Is the7/21−Isomer State ofS43Spherical?

Abstract

We report on the spectroscopic quadrupole moment measurement of the 7=2 À 1 isomeric state in 43 16 S 27 [E Ã ¼ 320:5ð5Þ keV, T 1=2 ¼ 415ð3Þ ns], using the time dependent perturbed angular distribution technique at the RIKEN RIBF facility. Our value, j Q s j¼ 23ð3Þ efm 2 , is larger than that expected for a singleparticle state. Shell model calculations using the modern SDPF-U interaction for this mass region reproduce remarkably well the measured j Q s j , and show that non-negligible correlations drive the isomeric state away from a purely spherical shape. Thanks to recent developments of intense radioactive beams, unexplored landscapes of the Ségré chart such as the structure of nuclei far from the stability line can now be investigated in detail. One of the most striking results from the two last decades is the modification or disappearance of the so-called ''magic numbers'' in exotic nuclei. The first evidence for such structure modification was observed in N ¼ 20 neutron-rich nuclei [1]. Similar information has been found for exotic nuclei around N ¼ 28 [2]. For the latter nuclei, the set of available theoretical [3,4] and experimental [5][6][7][8][9] data provide a coherent description of the gradual erosion of the N ¼ 28 gap and the onset of deformation from the spherical 48 Ca nucleus towards the neutron-rich and oblate deformed 42 Si nucleus [10]. Midway from these two extremes lie the sulfur isotopes of transitional nature, for which spherical or deformed shape coexistence is expected in 43;44 S mainly based on theoretical interpretations of recent experimental data [11,12]. However, no definitive experimental evidence assess shape coexistence in these isotopes. Within the shell model (SM) framework, shape transitions in this mass region reflect the strong increase of correlation energy while moving away from the stability line [13,14]. From a recent interpretation of in-beam -ray spectroscopy data in exotic Si isotopes, the aforementioned increase of the correlation energy was mainly ascribed to proton-neutron interactions [15]. The latter would be responsible for the inversion between natural (i.e., rather spherical) and intruder (i.e., deformed) configurations in both 43;44 S. For the 43 S isomeric state [E Ã ¼ 320:5ð5Þ keV, T 1=2 ¼ 415ð4Þ ns], the spin-parity J ¼ 7=2 À resulting from the natural orbital configuration ð f 7=2 Þ À1 can be inferred from the very good agreement of SM calculations with the recently measured magnetic moment [g exp ¼ À0:317ð4Þ, g SM ¼ À0:280] [12]. The SM furthermore predicts that this normal configuration coexists with the intruder prolate deformed 3=2 À ground state (g.s.).In order to verify this scenario in 43 S, two experimental observations are still missing: (i) evidence for the rotational band built on top of the suspected intruder prolate ground state of the nucleus and (ii) determination of the rather spherical nature of the isomeric state. In this Letter we report on the measurement of the spectroscopic quadrupole moment of the 7=2 À 1 isomeric state using the ...

Evolution of theN= 28 shell closure: a test bench for nuclear forces

“…More recently, the g factor of this isomer was measured [49] establishing its spin-parity values, I π = 7/2 − , and implying a rather spherical shape for this state. Nevertheless, the value of its quadrupole moment is larger than expected for a single-particle state [50], implying that it contains non-negligible correlations that drive the state away from a purely spherical shape, in agreement with SM calculations (see Figure 8). In addition, an intermediate-energy single-neutron knockout reaction was used to characterize other excited states of 43 S [51].…”

Lifetime Measurements and Triple Coexisting Band Structure in S43

“…A g-factor measurement proved the isomer to have the spin and parity 7/2 − [18], and suggested it to be built on the normalorder 1f 7/2 neutron-hole configuration, while the ground state has a neutron 2p 3/2 intruder nature [17]. The spectroscopic quadrupole moment of the isomeric state was later measured, and found to be larger than expected for a single-particle state [19]. Intruder neutron configurations, driven by the neutron-proton correlations, are suggested to contribute to its wave function [19].…”

Nature of Isomerism in Exotic Sulfur Isotopes

“…→ 940 keV) value, the structure of 7/2 − 1 has been less understood. Although an early analysis [9] proposed a quasi-spherical state, the large quadrupole moment in this state measured later [27], |Q| = 23(3) efm 2 , casts doubt on this interpretation. The AM-VAP analysis demonstrates that the ground state and the 7/2 − 1 state are dominated by K = 1/2 and K = 7/2, respectively, and that this K forbiddeness causes the isomerism in…”