Fission barrier, damping of shell correction, and neutron emission in the fission ofA∼200

Abstract

Decay of210 Po compound nucleus formed in light and heavy-ion induced fusion reactions has been analyzed simultaneously using a consistent prescription for fission barrier and nuclear level density incorporating shell correction and its damping with excitation energy. Good description of all the excitation functions have been achieved with a fission barrier of 21.9 ± 0.2 MeV. For this barrier height, the predicted statistical pre-fission neutrons in heavy-ion fusion-fission are much smaller than the experimental values, implying the presence of dynamical neutrons due to dissipation even at these low excitation energies (∼ 50 MeV) in the mass region A ∼ 200. When only heavy-ion induced fission excitation functions and the pre-fission neutron multiplicities are included in the fits, the deduced best fit fission barrier depends on the assumed fission delay time during which dynamical neutrons can be emitted. A fission delay of (0.8 ± 0.1 )×10−19 s has been estimated corresponding to the above fission barrier height assuming that the entire excess neutrons over and above the statistical model predictions are due to the dynamics. The present observation has implication on the study of fission time scale/ nuclear viscosity using neutron emission as a probe.The fission process involves most drastic rearrangements in nuclei, where both statistical and dynamical features, governed by the delicate interplay between the macroscopic (liquid drop) aspects and the quantal (shell) effects, are exhibited. One of the key questions in nuclear fission is: what is the maximum energy along the fission path (barrier height) [1]? The fission barrier has contributions from the macroscopic liquid drop part as well as from the microscopic shell effects. Accurate knowledge of the fission barrier height is vital not only to understand the heavy-ion induced fusion-fission dynamics and predictions concerning super-heavy nuclei, but also other areas, such as stellar nucleosynthesis and nuclear energy applications. The status of charged particle induced fission reactions has been reviewed recently [2] Experimental determination of the fission barrier height in mass A ∼ 200 continues to be a challenging problem. In this mass region, the fission barrier heights are much higher than the neutron separation energies and experimental cross sections around the fission barrier, being extremely low, are often not available. Large ground state shell corrections around the Z=82, N=126 brings in additional parameters in the investigation of fission in mass A ∼ 200 region.As shown in Fig. 1, the mass of a nucleus gets lowered from the liquid drop (LD) ground state due to negative shell correction energy (∆ n ). Knowledge of the shell correction at the saddle deformation (∆ f ) for A∼200 is obscure and most of the analyses assume ∆ f = 0. The shell correction reduces with increasing excitation energy and washes out at excitation energy of around 30-40 MeV. * kmahata@barc.gov.in † Raja Ramanna Fellow ‡ INSA Honorary Scientist A clear manifestation of this is...

Fission Dynamics Studies of Near Super-heavy Compound Nucleus $^{256}$Rf

“…The experimentally deduced average M pre n and M post n are further compared with the statistical model predictions for the 48 Ti+ 208 Pb system [11]. The fission barrier in the present calculations is obtained by including shell corrections to the liquid-drop nuclear mass [12]. Shell effects are also included in the nuclear level density which is used to calculate various decay widths of the CN.…”

Quest for consistent modelling of statistical decay of the compound nucleus

“…Fission hindrance is expected to impact not only the multiplicity of pre-scission neutrons and other light evaporated particles or photons but also the fission and evaporation residue (ER) cross-sections of fusion-fission reactions. It is observed that at least two or more input parameters in statistical model (SM) calculations, namely those defining the fission barrier, the level density parameter, the fission delay time and the dissipation coefficient are required to be adjusted for simultaneous fitting of both the fission/ER and the pre-scission neutron multiplicity excitation functions [3,4,[7][8][9]. Different values of parameter sets are found necessary for different systems.…”

Fission fragment mass distributions from Po210 and At213

“…However, recent dynamical calculation using stochastic Langevin equation [9] claimed that the observed angular anisotropy could be well explained with only macroscopic potential energy landscape without considering any shell effect at saddle point. Statistical model calculation [10] for the nucleus 210 Po, populated in light and heavy ion induced reactions, however could describe the excitation functions without the requirement of shell correction at the saddle, but required a huge fission delay to fit the pre-scission neutron multiplicity data in heavy ion induced reaction. Reanalysis [11] of the 210 Po data [8] with inclusion of multi-chance nature of fission was found to reduce the anisotropy anomaly.…”