Excitation energy dependence of the level density parameter close to the doubly magic ${}^{208}\mathrm{Pb}$

Neutron evaporation spectra have been measured from ${}^4\mathrm{He}+{}^{208}\mathrm{Pb}$ and ${}^4\mathrm{He}+{}^{209}\mathrm{Bi}$ reactions by using ${}^4\mathrm{He}$-ion beams of several bombarding energies. Excitation-energy dependence of the level density parameter has been studied for the two systems in the excitation energy range of $\sim 18$–50 MeV. For both the reactions an overall reduction of the asymptotic level density parameter with increasing excitation energy (temperature) is observed. The trend of the data was compared with the Thomas–Fermi model predictions and found to be in reasonable agreement. The value of the shell damping parameter has been extracted from the lowest-energy data in the case of ${}^{210,211}\mathrm{Po}$ and ${}^{211,212}\mathrm{At}$ nuclei close to the $Z=82$ and $N=126$ shell closure, and it was found to be consistent with the recent measurement in the vicinity of doubly magic ${}^{208}\mathrm{Pb}$ nucleus.

Figure 1

The experimental neutron energy spectra (symbols) at different angles in the center-mass-frame for the ${}^4\mathrm{He}+{}^{208}\mathrm{Pb}$ system at 60 MeV incident energy. The corresponding SM predictions are shown by dashed lines. The individual spectra have been scaled for better visualization.

Figure 2

The experimental neutron spectrum (symbols) along with the SM calculations (lines) using three $\gamma$ values (see text) for the ${}^4\mathrm{He}+{}^{208}\mathrm{Pb}$ system at 28 MeV incident energy.

Figure 3

Experimental neutron energy spectra (symbols) at different excitation energies along with the statistical model fits (continuous lines) for the ${}^4\mathrm{He}+{}^{208}\mathrm{Pb}$ system. The individual spectra have been scaled for better visualization.

Figure 4

Same as Fig. 3 but for the ${}^4\mathrm{He}+{}^{209}\mathrm{Bi}$ system.

Figure 5

Excitation-energy (temperature) dependence of the inverse level density parameter. The experimental data (symbols) are compared with the prediction of Eq. (7) (dashed line) and the TFA calculations (continuous line) of Ref. [17] (see text).