Linear momentum transfer in reactions between 140-MeVHe4ions and heavy nuclei

The fission of ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$, ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$, and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ induced by 140-MeV ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ ions has been studied in order to investigate the target-projectile reaction mechanism. The fission-fragment angular correlation technique was used to measure the linear momentum transfer in these reactions. It is found that an upper limit of (49.3≥4.4)% of the fission in ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ results from the complete fusion of target and projectile. The incomplete fusion component of the ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ fission cross section includes a large fraction of high linear momentum transfer events, presumably produced in reactions where preequilibrium decay and/or deep knockout reactions are highly probable. The fission of ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ and ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$ is found to proceed almost completely via a complete fusion mechanism at this energy. The complete fusion cross section for ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$ reactions with 140-MeV ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ ions is compared with that predicted from semiclassical collision models derived for heavy ion reactions.