Determination of ultralow contents of uranium and thorium in high-purity aluminium

A radiochemical separation procedure has been developed to determine ultralow concentrations of uranium and thorium in aluminum by neutron activation analysis. In this procedure the indicator nuclide for uranium, Np-239, is selectively separated from the activation products of all other elements and from the fission products. The radionuclide is sufficiently pure so that it can be measured by β-counting. Because of the high counting efficiency the sensitivity of detection is improved essentially. The indicator nuclide for thorium, Pa-233, is also separated with high purity, but it is not completely free of other radionuclides. Therefore, its activity is measured by gamma-ray spectrometry. The relatively simple separation consists of two steps: a selective ion-exchange separation for Np-239 from nitrate medium on only one column of Dowex-1X4 and extraction of Pa-233 with tri-n-octylphosphine oxide from the effluent. The chemical yields for uranium and thorium are 91.5 and 95.0% respectively. By this procedure detection limits of 0.0005 ng/g for uranium and 0.01 ng/g for thorium were obtained, using samples of 3 g, a thermal neutron flux of 3 · 1013 n/cm2s and an irradiation time of 1 day