Exploring the Low-$Z$ Shore of the Island of Inversion at $N = 19$

TL;DR

This study measures the unbound ground state energy of neutron-rich $^{28}$F using invariant mass spectroscopy, revealing minimal intruder configuration influence and defining the low-Z boundary of the N=19 island of inversion.

Contribution

First measurement of $^{28}$F ground state energy, confirming shell model predictions and clarifying the structure of N=19 isotones near the island of inversion.

Findings

$^{28}$F ground state energy at 220 keV above threshold.

Minimal role of pf shell intruder configurations in $^{28}$F.

Establishment of the low-Z boundary of the N=19 island of inversion.

Abstract

The technique of invariant mass spectroscopy has been used to measure, for the first time, the ground state energy of neutron-unbound $^{28}\textrm{F},$ determined to be a resonance in the $^{27}\textrm{F} + n$ continuum at $2\underline{2}0 (\underline{5}0)$ keV. States in $^{28}\textrm{F}$ were populated by the reactions of a 62 MeV/u $^{29}\textrm{Ne}$ beam impinging on a 288 $\textrm{mg/cm}^2$ beryllium target. The measured $^{28}\textrm{F}$ ground state energy is in good agreement with USDA/USDB shell model predictions, indicating that $pf$ shell intruder configurations play only a small role in the ground state structure of $^{28}\textrm{F}$ and establishing a low-$Z$ boundary of the island of inversion for N=19 isotones.