Bombarding-energy dependence of theBi209+Xe136reaction

Abstract

A previous study of the strongly damped reaction $^{209}\mathrm{Bi}$ + $^{136}\mathrm{Xe}$ at 1130 MeV has been extended to the lower bombarding energy of 940 MeV. With the same experimental technique, angular, energy, and atomic charge distributions and their correlations have been measured. The angular distribution of the reaction products is sideways peaked, but less strongly focused than at the higher bombarding energy. The energy spectrum extends far below the Coulomb energy calculated for touching spherical ions, indicating large deformations of the final fragments. The charge distributions are Gaussians approximately centered around the initial fragmentation. Examination of the correlations between experimental observables confirms that the energy-loss parameter is the most suitable quantity to describe the time evolution of the reaction. The relationship between energy loss and the width of the charge distribution is studied to gain information on the contribution of the nucleon exchange process to the total dissipated energy. The bombarding-energy dependence of this relationship suggests that the Pauli blocking of occupied single particle levels is an important effect, leading to a smaller dispersion of the fragment $Z$ distribution for a given energy loss than expected from a classical theory. A quantitative analysis establishes the nucleon exchange as the dominant mechanism for the dissipation of kinetic energy. With the aid of a phenomenological model, a decomposition of the reaction cross section in partial waves is performed. Classical trajectory calculations assuming spherical ions are compared to an empirically determined deflection function, energy loss, and interaction times. These calculations do not provide a consistent description of the experimental results, since the energy loss is systematically underestimated. From the interaction times and widths of the charge distributions, an angular momentum dependent proton number diffusion coefficient ${D}_{Z}(l)$ is derived, which shows a pronounced saturation behavior for angular momenta less than $\frac{2}{3}$ of the grazing angular momentum. The total probability for sequential fission of the targetlike fragment is determined to be 30% for all inelastic events. A simple model is presented which allows calculation of this probability on the basis of known fission properties of heavy elements.NUCLEAR REACTIONS $^{209}\mathrm{Bi}$ + $^{136}\mathrm{Xe}$, ${E}_{\mathrm{lab}}=940$ MeV; measured $\ensuremath{\sigma}(\ensuremath{\theta}, E, Z)$; strongly damped reaction; deduced correlations, interaction times, transport coefficients; comparison to trajectory calculations; yield of sequential fission.