Angular-Momentum Effects on Neutron Emission by Dy and Tb Compound Nuclei

We have studied as a function of energy three reactions producing 4.1-h ${\mathrm{Tb}}^{149g}$ from Tb compound nuclei and nine reactions producing Dy products from Dy compound nuclei. Incident particles were ${\mathrm{B}}^{10}$, ${\mathrm{B}}^{11}$, ${\mathrm{C}}^{12}$, and ${\mathrm{O}}^{16}$ of energy 4 to 10.4 MeV per amu. Measurements of the average recoil range give strong evidence that all these reactions proceed by compound-nucleus formation. We report angular distributions of the final heavy products for all these reactions. From angular-distribution data we deduce the average total energies of photons and neutrons for each reaction. In the Tb reactions the average total photon energy is always less than 12 MeV. In the Dy reactions the average total photon energy varies linearly with total available energy from nearly 0 to about 30 MeV. These large differences in total photon energy are attributed to differences in the angular momenta of the initial compound nuclei. The rate of increase of the average kinetic energy of all neutrons (from Dy systems) is approximately proportional to the square root of the excitation energy. The relationships between total photon (or neutron) energy and total available energy seem to be independent of the average angular momentum of all compound nuclei. These relationships vary systematically with the number of emitted neutrons.