Thermal Neutron Absorption Cross Sections of Boron and Gold

Abstract

The cross sections of boron and gold, which are widely used as standards for slow neutron measurements, have been determined as functions of wavelength. Total cross sections were measured in the wavelength range 1.5-10A with the Brookhaven slow chopper, the long wavelengths being used to minimize the effects of scattering. The absorption cross section of boron of normal isotopic constitution is 749\ifmmode\pm\else\textpm\fi{}4 barns at 2200 m/sec (1.80A); the variation of cross section with boron source is expected to be only about 1 percent. The measurement of the gold cross section is complicated by the deviation from $\frac{1}{v}$ resulting from a nearby resonance and by grain orientation effects in the scattering. The 2200-m/sec value, resulting from measurements at long wavelength together with a small correction for the 4.9-ev resonance, is 98.7\ifmmode\pm\else\textpm\fi{}0.6 barns.