two orders of magnitude12'. An obvious extension of the method to the detection of fast neutrons has been proposed»'4'6'. In this work, improvements in the self-activciion method are reported by combining a highly beta-sensitive TL phosphor with a highly effective activator, i.e., by mixing i TL phosphor with a material with high thermal neutron cross sections. 0.1% Dy-doped C«SO4 was choosen as the phosphor due to its high sensitivity to ionizing radiation as well as to the use with which it if prepared in the laboratory171. KBr was choosen as the activator due to relatively high thermal neutron activation crott sections for the ivgamma reactions: "Br to >0mBr(a6b; half-life: IB min), "Br to '°Br(2.9b; 4,6 h). and*'BrtotJBr(3.3b;35.9h|. Detailed experiments to find out the optimized phosphor activator proportion, the TL response to thermal neutrons, and the nuclidei responsible for the self-irradiition will be described. An estimai* of the minimum detectable fluenee will also be made. 0.1*4 Dy-doped CaSO« crystals were grown following the procedure described by Yameshite and coli'". Bafora usage the powder* ware selected for grain sites from 86 to 185 microns and annealed at eOOGfth In air. Reagent grade KBr powder was selected a* above, annealed at 400C/1 h, and than addad to CaSO4:Dy to be cold-pressed MMOIb/W) to discs of 1 mm thickness. All sample» were 1*1 Researeh work partially supported by IACA under contract ft. 14M/R2/RI and ComWo Nacional ea fnenje Nuclear. Approved for puDHeatKm M ICA mW% In December 1Í7*. evaluated for TL peak heights on a commercial 2000 A/B Harshaw thermoluminescent re Pre-selection of the pellet! were done by weighing end by glow curve analysis of samples exposed to ** Co gamma rays. The selected samples were exposed to n-gamme fields of • swimming pool Reactor at this Institute operating at 2 Mw producing fast fission neutrons plus the order of 10' R/h of photons and •bout 10'' n/cm'.s to 10'2 n/cm'.s at our irradiation positions. A 192Cf source immersed in a water tank with about 1.7 x 10' n/cma.j at a choosen irradiation position was also used for low exposures. The conventional activation of gold foils was the method used 10 determine the thermal neutron flux densities. The experimental procedure leading to the evaluation of the thermal neutron induced thermoluminescence was as follows: 1 — CaSO«:Dy + KBr samples are exposed to the mixed n-gamm» field during the irradiation 2-the irradiated sample is thermally annealed at 600C/15min to eliminate the defects produced by the gamma and neutron prompt dose; 3 — the sample is stored at room temperature for a period of 24 h to undergo self-irradiation; 4-the TL glow curve is then obtained giving a TL signal prpportional to the thermal neutron flux. The proportionality constant is determined by following the above sequence but exposing the specimen to a known thermal neutron flux density. Figure 1 shows the TL output of CaSO*:Dy + KBr pellets, obtained according to the experimental procedure described above, for various relative proportions of phosphor and activator. The total mass of each pellet was 180mg. The thermal neutron flux in the irradiation position was determined by the gold foil activation technique to ke 6.66x10" thermal neutrons/cm1 .s; the exposure time was 20 s. As the TL outputs of the different pellets was within less than 25%, the optimized proportion was choosen according to the one providing easiness of handling of the pellets: 100 mg of CaSO4:Dy and 80 mg of KSr. The TL glow curve for that pellet is shown in the insert Henceforth all the results are shown for the optimized pellets, and the TL signal is taken as the height of the higher amplitude TL peak. In Figure 2 the thermoluminescence response of CaSO* :0y + KBr to different thermal neutron fkiences is shown. For this work another irradiation position at the reactor was used, with a thermal neutron flux density of 6 x 10" neutrons/cm2.s. The upper curve was obtained for samples allowed to undergo self-irradiation for a period of 24 h. The bottom curve represents the second TL reading, namely, after obtaining the TL output resulting from the 24 h self-irradiation, the sample is stored for 1 h and its TL analysis is made again. These results show that even though the TL analysis is a destructive one, further measurements can be made. In Figure 3 similar results are shown for pellets exposed to the radiation field of • 400microgrims J"Cf source immersed In water. The fluence range was from 10'° to 10" thermal n/cm1, determined by gold foil activation techniques. Every irradiation has been done using one bare specimen and another wrapped in 1 mm Cd foil in order to deduce the contribution due to Intermediate neutrons. The minimum detectable flueno* «MM estimated to be of the order of 10*thermel neutrons/cm1. Succesflvt TL reading* at one hour intervala were carried out in • CeSO4 Dy • KBr pellet exposed to • mixed radiation field In the Reactor Station. The return summarized in Figure 4 snow • CaS04.Dy+KBr (180 mg) w 100 SOO »00 T(«C) 'c D XI ô I. O -J 1 1 1 0.5 1.0 2.0 mass ratio 1 - MMnducad tNrmolumlnflMOTM output of 180 mg prflfU of mlxtd CtóO4 Oy (phoíphor) md KBr (aetlraior) for wrylng tf» raltthw •mouim of pNwphor md tetlrator In tfw m»x- tun. Ttmmé mutron flux dmhy: 6.66 x 10*' n/em* J; kridlittvo tlmt: 20i; ttong* tfciM: 24 h. liMrt: typM glow eunt for • pdtot «vi* 1.28 mm ratio CaSC^Dy+KBr (l80mg) 10 TIME (s) MMnducod TL output of 180 mg p«<l«tt of &804£>V • KBr at • function of lrr«ltotlon «rot. Thimwi mutnm flu» dwMtty: 6J0 x 101' n/cm'*; itH-lmdlttion tbim: 24 h luppw «urw) «id 1 h afttr tta flrat rMdbMj (bottom ourw) CaStyDy+KBr (180mg) 100. I 30 100 NEUTRON FLUENCEdCTacm2) TL output of 180 m| p^M* of CtfO«Syf KBr it • function of tNnral iwutren fliMnoa. Th«ml imitren fkiM tftmity: 6J0x 10" n/om'j; Hor^t tbnt:24li CaSC^rDy+KBr (!80mg) A: 20 min 100. HALF-LIFE B:4.5h Lü L 10 20 30 TIME(h) - Hourly decay of tha itlMnducad TL In 180 mg palltti of CtS0«fiy • KBr decrease which was fitted to a sum of exponential decays. Only two half-lives could da evaluated with a fair accuracy: 20min and 4.5h, which were assigned to somBr and *°Br, respectively. The same identification process was used previously in other materials112'31. The main results suggest then the use of CaSO, 0.1 %Dy + KBr pellets and thermoluminesceno» as a complimentary technique in thermal neutron detection. RESUMO Foram foitoi Mudo* d* T»rmolumina«céncla (TU para • dataccfo de neutrons térmico» em amostra» da CaSO4:0,1% Dy + KBr compactadas a frio. A daNcçao bataia-w na autc-irradiacto da fósforo» da CaSO4:Dy paloa iaôtopot da bromo ativados pala axposiçfo am campo» mino» ntuiron-oama. REFERÊNCIAS BIBLIOGRÁFICAS* 1. MAYHUGH, M. R. & WATANABE. S. Fast neutron detection by phosphor activation. Him Phys., U 305-6, 1974. 1 MAYHUGH, M. R.; WATANABE, S.; MUCCILLO, R. Thtrmal neutron dosimetry by phosphor activation.
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