Significant improvements in construction of Ge detectors for ultra-sensitive gamma-ray spectrometry systems located underground have been made in recent years. A special attention in the construction of the spectrometer was paid on requirement to achieve as low as possible background rates at the lines of progenies in the decay chains of U-238 and Th-232. Radiopurity of construction materials used in underground experiments has been crucial for background. Underground installations require detectors made of radioactivity free materials with minimum contamination by natural radionuclides.
The aim of the present study was determination of impurities of uranium and thorium at ultra-low levels in electrolytic cooper by using radiochemical neutron activation analysis (RNAA) via their induced nuclides 239U/239Np and 233Pa, respectively. After irradiation of cooper together with uranium and thorium standards, various separation techniques were used for their separation from the matrix. For isolation of short-lived U-239 solvent extraction in combination of TBP and toluene was used, while for separation of Np-239 and Pa-233 extraction chromatography by using TEVA and TK-400 resins were applied. A special attention was paid on the estimation of radiochemical recovery, which was determined in each sample aliquot by using U-235, Np-238 and Pa-231. For quantification of induced nuclides and tracers used in, the Ge gamma-ray spectrometry was applied. Results obtained showed that electrolytic copper contains impurities of uranium and thorium in ultra-trace levels; up to 45pg/g (550 nBq/g) for uranium and up to 80 pg/g (330 nBq/g) for thorium. The electrolytic copper contains also impurities of Ag, Au, Sb, Se and Zn as observed using the k0 INAA technique.