A direct measurement of the $^6$Li(n,t)$^4$He cross section at sub-thermal neutron energy

The thermal neutron capture cross section for the $^6$Li(n,t)$^4$He reaction is an important neutron cross section standard. Yet few measurements of it have been performed and the ENDF/B-VII recommended value of $(938.5 \pm 1.3)$ b is based heavily on measurements performed at higher energies. The first absolute, direct measurement of the $^6$Li(n,t)$^4$He cross section at sub-thermal neutron energy has been performed at the NIST Center for Neutron Research. An alpha-gamma counter was used to measure the absolute neutron fluence of a monoenergetic neutron beam to sub-0.1\,\% precision. The alpha-gamma counter used a thick, totally absorbing target of $^{10}$B-enriched boron carbide. The rate of absorbed neutrons was determined by counting the 478 keV $^{10}$B(n,$\gamma$)$^7$Li gamma rays with calibrated high-purity germanium detectors. Simultaneously, the absolute rate of neutron-induced charged particles was measured for three thin $^6$Li targets of known density with a defined solid-angle counter. Using the known density of the $^6$Li targets and measurements of the rate of charged particles from the $^6$Li targets, the fluence of the neutron beam, and the energy of the neutron beam, we determine the $^6$Li(n,t)$^4$He cross section at $E_n=3.3$ meV to 0.3\,\% uncertainty.