Neutron Emission from theZn64Compound Nucleus Formed in Two Ways:p+Cu63andα+Ni60

Abstract

The ${\mathrm{Zn}}^{64}$ compound nucleus was formed at an excitation energy of 33.3 MeV using tow different target-projectile systems: 26.0-MeV protons on ${\mathrm{Cu}}^{63}$ and 31.3-MeV $\ensuremath{\alpha}$ particles on ${\mathrm{Ni}}^{60}$. Neutrons were detected with nuclear emulsions using the internal-radiator method. Exposures were made simultaneously at the following angles: 18, 33, 90, 147, and 162\ifmmode^\circ\else\textdegree\fi{}. The c.m. energy spectra (2-15 MeV) of the two systems are very nearly the same. For both systems symmetry about 90\ifmmode^\circ\else\textdegree\fi{} in the c.m. angular distributions is seen up to about 6 MeV followed by a forward peaking at higher energies. The symmetric (compound-nuclear) parts of the angular distributions differ significantly in anisotropy. For the $p+{\mathrm{Cu}}^{63}$ system the distribution is essentially isotropic while for the $\ensuremath{\alpha}+{\mathrm{Ni}}^{60}$ system we have an average anisotropy of about 1.4. We also observe for the $\ensuremath{\alpha}+{\mathrm{Ni}}^{60}$ system a slight but significant increase in anisotropy with energy of the emitted neutron (1.290 \ifmmode\pm\else\textpm\fi{} 0.047 at 2 MeV to 1.649 \ifmmode\pm\else\textpm\fi{} 0.119 at 6 MeV). The changes in average anisotropy, as we change target-projectile systems, are basically consistent with multiple-emission calculations using a rigid-sphere moment of inertia (${r}_{0}=1.22$ F). The change in anisotropy with emission energy is not seen in the calculations. It is estimated that, for the $p+{\mathrm{Cu}}^{63}$ and $\ensuremath{\alpha}+{\mathrm{Ni}}^{60}$ systems, direct-reaction neutrons constitute 5 and 3% of the total observed neutron spectra.