No signature of the saturation of giant dipole resonance width in medium-mass nuclei

Abstract

An experimental study on the temperature ($T$) dependence of giant dipole resonance (GDR) width was performed for the medium mass nucleus $^{74}\mathrm{Kr}$ in the range of $T\ensuremath{\approx}2--2.5$ MeV at an average angular momentum of $26\ensuremath{\hbar}$ using the $^{16}\mathrm{O}+^{58}\mathrm{Ni}$ fusion reaction. The emitted high-energy $\ensuremath{\gamma}$ rays and evaporated neutrons were measured in coincidence with low-energy discrete $\ensuremath{\gamma}$-ray multiplicities. The GDR parameters, nuclear level density parameter, and nuclear temperature were determined by the statistical model analysis of the high-energy $\ensuremath{\gamma}$-ray spectra as well as evaporated neutron spectra. The measured GDR width is found to increase monotonically with temperature, in contradiction with the recent observation of the width saturation in $^{88}\mathrm{Mo}$. Comparisons of the measured data with predictions of the adiabatic thermal shape fluctuation model and its refined version, the critical temperature included fluctuation model, are presented and discussed.