Low-lying structure and shape evolution in neutron-rich Se isotopes

Abstract

Neutron-rich 88,90,92,94 Se isotopes were studied via in-beam γ -ray spectroscopy after nucleon removal reactions at intermediate energies at the Radioactive Isotope Beam Factory. Based on γ -γ coincidence analysis, low-lying excitation level schemes are proposed for these nuclei, including the 2 + 1 , 4 + 1 states and 2 + 2 states at remarkably low energies. The low-lying 2 + 2 states, along with other features, indicate triaxiality in these nuclei. The experimental results are in good overall agreement with self-consistent beyond-mean-field calculations based on the Gogny D1S interaction, which suggests both triaxial degree of freedom and shape coexistence playing important roles in the description of intrinsic deformations in neutron-rich Se isotopes.

Metastable States of Se92,94 : Identification of an Oblate K Isomer of Lizarazo1, Söderström2, Werner3 et al. 2020Phys. Rev. Lett.Self Cite

“…A spectrum of events with t γ ≥ 70 μs was scaled and subtracted to further reduce the environmental background component. All transitions so far reported [10,36] for this nucleus were observed, in addition to new transitions with energies of 66.8(3), 351.9(3), and 1250.2(3) keV. The half-lives inferred from the time distribution of the individual transitions were consistent with each other within statistical uncertainties, indicating the decay of one common isomeric state.…”

“…Similar to the procedure for 92 Se, a normalized background was subtracted from events within the time window 9 ≤ t γ ≤ 23 μs. In addition to the transitions reported in [10], new transitions with energies of 495, 752, 822, and 1180 keV were observed. Following the analysis steps as for 92 Se, a single isomeric state was found with T 1=2 ¼ 0.68ð5Þ μs.…”

“…Nevertheless, for the majority of nuclei, the simplistic interpretation of the shape is not possible, as these observables take intermediate values between the benchmark limits, implying softness of the nuclear shapes or mixing of nuclear wave function components with different deformations. In the case for the neutron-rich 86−94 Se 52−60 isotopes, the available data so far unveil a rather smooth systematic behavior beyond the N ¼ 50 shell closure [7][8][9][10]. The excitation energies of the first J π ¼ 2 þ states, Eð2 þ 1 Þ, decrease gradually from 704 ( 86 Se 52 ) to 470 keV ( 94 Se 60 ), which is one possible indicator of an increase of collectivity with increasing neutron number.…”

“…The deexcitation of the products following nucleon-knockout reactions was studied via in-beam γ-ray spectroscopy using the NaI(Tl) detector array for low intensity radiation (DALI2) detector array [28], for details see Refs. [10,13,[29][30][31][32][33]. The selenium nuclei discussed here were produced both at the production and the secondary targets.…”

Sequential Nature of (p,3p) Two-Proton Knockout from Neutron-Rich Nuclei

“…Thus, only bound final states were measured. Details of the experimental campaigns can be found in [24,[35][36][37][38][39][40][41][42]. Events considered for the cross section determination were triggered by a thin plastic scintillator few meters upstream the hydrogen target.…”