β decay ofNa32

Abstract

The β-decay of 32 Na has been studied using β-γ coincidences. New transitions and levels are tentatively placed in the level scheme of 32 Mg from an analysis of γ -γ and β-γ -γ coincidences. The structure of nuclei along the valley of stability is well understood in terms of the traditional magic numbers, corresponding to large gaps in single-particle energy levels of nucleons in realistic mean-field theories including a spin-orbit interaction. The evolution of the shell closures far from stability is, however, a subject of intense scrutiny. In particular, experimental studies have shown a breaking of magicity of the N = 20 [1][2][3] and the N = 28 [4,5] configurations for neutron-rich nuclei. Key nuclei in their respective regions, such as 32 Mg and 44 S, display large quadrupole collectivity arising from their prolate deformation. Over the years (and with the steady increase of computing power), shell model calculations have led to a better understanding of the magicity disappearance in certain regions of the nuclear chart (for a recent review, see Ref.[6] and references therein). In the case of the N = 20 shell closure, the so-called "island of inversion" around 32 Mg is due to low energy fp-intruder states competing with the standard sd-orbitals, giving rise to the deformed configurations.Despite many experimental studies, significant features of the 32 Mg level scheme are still unknown. In fact, only the first excited state at 885 keV is firmly assigned as a 2 + state [7]. The other excited states have, at best, only received a tentative assignment. Of interest is the location of the 4 + state to shed

Merging of the islands of inversion atN=20andN=28

“…This would establish the limit of the N=20 "island of inversion". However, as we shall see in the next section, the very large depopulation of the 0f 7/2 orbit in 36 Mg indicates that before leaving the N=20 "island of inversion" we enter the N=28 "island of inversion", and that both islands are actually merged in a single one. These arguments apply as well to the 3p-3h excitations in the N=21 isotopes, that we expect very low in energy.…”

“…It follows from the above discussion that the configuration with four neutrons in the pf -shell and two neutron holes in the sd-shell maximizes the quadrupole moment and, a fortiori the quadrupole correlation energy. Therefore one should expect the 2p-2h configuration to be also dominant in 34 Mg. On the contrary one expects the 0p-0h one to begin taking over in 36 Mg. This would establish the limit of the N=20 "island of inversion".…”

“…Their respective energy gains are 5 MeV and 16 MeV and the underlying structures correspond to the K=3 − and K=2 − band-heads as expected from the Nilsson diagrams for β=0.15 and β=0.4. The 4p-4h state of of 32 Mg has an academic interest in itself even if the states belonging to its rotational band do not manifest themselves openly in the low energy spectrum, (as do their cousins in the superdeformed bands of 36 Ar and 40 Ca [23][24][25]) because of its strong mixing with the 0p-0h and 2p-2h spherical and deformed states. It may well happen that they could become yrast at some higher spin, but the threshold for neutron emission is not very high, and the experiments to find them are probably hopeless.…”

“…Since the early beta decay experiments at Isolde [35] it is known that in 32 Mg there are many states, mostly of negative parity, above the 4 + . They have been explored more recently via the 32 Na beta decay [36,37] or in (p,p') experiments [38]. Ref.…”

Multiparticle-multihole states in31Mg and33Mg: A critical evaluation

“…Weakly populated levels in the 31 Na decay appear at 945 and 1029 keV (thin black lines). The levels at 461, 945, 1154, and 1390 keV are populated via the βdelayed neutron decay from 32 Na [19,26,27] (dashed red lines).…”