Mechanism for the disassembly of excitedO16projectiles into four alpha particles

The decay of excited ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ projectiles into the four alpha particle exit channel has been investigated. The projectiles, with bombarding energies of E/A=25 MeV, were excited through peripheral interactions with ${}_{}{}^{159}{}_{}{}^{}\mathrm{Tb}$ target nuclei. A 4π counter was used to detect both the projectile alpha particles and the light charged particles evaporated from the target nucleus. Criteria for selecting true ${}_{}{}^{16}{}_{}{}^{}\mathrm{\alpha }$ events with the minimum contamination from incorrectly identified events were examined. The distribution of relative angles between the four alpha particles in their center of mass frame was compared to simulations of ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ decay by sequential decay mechanisms. The relative angles were shown to be consistent with a sequential evaporation mechanism if a nonzero angular momentum is assumed for the excited projectile. The momentum distribution of the alpha particles in the reaction plane was examined and found to be anisotropic for high projectile excitation energies. This anisotropy was shown to be consistent with Coulomb interactions between the first emitted alpha particle and the target nucleus, which would indicate that the lifetime of the projectile is very short, on the order of ${10}^{\mathrm{-}22}$ s.