Pre-scissionHe4multiplicity in theF

Abstract

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

Evaporation of charged particles from highly deformed nucleus

“…The necessity to shift the emission-barrier height relative to the barrier * JAERI Research Fellow; present address: GSI, W-6100 Darmstadt, FRG for the capture reaction was demonstrated in [8, 91. Another interesting feature seen in the data of [8,91 is the observation of very low-energy component alphas in between the evaporation peak from the compound nuclei and the fragment peak. This component, viz.…”

“…(12) A noteworthy point of this expression is that it has nothing to do with the distribution of time during which the system stays in the ground configuration or in the saddle-point configuration. The effect of such distribution is included implicitly in the level density of the residue through (8).…”

“…Fig. 6 of [8], may originate from ternary fission process. But it might originate from quite different process as well.…”

“…We should be careful in considering excess of low-energy charged particles from the compound-nucleus reactions because, for example, low-energy charged particles may come flom deep-inelastic collisions. In this respect, measuring of alpha particles in coincident with the fission fragment, as in [8,9], where the ambiguity is eliminated, is very useful. The necessity to shift the emission-barrier height relative to the barrier * JAERI Research Fellow; present address: GSI, W-6100 Darmstadt, FRG for the capture reaction was demonstrated in [8, 91.…”

“…6 of [8], it is quite helpful to have a closed-form expression for the emission rate from the deformed nucleus. In the absence of a closed-form expression, Monte Carlo method or some approximate methods based on the emission width expression for the spherical nucleus are used as in past analyses.…”