Emission of Alpha Particles from Nuclei Having Large Angular Momenta

The statistical theory of nuclear de-excitation predicts $\alpha$-particle energy spectra having important features that were missed in earlier, less complete calculations. To a good approximation, the $\alpha$ spectrum is composed of three qualitatively different subspectra. For ${\mathrm{Dy}}^{156*}$ compound nuclei formed by ${\mathrm{Ce}}^{140}$+${\mathrm{O}}^{16}$ at 90 MeV (lab), these subspectra have their respective maxima at 17, 12, and 7.5 MeV. The 7.5-MeV subspectrum should be resolvable into a group of sharp lines. The crucial roles of the lowest excited state at every angular momentum (the yrast levels), and of the competition with neutron and with dipole and quadrupole $\gamma$-ray emission, are stressed. Simple formulas are derived for estimating the energies at the maxima of the two lowest-energy subspectra. Since the $\alpha$-particle subspectra are predictions of the most widely used version of the statistical model of nuclear de-excitation, a failure to observe them would be important. If they are observed, the experimental data should provide information about several nuclear properties heretofore inaccessible.