Determination of indium in rocks by neutron activation analysis

There have been few reliable data of indium contents in silicate rock samples on account of the extremely low concentrations of the element. Neutron activation analysis has been developed for determining indium in rocks to obtain more reliable and comprehensive geochemical figures about indium. Sample (ca. 0.4 g) and reference standard (0.1μg In) were irradiated for one hour in the TRIGA Mark II Reactor of Rikkyo University with the neutron flux of 5×1011n/cm2/sec. After the irradiation, the sample was decomposed by NH4F·HF-NH4NO3 fusion in the presence of indium carrier (20 mg). The induced radionuclide from indium was separated and purified from the matrix by hydrobromic acid-ether extraction, followed by CuS and SiO2 scavenging procedures. Finally, the indium was precipitated as sulfide and the β-activity of the purified isotope 116mIn was counted. The total time required for the completion of the whole procedure was one hour and a half, and the average chemical yield was 50%. Some of the typical results obtained were summarized as follows (ppm In): G-1 0.026; W-1 0.057; Granites 0.042; Andesites 0.050; Basalts 0.083; Peridotites 0.025. Errors due to self-shielding and competing nuclear reactions 116Sn(n, p)116In were also discussed.