Upbend and M1 Scissors Mode in Neutron-rich Nuclei --- Consequences for r-process $(n,\gamma )$ Reaction Rates

A C Larsen ,A Bracco,M Wiedeking,S Frauendorf,F Naqvi,S Leoni,A Spyrou,F Giacoppo,F Camera,S Siem ,S N Liddick ,M Guttormsen ,A Voinov ,Stéphane Goriely ,L A Bernstein ,H T Nyhus ,A Görgen ,S Harissopulos ,T Renstrøm ,T K Eriksen ,S J Rose ,B A Brown ,R Schwengner ,D L Bleuel ,G M Tveten

doi:10.5506/aphyspolb.46.509

Abstract

An enhanced probability for low-energy γ-emission (upbend, Eγ < 3 MeV) at high excitation energies has been observed for several light and medium-mass nuclei close to the valley of stability. Also the M1 scissors mode seen in deformed nuclei increases the γ-decay probability for low-energy γ-rays (Eγ ≈ 2–3 MeV). These phenomena, if present in neutron-rich nuclei, have the potential to increase radiative neutron-capture rates relevant for the r-process. Furthermore, the experimental and theoretical status of the upbend is discussed, and preliminary calculations of (n,γ) reaction rates for neutron-rich, mid-mass nuclei including the scissors mode are shown.

Highlights

Resonance-like structures in the γ-ray strength function, which is a measure of the average, electromagnetic decay properties of the nucleus, may reveal the underlying nuclear structure and shed light on decay modes at high excitation energy
A new and very promising technique has been developed at NSCL/MSU by Spyrou et al [13] to measure level density and γ-ray strength in neutronrich nuclei by β-decay tagged γ-ray spectra (β-Oslo method)
It is clear that the low-energy part of the γ-ray strength function (γSF) of neutron-rich nuclei needs to be experimentally investigated in order to verify the existence of the upbend and the scissors mode

Summary

Introduction

Resonance-like structures in the γ-ray strength function (γSF), which is a measure of the average, electromagnetic decay properties of the nucleus, may reveal the underlying nuclear structure and shed light on decay modes at high excitation energy. The unexpected enhanced decay strength at low γ-ray energies (the upbend, Eγ < 3 MeV) found in f p-shell [1, 2] and A ∼ 90– 100 nuclei [3, 4] represents a recent finding in that respect. If this upbend is present in neutron-rich nuclei, their (n, γ) reaction rates could be significantly enhanced [5]. The present knowledge of the upbend is presented, and for the scissors mode preliminary reaction-rate calculations for neutron-rich nuclei in the region A ∼ 60–180 are shown

The low-energy upbend

The scissors mode

Summary and outlook