Determination of trace impurities in gallium arsenide by NAA

GaAs is not an ideal matrix for INAA because elements yielding activation products with half-lives up to about 5 d cannot be measured due to the interference by72Ga and76As (t1/2=14.1 h and 26.4 h, respectively). The measurement of radionuclides with longer half-lives is interfered with by74As (t1/2=17.7 d), generated by fast neutrons. However, using an irradiation facility with a very low flux of fast neutrons, in which the generation of74As is minimal, five elements could be determined in GaAs (Cr, Co, Zn, Ag, and Hg). For 27 elements the detection limits were below 1 μg/g and for ten of them below 10 ng/g. The determination of nitrogen in GaAs has been carried out using the (n, p)-reaction on14N, which is induced by thermal neutrons. The activation product,14C, can be effectively separated and purified via14CO2 and counted with high efficiency in a liquid scintillation counter, and nitrogen can be determined with fairly low detection limits if sufficiently high neutron fluxes and long irradiation times are applied. The procedure described is based on a reactor irradiation with a thermal flux of 2·1014 n·cm−2·s−1 for 51 days. 0.16±0.09 μg/g N in GaAs were determined and the detection limit was about 3 ng/g.