Study on Evaluation of Internal Exposure Dose

Miyahara, H.*; Narita, N.*; Ikeda, Keiichi*; Kato, Y.*; Fujiki, K.*; Momose, Takumaro  ; Tasaki, Takashi; Kurihara, Osamu; Hayashi, Naomi

To estimate the internal exposure dose, it is important to determine the radioactivity (disintegration rate) and distribution of radioactive material. A coincidence method using two large HPGe detectors is applied to achieve this aim. List date from two HPGe detectors with relative efficiencies of 60 % were accumulated by coincidence apparatus using two-dimensional data-acquisition system and spectra required for analysis were obtained by filtering after measurement. The disintegration rate was calculated from the intensities of cascade rays and coincidence counting rates determined by the spectra. Furthermore, chance coincidence could be corrected in this method. Some sources of Na, Co, and Cs, that the disintegration rates were determined within uncertainties of 1 % by 4- coincidence counting were measured in various were evaluated in comparison with true disintegration rates. The calculated values for point sources of three nuclides agreed within 10 % and did not depend on position. Sources with disintegration rates between 10kBq to 300 kBq showed agreement within 10 %. To estimate source position peak intensity ratio was studied for the ratio of inverse squared distance of source and detector. As results, linear relation was recognized between them, and showed possibility of position estimation. Measurement of three mixed sources gave the corrected values within 20 % because of complex spectrum.