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

Electric monopole (E 0) transitions are a highly sensitive probe of the charge distribution of an atomic nucleus. A large E 0 transition strength (ρ 2 (E 0)) is a clear indicator of nuclear shape coexistence. In the region between doubly magic 40 Ca and 56 Ni, E 0 transitions have never been observed in the Ti or Cr isotopes, nor in the heavier iron isotopes (56,58 Fe). We have performed the first measurements of the E 0 transitions in 52 Cr 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 52 Cr, including the first observation of the E 0 transition from the first-excited 0+ state in 52 Cr, in both electron-positron pairs and conversion-electron spectroscopy. The preliminary values for the E 0 strength in the 1531keV 2+ → 2+ transition in 52 Cr is ρ 2 (E 0) × 103 = 470(190), and for the 1728-keV 23+ → 21+ transition, it is ρ 2 (E 0) 103 = 1800(1200). The large E 0 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 E 0 strengths have large uncertainties. More precise determinations of relevant spectroscopic quantities, such as the state lifetimes and transition mixing ratios for mixed M 1 + E 2 transitions, are needed to determine the E 0 strength more precisely.