Effects of Pair Correlations in Statisticalγ-Decay Spectra

Abstract

Statistical y-decay spectra from excited nuclear states are calculated.The spectra reflect the perturbation of the level density by pair correlations and, hence, the stepwise weakening of the pair correlations with increasing quasiparticle number. The level density is obtained by counting manyquasiparticle states from a self-consistent BCS calculation or from a diagonalization of particle numberprojected states. The calculated spectra resemble measured spectra from the decay out of superdeformed bands, but have only -70% of the measured intensity. PACS numbers: 21.10.Ma, 21.60.Ka, 23.20.Lv, 27.80.+w Pair correlations are essential for describing many properties of the low-lying states of nuclei. By applying methods developed to describe superAuidity and superconductivity in condensed matter physics, one can explain the observed moments of inertia of ground state rotational bands, as well as the occurrence of a gap in the energy spectrum of even nuclei between the ground and excited states [1]. With increasing temperature, pair correlations become weaker, and are finally completely quenched, in analogy with the superAuid to normal phase transition in liquids. Because of the mesoscopic (i.e., finite) nature of the nucleus, pairing correlations can be appreciably re-

Pairing correlations and transitions in nuclear systems

“…From this simple model the gap decreases monotonically to zero at a critical temperature of T c ∼ 0.5 ∆. However, if particle number is projected out [27,28], the decrease is significantly delayed. The predicted decrease of pair correlations takes place over several MeV of excitation energy [28].…”

Excitation energies and spins of the yrast superdeformed band inHg191

“…7 shows a region with depleted yield between 2.3 and 3.3 MeV. Following Døssing et al [37], this depletion can be explained by the reduction in level density in the interval from the ND yrast line up to the energy required to break the first pair of neutrons or protons. In Ref.…”

“…In Ref. [37] it is seen that the width of the depleted region in the γ spectrum is around 1.5 times the average pairing gap. The depleted region (which is most clearly defined by the decay-out transitions with ∆I = 1h) occurs between 2.3 and 3.3 MeV, implying a pair gap of about 0.7 MeV.…”

Superdeformation in the A=150 and A=190 regions

“…The decay spectrum also contains a noticeable bump between 1.3 and 2.2 MeV which sits on top of the statistical spectrum. A recent calculation by Dossing based on a model which treats self-consistently the weakening of pair correlations with increasing number of quasiparticle excitations reproduces the decay spectrum including this bump [75]. In the calculation, the bump arises from a combination of transitions from the sequential steps of the de-excitation cascade and from the last step of the decay where the statistically fed continuum states must cross the pair gap to the ground band.…”