Measurement of Fast Neutron Capture Cross Sections of Interest in Fast Neutron Activation Analysis

Abstract

The work herein describes the absolute measurement of fast neutron capture cross-sections of interest in activation analysis. In particular, the Cu63 (n2n)Cu62 cross-section, which is widely used as a standard for relative measurements, has been remeasured, and the value obtained compared with previous absolute measurements and with the statistical theory of cross-sections. The (n2n) cross-sections have also been measured for Cu65 and Mo92, and the (nα) cross-section for Al27, The 14. Mev neutrons employed were obtained from the T(dn)He4 reaction. Deuterons were accelerated, using a 500 kV Van de Graaff accelerator, and allowed to impinge on a tritium titaniun target. The neutron flux was measured using an associated particle technique in which the helium nuclei given off from the T(dn)He4 reaction were recorded using a silicon surface barrier detector. The work was latterly transferred to a SAMES accelerator of maximum working voltage 150 kV, and the measurements compared. The samples to be irradiated were located close to the back of the tritium target and in the same plane. The effect of the finite neutron source size and its non uniformity was determined theoretically. The absolute activity of the samples was determined by gamma counting using a 3" x 3" NaI(T1) scintillation spectrometer.