An investigation into nuclear shape coexistence through high precision conversion electron and electron-positron pair spectroscopy

Abstract

Nuclear shape coexistence is a phenomenon where nuclei can exhibit different shapes at similar excitation energy, and, although known for decades, it remains poorly understood. It appears to be ubiquitous across the nuclear chart and seems to be related to the emergence of collectivity in atomic nuclei. Electric monopole transitions are a unique probe of shape coexistence since they are strongly related to shape change and configuration mixing; the E0 transition strength is large when there is a significant change in the mean-square charge radius of the nucleus and when there is mixing between the configurations. However, there is a paucity of measured E0 transition strengths across the nuclear chart. Electronic factors are an essential piece in the determination of E0 transition strengths. A new tabulation of E0 electronic factors is reported for conversion electrons from elements of Z=5 to Z=126 and for internal pair formation (IPF) in elements from even Z=4 to Z=100. The conversion electron tabulation covers all atomic shells from K to R2 and transition energies up to 6 MeV, while the IPF tabulation extends to 8 MeV. The conversion electron tabulation has been newly calculated according to a Relativistic Hartree-Fock-Slater approach, while the IPF calculations are based on the Wilkinson model with Coulomb correction. The new tabulation is compared to extant published tabulations and the first evaluation of E0 electronic factors compared to experimental shell ratios is made and presented. The new tabulation is significantly better than existing tabulations in both accuracy and coverage. The E0 transition depopulating the 6432-keV first-excited 0+ state in 24Mg has been observed for the first time. Electron-positron pair and gamma-ray spectroscopy at the ANU with the Super-e pair spectrometer gives a large E0 transition strength of 380(70) milliunits, suggesting significant shape change between the ground state and the first-excited 0+ state. The two-state mixing model implies a quadrupole deformation of beta2=~1 for the first-excited 0+ state, providing evidence for superdeformation in 24Mg, in concordance with theoretical predictions. This result agrees with previous inelastic electron scattering data and motivates further measurements on 24Mg to confirm the possible superdeformed nature and search for new E0 measurements in other N=Z nuclei. While shape coexistence in the N=28 region has been theoretically predicted and large E0 strengths have been observed in the Fe and Ni isotopes, the Ti and Cr isotopes have not been previously investigated. Searches in 50,52Cr have been performed via gamma-ray, conversion-electron, and electron-positron spectroscopy. No significant E0 strength in 50Cr was observed in either mixed or pure E0 transitions and limits have been set. E0 transition strengths of 350+100-80 and 870+490-400 mmilliunits have been determined for the 1530.7-keV and 1727.5-keV mixed M1+E2+E0 transitions in 52Cr, respectively. Conversion electrons and electron-positron pairs for the 2646.9-keV E0 transition from the first-excited 0+ state in 52Cr were measured but without a state lifetime, an E0 transition strength could not be determined; however, an X(E0/E2) value of 0.451(35) has been determined. Large E0 transition strengths in 52Cr strongly support the presence of shape coexistence, in agreement with shall model expectations of 2p-2h neutron excitations across the N=28 shell gap driving the deformation. Further measurements of mixing ratios and lifetimes are needed to make more firm conclusions on the structure of these nuclei. This result also motivates the need for more E0 measurements in this regions, for example the Ti isotopes. In summary, significant advancements in measuring E0 transitions in light nuclei have been made. These new experimental results and theoretical calculations are expected to motivate future work on E0 transitions both in light nuclei and across the nuclear chart.