V. S. Ramamurthy

Macroscopic systematics of nuclear level densities

Abstract In a number of recently proposed semi-empirical nuclear level density formulae, a macroscopic-microscopic approach similar to that used in nuclear potential energy calculations has been successfully used. While the underlying macroscopic part in these formulae is the well-known Bethe expression, these fomulae differ in the treatment of the microscopic part, namely the shell and pairing effect. In the present work, we have carried out a study of the macroscopic features of the nuclear level densities on the basis of the independent-particle model of the nucleus. It was found that even after taking into account shell and pairing effects, large deviations from the predictions of the Bethe expression with a level denisty parameter a smoothly dependent on the mass number ( a = αA ) are present. In particular, the level density parameter a was found to be strongly dependent on the last nucleon separation energies. A simple parametrisation of a including its dependence on the separation energies is proposed. The experimental data on the neutron resonance spacings of spherical nuclei have been analysed on the basis of this parametrisation. It is shown that the predicted isospin dependence of a is consistent with the available experimental data. The deformation dependence of a is also discussed.

Statistical properties of excited nuclei and the determination of the ground-state shell correction energies

It is shown that the ground-state shell correction energies of nuclei can be obtained from a study of the high temperature behaviour of their thermodynamic properties without recourse to Strutinskys smearing procedure.

Mean distribution of single-particle levels in thin-skinned potential wells and the macroscopic level density parameters of nuclei

Abstract A general leptodermous expansion for the density of single-particle levels in thin-skinned potential wells is derived and is used to study the finite size corrections to the macroscopic level density parameters of nuclei. With droplet model values for the potential parameters, the calculated level density parameters for nuclei along the β-stability line show systematic deviations from the experimental estimates. Possible reasons for these deviations are also discussed.

Classical microscopic description of particle cluster collisions: application to heavy ion collisions

A classical microscopic description of the collision between two bound particle clusters, interacting via a suitable two body force is presented with a view to extend the analogy to nuclear collisions. It is shown that with a proper choice of the parameters of the two body force, the model calculations can bring out qualitatively all the essential features of low energy heavy ion collisions such as complete fusion, deep inelastic scattering and nucleon transfers. The model avoids some of the limitations of purely hydrodynamic descriptions connected with the shape parametrization, compressibility and viscosity effects, etc.

Prompt gamma ray multiplicity distributions in spontaneous fission of252Cf

The shape parameters of the multiplicity distribution of prompt gamma rays emitted in spontaneous fission of252Cf were obtained using the multiple coincidence technique. The multiplicity distribution is well represented by a Gaussian distribution. Assuming the average number of prompt gamma rays emitted per fission to be 10.3, the standard deviation of the multiplicity distribution was estimated to be 4.2±0.4. The variation of the standard deviation of the multiplicity distribution has also been obtained as a function of kinetic energy of one of the fragments and was found to exhibit a strong zigzag dependence on the single fragment kinetic energy. The results have been discussed on the basis of the emission mechanism of prompt gamma rays in fission.

Evidence of massive cluster transfers in {sup 19}F+{sup 232}Th reaction at near barrier energies

Yields and angular distributions of the transfer reaction products have been measured in the {sup 19}F+{sup 232}Th reaction at bombarding energies near the Coulomb barrier. The transfer probabilities were calculated ({ital P}{sub tr}) from the measured differential cross sections at the grazing angle, for the most dominant channels for given ({Delta}{ital N}) transfers. The values of {ital P}{sub tr} show, in general, an exponential dependence on the ground state {ital Q} value ({ital Q}{sub 0}) of the reaction and the large cross sections observed for 1{alpha} and 2{alpha} transfer channels are largely accounted by the {ital Q}-value variations. However, the transfer channels of {Delta}{ital N}=5, 9, and 12 leading to {sup 14}C, {sup 10}Be, and {sup 7}Li products, respectively, show strong enhancements in the transfer probabilities. The large cross sections in these channels may imply simultaneous correlated transfers of ({alpha}{ital p}), ({alpha}{ital p},{alpha}), and (3{alpha}) clusters from the {sup 19}F projectile to the target.

Microscopic calculation of mass transport coefficients in heavy ion collisions

We present results of microscopic calculations of the drift and diffusion coefficients in deep inelastic heavy ion collisions. Assuming the two nuclei to interact weakly, we make use of first-order perturbation theory (Golden Rule) for nucleon transfer and take into account the Pauli blocking effect. The nucleon transfer probability from one nucleus to the other is obtained using single particle wave functions. The results obtained are consistent with empirical estimates.

Variation of Kinetic energy distributions of fission fragments with bombarding energy in 4HE induced fission of 238U at medium energies

Abstract The kinetic energy and mass distributions of fission fragments in 4He induced fission of 238U were studied in the bombarding energy range of 32 to 56 MeV. The variation in the average fragment kinetic energy E k with bombarding energy has been obtained as a function of fragment mass. It is seen that E k increases strongly with bombarding energy and the slope d E k / d E α has a strong dependence on the fragment mass. This result indicates that both the shell effects and the saddle to scission dynamics may play a role in determining the scission configuration at these excitation energies.

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.

Frictional effects on post-scisson dynamics and fission fragment kinetic energies

Effect of friction on the post-scission motion of fission fragments is studied. It is shown that, with values of the frictional constant in the neighbourhood of those obtained from heavy-ion reaction data, no significant pre-scission kinetic energy can exist.