The ${\mathrm{Li}}^6(p, \alpha ){\mathrm{He}}^3$ reaction has been studied in the energy range $0.6\le E_p\le 2.9$ Mev by observing the magnetically analyzed ${\mathrm{He}}^3$ and $\alpha$ particles at laboratory angles of 20° and 60°. By inferring the $\alpha$ yield in the backward direction from the ${\mathrm{He}}^3$ yield observed at the corresponding forward angle, it was possible to construct the angular distributions of the $\alpha$ particles. Two resonances were found in the energy range studied, corresponding to a low-energy (≈1 Mev) $s$-wave state and a $p$-wave state near 1.85 Mev. A single-level resonance formula for a $J=\frac{5}{2^{-}}$, $p$-wave state plus a smoothly varying background has been fitted to the total cross-section curve in the region of the 1.85-Mev resonance and the following resonance parameters extracted: $E_{\lambda }=2.20\mathrm{}$ Mev, ${\mathrm{Be}}^{7*}$=7.80 Mev, ${{\theta }_p}^2=0.24\mathrm{}$, ${{\theta }_{\alpha }}^2=0.048\mathrm{}$. A similar analysis was made of the data of Blair and Holland on the mirror reaction, ${\mathrm{Li}}^6(n, \alpha ){\mathrm{H}}^3$, and the following parameters obtained: $E_{\lambda }=0.43\mathrm{}$ Mev, ${\mathrm{Li}}^{7*}$=7.68 Mev, ${{\theta }_n}^2=0.26\mathrm{}$, ${{\theta }_{\alpha }}^2=0.0085\mathrm{}$. The close agreement between the reduced proton and neutron widths obtained from these mirror reactions indicates that the ${\mathrm{Be}}^7$ level at 7.80 Mev and the ${\mathrm{Li}}^7$ level at 7.68 Mev are mirror states with $J=\frac{5}{2^{-}}$. This conclusion agrees with that previously reached by Bashkin and Richards. The $\alpha$-particle angular distributions in the energy range $0.6\le E_p\le 2.5$ Mev can be qualitatively explained on the basis of two interfering levels with $J=\frac{3}{2^{+}}$ (at $E_p\cong 1.0$ Mev) and $J=\frac{5}{2^{-}}$ (at $E_p\cong 1.85$ Mev). The angular distribution of the $\alpha$ particles from the ${\mathrm{Li}}^6(p, \alpha ){\mathrm{He}}^3$ reaction at $E_p=2.91\mathrm{}$ Mev cannot be described in terms of $s$ and $p$ waves alone and suggests either that angular momenta greater than one are becoming effective in the formation of the compound nucleus at the higher bombarding energies or that a direct interaction process is taking place.

• Received 13 August 1956

DOI:https://doi.org/10.1103/PhysRev.104.1402