Abstract
The structure of $^{34}$Si was studied through $\gamma$ spectroscopy separately in the $\beta^-$ decays of $^{34}$Mg and $^{34}$Al at the ISOLDE facility of CERN. Different configurations in $^{34}$Si were populated independently from the two recently identified $\beta$-decaying states in $^{34}$Al having spin-parity assignments $J^\pi = 4^-$ dominated by the normal configuration $\pi (d_{5/2})^{-1} \otimes \nu (f_{7/2})$ and $J^\pi = 1^+$ by the intruder configuration $\pi (d_{5/2})^{-1} \otimes \nu (d_{3/2})^{-1}(f_{7/2})^{2}$. The paper reports on spectroscopic properties of $^{34}$Si such as an extended level scheme, spin and parity assignments based on log($ft$) values and $\gamma$-ray branching ratios, absolute $\beta$ feeding intensities and neutron emission probabilities. A total of 11 newly identified levels and 26 transitions were added to the previously known level scheme of $^{34}$Si. Large scale shell-model calculations using the {\sc sdpf-u-mix} interaction, able to treat higher order intruder configurations, are compared with the new results and conclusions are drawn concerning the predictive power of {\sc sdpf-u-mix}, the $N=20$ shell gap, the level of mixing between normal and intruder configurations for the 0$_1^+$, 0$_2^+$ and 2$_1^+$ states and the absence of triaxial deformation in $^{34}$Si.
Highlights
Nuclear deformation and shape coexistence have been topics of interest in nuclear structure research for more than five decades [1]
The present paper addresses some aspects of the 34Si nucleus structure that remain poorly known, that are still questionable, and that are essential benchmarks for theoretical calculations on doubly-magic nuclei in general: the amount of mixing between the first two 0+ states, the size of the N =
Ref. [13]), and the possible existence of trixiality, as predicted by the Gogny D1S and SDPF-M interactions and suggested experimentally in Ref. [20]. To achieve these challenging goals, we have studied the combined β decay of the 34Mg and 34Al nuclei, that has the advantage of providing information on positive and negative parity states in 34Si over a broad range of energy up to the neutron emission threshold, Sn = 7.514(15) MeV
Summary
Nuclear deformation and shape coexistence have been topics of interest in nuclear structure research for more than five decades [1]. For particular Z and N values, the correlation energy in these intruder configurations (quadrupole and pairing energy) is higher than in the case of normal configurations (no particle-hole excitations) This effect, combined with a lowered shell gap, leads to ground states in both even-even and odd-mass nuclei that have strongly correlated states [10]. Due to its closedshell Z = 14, N = 20 character, 34Si has the properties of a doubly-magic spherical nucleus (e.g., high 2+ energy [12], low B(E 2; 21+ → 01+) value [18], drop in Sn value after N = 20), but lies at the verge of the “island of inversion,” where nuclei are deformed in their ground state configuration It follows that deformed configurations, shape coexistence [19], and possibly triaxial shapes are present already among the few first excited states in 34Si [20]. A total of 11 newly identified levels with tentative spin and parity assignments and 26 transitions were added to the previously known level scheme of 34Si
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