Excitation Curves forαParticles fromB11Bombarded with Protons

The spectra of the $\alpha$ particles emitted from ${\mathrm{B}}^{11}$ under proton bombardment have been investigated by means of a 180° double-focusing magnetic spectrometer. In the range of bombarding energies from 0.3 Mev to 1.8 Mev, the $\alpha$ group leading to the first excited state of ${\mathrm{Be}}^8$ exhibits the same resonances, at 0.67 Mev and 1.4 Mev, as does the proton capture process, whereas the $\alpha$ group leading to the ground state of ${\mathrm{Be}}^8$ is resonant only for the higher energy. The approximate values for the cross sections at the maxima, are in the former case, 600 millibarns and 150 millibarns, respectively, and, in the latter, 6 millibarns. Spin and parity assignments of $2^{-}$ and $1^{-}$ for the two levels, respectively, are compatible with these results.