Recent studies in heavy ion induced fission reactions

Abstract

Nuclear fission process involves large scale shape changes of the nucleus, while it evolves from a nearly spherical configuration to two separated fission fragments. The dynamics of these shape changes in the nuclear many body system is governed by a strong interplay of the collective and single particle degrees of freedom. With the availability of heavy ion accelerators, there has been an impetus to study the nuclear dynamics through the investigations of nucleus-nucleus collisions involving fusion and fission process. From the various investigations carried out in the past years, it is now well recognized that there is large scale damping of collective modes in heavy ion induced fission reactions, which in other words implies that nuclear motion is highly viscous. In recent years, there have been many experimental observations in heavy ion induced fission reactions at medium bombarding energies, which suggest possible occurrence of various non-equilibrium modes of fission such as quasi-fission, fast fission and pre-equilibrium fission, where some of the internal degrees of freedom of the nucleus is not fully equilibrated. We have carried out extensive investigations on the fission fragment angular distributions at near barrier bombarding energies using heavy fissile targets. The measured fragment anisotropies when compared with the standard saddle point model (SSPM) calculations show that for projectile-target systems having zero or low ground state spins, the angular anisotropy exhibits a peak-like behaviour at the sub barrier energies, which cannot be explained by the SSPM calculations. For projectiles or targets with large ground state spins, the anomalous peaking gets washed out due to smearing of the Ã-distribution by the intrinsic entrance channel spins. Recently studies have been carried out on the spin distributions of fission fragments through the gamma ray multiplicity measurements. The fission fragments acquire spin mainly from two sources: (i) due to rigid rotation of the nascent fragments at scission and (ii) due to statistical excitation of the spin bearing collective modes in the fissioning nucleus. One of the collective modes -the tilting mode depends on the à quantum number and is responsible for the emission angle dependence of fragment spin. In our studies, we have shown conclusively that the collective statistical spin modes get strongly suppressed for high à values corresponding to large rotational frequencies along the fission axis. These results bring out the importance of the dynamical effects in the heavy ion induced fusion-fission reactions. The present article will review the work carried out on the above aspects in heavy ion fission reactions as well as on the fission time scales, and some of the recent studies on the mass-energy correlations of fission fragments at near-barrier bombarding energies.

Non-compound-nucleus fission events and standard saddle-point statistical model

“…V b ≤ 1.2 energy range is approximately 44%, this contribution for the 32 S + 208 Pb system over the same energy range is obtained 50%. The predicted value of average contributions of NCNF for the 40 Ar + 208 Pb system over the 1.05 ≤ Ec.m.…”

“…In recent years, heavy-ion induced fission fragments angular distribution measurements performed at below to above barrier energies have generated much interest due to the failure of the predictions of the SSPSM for heavy-ion induced fission of actinide targets [12,40,41].…”

Recent Developments in Nanofabrication Using Ion Projection Lithography

“…Practically, this level of ion energy is beyond the useful range for nanofabrication and will not be discussed here. Some recent developments in this subject, especially in heavy‐ion reactions, may be found in the publication by Choudhury and Kicinska‐Habior et al 13. 14…”

Investigation of fission fragments average spin based on four dimensional Langevin dynamical model

“…Various shape parameterizations have been adopted to describe the family of shapes that the nucleus spans as it deforms from the ground state to the saddle and scission configurations [7]. Based on these studies the fission barrier parameters such as the barrier height and saddle point moments of inertia as a function of Z 2 /A and angular momentum, have been calculated [8][9][10]. It is seen that for heavy actinide nuclei (Z 2 /A = 35), the fission barrier is in the range of 5 to 6 MeV for zero angular momentum and goes to zero when the angular momentum increases in the range of 30-40 . This has an important bearing on the study of heavy ion induced fusion-fission reactions, since the compound nuclei formed in heavy ion reactions at above-barrier energies have very large angular momenta.…”