Using nuclear structure to study the vaporization of hot nuclei

Many experiments on the gamma decay of highly excited nuclei show the persistence of the giant dipole resonance as a collective mode even under rather extreme conditions. The theory of these resonances predicts that they should essentially retain the properties they have in the ground state to quite high excitation. The average resonance energy may be studied in mean-field theory and is found to change less than 5% for temperatures as high as approx.1.5 MeV. The spreading of the resonance has recently been calculated for nuclei at finite temperatures and rotational frequencies. The damping is found to increase by an insignificant amount in the measured temperature range, except when the nucleus changes deformation. The authors argue here that the stability of the dipole may be used to advantage in the study of other properties of nuclei at high excitation. For example, given that a compound nucleus is formed in a heavy-ion reaction, the dipole branching ratio is very sensitive to the statistical properties of the nucleus. The branching ratio allows a more sensitive measurement of the level density parameter at high excitation than would be otherwise available