PRE-SCISSION HE-4 MULTIPLICITY IN THE F-19+AU-197 REACTION

Pre- and post-scission He4 particle multiplicities for the F19+197Au reaction in the excitation energy range of 43 to 90 MeV have been measured in coincidence with fission fragments. The coincident He4 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 He4 particles measured at backward angles is shifted towards lower energies by 2 MeV compared to a statistical model calculation performed by assuming He4 emission from a spherical compound nucleus. The observed pre-scission He4 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 He4 multiplicity is reproduced by the calculation without taking into account the delayed onset of fission if the reduced emission barrier for He4 is assumed in the calculation. The emission mechanism of the pre-scission He4 is discussed. © 1990 The American Physical Society.