Evidence for the reduction of nuclear level density away from the $\beta$-stability line

The isospin dependence of nuclear level density has been investigated by analyzing the spectra of evaporated neutrons from excited ${}^{116}\mathrm{Sn}$ and ${}^{116}\mathrm{Te}$ nuclei. These nuclei are populated via $p+{}^{115}\mathrm{In}$ and ${}^4\mathrm{He}+{}^{112}\mathrm{Sn}$ reactions in the excitation energy range of 18–26 MeV. Because of low excitation energy, the neutron spectra are predominantly contributed by the first-chance decay leading to the $\beta$-stable ${}^{115}\mathrm{Sn}$ and neutron-deficient ${}^{115}\mathrm{Te}$ as residues for the two cases. Theoretical analysis of the experimental spectra has been performed within the Hauser-Feshbach formalism by employing different models of the level-density parameter. It is observed that the data could only be explained by the level-density parameter that decreases monotonically when the proton number deviates from the $\beta$-stable value. This is also confirmed by performing a microscopic shell-model calculation with the Woods-Saxon mean field. The results have strong implications on the estimation of the level density of unstable nuclei and calculation of astrophysical reaction rates relevant to $r$ and $rp$ processes.

Figure 1

Experimental neutron double differential spectra (filled circles) for the $p+{}^{115}\mathrm{In}$ reaction measured at ${155}^{\circ }$ for the incident proton energies of (a) 9 MeV and (b) 12 MeV. Lines are the predictions of talys using three different parametrizations of the level-density parameter (see the text). The arrows show the positions above which the spectra are entirely determined by the first-chance emission.

Figure 2

Experimental neutron spectra (filled circles) for the ${}^4\mathrm{He}+{}^{112}\mathrm{Sn}$ reaction at ${155}^{\circ }$ along with the predictions of talys with phenomenological (a) GC, (b) BSFG, and (c) GSM level densities using the standard $\alpha A$ form of the level-density parameter. The shaded regions in plots (a)–(c) correspond to $\pm 15\%$ variation in the standard value of $\alpha$ for each model. (d) The experimental spectrum compared with the talys calculation using microscopic HFBCS (continuous line), and HFB $+$ C (dashed line) level densities as inputs.

Figure 3

The same as Fig. 1 but for the ${}^4\mathrm{He}+{}^{112}\mathrm{Sn}$ reaction.

Figure 4

The prediction of the $Z-Z_0$ form [Eq. (4)] of the level-density parameter (continuous line) is compared with a microscopic shell-model calculation (dashed line). The shaded region corresponds to the theoretical uncertainty in the shell-model prediction. The nuclei investigated in the present work are indicated by the symbols.