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Highly sensitive neutron activation analysis of uranium and thorium in high quality silica and aluminium has been investigated using the Japan Materials Testing Reactor (JMTR), having a thermal neutron flux higher than 1014 n/cm2/s. In order to determine ultra-low contents of uranium and thorium,239Np and233Pa as activation products were separated by using anion exchange and LaF3 coprecipitation methods. As a result, a number of interfering radioactive isotopes containing double neutron capture product such as183Ta were removed completely from the isolated239Np and233Pa fraction and the detection limits for uranium and thorium were found to be 2·10−12 g and 4·10−13 g, respectively.

Neutron activation analysis for thorium and uranium in high purity silica and aluminum was investigated. The activation products 233Pa and 239Np for the determination of thorium and uranium were separated from majority of interfering radioactivity by a method combined with anion exchange and LaF3 coprecipitation. It was found that the analysis with detection limit of several ppt for both uranium and thorium was possible when a high flux reactor was used. The method is useful for the determination of uranium and thorium in semiconductor grade silica and aluminum.