The ¹⁰B(n,₀)/¹⁰B(n,₁) Branching Ratio

The ¹⁰B(n,₀)/¹⁰B(n,₁) branching ratio has been measured at the Geel linear accelerator based time-of-flight spectrometer in the incident neutron energy range from 0.1 keV up to 2 MeV. A twin Frisch-grid ionization chamber has been used with two very thin ¹⁰B samples mounted back-to-back on the common cathode. This type of ionization chamber made it possible to measure both the energy and the angular distribution of the emitted reaction products (alpha particles and ⁷Li nuclei) with a clear separation of both reaction channels: emission to the ground state (₀) and first excited state (₁). The branching ratio ¹⁰B(n,₀)/¹⁰B(n,₁) was found to be in good agreement with the ENDF/B-VI evaluation up to ~1 MeV incident neutron energy. At higher energies (>1 MeV), a clear deviation is observed. The present branching ratio data have been entered into the ongoing International Atomic Energy Agency Coordinated Research Project on "Improvement of the Standard Cross Sections for Light Elements." A preliminary R-matrix calculation reproduces the measured branching ratio in the whole energy range up to ~2 MeV.