The dynamics of fission has been studied by solving Euler-Lagrange equations with dissipation generated through one and two body nuclear friction. The average kinetic energies of the fission fragments, prescission neutron multiplicities and the mean energies of the prescission neutrons have been calculated and compared with experimental values and they agree quite well. A single value of friction coefficient has been used to reproduce the experimental data for both symmetric and asymmetric splitting of the fissioning systems over a wide range of masses and excitation energies. It has been observed that a stronger friction is required in the saddle to scission region as compared to that in the ground state to saddle region.

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Prescission neutron multiplicities have been calculated in the framework of a simple, dynamical model of fission. The fission trajectories have been calculated by solving Euler-Lagrange equations with dissipation generated through two-body nuclear viscosity. Systematic study of the relationship between the prescission neutron multiplicities and nuclear viscosity has been made in the range of mass 150-200 and incident energy 4 -13 MeV/nucleon. The values of the viscosity coefficients which are used to predict the observed prescission neutron multiplicities follow a global relation in the region of mass and energy studied.PACS number͑s͒: 25.70.Jj, 24.75.ϩi, 25.70.Gh

The inclusive energy distributions of fragments with Z ≥ 3 emitted from the bombardment of 12 C by 20 Ne beams with incident energies between 145 and 200 MeV have been measured in the angular range θ lab ∼ 10 • -50 • . Damped fragment yields in all cases have been found to be characteristic of emission from fully energy equilibrated composites; for B, C fragments, average Q-values, < Q >, were independent of the centre of mass emission angle (θc.m.), and the angular distributions followed ∼1/sinθc.m. like variation, signifying long life times of the emitting di-nuclear systems. Estimation of total yields of these fragments have been found to be much larger compared to the standard statistical model predictions of the same. This may be indicative of the survival of orbiting like process in 12 C + 20 Ne system at these energies.

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The energy spectra of α-particles have been measured in coincidence with the evaporation residues for the decay of the compound nucleus 31 P, produced in the reaction 19 F (96 MeV) + 12 C. The data have been compared with the predictions of the statistical model code CASCADE. It has been observed that significant deformation effect in the compound nucleus need to be considered in order to explain the shape of the evaporated α-particle energy spectra.One of the main motivations of the low energy heavy ion reactions studies has been to extract informations on the statistical properties of the hot, rotating nuclei. The informations on the main ingredients of the statistical description, i.e., the nuclear level densities and the barrier transmission probabilities, are usually obtained from the study of the evaporated light particle spectra. The validity of the statistical model depends crucially on the successful description of the light particle emission data and the model, so far, has been overwhelmingly successful in explaining a wide variety of nuclear reaction data in low energy regime. In this perspective, recent studies on the evaporated α-particle energy spectra has evoked a lot of interest (see, for example, ref.[1] and references therein). It has been observed that the standard statistical model calculations failed to predict the shape of the evaporated α-particle energy spectra satisfactorily. A large number of experiments have been performed to study this anomaly over a wide range of compound nuclear masses A CN in the range of ∼60-170 [1-6], and in all cases it has been found that the average energies of the measured α-particle energy spectra are much lower than the corresponding theoretical predictions. Several attempts have been made in the past few years to explain this anomaly. Some of the authors [2,3] argued that the discrepancy was due to the lowering of the emission barriers of the hot nuclei as compared to the fusion barriers for the corresponding relatively 'cold' inverse absorption channels as a result of the excitation energy and angular momentum dependent deformation of the emitting system in the former. On the other hand, there is another group of authors who claim that the anomaly may be well explained by incorporating spin dependent level density in the standard statistical model prescription and emission barriers need not be changed [4,5]. Moreover, it has also been observed that the magnitude of the discrepancy has some entrance channel dependence [6], the discrepancy being more for the more symmetric entrance channels. This is indicative of the fact that the magnitude of the phenomena may also be linked with the entrance channel dynamics of the system. Intuitively, the shape of the α-particle spectra would be affected by the deformation of the equilibrating system if it remains deformed over a time scale comparable to the mean life time of α emission. For heavier nuclei (A CN > 60), the theoretical calculations of shape equilibration time, using the code HICOL [1,7] show that these ti...

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The observed saturations of temperature and of linear momentum transfer per incident nucleon in intermediate-energy nuclear reactions are studied in the model of promptly emitted particles. We demonstrate that only with the inclusion of two-body collisions is very good agreement with experimental data obtained.PACS numbers: 25.70.Jj Extensive experimental studies have recently been made to understand the phenomena of incomplete linear momentum transfer in fusionlike nucleus-nucleus reactions. ,_3 A number of models 4 " 9 have been used with varying degrees of success to explain the basic features of the data like (a) approximate universality of the scaling of fractional momentum transfer with heavy-ion projectile mass 10,11 and (b) apparent saturation of that momentum transfer in the range of 170-220 MeV/c per incident nucleon. i1213 A recent measurement of nuclear "temperature" in 60-MeVA4 Ar + Au reactions 14 has also indicated that there may be some kind of dynamical limitation to the excitation-energy storage and vis-a-vis temperature in the residual system. A schematic model prediction 2 for the linear momentum transfer and detailed Landau-Vlasov calculations for the excitationenergy storage in the nucleus 15 both point to the fact that the origin of the saturation, either in linear momentum transfer per incident nucleon (PT/A) or in nuclear temperature (70, may lie in the partial breakdown of mean-field effects and gradual dominance of two-body collisions, which is supported by a number of experimental observations * where fusion cross sections are found to decrease with an increase in bombarding energy.To have a simple but transparent understanding of the importance of two-body collisions in intermediate-energy fusionlike reactions, we have undertaken a detailed dynamical calculation of the linear momentum transfer and excitation-energy deposition in nuclei using a realistic model of promptly emitted particles (PEP's) 16,17 where the effect of two-body collisions has been explicitly taken into account. The effects of dynamically changing momentum distributions due to energy deposition are also taken care of by simulation of excitation effects through temperature. 17 This model has been quite successful in explaining the angular distributions of emitted nucleons in heavy-ion reactions 17 as well as the dependence of velocities of fused residues on entrance-channel mass asymmetry. 18 A similar piece of work done by