First lifetime investigations of $N>82$ iodine isotopes: The quest for collectivity

We report on spectroscopic information and lifetime measurements in the neutron-rich I 135 , 137 , 139 isotopes. This is the first lifetime data on iodine isotopes beyond N = 82 . Excited states were populated in fast neutron-induced fission of U 238 at the ALTO facility of IJCLab with the LICORNE neutron source and detected using the hybrid ν -ball spectrometer. The level schemes of the I 135 , 137 , 139 isotopes are revised in terms of excited states with up to maximum spin-parity of ( 33 / 2 + ) , populated for the first time in fast neutron-induced fission. We provide first results on the lifetimes of the ( 9 / 2 1 + ) and ( 13 / 2 1 + ) states in I 137 and I 139 , and the ( 17 / 2 1 + ) state in I 137 . In addition, we give upper lifetime limits for the ( 11 / 2 1 + ) states in I 135 − 139 , the ( 15 / 2 1 + ) state in I 137 , the ( 17 / 2 1 + ) state in I 139 , and reexamine the ( 29 / 2 1 + ) state in I 137 . The isomeric data in I 135 are reinvestigated, such as the previously known ( 15 / 2 1 + ) and ( 23 / 2 1 − ) isomers with T 1 / 2 of 1.64(14) and 4.6(7) ns, respectively, as obtained in this work. The new spectroscopic information is compared to that from spontaneous or thermal-neutron induced fission and discussed in the context of large scale shell-model (LSSM) calculations for the region beyond Sn 132 , indicating the behavior of collectivity for the three valence-proton iodine chain with N = 82 , 84 , 86 .