Alleviating the inconsistencies in modelling decay of fissile compound nuclei

Abstract

Despite remarkable success of the statistical model (SM) in describing decay of excited compound nuclei (CN), reproduction of the observables from heavy ion-induced fusion-fission reactions is often quite challenging. Ambiguities in choosing the input parameters, lack of clarity about inclusion of various physical effects in the model and contradictory requirements of input parameters while describing different observables from similar reactions are among the major difficulties of modelling decay of fissile CN. This work attempts to overcome the existing inconsistencies by inclusion of important physical effects in the model and through a systematic analysis of a large set of data over a wide range of CN mass (ACN). The model includes shell effect in the level density (LD) parameter, shell correction in the fission barrier (B f ), effect of the orientation degree of freedom of the CN spin (Kor), collective enhancement of level density (CELD) and dissipation in fission. Input parameters are not tuned to reproduce observables from specific reaction(s) and the reduced dissipation coefficient (β) is treated as the only adjustable parameter. Calculated evaporation residue (ER) cross sections (σER), fission cross sections (σ fiss ) and particle, i.e. neutron, proton and α-particle, multiplicities are compared with data covering ACN = 156 -248. The model produces reasonable fits to ER and fission excitation functions for all the reactions considered in this work. Pre-scission neutron multiplicities (νpre) are underestimated by the calculation beyond ACN ∼ 200. An increasingly higher value of β, in the range of 2 -4 ×10 21 s −1 , is required to reproduce the data with increasing ACN. Proton and α-particle multiplicities, measured in coincidence with both ERs and fission fragments, are in qualitative agreement with model predictions. The present work mitigates the existing inconsistencies in modelling statistical decay of the fissile CN to a large extent. Contradictory requirements of fission enhancement, by scaling down the fission barrier, to reproduce σER or σ fiss and fission suppression, by introducing dissipation in the fission channel, to reproduce νpre for similar reactions have now become redundant. There are scopes for further refinement of the model, as is evident from the mismatch between measured and calculated particle multiplicities in a few cases.