Determination of U and Th in tungsten by a radiochemical neutron activation analysis

Tungsten attracts attention, by its high melting point and low electron migration, as an ultimate substitute for aluminum used for the gate material of semiconductors. The impurities of uranium and thorium should be restricted as low as possible for this purpose. About 100 mg of 3N and 5N tungsten samples were irradiated with two comparators of gold and cobalt at TRIGA Mark 3 reactor for 20 hours. The sample, which was finally dissolved in the mixture of 10 ml of 0.1 N hydrochloric acid and 10 ml of 1 N hydrofluoric acid, was passed through anion exchange resin (Dowex 1 x 8). The column was then eluted with 50 ml of the mixture of 0.1 N hydrochloric acid and 1 N hydrofluoric acid, and subsequently with 150 ml of the mixture of 1 N hydrochloric acid and 1 N hydrofluoric acid. The peak identification and activity determination of each nuclide in the spectra of the two eluates were done by the program of PIAD (Peak Identification and Activity Determination), and then the content of each element was calculated by a single comparator method using two comparators. Na, K and As were analyzed in the first eluate, and U, Th and Cr were done in the second eluate, and recovery of each element was over 95%. In the second eluate, 150 ml should be used for the separation of protactinium. The recovery of neptunium was drastically decreased as the content of thorium was increased. This may be due to the fact that neptunium is coprecipitated with insoluble complex of thorium tetra fluorides. However, the coprecipitation effect was negligible if the content of thorium was less than 100 ppm. The contents of uranium and thorium are 30 ppb and 300 ppb, respectively, for 3N tungsten, and less than detection limits for 5 N tungsten. The detection limits of these two elements are 4.0 ppb and 1.2 ppb, respectively, in this experimental conditions