Abstract
Methods to determine the uranium isotopic ratio 235U/238U by neutron activation analysis were examined. Uranium isotopic ratio in several environmental samples containing small amount of uranium was determined by these methods.After isolated uranium samples were irradiated with thermal neutron flux at 3.5×1012n/cm2·sec for one hour, the radioactivity of 99Mo+99mTc produced from 235U fission and that of 239Np induced from 238U were measured by the following two methods to determine the isotopic ratio 235U/238U.1. Energy spectrum analysis of γ-rays with a Ge (Li) semi-conductor detector.2. Measurement of the radioactivity with a 2π gas-flow counter and a NaI (T1) scintillation counter after radiochemical isolation by solvent extraction.Comparing above two methods, it was found that β-counting using 2π gas-flow counter and γ-spectrometry gave preferable results with fluctuation below 5% to γ-counting using NaI (T1) scintillation counter. The isotopic ratios 235U/238U in environmental soil samples collected at Ningyo pass and its neighboring places in Okayama Prefecture and at Muro in Nara Prefecture were found to be in the range of 0.0062-0.0075, whereas the ratio in uranyl nitrate hexahydrate (commercial reagent) was 0.0076±0.0004.
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