Heavy Ion Induced Fission Reactions Research Articles

Development of Micro-Pattern Gaseous Detectors for Nuclear Reaction Studies

One of the frontiers of today’s nuclear physics research is the synthesis of Super Heavy Elements (SHE). Fusion-fission dynamics, namely the competition between quasi fission and fusion is one of the key challenges to optimize the SHE. To have an insight into the dynamics, one requires the study of fission fragment mass and angular distribution near barrier energies for heavy-ion induced fission reactions. Recent successful installation of linear accelerators in India offers a unique opportunity to study the dynamics of nuclear reactions and formation process of SHE. For the effective utilization of these current, as well as upcoming facilities, development of novel detectors to study reaction dynamics, formation process of SHE with heavier projectiles and higher beam energies is needed. Gaseous detectors have undergone a rapid improvement in terms of spatial, temporal and energy resolution, rate capability, radiation hardness, ion feedback etc., ushering in a new genre of micro-structured devices based on semi-conductor technology, commonly known as Micro-Pattern Gaseous Detectors (MPGDs). Although many of the MPGD structures were primarily developed for high-rate tracking of charged particles in high energy physics experiments, stability of operation, simplicity of construction and relatively low cost make these detectors suitable for other applications, such as low-energy nuclear physics experiments. The present activities encompass a detailed evaluation of the operational conditions of Micromesh-Multi Wire and THGEM-Multi Wire hybrid detector operated in low-pressure isobutane gas with a view to optimizing their use in the detection of charged particles and fission fragments.

The Effect of Stiffness Parameter on Mass Distribution in Heavy-Ion Induced Fission

The stiffness parameter of the composite system has been studied for several heavy-ion induced fission reactions without the contribution of non-compound nucleus fission events. In this research, d...

Effective moment of inertia for several fission reaction systems induced by nucleons, light particles, and heavy ions

The effective moment of inertia of the compound nucleus is calculated for several fission reaction systems induced by nucleons, light particles, and heavy ions. The determination of this quantity for these systems is based upon the comparison between the experimental data of the fission fragment angular distributions as well as the prediction of the standard saddle-point statistical model. For the systems, the two cases, namely with and without neutron emission corrections, were considered. Afterward, our theoretical results are compared with the data obtained from the rotational liquid drop model as well as the Sierk model and satisfactory agreements were found. We also calculated the quadrupole deformation and mass asymmetry parameters of the fissioning nuclei at the saddle point by two methods, the first method is based on an ellipsoid shape assumption for the fissioning nucleus, and the other is based on dinuclear system model, since in heavy ion induced fission reactions, the compound nuclei which undergo fission can be considered as states of two touching clusters (dinuclear system). Afterward, the results of these two methods were compared and satisfactory agreements were found between them.

Non-equilibrium processes in Heavy-ion induced Fission Reactions

Heavy ion induced reactions offer a wide range of possibilities in exploring the collective behavior of nuclei and nuclear processes. Recent years have seen a spurt in the experimental and theoretical studies to understand the mechanism of the fission process in heavy target projectile systems, arising from the interest to produce elements in the superheavy region. A number of new features are observed experimentally with respect to the kinetic energy, mass and angular distributions of fission fragments and their correlations, implying prevalence of non-equilibrium phenomena in the fission process in many target projectile systems. There are many theoretical attempts both in terms of static potential energy considerations and dynamics to understand the important degrees of freedom that govern the evolution of the nuclear system from initial interaction to the final fission stages. Depending on the degree and nature of equilibration, various processes such as fast-fission, quasi-fission and pre-equilibrium fission have been invoked that can compete with the fully equilibrated compound nuclear fission process. In what we call a C-T Fissility plot, we show the remarkable dependence of the onset of these non-equilibrium processes on the entrance channel parameters.

Application of pulse shape discrimination in Si detector for fission fragment angular distribution measurements

Pulse shape discrimination (PSD) with totally depleted transmission type Si surface barrier detector in reverse mount has been investigated to identify fission fragments in the presence of elastic background in heavy ion-induced fission reactions by both numerical simulation and experimental studies. The PSD method is compared with the other conventional methods adopted to identify fission fragments with solid-state detectors such as ΔE–E telescope and single thin ΔE detector and the data for the10B +232Th fission reaction are presented. Results demonstrate the usefulness of a single transmission-type surface barrier detector for the identification of fission fragments and projectiles like heavy ions

Time of flight (TOF) spectrometer for accurate measurement of mass and angular distribution of fission fragments in heavy ion induced fission reactions

The performance of a dual time of flight (TOF) spectrometer using two 24 cm×10 cm area X – Y position sensitive Multi-Wire Proportional Counters (MWPC) is reported. Good separation of compound nuclear fission channel from elastic, quasi-elastic and non-compound fission channels have been achieved. Time calibration of the spectrometer using a technique, dependent on the detector independence of the mass or kinetic energy distributions gave a three times better mass resolutions compared to the conventional calibration technique using elastically scattered projectiles in a heavy ion induced fission reaction. Detail experimental methods in determining fission fragment mass and angular distribution have been presented and discussed.

Recent studies in heavy ion induced fission reactions

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 K-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 K 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 K 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.

Suppression of collective spin excitations for high-Kstates in heavy ion induced fission reactions

Spin distributions of fission fragments have been measured in ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O},$ ${}^{19}\mathrm{F}{+}^{209}\mathrm{Bi},$ and ${}^{232}\mathrm{Th}$ reactions at near and above barrier energies. It is found that the collective spin modes are suppressed for fragment emission along $90\ifmmode^\circ\else\textdegree\fi{}$ in comparison to the forward-backward emission angles. The suppression factor exhibits a universal scaling with respect to the rotational frequency of the fissioning nucleus along the fission symmetry axis. The analysis of data from the present measurements, as well as those available from the literature, shows that the suppression of collective spin degrees of freedom has the form ${\mathrm{S}}_{\mathrm{coll}}(\ensuremath{\theta})=\mathrm{exp}(\ensuremath{-}\ensuremath{\Delta}E(\ensuremath{\theta})/T){\mathrm{S}}_{\mathrm{coll}}(0\ifmmode^\circ\else\textdegree\fi{}),$ where $\ensuremath{\Delta}E(\ensuremath{\theta})$ is the excitation energy in the tilting mode characterized by the K quantum number.

Fragment spin as a function of the mass asymmetry in heavy ion induced fission reactions

The total average spins of fission fragments were measured as a function of fragment mass in ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{209}\mathrm{Bi}$ and ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{232}\mathrm{Th}$ reactions at bombarding energies near and above the Coulomb barrier. The mass dependence of the fragment spin for the ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{232}\mathrm{Th}$ reactions is observed to be in marked contrast to that for the ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{209}\mathrm{Bi}$ reactions. The present results have been analyzed within the framework of the statistical model taking into account the finite relaxation times for the equilibration of the collective-spin modes. The observed features in ${}^{12}\mathrm{C},$ ${}^{16}\mathrm{O}{+}^{232}\mathrm{Th}$ reactions can be ascribed to incomplete equilibration of the collective-spin modes where the mass relaxation at extreme mass asymmetries terminates before full statistical equilibration of the collective-spin modes is reached.

Heavy-ion induced fission reactions at near-barrier energies — what have we learnt?

Systematic studies of heavy-ion induced fission reactions at near-barrier energies carried out in the last decade have brought out many interesting aspects of fission process in general. The recent experimental findings which show dependence of fission fragment angular distributions on entrance channel, shape, size and spin of the interacting nuclei and shell closure of the intermediate compound nucleus are summarised in the present paper.

Studies of fragment spin as a function of fragment mass in heavy ion induced fission reactions

Measurement on fragment spin as a function of its mass has been carried out over a wide range of bombarding energy in 12 C, 16 O + 209 Bi and 12 C, 16 O + 232 Th reactions. The mass dependence of fragment spin for 12 C, 16 O + 232 Th reactions is observed to be in marked contrast to that for the 12 C, 16 O + 209 Bi reactions. The present results have been analysed within the statistical model by taking into account the finite relaxation time for the equilibration of the collective spin modes. It is found that the observed feature in 12 C, 16 O + 232 Th reactions are characteristic of incomplete equilibration of the collective spin modes where the mass relaxation process at extreme mass asymmetry terminates before full statistical equilibration of the collective spin modes are reached.

Pre-equilibrium fission: A new decay channel for composite systems formed in fusion reactions

Abstract When the fused composite system in a heavy ion collision has a fission barrier height comparable to its temperature, it can undergo fission in a very short interval of time of the order of 10-21s before the formation of a fully equilibrated compound nucleus. In analogy with the well known pre-equilibrium particle emission in similar time scales, we call these fission events pre-equilibrium fission. Experimental signatures of this new mode of fission are demonstrated from an analysis of fragment angular distributions in a number of heavy ion induced fission reactions.

Effect of fission following incomplete fusion on the interpretation of fission fragment angular distributions in heavy-ion fusion reactions.

It is shown that with the inclusion of fission following incomplete fusion in addition to the normal equilibrium fission following compound nucleus formation and the recently proposed pre-equilibrium fission, one can understand in a quantitative way the host of apparently anomalous fragment angular distributions measured in a number of heavy ion induced fission reactions involving the highly fissile actinides as one of the collision partners.

Fusion-fission angular distributions: A new probe of fast fission fractionation in nucleus-nucleus collisions

Fragment angular distributions in heavy ion-induced fission reactions have been analysed in terms of a two component model—fission following compound nucleus formation and fast fission events. It is seen that, contrary to the general assumption, fast fission competes with compound nucleus fission even when the composite system is formed with a spin less than the rotating liquid drop model limit for vanishing fission barrier.