RADIOCHEMICAL NEUTRON-ACTIVATION ANALYSIS FOR ULTRATRACE URANIUM AND THORIUM IN HIGH-PURITY SILICA

It is well known that alpha-particle emission from traces of uranium or thrium in semiconductor materials causes damages to semiconductor function. To produce high quality semiconductor materials, it is necessary to develop highly sensitive analytical methods for uranium and thorium. High quality silica with low uranium and thorium content was analyzed by radiochemical neutron activation analysis. The samples were irradiated by the Musashi Institute of Technology Reactor (thermal neutron flux 10(12)n cm-2 s-1) and were subjected to anion exchange chromatography and a LaF3 coprecipitation method. By this method, many interfering radioactive isotopes, Na-24, K-42, Sb-122, Ta-182 were removed by anion exchange chromatography, and W-187 was removed by the LaF3 coprecipitation method. The lower limits of the determination were found to be 3 ppt (10(-12) g/g) and 6 ppt (10(-12) g/g) for uranium and thorium, respectively. This method is useful for the determination of uranium and thorium in high purity silica as semiconductor materials.