Angular momentum dependence of the nuclear level density in the $A\approx 170$–200 region

Neutron evaporation spectra along with $\gamma$ multiplicity has been measured from ${}^{201}\mathrm{Tl}{}^{*},{}^{185}\mathrm{Re}{}^{*}$, and ${}^{169}\mathrm{Tm}{}^{*}$ compound nuclei at the excitation energies of $\sim 27$ and 37 MeV. Statistical model analysis of the experimental data has been carried out to extract the value of the inverse level density parameter $k$ at different angular-momentum $\left(J\right)$ regions corresponding to different $\gamma$ multiplicities. It is observed that, for the present systems the value of $k$ remains almost constant for different $J$. The present results for the angular-momentum dependence of the nuclear level density (NLD) parameter $\tilde{a}(=A/k)$, for nuclei with $A\sim 180$ are quite different from those obtained in earlier measurements in the case of light- and medium-mass systems. The present study provides useful information to understand the angular-momentum dependence of the NLD at different nuclear mass regions.

Figure 1

Schematic illustration of the experimental setup used in the present experiment.

Figure 2

Experimental neutron energy spectra (symbols) for different folds along with the theoretical fits (continuous lines) made by using the statistical model code cascade for the ${}^4\mathrm{He}+{}^{197}\mathrm{Au}$ system at 40 MeV incident energy.

Figure 3

Same as Fig. 2 but for the ${}^4\mathrm{He}+{}^{181}\mathrm{Ta}$ system at (a) 40 MeV and (b) 30 MeV incident energies.

Figure 4

Same as Fig. 2 but for the ${}^4\mathrm{He}+{}^{165}\mathrm{Ho}$ system at (a) 40 MeV and (b) 28 MeV incident energies.

Figure 5

Variation of the inverse level density parameter with average angular momentum corresponding to different $\gamma$ folds in the case of (a) ${}^4\mathrm{He}+{}^{165}\mathrm{Ho}$ (b) ${}^4\mathrm{He}+{}^{181}\mathrm{Ta}$, and (c) ${}^4\mathrm{He}+{}^{197}\mathrm{Au}$ systems.

Figure 6

Variation of the inverse level density parameter with average angular momentum for the ${}^4\mathrm{He}+{}^{93}\mathrm{Nb}$ (filled triangle) and ${}^4\mathrm{He}+{}^{58}\mathrm{Ni}$ (filled circle) systems as observed from the analysis of the (a) neutron, (b) proton, and (c) $\alpha$-particle spectra and (d) for the ${}^4\mathrm{He}+{}^{115}\mathrm{In}$ system as observed from the analysis of the neutron evaporation spectra at $E_{\mathrm{lab}}=30$ MeV (filled triangles) and $E_{\mathrm{lab}}=40$ MeV (filled circles). Data were taken from Refs. [2, 3], respectively.