Low-spin States in Sn118 Populated by the Radiative Capture of Thermal Neutrons

Background: gamma -ray spectroscopy studies of the Sn isotopes provide important information on nuclear structure and shell evolution across the long isotopic chain between the doubly-magic Sn-100 and Sn-132 nuclei. These studies also offer great value to test and tune nuclear models which can then be applied to other regions of the nuclear chart. Purpose: We aim to expand the level scheme of Sn-118 by populating low-spin states in the range of 3-5 MeV and determine their angular momentum for the possible connection of these states to pygmy quadrupole Resonances, a new phenomenon observed in the neighboring Sn-112,Sn-114,Sn-124 isotopes as a resonance-like structure in the 3-5 MeV range. Method: Excited states in Sn-118 were populated via the Sn-117(n, gamma)Sn-118 reaction at the Institut Laue-Langevin in Grenoble, France. The FIssion Product Prompt gamma-ray Spectrometer (FIPPS), an array of eight n -type high purity Germanium clover detectors augmented with eight additional clover detectors from IFIN "Horia Hulubei" were used to detect gamma rays from excited states in Sn-118. The array provides a superior efficiency for gamma -ray detection and nearly 4 pi coverage for the measurements of angular correlations for spin assignment of excited nuclear levels. Results: Through gamma -gamma coincidences, 112 excited states were identified with 57 being newly placed in the level scheme. From these states, 567 gamma -ray transitions were observed with 501 being newly identified. Many levels were identified in the 3-5 MeV region. Further, an indirect measurement of the E0 transition which decays from the 0(3)(+) state to the 2p -2h, 0(2)(+) state was performed and the q(K)(2)(E0/E 2) and X(E0/E2) for this transition were determined to be 12.7(11) and 6.3(5), respectively. The 10(3) x rho(2)(E0) was determined to be >38 based on a half-life limit of <200 ps of the 2057-keV, 0(3)(+) level. Conclusions: The abundant spectroscopic information on Sn-118 obtained in the present experiment is an important input to the theoretical description of nuclei in the region and highlights the capabilities of the FIPPS array at ILL in conjunction with neutron capture reactions. Many states identified in the 3-5 MeV region could very likely have J = 2(+) and contribute to the pygmy quadrupole resonances.