The thermal cross section for the reaction ${}^{10}\mathrm{B}+\mathit{n}\to {}^3\mathrm{He}+{}^4\mathrm{He}+{}^4\mathrm{He}$ is determined to be 4.47±0.15 mb by neutron irradiation of ${\mathrm{H}}_3$${\mathrm{BO}}_3$ followed by measurement of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ (from decay of ${}_{}{}^3{}_{}{}^{}\mathrm{H}$) and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in a static mass spectrometer. Some samples contained boron of normal isotopic composition and some were enriched in ${}_{}{}^{10}{}_{}{}^{}\mathrm{B}$, and irradiations were carried out in a highly thermalized neutron flux and in the core of a light-water-moderated reactor.

• Received 28 December 1988

DOI:https://doi.org/10.1103/PhysRevC.39.1633