Structure of neutron-rich ³¹Mg by β-decay spectroscopy of spin-polarized ³¹Na

 The structure of neutron-rich nuclei around the N=20 “island of inversion” has been attracting much attention because of the prevalence of intruder nature and persistence of spherical structure. Recent theories predicted possible shape coexistence in Mg isotopes due to subtle competitions between the spherical mean field and the nuclear correlation, which causes deformation. Even after a variety of experimental studies, the experimental information, in particular spin-parity of excited states, is not yet available in these nuclei. Such a situation prevents us from discussing the nuclear structure.
 We have successfully assigned spins and parities of the Mg levels by observing asymmetric β-decay of spin-polarized Na in coincidence with the delayed γ rays and/or neutrons at TRIUMF. In ³¹Mg, we assigned the spins of 8 positive-parity levels, including 3 neutron-unbound states, together with intensities of γ, β decay, and neutron emission. This detailed information enabled us to compare the experimental results with the theoretical ones on a level-by-level basis. As a result, we found four types of rotational bands with different deformation and some levels with spherical natures, at low excitation energy. These results show clear evidence for the shape coexistence in ³¹Mg.