Puzzle of collective enhancement in the nuclear level density

The collective rotational enhancement in the nuclear level density (NLD) arising due to nuclear deformation is still not well understood due to sparse experimental findings. To address this issue, angular momentum (J) gated neutron, proton and GDR γ-ray spectra have been measured from two deformed nuclei (¹⁶⁹Tm and ¹⁸⁵Re) and one near spherical nucleus (²⁰¹Tl) by populating them around 26 MeV excitation energy. An enhanced yield compared to statistical decay is observed in all the three spectra (p, n, γ) for both the deformed nuclei but only statistical decay for near spherical nucleus. Intriguingly, the relative enhancement factors determined independently from all the spectra are very similar (≈10) for both the deformed nuclei. Moreover, the results indicate that the fadeout of the collective enhancement does not dependent strongly on the nuclear ground state deformation which is in stark contrast to the expectations of the phenomenological as well as microscopic calculations. The possible reasons for this discrepancy are discussed.