Pigmy resonance in monopole response of neutron-rich Ni isotopes ?

TL;DR

This study investigates the monopole response of neutron-rich Ni isotopes, especially $^{68}$Ni, using a continuum RPA approach with Skyrme interactions, revealing broad monopole strength rather than sharp resonances.

Contribution

It provides a continuum RPA analysis of monopole strength in neutron-rich Ni isotopes, emphasizing the importance of proper continuum treatment and challenging the interpretation of observed pigmy resonances.

Findings

Broad monopole strength shoulder above particle threshold

Sharp monopole peaks are unlikely in $^{68}$Ni at 12.9 MeV

Continuum effects are crucial for accurate monopole response

Abstract

The RPA monopole strength as well as the unperturbed particle-hole excitation strength is studied, in which the strength in the continuum is properly treated without discretizing unbound particle spectra. The model is the sef-consistent Hartree-Fock calculation and the RPA Green's function method with Skyrme interactions. Numerical examples are the Ni-isotopes, especially $^{68}_{28}$Ni$_{40}$, in which an experimental observation of low-lying socalled "pigmy resonance" with an appreciable amount of monopole strength at 12.9 $\pm$ 1.0 MeV was recently reported. In the present study it is concluded that sharp monopole peaks with the width of the order of 1 MeV can hardly be expected for $^{68}$Ni in that energy region. Instead, a broad shoulder of monopole strength consisting of neutron excitations to non-resonant one-particle states (called "threshold strength") with relatively low angular-momenta $(\ell, j)$ is obtained in the continuum energy region above the particle threshold, which is considerably lower than that of isoscalar giant monopole resonance. In the case of monopole excitations of $^{68}$Ni there are no unperturbed particle-hole states below 20 MeV, in which the particle expresses a neutron (or proton) resonant state. It is emphasized that in the theoretical estimate a proper treatment of the continuum is extremely important.