Damping of the Giant Resonance in Heavy Nuclei

In heavy nuclei the damping of the giant resonance is due to thermalization of the energy rather than to direct emission of particles; the latter process is strongly inhibited by the angular-momentum barrier. The thermalization proceeds via inelastic collisions leading from the particle-hole state to two-particle-two-hole states. In heavy nuclei, several hundred such states are available at the energy of the giant dipole resonance. The rather large width of the giant resonance arises from the addition of many small partial widths of channels leading to the different two-particle-two-hole states. Both the density of the two-particle-two-hole states and the mean value of the interaction matrix elements between the particle-hole and two-particle-two-hole states are evaluated in a simplified square-well shell model. In a given nucleus the energy dependence of the widths is determined mainly by the density of states; the $A$ dependence is determined mainly by the size of the matrix elements. For $A\approx 200$, we find $0.5 \mathrm{MeV}\le \Gamma \le 2.5$ MeV. The uncertainty in this value comes mostly from the uncertainty in the strength of the interaction. Representing the energy dependence of the width by a power law we find for the exponent the value ∼1.8.