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

Electric monopole (E0) transitions are a highly sensitive probe of the charge distribution of an atomic nucleus. A large E0 transition strength (rho(2)(E0)) is a clear indicator of nuclear shape coexistence. In the region between doubly magic Ca-40 and Ni-56, E0 transitions have never been observed in the Ti or Cr isotopes, nor in the heavier iron isotopes (Fe-56,Fe-58). We have performed the first measurements of the E0 transitions in Cr-52 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 Cr-52, including the first observation of the E0 transition from the first-excited 0(+) state in Cr-52, in both electron-positron pairs and conversion-electron spectroscopy. The preliminary values for the E0 strength in the 1531-keV 2(2)(+) -> 2(1)(+) transition in Cr-52 is rho(2)(E0) x 10(3) = 470(190), and for the 1728-keV 2(3)(+) -> 2(1)(+) transition, it is rho(2)(E0) x 10(3) = 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.