Identification of the crossing point at = 21 between normal and intruder configurations

Lic, R.*; Rotaru, F.*; Borge, M. J. G.*; Grvy, S.*; Negoita, F.*; Poves, A.*; Sorlin, O.*; Andreyev, A. N.; Borcea, R.*; Costache, C.*; De Witte, H.*; Fraile, L. M.*; Greenlees, P. T.*; Huyse, M.*; Ionescu, A.*; Kisyov, S.*; Konki, J.*; Lazarus, I.*; Madurga, M.*; Mrginean, N.*; Mrginean, R.*; Mihai, C.*; Mihai, R. E.*; Negret, A.*; Page, R. D.*; Pakarinen, J.*; Pascu, S.*; Pucknell, V.*; Rahkila, P.*; Rapisarda, E.*; erban, A.*; Sotty, C. O.*; Stan, L.*; Stnoiu, M.*; Tengblad, O.*; Turturic, A.*; Van Duppen, P.*; Wadsworth, R.*; Warr, N.*

The decay of Mg was used to study the Al nucleus through spectroscopy at the Isotope Separator On-Line facility of CERN. Previous studies identified two -decaying states in Al having spin-parity assignments =4 dominated by the normal configuration (d5/2) (f7/2) and = 1 by the intruder configuration (d5/2) (d3/2)(f7/2). Their unknown ordering and relative energy have been the subject of debate for the placement of Al inside or outside the = 20 "island of inversion". We report here that the 1 intruder lies only 46.6 keV above the 4 ground state. In addition, a new half-life of =44.9(4)ms, that is twice as long as thepreviously measured 20(10) ms, has been determined for Mg. Large-scale shell-model calculations with the recently developed SDPF-U-MIX interaction are compared with the new data and used to interpret the mechanisms at play at the very border of the = 20 island of inversion.