J.P. Lestone, J.F. Liang, D.J. Prindle, A.A. Sonzogni, J.P.S. van Schagen and R. Vandenbosch

Recently, Fabris et al.¹ have measured -particles in coincidence with evaporation residues (ER) in ¹⁹F + ¹⁸¹Ta reactions with beam energies from 90 to 140 MeV. From these -data they concluded the Fermi-gas level density parameter decreases dramatically from a = A/8.3 MeV^-1 at a thermal excitation energy of U=20 MeV to a = A/12 MeV^-1 at U=100 MeV. Such a sharp drop in the level density parameter would give a much stronger increase in nuclear temperature, and thus particle emission rates, with increasing excitation energy than previously believed. If true, the claims of Fabris et al. would seriously affect the conclusions of many recent studies where the properties of particle emission from hot fissioning systems are used to estimate the time scales of heavy-ion fission reactions. We have measured the proton and -emission spectral shapes at 90° and 160° to the beam direction in coincidence with ER from the reactions 150 MeV and 190 MeV ¹⁹F + ¹⁸¹Ta. Our center of mass spectra are not consistent with the experimental results of Ref. 1. At 160° to the beam, the influence of non-equilibrium emission and ER detection efficiencies on our observed spectral shapes are expected to be small. Fig. 3.7-1 shows our 160° proton and -spectra converted into the center of mass reference frame. The solid lines show statistical model calculations of the corresponding spectral shapes with an inverse level density parameter K=A/a which increases linearly with excitation energy from a value of K=8.1 MeV at U=0 MeV to K=9.2 MeV at U=100 MeV. The standard optical model emission barrier heights were lowered to reproduce the measured peak positions. The disagreement at the highest kinetic energies cannot be fixed with a simple change in the above-mentioned excitation energy dependence of the level density parameter without producing a significantly poorer fit to the data in the region several MeV above the peak positions. The dashed lines show calculations with the excitation energy dependence of the level density parameter as suggested by Fabris et al. The dependence of the level density parameter on excitation energy suggested by our data is consistent with recent theoretical calculations^2,3 while the results of Fabris et al. are not.

Fig. 3.7-1. Center of mass kinetic energy spectra for proton and emission at _lab=160° in coincidence with ER in 150 MeV and 190 MeV ¹⁹F + ¹⁸¹Ta reactions (solid circles). The solid and dashed lines shown statistical model calculations (see text).