Experimental signature of collective enhancement in nuclear level density

We present a probable experimental signature of collective enhancement in the nuclear level density (NLD) by measuring the neutron and the giant dipole resonance (GDR) $\gamma$ rays emitted from the rare-earth ${}^{169}\mathrm{Tm}$ compound nucleus populated at 26.1 MeV excitation energy. An enhanced yield is observed in both neutron and $\gamma$-ray spectra corresponding to the same excitation energy in the daughter nuclei. The enhancement could only be reproduced by including a collective enhancement factor in the Fermi gas model of NLD to explain the neutron and GDR spectra simultaneously. The experimental results show that the relative enhancement factor is of the order of 10 and the fadeout occurs at $\sim 14$ MeV excitation energy, much before the commonly accepted transition from deformed to spherical shape. We also explain how the collective enhancement contribution changes the inverse level density parameter $k$ from 8 to 9.5 MeV observed recently in several deformed nuclei.

Figure 1

Schematic view of the experimental setup.

Figure 2

(a) Experimental neutron spectra measured at two angles are compared with each other. (b) Experimental neutron spectrum is compared with the cascade calculations. (c) Experimental $\gamma$ spectra measured at two angles are compared with each other. (d) Experimental $\gamma$ spectrum is compared with the cascade calculations plus bremsstrahlung component. The enhancement in the spectra and the contributions from different nuclei are shown with arrows.

Figure 3

Solid dots represent the neutron spectra (no enhancement in NLD) as calculated from cascade with $k=9.5$ MeV for the reaction ${}^4\mathrm{He}(E_{\mathrm{lab}}=40\mathrm{MeV})+{}^{165}\mathrm{Ho}$ studied earlier [25]. The same calculation (solid lines) but with $k=8.0$ MeV and (a) no enhancement in NLD of any nuclei and (b) enhancement in NLD of all the three nuclei.

Figure 4

Enhanced level densities of (a) ${}^{169}\mathrm{Tm}$ and (b) ${}^{168}\mathrm{Tm}$ at $11\hslash$ and (c) ${}^{167}\mathrm{Tm}$ at $16\hslash$ are shown as used in the cascade code. The Fermi gas level density is also displayed for comparison. The level densities are not in absolute scale because they are not normalized to experimental data. The level density of ${}^{170}\mathrm{Yb}$ is compared with ${}^{169}\mathrm{Tm}$. (d) Relative enhancement factors extracted as a function of excitation energy for three nuclei.