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

We report on spectroscopic information and lifetime measurements in the neutron-rich ^135,137,139I isotopes. This is the first lifetime data on iodine isotopes beyond N=82. Excited states were populated in fast neutron-induced fission of ²³⁸U at the ALTO facility of IJCLab with the LICORNE neutron source and detected using the hybrid ν-ball spectrometer. The level schemes of the ^135,137,139I 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 ¹³⁷I and ¹³⁹I, and the (17/2⁺₁) state in ¹³⁷I. In addition, we give upper lifetime limits for the (11/2⁺₁) states in ^135-139I, the (15/2⁺₁) state in ¹³⁷I, the (17/2⁺₁) state in ¹³⁹I, and reexamine the (29/2⁺₁) state in ¹³⁷I. The isomeric data in ¹³⁵I 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 ¹³²Sn, indicating the behavior of collectivity for the three valence-proton iodine chain with N=82,84,86.