Limits of collective motion in hot nuclear matter

Abstract

High-energy \ensuremath{\gamma}-ray spectra have been measured in coincidence with products from the reaction $^{40}\mathrm{Ge}$ at 15 and 24 MeV/nucleon. The evolution of the giant dipole resonance (GDR) is studied as a function of excitation energy (${E}^{\mathrm{*}}$). Up to ${E}^{\mathrm{*}}$\ensuremath{\approxeq}300 MeV the measured GDR strength is consistent with full collectivity. At higher ${E}^{\mathrm{*}}$ a strong inhibition of the GDR \ensuremath{\gamma} decay is observed. At the highest transition energies, the spectra are consistent with bremsstrahlung emission associated with nucleon-nucleon collisions.