Neutron pickup by alpha particles to unbound states

Two conspicuous peaks are observed in the two-dimensional coincidence spectra of $\alpha$ particles and neutrons emitted in bombardments of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, Rh, ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, and U with 90 MeV $\alpha$ particles. Through their energy-angle correlation these peaks are identified as arising from the breakup products of ${}_{}{}^5{}_{}{}^{}\mathrm{He}$ (g.s.) formed in the ($\alpha ,{}_{}{}^5{}_{}{}^{}\mathrm{He}(\mathrm{g}.\mathrm{s}.\mathrm{})$) reaction. A somewhat less prominent peaking of events is identified as the signature for the breakup of ${}_{}{}^6{}_{}{}^{}\mathrm{H}\mathrm{e}*$(1.8 MeV) formed by two-neutron pickup. A distorted-wave Born-approximation calculation was carried out for the ($\alpha ,{}_{}{}^5{}_{}{}^{}\mathrm{He}(\mathrm{g}.\mathrm{s}.\mathrm{})$) reaction on ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$ and was found to fit the measurements. The large differential cross section for the ($\alpha ,{}_{}{}^5{}_{}{}^{}\mathrm{He}(\mathrm{g}.\mathrm{s}.\mathrm{})$) reaction confirms an earlier interpretation of the somewhat square structure observed in the spectra of inelastically scattered $\alpha$ particles.