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Structure of the neutron-rich nucleus $^{30}\mathrm{Mg}$, which is located close to the region of the $N=20$ ``island of inversion,'' has been investigated by $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy of spin-polarized $^{30}\mathrm{Na}$. From the detailed $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence relations, nine new states have been established in $^{30}\mathrm{Mg}$. The spin-polarization of $^{30}\mathrm{Na}$ enabled firm spin-parity assignments for seven states in $^{30}\mathrm{Mg}$ by means of the spatial asymmetry of $\ensuremath{\beta}$-ray counts in coincidence with the $^{30}\mathrm{Mg}\phantom{\rule{4pt}{0ex}}\ensuremath{\gamma}$ rays. Based on the experimental information, it is shown that various characteristic structures coexist in $^{30}\mathrm{Mg}$: (i) the ground-state vibrational-like band, (ii) the deformed band built on the ${0}_{2}^{+}$ state at 1.788 MeV, (iii) the $\ensuremath{\gamma}$-vibrational band built on the ${2}_{3}^{+}$ state at 3.543 MeV, and (iv) four states around 5 MeV associated with large $\ensuremath{\beta}$-transition probabilities: ${1}^{+}$ and ${2}^{+}$ states with spherical nature, ${1}^{+}$ and ${2}^{+}$ states with collective nature. The experimental results are compared with three theoretical calculations of the CHFB $+$ LQRPA, the AMD $+$ GCM, and the large-scale shell model with the EEdf1 interaction. All these theories reasonably reproduce the three bands.

In-flight fission of a 345 MeV per nucleon 238U primary beam on a 2 mm thick 9Be target has been used to produce and study the decays of a range of neutron-rich nuclei centred around the doubly mid-shell nucleus 170Dy at the RIBF Facility, RIKEN, Japan. The produced secondary fragments of interest were identified eventby- event using the BigRIPS separator. The fragments were implanted into the WAS3ABI position sensitive silicon active stopper which allowed pixelated correlations between implants and their subsequent β-decay. Discrete γ-ray transitions emitted following decays from either metastable states or excited states populated following beta decay were identified using the 84 coaxial high-purity germanium (HPGe) detectors of the EURICA spectrometer, which was complemented by 18 additional cerium-doped lanthanum bromide (LaBr3)

The structure of excited states in the neutron-rich nucleus 31 Mg, which is in the region of the “island of inversion” associated with the neutron magic number N = 20 , is studied by β – γ spectroscopy of spin-polarized 31 Na. Among the 31 Mg levels below the one neutron separation energy of 2.3 MeV, the spin values of all five positive-parity levels are unambiguously determined by observing the anisotropic β decay. Two rotational bands with K π = 1 / 2 + and 1 / 2 − are proposed based on the spins and energies of the levels. Comparison on a level-by-level basis is performed between the experimental results and theoretical calculations by the antisymmetrized molecular dynamics (AMD) plus generator coordinate method (GCM). It is found that various nuclear structures coexist in the low excitation energy region in 31 Mg.

The level structure of $^{31}\mathrm{Mg}$, which is located close to the region of the $N$ = 20 ``island of inversion'' in the nuclear chart, has been studied by $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy with spin-polarized $^{31}\mathrm{Na}$. In $^{31}\mathrm{Mg}$, shape coexistence is expected as a result of subtle competition between the spherical mean field and the nuclear correlation which favors deformed configurations. In the present work, our unique method utilizing the anisotropic $\ensuremath{\beta}$ decay of spin-polarized $^{31}\mathrm{Na}$ enables us to firmly assign the spins of all positive-parity excited levels in $^{31}\mathrm{Mg}$ below the neutron separation energy at 2.3 MeV. Furthermore, by constructing a very detailed decay scheme, including two newly found levels, the spins of negative-parity levels are restricted. The examination of the spectroscopic information shows that the deformed rotational bands with ${K}^{\ensuremath{\pi}}=1/{2}^{+}$ and $1/{2}^{\ensuremath{-}}$, which have very similar structures to those observed in a higher excitation energy region of $^{25}\mathrm{Mg}$, appear as the ground-state and low-lying bands, respectively, in $^{31}\mathrm{Mg}$. The experimental levels of $^{31}\mathrm{Mg}$ are compared, on the level-by-level basis, with two types of theoretical calculations. These are, first, the antisymmetrized molecular dynamics (AMD) plus generator coordinate method (GCM) and, second, the shell model with the EEdf1 interaction, which is microscopically derived from chiral effective field theory. It is understood that 8 levels among the experimental 11 levels are the members of four types of largely deformed rotational bands and 2 levels are of spherical nature. The $1/{2}^{+}$ 2.244-MeV level is successfully reproduced by the shell-model calculation with a dominant 4p4h configuration. The present work clearly demonstrates that various structures coexist in a low excitation energy region of $^{31}\mathrm{Mg}$.

Physics letters / B 762, 404 - 408 (2016). doi:10.1016/j.physletb.2016.09.058, Published by North-Holland Publ., Amsterdam

In this contribution the low-excitation structural properties of the doubly mid-shell nucleus 170Dy are discussed, with a special empasis on the evolution of the ground state rotational band within the dysprosium isotopic chain. Recent results from an experiment with the EURICA setup at RIKEN are shown in the context of previous measurements at the PRISMA+CLARA as well as the PRISMA+AGATA setups at Laboratori Nazionali di Legnaro. A brief outlook on future planned measurements is also given.

We present information on the excited states in the prolate-deformed, neutron-rich nuclei 165;167Tb100;102. The nuclei of interest were synthesised following in-flight fission of a 345 MeV per nucleon 238U primary beam on a 2 mm 9Be target at the Radioactive Ion-Beam Factory (RIBF), RIKEN, Japan. The exotic nuclei were separated and identified event-by-event using the BigRIPS separator, with discrete energy gamma-ray decays from isomeric states with half-lives in the s regime measured using the EURICA gamma-ray spectrometer. Metastable-state decays are identified in 165Tb and 167Tb and interpreted as arising from hindered E1 decay from the 7 2 [523] single quasi-proton Nilsson configuration to rotational states built on the 3 2 [411] single quasi-proton ground state. These data correspond to the first spectroscopic information in the heaviest, odd-A terbium isotopes reported to date and provide information on proton Nilsson configurations which reside close to the Fermi surface as the 170Dy doubly-midshell nucleus is approached.

Physics letters / B 760, 641 - 646 (2016). doi:10.1016/j.physletb.2016.07.057, Published by North-Holland Publ., Amsterdam