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The half-life of the 4 + 2 state of the 2p − 2h proton intruder band of 116 Sn has been measured for the first time with the fast-timing technique. The lifetime of T 1/2 = 29(10) ps leads to a transition probability B(E2; 4 + 2 → 2 + 2 ) = 40(13) Weisskopf units, which is in very good agreement with the large B(E2; 2 + 2 → 0 + 3 ) value measured previously and provides strong support for the 0 + 3 state as the band-head of the intruder configuration and for the shape coexistence in the semi-magic 116 Sn nucleus. IBM-2 calculations with mixing reproduce the experimental data very well, giving insight into the degree of mixing between the different states.suggested that they were built on intruding 2p − 2h proton excitations across the Z = 50 closed shell. Subsequent detailed spectroscopy using light-ion reactions and detecting both the γ-ray and conversion electron decays of the low-lying 0 + and 2 + levels, [9][10][11], provided firm evidence, summarized by Julin [12], for the deformed shapes of the intruder bands through the observation of enhanced E0 transitions.

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Delayed γ rays from neutron-rich A = 97 fission fragments were measured using the Lohengrin spectrometer at the high-flux reactor of the Institut Laue-Langevin in Grenoble. Several lifetimes of excited states in 97 Sr were measured using the fast-timing technique. The nucleus 97 Sr exhibits shape coexistence and is located exactly at the border of the spherical (N 58) and deformed (N 60) ground-state deformation. It is of particular interest to study the shape-coexisting structures at the spherical-deformed border (N = 59). The determined lifetimes within this work are compared to an interacting boson-fermion model calculation that is based on the microscopic energy density functional to provide a better understanding of the spherical-deformed border in strontium isotopes.

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