Long Wavelength Crystal Spectrometer and the Neutron Absorption Cross Sections of Gold and Boron

The absorption cross sections of gold and boron have been measured at long neutron wavelengths with a single crystal spectrometer. Mica along with microcrystalline filters of Be and BeO was used as a monochromator for the wavelength range from 4 to 8.75 Å, and for longer wavelengths a mechanical monochromator was used to remove second and higher order neutrons. Neutron beams with negligible higher order contamination were obtained with a wavelength resolution Δλ/λ of 0.018. The total cross section of gold for wavelengths from 5 to 11.5 Å is σ_t = (54.56 ± 0.09) λ (0.46 ± 0.67) barns. Evaluation of the thermal neutron (2200 m/sec) absorption cross sections gave (98.8 ± 0.3) barns for gold and (7.56 ± 6) barns for boron in glass plates for use as secondary standards.