Continuum random-phase approximation for gamma transition between excited states in neutron-rich nuclei

TL;DR

This paper extends the continuum random phase approximation (cRPA) to describe gamma transitions from excited states in neutron-rich nuclei, enabling better understanding of decay pathways including to low-lying states.

Contribution

The study introduces an extended cRPA scheme with a non-local operator to model gamma transitions from excited states, including decay to low-lying states, in neutron-rich nuclei.

Findings

Demonstrated the scheme on $^{140}$Sn for various multipolarities.

Found cases where decay to low-lying states dominates or is comparable to decay to the ground state.

Highlighted the importance of collectivity and continuum orbits in transition dynamics.

Abstract

A characteristic feature of collective and particle-hole excitations in neutron-rich nuclei is that many of them couple to unbound neutron in continuum single-particle orbits. The continuum random phase approximation (cRPA) is a powerful many-body method that describes such excitations, and it provides a scheme to evaluate transition strengths from the ground state. In an attempt to apply cRPA to the radiative neutron capture reaction, we formulate in the present study an extended scheme of cRPA that describes gamma-transitions from the excited states under consideration, which decay to low-lying excited states as well as the ground state. This is achieved by introducing a non-local one-body operator which causes transitions to a low-lying excited state, and describing a density-matrix response against this operator. As a demonstration of this new scheme, we perform numerical calculation for dipole, quadrupole, and octupole excitations in $^{140}$Sn, and discuss E1 and E2 transitions decaying to low-lying $2^{+}_{1,2}$ and $3^{-}_{1}$ states. The results point to cases where the branching ratio to the low-lying states is larger than or comparable with that to the ground state. We discuss key roles of collectivity and continuum orbits in both initial and final states.