Neutron-rich $N=22$, 24, 26 magnesium isotopes were studied via in-beam $\gamma$-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory following secondary fragmentation reactions on a carbon target at $\approx 200\mathrm{MeV}/\mathrm{\text{nucleon}}$. In the one- and two-proton removal channels from ${}^{39}\mathrm{Al}$ and ${}^{40}\mathrm{Si}$ beams, two distinct $\gamma$-ray transitions were observed in ${}^{38}\mathrm{Mg}$, while in the one-proton removal reaction from ${}^{37}\mathrm{Al}$ a new transition was observed in addition to the known $2_1^{+}\to 0_{\mathrm{g}.\mathrm{s}.}^{+}$ decay. From the experimental systematics and comparison to theoretical predictions it is concluded that the transitions belong to the $2_1^{+}\to 0_{\mathrm{g}.\mathrm{s}.}^{+}$ and $4_1^{+}\to 2_1^{+}$ decays in ${}^{36}\mathrm{Mg}$ and ${}^{38}\mathrm{Mg}$, respectively. For ${}^{34}\mathrm{Mg}$, previously reported $2_1^{+}$ and $4_1^{+}$ level energies were remeasured. The deduced $E(4_1^{+})/E(2_1^{+})$ ratios for ${}^{34,36,38}\mathrm{Mg}$ of 3.14(5), 3.07(5), and 3.07(5) are almost identical and suggest the emergence of a large area of deformation extending from the $N=20$ to the $N=28$ shell quenching.

• Received 25 August 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.212502