Evidence for shape coexistence in ⁵²Cr through conversion-electron and pair-conversion spectroscopy

Abstract

Electric monopole (E0) transitions are a highly sensitive probe of the charge distribution of an atomic nucleus. A large E0 transition strength (ρ2(E0)) is a clear indicator of nuclear shape coexistence. In the region between doubly magic 40Ca and 56Ni, E0 transitions have never been observed in the Ti or Cr isotopes, nor in the heavier iron isotopes (56,58Fe). We have performed the first measurements of the E0 transitions in 52Cr via conversion-electron and pair-conversion spectroscopy using the Super-e spectrometer at the Australian National University Heavy Ion Accelerator Facility. We present the first spectra obtained for 52Cr, including the first observation of the E0 transition from the first-excited 0+ state in 52Cr, in both electron-positron pairs and conversion-electron spectroscopy. The preliminary values for the E0 strength in the 1531keV 2+ → 2+ transition in 52Cr is ρ2(E0) × 103 = 470(190), and for the 1728-keV 23+ → 21+ transition, it is ρ2(E0) 103 = 1800(1200). The large E0 strengths observed are consistent with shape coexistence in this region. However, despite the relatively precise observation of the conversion-electron and electron-positron pair intensities, the E0 strengths have large uncertainties. More precise determinations of relevant spectroscopic quantities, such as the state lifetimes and transition mixing ratios for mixed M1 + E2 transitions, are needed to determine the E0 strength more precisely.

To Shell Model, or Not to Shell Model, That Is the Question

“…A leading factor is lack of stable targets for multi-nucleon transfer reactions. However, an initiative is underway, with a highresolution internal-pair spectrometer, the Super-e, at the ANU to explore the occurrence of E0 transitions in this mass region [128,129,[233][234][235][236]. However, there is evidence that shape coexistence is present in the N = 28 isotones 54 Fe and 52 Cr from transfer reactions: this is placed in a broad framework in Figure 60.…”

The Heavy Ion Accelerator Facility: Research Achievements and Aspirations

“…More recently, pair spectroscopy has been used to study E0 transitions in relatively low-Z nuclei. Examples are 0 + → 0 + transitions in the N = Z nuclei 24 Mg and 40 Ca, and a systematic study of E0 transition strengths near N = 28 and Z = 28 that is finding that strong E0 transitions occur only in the semimagic isotopes [39][40][41].…”