H. "Ohuchi

Y. Kondo

Y." Asakura

National Institute for Fusion Science (NIFS-Repository)

Separation method for measurement of radiation dosefrom tritium in high gamma-ray radiation fields byusing an imaging plateOhuchi, H. (Pharm. Sci., Tohoku Univ.)Kondo, Y. (Ishinomaki Univ.)Asakura, Y.Tritium measurement is important from bothviewpoint of tritium accountancy of fusion reactors andstudies on tritium behavior in plasma facing materials. Inparticular tritium retention in plasma facing materials isone of the most important safety issues in fusion reactors.In the fuel- processing systems of deuterium-tritium (DT)-fusion facilities, gamma radiation deriving fromcomponents activated by neutron seriously affects tritiummeasurement. Separation measurement method of radiationdose from tritium and those from other nuclides is required.The main nuclides produced by neutron activation fromMo, Ni, Fe, and Cu contained in vacuum vessel (SS316)and superconducting magnet are 99Mo, 58Co, 57Co, 54Mn,and 60Co in Large Helical Device facility.1) It was assumedthat the same nuclides are produced by neutron activationin fusion reactors as well. Because of its long half-life(5.27 y) and high gamma emission energy (1.173 and1.333 MeV), 60Co would be a main issue for tritiummeasurement.In the present study, an imaging plate (IP) wasapplied to measure tritium detecting bremsstrahlung X-raysgenerated by the interaction between the beta particles fromtritium and matter. Recently, the tritium IP technique hasbeen successfully applied to determine the surface tritiumdistribution on plasma facing tiles.2) The IP made ofeuropium-doped BaFBr(I), a photostimulated luminescence(PSL) material, is a two-dimensional radiation sensor. ThisIP has many excellent properties for this purpose, includinga high sensitivity, a wide dynamic range over five orders ofmagnitude, and a high degree of spatial resolution.3) ThePSL response of the IP has a peak at 20-50 keV andsteeply decreases towards higher energy, falling by onehundredth at around 1 MeV, because the photostimulablephosphor is composed of elements with relatively highatomic numbers. Utilizing this strong dependence of thePSL sensitivities on energy, tritium/ 60Co separationmeasurement of radiation dose was attempted by using theIP.Measurements were conducted by using four smallborosilicate glass tubes, filled with pure tritium gas of12.5, 25, 50, and 100 MBq, respectively as the tritiumsources that generate bremsstrahlung X-rays. A 60Co  point source was used as a gamma-ray source. A BAS-MStype-IP, manufactured by FUJIFILM Co., Ltd., was used todetect bremsstrahlung X-rays from tritium. The IP wasirradiated for 1 h with the tritium sources and the 60Cosource at dose rate in the range 0.0013 to 9.22 �Gy/m,simultaneously. An ionization chamber, the EXRADINmodel A-6 of 800 ml effective volume was used tomonitor dose rate during irradiation with the 60Co source.Dose rate attributed to natural radiation was estimated to be0.0013 �Gy/m. The IP was read out by using theBAS5000 fabricated by FUJIFILM Co., Ltd. The PSLvalues from tritium were obtained by subtracting thosewith the 60Co gamma ray from total PSL values.   Fig.1 shows a relationship between PSL values fromthe tritium source and dose rate of irradiation with the 60Cosource (�Gy/m). It was found that the effect of irradiationwith the 60Co gamma rays to the PSL values, obtained byirradiated with tritium of 12.5 MBq, was negligible up todose rate of 4.380 �Gy/m (3,370 times as high as naturalradiation level). There was only 7.0% difference of the PSLvalues, obtained by irradiated with the 100 MBq tritiumsource, between dose rate of 0.0013 and 9.22 �Gy/m. Wecan obtain separate PSL values for tritium and 60Co byattenuating bremsstrahlung X-rays induced by the tritiumbeta-rays using a thin filter such as aluminum or nickel foil.The IP tritium measurement method can be a promisingcandidate to measure tritium in high gamma-ray radiationfields.     Fig. 1.  Relationship between PSL values from thetritium source and dose rate of irradiation with the60Co source (�Gy/m).1)  Handa, H. et al.: Fusion Eng. Des. 17 (1991) 3352)  Tanabe, T. et al.: J.Nucl. Mater. 478 (2003) 3133)   Miyahara, J. et al.: Nucl. Instr. and Meth. A 246 (1986)       572????????????????????????????????????? ??????? ??????? ??????? ??????? ????????????????????????????????????????????????????????????????????????????????????????????????????????????????297§7. eparation Method for M asurement of Radia on Dose fro  Tritium in HighGamma-ray Radiation Fields by Using  an Imaging PlateOhuchi, H. (Pharm. Sci., Tohoku Univ.),  Kondo, Y. (Ishinomaki Univ.),  Asakura, Y.

Separation Method for Measurement of Radiation Dose from Tritium in High Gamma-ray Radiation Fields by Using an Imaging Plate

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