Residual radioactivity measurement for high purity silicon irradiated by pile-neutron

Residual radioactivity measurement is described for high purity silicon (purer than ten-nine) irradiated by pile-neutron to produce n-type silicon semiconductor. The silicon samples were irradiated in four different atomic reactors with thermal neutron fluxes of 0.12, 1.48, 1.8 and 5.5 x 1013 n/cm2.sec. After irradiation, the sample was dissolved in HF-HNO3, and the solution dried. The residual radioactivity was measured by a 2π gas-flow low background GM counter. The only active species found was 32P, which was formed by the consecutive reactions of 30Si(n, γ)31Si→31P followed by 31P(n, γ)32P. The observed production rate for 32P was in good agreement with the calculated one. This technique can be applied to estimate amounts of not only doped phosphorus but impurities such as gold in irradiated silicon. Tail part of a float-zone silicon rod contained about 0.01 atomic ppb of Au, but no activity was observed in its middle part. The present technique is suitable for checking and characterizing silicon rods before their neutron-doping. (auth.)