Decay of hot, high-spin nuclei produced in ⁶Li-induced fusion reactions

Abstract

The decay of compound nuclei formed at high excitation and angular momentum in ⁶Li bombardment of ¹⁸¹Ta, ^194,198Pt, ¹⁹⁷Au, and ²⁰⁸Pb has been investigated in a coherent program of experiments and statistical model analysis. The measured decay properties include absolute cross sections for fusion and for all major competing decay modes, fission-fragment angular distributions, charged-particle energy spectra, and (⁶Li, xn) residue mass distributions. The measurements are compared with highly constrained calculations in which all nuclear structure parameters are fixed to values consistent with the rotating-liquid-drop and noninteracting Fermi-gas models. Possible ambiguities in interpreting the experimental results, and the likely influence of neglected effects on the statistical model calculations, are explored. In order to obtain good quantitative agreement with all measurements it is necessary to simulate in the calculations the effects of preequilibrium nucleon emission in the early stages of the decay, and it is furthermore helpful to allow the collective enhancements of deformed-nucleus level densities to fade out with increasing temperature. Within existing uncertainties in the statistical model treatment, reproduction of the experimental results does not require any appreciable macroscopic or microscopic corrections to the nuclear structure input parameters. Several experimental methods for studying more selectively the fission and particle decay of low-excitation, high-spin nuclei, and thereby enhancing sensitivity to microscopic structure contributions, are suggested.