Evaporation of Charged Particles from Highly Excited Compound Nuclei

Abstract

Energy spectra and angular distributions of alpha particles and protons emitted in the bombardment of Ni targets with 160-Mev oxygen ions have been measured. The initial compound nucleus is characterized approximately by $Z=36$, $A=75$, total excitation energy=125 Mev, and rms $\mathrm{angular}\mathrm{momentum}=50\ensuremath{\hbar}$. The energy distributions have the shapes of evaporation spectra although they are displaced by several Mev toward lower energies compared to calculated evaporation spectra. The angular distributions are approximately symmetric about 90\ifmmode^\circ\else\textdegree\fi{} and sharply peaked in the forward and backward directions. At all energies there is some excess of particles in the forward direction and at high energies the distributions are predominantly forward peaked. Most of the particles are believed to be evaporated from compound nuclei with angular distributions determined by the high angular momenta of the emitting systems. The angular distributions can be fitted by a theory with a single adjustable parameter which is related to the variation of level density with angular momentum and the incoming and outgoing angular momenta. The total cross section for alpha-particle production is 2.6\ifmmode\pm\else\textpm\fi{}0.6 barns and for protons 4.0\ifmmode\pm\else\textpm\fi{}1.0 barns. These cross sections are larger than the estimated cross section for compound nucleus formation, indicating that several charged particles are evaporated per interaction. About half the collision cross section is believed to result in compound nucleus formation.