Pre-scissionHe4multiplicity in theF19+197Au reaction

Pre- and post-scission ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ particle multiplicities for the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ ${+}^{197}$Au reaction in the excitation energy range of 43 to 90 MeV have been measured in coincidence with fission fragments. The coincident ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ particles measured at backward angles are accounted for by evaporation from a compound nucleus and fission fragments. The most probable center-of-mass energy of the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ particles measured at backward angles is shifted towards lower energies by ≊2 MeV compared to a statistical model calculation performed by assuming ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ emission from a spherical compound nucleus. The observed pre-scission ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ multiplicity as a function of excitation energy is compared to a set of statistical-model calculations which also included the delayed onset of fission. The comparison shows that the observed energy dependence of the pre-scission ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ multiplicity is reproduced by the calculation without taking into account the delayed onset of fission if the reduced emission barrier for ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is assumed in the calculation. The emission mechanism of the pre-scission ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is discussed.