Mechanisms of production of fast particles withZ=1,2inLi6-induced reactions far above the Coulomb barrier

Abstract

Various mechanisms for the production of fast, beam-velocity particles have been explored using a 12.5 MeV/nucleon ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ beam, with special emphasis on results from a ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ target. Energy spectra and angular distributions of fast, $Z=1,2\mathrm{}$ products revealed an $A^{\frac{1}{3}}$ target mass dependence of the individual total yields which is characteristic of the surface mechanisms. Relative fragment yields were essentially independent of the target mass and appear to depend upon the momentum distributions of the different cluster substructures of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$. Particle-$\gamma$ coincidences reflected the $Q$-optimum transfer of a beam-velocity $\alpha$ or $d$ fragment, initiating a (fragment, $\mathrm{xn}\gamma$)-like reaction, and which accounts for approximately 13% of the singles beam-velocity particle production. In-plane $\alpha -d$ coincidences could be divided roughly equally between two-step (sequential) and one-step projectile breakup. Alpha-proton coincidences suggested the strong presence of neutron transfer, forming unstable ${}_{}{}^5{}_{}{}^{}\mathrm{Li}$ which subsequently breaks up into $\alpha +p$. The cross section from correlated $\alpha +d$ and $\alpha +p$ production accounts for at least 50% of the singles fast-particle production. Other possible mechanisms are also discussed.