Alpha-particle breakup at incident energies of 20 and 40 MeV/nucleon

The breakup of alpha particles at incident energies of 20 and 40 MeV/nucleon on ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{58}{}_{}{}^{}\mathrm{Ni}$, ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, and ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ has been studied. It was found that the breakup cross section decreases rapidly with increasing angles and increases with increasing target mass and incident energy. The total breakup yield, summed over all charged fragments, is ∼15-35% of the alpha-particle total reaction cross section, and has an approximate $A^{\frac{1}{3}}$ dependence. The ratios of breakup yields among different fragments are approximately $p:d:t:{}_{}{}^3{}_{}{}^{}\mathrm{He}\approx 13:3:1:2$, and are roughly independent of the incident energy and the target nucleus. These features suggest that the alpha-particle fragmentation is a peripheral process and is dominated by the properties of the incident projectile. A simple plane-wave alpha-particle breakup model gives a rather good description to the experimental data. In addition to the breakup deuteron peak at half of the beam energy, a second peak at quarter of the beam energy (or the same energy as the breakup proton peak) is observed. This peak might be due to a two-step breakup-pickup process.