Unexpected distribution of $\nu1f_{7/2}$ strength in the calcium isotopes at $N$=30

TL;DR

This study examines the unexpected distribution of neutron $1f_{7/2}$ strength in calcium isotopes at N=30, revealing discrepancies with existing shell-model interactions and highlighting the fragmentation of nuclear strength to higher states.

Contribution

It provides new experimental data on neutron spectroscopic strengths in calcium isotopes and challenges current shell-model interactions by showing their inability to fully explain the observed strength distribution.

Findings

Reduced strength in $^{49}$Ca compared to $^{47}$Ca

Fragmentation of $l=3$ strength to higher-lying states

Inconsistency with GXPF1 interaction predictions

Abstract

The calcium isotopes have emerged as an important testing ground for new microscopically derived shell-model interactions, and a great deal of focus has been directed toward this region. We investigate the relative spectroscopic strengths associated with $1f_{7/2}$ neutron hole states in $^{47, 49}$Ca following one-neutron knockout reactions from $^{48,50}$Ca. The observed reduction of strength populating the lowest 7/2$^{-}_{1}$ state in $^{49}$Ca, as compared to $^{47}$Ca, is consistent with the description given by shell-model calculations based on two- and three-nucleon forces in the neutron $pf$ model space, implying a fragmentation of the $l$=3 strength to higher-lying states. The experimental result is inconsistent with both the GXPF1 interaction routinely used in this region of the nuclear chart and with microscopic calculations in an extended model space including the $\nu1g_{9/2}$ orbital.