Gamma-Ray Emission from Compound Nucleus Reactions of Helium and Carbon Ions

Abstract

The assumption that gamma emission does not occur in compound nucleus reactions until the excitation energy falls below the particle emission threshold was tested by direct observation of the gamma rays. Bombardments of Ba, Ho, Co, and Ta targets were carried out with helium ions; Te and V were bombarded with ${\mathrm{C}}^{12}$ ions. For the carbon ion reactions, in which the average angular momentum induced by the projectile ranged from $17\ensuremath{\hbar}$ to $38\ensuremath{\hbar}$, the total energy release in gamma rays exceeded the neutron binding energy, in contradiction to the assumption. This was not so for any alpha-particle reaction. A comparison of the gamma yields for the same compound nucleus, as produced by alpha particles and carbon ions at the same excitation energy, indicates that the enhancement is an angular momentum effect, independent of excitation energy.The magnitude of the gamma yield enhancement is in agreement with observed displacements of heavy ion excitation functions to higher bombarding energy.The gamma yield was measured at 45\ifmmode^\circ\else\textdegree\fi{} and 90\ifmmode^\circ\else\textdegree\fi{} to the beam direction. Both the helium ion bombardments at 45 MeV and carbon ions incident on ${\mathrm{V}}^{51}$ at comparable energy produced large anisotropies in the gamma-ray emission, with maxima in the direction required by quadrupole transitions. The photon emission was more nearly isotropic for alpha bombardments at 28 MeV and for ${\mathrm{C}}^{12}$ on Te at 115 MeV. The average photon energy ranged from 1.0 to 1.6 MeV.These observations are consistent with gamma emission in collective transitions and are inconsistent with a statistical model of the deexcitation process, which predicts dipole emission, small anisotropy, and an average gamma energy twice that observed. The lack of quadrupole anisotropy for the Te target may result from a high-spin limit to the collective motion. The energy spectra suggest that the collective transitions may be vibrational, with rotational transitions occurring mainly in nuclei deformed in the ground state. Except for compound nuclei of $A\ensuremath{\sim}60$, good agreement was obtained between the observed anisotropy and a model based on a gamma cascade through levels of specified rather than randomly chosen spin.