Re-examining the transition into the N=20 island of inversion: structure of $^{30}$Mg

TL;DR

This study investigates the structure of $^{30}$Mg near the N=20 island of inversion using neutron removal reactions, revealing unexpected weak intruder state strength and identifying negative parity levels, challenging existing shell-model assumptions.

Contribution

It provides new experimental data on $^{30}$Mg's energy levels and spin-parity assignments, and compares these with novel shell-model calculations, highlighting complex cross-shell effects.

Findings

Weak 0$^{+}_{2}$ level strength challenges conventional intruder models.

First identification of negative parity levels in $^{30}$Mg.

Complex cross-shell effects influence $^{30}$Mg structure more than previously thought.

Abstract

Intermediate energy single-neutron removal from $^{31}$Mg has been employed to investigate the transition into the N=20 island of inversion. Levels up to 5~MeV excitation energy in $^{30}$Mg were populated and spin-parity assignments were inferred from the corresponding longitudinal momentum distributions and $\gamma$-ray decay scheme. Comparison with eikonal-model calculations also permitted spectroscopic factors to be deduced. Surprisingly, the 0$^{+}_{2}$ level in $^{30}$Mg was found to have a strength much weaker than expected in the conventional picture of a predominantly $2p - 2h$ intruder configuration having a large overlap with the deformed $^{31}$Mg ground state. In addition, negative parity levels were identified for the first time in $^{30}$Mg, one of which is located at low excitation energy. The results are discussed in the light of shell-model calculations employing two newly developed approaches with markedly different descriptions of the structure of $^{30}$Mg. It is concluded that the cross-shell effects in the region of the island of inversion at Z=12 are considerably more complex than previously thought and that $np - nh$ configurations play a major role in the structure of $^{30}$Mg.