Extension of theN=40Island of Inversion towardsN=50: Spectroscopy ofCr

Abstract

We report on the measurement of the first 2 + and 4 + states of 66 Cr and 70,72 Fe via in-beam γ-ray spectroscopy. The nuclei of interest were produced by (p, 2p) reactions at incident energies of 260 MeV/nucleon. The experiment was performed at the Radioactive Isotope Beam Factory, RIKEN using the DALI2 γ-ray detector array and the novel MINOS device, a thick liquid hydrogen target combined with a vertex tracker. A low-energy plateau of 2 + 1 and 4 + 1 energies as a function of neutron number was observed for N≥38 and N≥40 for even-even Cr and Fe isotopes, respectively. State-of-the-art shell model calculations with a modified LNPS interaction in the pf g 9/2 d 5/2 valence space reproduce the observations. Interpretation within the shell model shows an extension of the Island of Inversion at N=40 for more neutron-rich isotopes towards N=50. Atomic nuclei are the place of a complex interplay between single-particle configurations and correlations which strongly determine their quantum coherent wavefunctions. All over the nuclear chart, the so-called magic numbers of nucleons define boundaries of large areas of deformation. This picture, mainly established for stable nuclei and neighbors, is re-examined at the light of new available nuclei with an unbalanced proton-to-neutron ratio, with the underlying question of the persistence or evolution of magic numbers [1,2]. Specific terms of the nuclear interaction can induce the formation of shell gaps or the lowering of relative orbital energies which, combined with correlations, sometimes lead to energetically favored intruder states as the ground state configuration [3][4][5][6][7][8]. Regions where two-particle two-hole (2p2h) configurations are favored over normally-filled orbitals by quadrupole correlations have been termed as Islands of Inversion (IoI) [9][10][11]. The N=20 IoI in the vicinity of 32 Mg has provided unique information on shell evolution [12]. This IoI does not show any decrease in collectivity for Mg isotopes at N > 24 and merges with the N=28 deformation region [8,13]

Triaxiality of neutron-rich Ge84,86,88 from low-energy nuclear spectra

“…A calibration error of 1.5 keV and an energy resolution of 9% (6%) FWHM at 662 keV (1.332 MeV) was obtained, in agreement with the analyses from Refs. [36,38,39]. The Doppler-corrected γ -ray spectra of 84,86,88 Ge are shown in Fig.…”

Shape and shell structure of lighter ( N≲90 ) neutron-rich nuclei based on a phenomenological Woods-Saxon potential

“…As is already written in Introduction, an approximate ratio N/Z of the even-even core of the observed heaviest odd-N nuclei of 6 C, 8 O, 10 Ne, and 12 Mg isotopes is about 2. Thus, in order to see the systematic shell-structure of neutrons in neutron drip line nuclei, in Fig.…”

“…Since the calculated shell-structure of these two nuclei is very similar, in the following the numerical result of only 70 Fe is presented. The recent experimental information [7,8] indicates that even-even Cr isotopes with N=38-42 and even-even Fe isotopes with N=40-46 are deformed, based on both the observed very low energies of the first excited state which is assumed to be 2 + in some cases and the observed ratios of E(4 + 1 )/E(2 + 1 ) where the second excited state is assumed to be 4 + in some cases. An interesting question is whether or not Cr isotopes with N>42 and Fe isotopes with N>46 are also deformed.…”

“…[8] it was wondered whether or not the deformation extends to N=50 in the 24 Cr and 26 Fe isotopes, though in Ref. [8] no results of shell-model calculations were given for those isotopes, for which experimental data have not been obtained. In my present work I conclude that the deformation continuously up to N=50 is unlikely.…”

“…Third, as an example of going one step further from the shell structure related to the spherically symmetric potential, taking the shell structure obtained for a quadrupole deformed potential I study the nuclei 70 26 Fe 44 and 66 24 Cr 42 , for which the ratio is N/Z ≈ 1.7, though those nuclei are a bit too light to have something to do with the r-process. Those nuclei were recently studied intensively both experimentally [7][8][9] and theoretically and have a typical shell structure which indicates the possibility of having deformed shape similar to the shell structure of nuclei with N≈20 called the "island of inversion (IoI)". My intention is to look for a general basic sign of deformation appearing already in one-particle spectra of the spherical one-body potential before going to complicated microscopic calculations which necessarily contain various parameters.…”