Determination of ultra trace uranium and thorium in high purity aluminium by NAA

Uranium and thorium in high purity aluminium samples were determined by radiochemical NAA using an ion exchange chromatography. The samples irradiated at the Musashi Research Reactor, were dissolved in HCl or HCl-HNO3 = 3 : 1 solution. The solution prepared to be 9 M in HCl, were loaded onto the column of anion exchange resin (Muromac 1 x 8, 100 ∼ 200 mesh, Cl--form). 239Np and 233Pa which were formed from U and Th, respectively were absorbed on the resin. After washing the resin with 9 M HCl to remove the activated Al, Na, Sm and Sc, Pa was eluted by 9 M HCl-0.05 M HF solution, Np was successively eluted by 9 M HCl-2 M HF solution. Gamma-ray spectra of Pa and Np were measured with a Ge detector connected with a multichannel analyzer. As a result, 17 ppt and 8 ppt were obtained as the lowest content for U and Th, respectively in the ultra high purity Al. (author)