Experimental signature of collective enhancement in nuclear level density

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) γ 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 γ 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 ∼ 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.

Total absorption spectroscopy of the β decay of Zr101,102 and TcDombos1, Spyrou2, Naqvi3 et al. 2021Phys. Rev. C9000View full textAdd to dashboardCite

“…This procedure is repeated for all pseudolevels within the quasicontinuum. Other implementations of the statistical model to create γ -ray cascades can be found in DECAYGEN [56], DEGEN [57], CASCADE [58][59][60][61], γ DEX [62], and RAINIER [63].…”

Detailed statistical model analysis of observables from fusion-fission reactions

“…The values of E cr and E are taken as 40 and 10 MeV, respectively, which were obtained by fitting yields from projectile fragmentation experiments [95]. The effect of damping of CELD with increasing excitation energy has also been experimentally observed in evaporation spectra of neutrons and high energy photons [97,98].…”

Nuclear level densities away from line of β-stability

“…The (Z − Z 0 ) dependence of the level densities may be quite sensitive to the single-particle energies which may change drastically as one moves away from the β-stability line. Consequently, the other microscopic effects such as collective enhancement due to rotational and vibrational states [21][22][23][24][25][26][27][28][29], pairing and shell corrections [30][31][32][33][34][35][36] will also be crucial in determining the behavior of level density around Z 0 [12,21,37]. One of the ways to include various microscopic effects in the level density parameter is through the semi-classical approach.…”