Lifetime measurements of excited states in 17C: Possible interplay between collectivity and halo effects

Lifetime measurements were performed on low-lying excited states of the neutron-rich isotope C17 using the recoil shadow method. The γ-decay mean lifetimes were determined to be 583±21(stat)±35(syst) ps for the first excited state at 212 keV and 18.9±0.6(stat)±4.7(syst) ps for the second excited state at 333 keV. Based on a comparison with the empirical upper limits for the electromagnetic transition strengths, these decays are concluded to be predominantly M1 transitions. The reduced M1 transition probabilities to the ground state were deduced to be (1.0±0.1)×10−2μN2 and (8.2−1.8+3.2)×10−2μN2, respectively, for the first and second excited states. The strongly hindered M1 strength as well as the lowered excitation energy represents unique nature of the 212-keV state.