Cross sections for the reaction ${\mathrm{B}}^{11}(n, \gamma ){\mathrm{B}}^{12}$ were measured by an activation technique for neutron energies from 139 to 2325 kev and at thermal energy. Resonances in the reaction were observed at neutron energies of 430, 1030, 1280, and 1780 kev. Since the resonance at 1030 kev did not correspond to a known level in ${\mathrm{B}}^{12}$, the total neutron cross section for boron was measured with 4-kev resolution in the neutron energy region between 950 and 1350 kev. An $S$-wave resonance was observed in the total cross section at 1027±10 kev. This resonance corresponds to a level in ${\mathrm{B}}^{12}$ at an excitation energy of 4.31 Mev; it most probably has spin and parity of $1^{-}$, and a width of 10±4 kev. From an analysis of the capture cross section resonances, the radiation widths of the levels were found to be 0.3 ev at 430 kev, 0.3 ev at 1027 kev, 0.2 ev at 1280 kev, and 0.9 ev at 1780 kev, with estimated uncertainties of 50%. The measured value of the ${\mathrm{B}}^{11}(n, \gamma ){\mathrm{B}}^{12}$ cross section for thermal neutrons was 5±3 mb.

• Received 2 October 1961

DOI:https://doi.org/10.1103/PhysRev.125.1334