Abstract
Thorium dioxide, previously freed of uranium and protactinium contamination by means of ion-exchange, was irradiated for 70.5 and 140 hr respectively in the core and graphite thermal column positions of the KUR (Kyoto University Reactor). After being cooled for more than 11/2yr, the 233U produced was chemically purified by ion exchange, and the resulting 223U specimens were electro-deposited on a stainless steel plate for submission to a-spectrometric measurement with a silicon surface barrier detector. From the α-spectra thus obtained, the 232U to 233U atomic ratios of the specimens were evalu-ated and compared with the calculated values. This revealed that the rate of formation of 232U depended largely on the 232Th(n, 2n)231Th reaction by fast neutrons, and conse-quently, on the neutron energy distribution prevailing at the position of irradiation. The purest 233U sample obtained from the thorium target, irradiated with pure thermal neu-trons in the graphite thermal column, yielded a 232U to 233U atomic ratio of 3.02×10-9, which corresponded to 6.62 × 10-6 in α-activity ratio.
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