Nuclear structure of 118Sn studied through beta decay of 118In and thermal neutron capture of 117Sn(n, γ)

The works presented here focus on the semi-magic, stable nucleus 118Sn and the nuclear structure within. Two very different reactions were performed to populate excited states in 118Sn: β-decay of 118In and the thermal neutron capture, 117Sn(n, γ) 118Sn. In the β -decay experiment performed at TRIUMF, the 5 + isomer of 118In decayed to excited states in 118 Sn which subsequently decayed via γ-ray emission towards the ground state. The γ rays were detected with the GRIFFIN spectrometer and a total of 10 9 events were recorded in γ-γ coincidence. A total of 99 transitions from 23 levels were observed with one level and 49 transitions being newly observed. The focus of the analysis was on observing states with a collective proton two-particle two-hole, 2p-2h, excitation across the Z = 50 shell closure that gives rise to deformed states and shape coexistence. From the 2p-2h, 2+2 state, a known 284.5-keV transition was resolved from a triplet centred at 285.2 keV. A reduction in the intensity, compared to literature, was observed for this transition which led to a reduced E2 transition probability of 21(4) W.u. from the previous 39(7) W.u.—suggesting less collectivity. The neutron capture experiment was performed at the Institut Laue-Langevin where the enriched 117Sn target was bombarded with a high flux of thermal neutrons at the centre of the FIPPS array. Over 10 10 γ-γ coincidence events were recorded. A total of 423 γ-ray transitions were placed to the level scheme of 96 states. From these, 357 transitions and 46 levels were newly observed. Primary γ-ray transitions from the capture state populated 33 potential J = 2+ states in the 3–5 MeV region. A level density consisting of 2+ states in this energy region is of interest for the study of the pygmy quadrupole resonances (PQR). The spins of these states were not firmly established and no conclusive evidence for the PQR was observed. However, the measured level energies could help constrain future calculations for these states.