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

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 $T\approx 2–2.5$ MeV at an average angular momentum of $26\hslash$ using the ${}^{16}\mathrm{O}+{}^{58}\mathrm{Ni}$ fusion reaction. The emitted high-energy $\gamma$ rays and evaporated neutrons were measured in coincidence with low-energy discrete $\gamma$-ray multiplicities. The GDR parameters, nuclear level density parameter, and nuclear temperature were determined by the statistical model analysis of the high-energy $\gamma$-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.

Figure 1

Plot of angular momentum gated ($\langle \mathrm{J}\rangle \approx 26\hslash$) neutron spectra, measured at backward (yellow circle) and forward angles (red circle) along with statistical model cascade calculation (solid blue line) and hipse calculation (thin green line). The spectra at different energies are scaled for better visualization.

Figure 2

Measured high-energy $\gamma$-ray spectra and divided plots along with best fitted cascade calculations (blue continuous lines) at incident energies of 116, 140, and 160 MeV. In the left panel, bremsstrahlung components are shown (cyan dashed line). In the right panel, model calculations based on the fluctuation of nuclear shape are shown. The pink dashed line represents the adiabatic TSFM calculation. The red dotted line and the green solid line correspond to the pTSFM [40] and CTFM [11] calculations, respectively. All the model calculations were performed at $J=26\hslash$ (see text).

Figure 3

Plot of GDR width with temperature. Top panel: GDR widths for ${}^{88}\mathrm{Mo}$ [21], ${}^{86}\mathrm{Mo}$ [37], ${}^{92}\mathrm{Mo}$ [36], and ${}^{100}\mathrm{Mo}$ [36] along with prediction of pTSFM [40] (see Ref. [21] for details). Bottom panel: The red filled circles are the values for ${}^{74}\mathrm{Kr}$ deduced from the present work. Experimental data for ${}^{76}\mathrm{Kr}$ [39] (pink open circle) are from previous works. The pink dashed line represents the adiabatic model of TSFM calculation. The red dotted line and the green solid line corresponds to the pTSFM [40] and CTFM [11] calculations, respectively. All the model calculations were performed at $J=26\hslash$.