Prompt gamma as a fluence rate monitor in neutron beam experiments

Abstract

An accurate measurement of the neutron lifetime requires a determination of neutron fluence rate to an accuracy of a few tenths of a percent. The10B(n,α)7 Li reaction offers the possibility of achieving this uncertainty. The thermal cross section is large and its departure from 1/v behavior is about 3 parts in 10000. The principal alpha branch is to the first excited state of7Li which then decays by emission of a 478 keV gamma ray. The measurement of the gamma branch can be made with boron samples that totally absorb thermal neutrons, allowing greater sensitivity and eliminating the uncertainty of target thickness. The absolute efficiency of the gamma detector can be determined by an alpha-gamma coincidence technique. Preliminary investigations of this method are presented with a discussion of the problems that must be overcome to achieve the desired uncertainty.