Mass Asymmetry Coordinate Research Articles

Evidence of incomplete relaxation in the reaction Ag+40Ar at 288 and 340 MeV bombarding energies

Abstract The particles emitted in the reaction induced by 40 Ar on natural Ag at 288 and 340 MeV bombarding energy have been studied. The fragments have been identified in atomic number, their kinetic energy distribution and their angular distributions have been measured. The kinetic energy spectra show two components: a high-energy component related to the beam energy, or “quasi-elastic” component, and a low kinetic energy component, close to the Coulomb energy called “relaxed” component. The relaxed component is present at all angles and for all particles. The quasi-elastic component is present close to the grazing angle for atomic numbers close to that of the projectile. The relaxed cross section increases with atomic number for Z > 9. The increase in cross section is sharper for the lower bombarding energy. The angular distributions are forward peaked, in excess of 1 sin θ for all the measured atomic numbers. The forward peaking is larger for particles close in Z to the projectile. The results are interpreted in terms of characteristic times associated with a short-lived intermediate complex. The cross sections and angular distributions are satisfactorily reproduced on the basis of a model accounting for a diffusion process occuring along the mass asymmetry coordinate of the intermediate complex.

Studies in the liquid-drop theory of nuclear fission

In connection with nuclear fission we study the division of an idealized charged drop, using a simplified version of the liquid-drop model. The degrees of freedom essential to a discussion of the division of a charged drop and the separation of the fragments to infinity are taken into account: a fragment-separation coordinate, a mass-asymmetry coordinate, a deformation coordinate for each fragment and rotational coordinates for each fragment. To specify fragment deformation, the fragments are represented by spheroids; a nucleus prior to division is represented by two overlapping spheroids. The Hamiltonian for the idealized system consists of a sum of surface, Coulomb and kinetic energies. A study of the saddle-point energies and shapes calculated in this two-spheroid approximation indicates that the approximation is most useful for discussing the fission of elements lighter than about radium. On the basis of this model, we calculate probability distributions for certain observable characteristics of fission fragments at infinity — their total translational kinetic energy, mass, individual excitation energies and individual angular momenta. This is done by applying standard static, dynamical and statistical methods to the Hamiltonian for the system. The present treatment, for the most part, is classical; quantum mechanics is considered only in the statistical-mechanics discussion of the behaviour of the system near the saddle point. The predictions of the model are compared with existing experimental data for distributions in fragment mass and total translational kinetic energy for nuclei lighter than radium. The comparisons are made without the use of any adjustable parameters. The theory is capable of accounting for the magnitudes of the most probable values and widths of the experimental distributions, as well as some, but not all, finer details of the distributions. The dependence of the experimental distributions upon nuclear temperature, and the dependence of the experimental most probable kinetic energies upon the fissility parameter are also approximately reproduced by the calculations.