Experimental signature of collective enhancement in nuclear level density

TL;DR

This study provides experimental evidence of collective enhancement in nuclear level density by measuring neutron and GDR gamma rays from $^{169}$Tm, revealing a significant enhancement factor and its fadeout at 14 MeV excitation energy.

Contribution

First experimental identification of collective enhancement in nuclear level density using neutron and gamma-ray spectra from a rare earth nucleus.

Findings

Enhanced neutron and gamma-ray yields at the same excitation energy.

Collective enhancement factor of about 10 in the Fermi gas model.

Fadeout of enhancement at approximately 14 MeV excitation energy.

Abstract

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 $^{169}$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.