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

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 ${\hat{I}}^3$-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.

Physics Letters B, Volume 779, 10 April 2018, Pages 124-129