Spectroscopy and high-spin structure of 210Fr: Isomerism and potential evidence for configuration mixing

The structure of Fr-210 has been established up to an excitation energy of similar to 5.5 MeV and spins of similar to 25h, via time-correlated gamma-ray spectroscopy and using the Au-197(O-18, 5n)Fr-210 reaction with pulsed beams at an energy of 97 MeV. A significantly different level scheme has been obtained compared to previous publications. Several isomers are reported here, including a J(pi) = (23)(+), tau = 686(9)-ns state at 4417 keV and a 10(-), 29.8(11)-ns state at 1113 keV. The former isomer has been associated with the pi(h(9/2)(3)i(13/2)(2))v(p(1/2)(-2)f(5/2)(-1)) configuration and decays via proposed E3 transitions with strengths of 8.4(3) and 21.2(8) W.u. There are only very few known cases of a high-spin isomer decaying via two parallel E3 transitions. Indeed, this is not seen in other Fr nuclei, and consequently these strengths differ from related decays in the neighboring isotopes. However, by examining the systematics of E3 transitions in trans-lead nuclei, we suggest that the weaker of the two transitions decays to a mixed 20-state. Systematics of the 10-isomer are also discussed. Comparisons are made between the observed spectrum of states and those predicted from semiempirical shell-model calculations.