Probing the $N=14$ subshell closure: $g$ factor of the ${}^{26}$Mg(2${}_1^{+}$) state

Abstract

The first-excited state $g$~factor of ${}^{26}$Mg has been measured relative to the $g$ factor of the ${}^{24}$Mg($2_1^{+}$) state using the high-velocity transient-field technique, giving $g=+0.86\pm 0.10$. This new measurement is in strong disagreement with the currently adopted value, but in agreement with the $sd$-shell model using the USDB interaction. The newly measured $g$ factor, along with $E(2_1^{+})$ and $B(E2)$ systematics, signal the closure of the $\nu d_{5/2}$ subshell at $N=14$. The possibility that precise $g$-factor measurements may indicate the onset of neutron $pf$ admixtures in first-excited state even-even magnesium isotopes below ${}^{32}$Mg is discussed and the importance of precise excited-state $g$-factor measurements on $sd$~shell nuclei with $N\ne Z$ to test shell-model wavefunctions is noted.

8 pages, 5 figures