Search for uranium in high purity silicon

Detection of small quantities of uranium in silicon wafers has been carried out by neutron activation followed by observation of fission product¹⁴⁰La. Irradiations of about one week were made at a flux of 6·10¹⁴n cm⁻² s⁻¹ and the activity of the 1596 keV line was determined. Counting rates of as low as 1 count per minute have been observed. This indicates uranium concentrations of about 5·10¹⁰ atoms per cubic centimeter of silicon or about 0.01 mg/g, assuming activity from other fissionable nuclides to be negligible.