Gamma-widths, lifetimes and fluctuations in the nuclear quasi-continuum

M Guttormsen,V W Ingeberg,G M Tveten,A C Larsen,S Siem,A Görgen,L E Kirsch,T Renstrøm,L Crespo Campo,F Zeiser,J E Midtbø

doi:10.1051/epjconf/201817806001

Abstract

Statistical γ-decay from highly excited states is determined by the nuclear level density (NLD) and the γ-ray strength function (γSF). These average quantities have been measured for several nuclei using the Oslo method. For the first time, we exploit the NLD and γSF to evaluate the γ-width in the energy region below the neutron binding energy, often called the quasi-continuum region. The lifetimes of states in the quasi-continuum are important benchmarks for a theoretical description of nuclear structure and dynamics at high temperature. The lifetimes may also have impact on reaction rates for the rapid neutron-capture process, now demonstrated to take place in neutron star mergers.

Highlights

Nature displays a huge span of lifetimes, from the birth and death of stars to the population and decay of states in the micro-cosmos
In the world of quantum physics, unstable states are associated with an energy width Γ, which is related to the lifetime through τΓ =
Both quantities depend on available final states and the γ strength into these states

Summary

Introduction

Nature displays a huge span of lifetimes, from the birth and death of stars to the population and decay of states in the micro-cosmos. When the number of states reaches 100-1000 levels per MeV, detailed spectroscopy becomes almost impossible and less useful In this quasi-continuum region, the NLD and the average γ-ray strength function (γSF) become fruitful concepts. These two quantities replace the accurate position of initial and final states and the transition probabilities between them in conventional discrete spectroscopy. The Oslo method has provided NLDs and γSFs for many nuclei in the vicinity of the β-stability line1 From these observables, lifetimes, γ widths, and fluctuations can be explored in the quasicontinuum. For γ energies around 3 MeV, the corresponding increase in γ strength is only one order of magnitude This makes sense, because the NLD is fundamentally a combinatorial problem of the number of active quasi-particles, while the electric-dipole γ strength scales linearly with the number of protons. The 64 silicon particle telescopes of SiRi are placed in the vacuum chamber at the center of CACTUS

The Oslo Method

The evaluation of γ width and lifetime

Results and discussion