Measurement of Neutron Cross Sections with a Crystal Spectrometer

Abstract

By use of the monoenergetic beams of neutrons diffracted from the (100) planes of lithium fluoride, survey measurements of the total neutron cross section of a series of elements have been made. The strong diffracted beams were obtained by placing a single crystal in the high flux of neutrons emitted by the heavy water moderated pile at the Argonne Laboratory. Observations on this group of elements, which includes several strongly absorbing rare earths, and elements commonly used as neutron filters and detectors, have been made in the range of energy between 0.04 and 65.0 electron volts. The method permits energy dependent cross-section measurement by activation or by transmission. However, all values reported were obtained by calculation from observed transmission data because the range of available intensities in the diffracted beam was best measured with a boron counter. Correction methods for resolution and order effects have been developed which are valid for low neutron energies. The following absorption levels have been observed: Rh, 1.28 ev; Au, 5.4 ev; Ir, 0.635, 1.35, and 6.0 ev; Gd, 0.031 ev; Sm, 0.096, 10.0, and 33.0 ev; Eu, 0.465, 3.3, 9.2, and 22.0 ev. Resonance levels at energies less than zero were found in Dy at -1.01 ev and in Eu at -0.011 ev.