Orbital dependent nucleonic pairing in the lightest known isotopes of tin

By studying the Xe109→Te105→Sn101 superallowed α-decay chain, we observe low-lying states in Sn101, the one-neutron system outside doubly magic Sn100. We find that the spins of the ground state (J=7/2) and first excited state (J=5/2) in Sn101 are reversed with respect to the traditional level ordering postulated for Sn103 and the heavier tin isotopes. Through simple arguments and state-of-the-art shell-model calculations we explain this unexpected switch in terms of a transition from the single-particle regime to the collective mode in which orbital-dependent pairing correlations dominate.