Compound Nuclear Shell Effects in 14-MeV (n, α) Reactions

Smoothed curves drawn with suitable selection of 14-MeV reaction cross-section data reveal distinct shell effects. The cross section dips down at the proton shell and subshell closure positions throughout the mass region. Within a shell, the odd-even nucleon effects tend to disappear and the cross section is seen to be a very slowly decreasing function of the mass number. Across the shells, in addition to abrupt discontinuities, there is a gradual decrease of the cross sections from smaller to larger shells. These effects can be compared with various recent statements of the statistical level density. In the present work, the simple Bloch-Rosenzweig model of the shell-dependent form of the level density has been used and developed to study the effects. Results of computation reproduce the observed shapes faithfully up to $Z<\mathrm{}50\mathrm{}$; with plausible estimates about the combinatorial degeneracy in a major shell, good agreement with the assumption of the validity of the compound nuclear reaction at high excitation is obtained.