Search for in-band transitions in the candidate superdeformed band in Si-28

Background: Superdeformed (SD) bands are suggested by theory around Ca-40 and in lighter alpha-conjugate nuclei such as Mg-24, Si-28, and S-32. Such predictions originate from a number of theoretical models including mean-field models and antisymmetrized molecular dynamics (AMD) calculations. While SD bands have been identified in Ca-40 and its near neighbors, evidence of their existence in the lighter, midshell nuclei is circumstantial at best. The key evidence of superdeformation would be the observation of transitions with high B(E2) transition strengths connecting states in a rotational sequence. This is challenging information to obtain since the bands lie at a high excitation energy and competition from out-of-band decay is dominant. Purpose: The purpose of the present study is to establish a new methodology to circumvent the difficulties in identifying and quantifying in-band transitions through directly populating candidate states in the SD band in Si-28 through inelastic alpha scattering, selecting such states with a spectrometer, and measuring their gamma-ray decay with a large array of high-purity germanium detectors, allowing direct access to electromagnetic transition strengths. Methods: Excited states in Si-28 were populated in the Si-28(alpha, alpha') reaction using a 130-MeV He-4 beam from the K140 AVF cyclotron at the Research Center for Nuclear Physics. Outgoing alpha particles were analyzed using the Grand Raiden spectrometer positioned at an angle of 9.1 degrees to favor the population of states with J approximate to 4. Coincident gamma rays were detected with the CAGRA array of 12 HPGe clover detectors augmented by a set of four large LaBr3 detectors. Results: Data analysis showed that it was possible to identify additional low-energy transitions in competition with high-energy decays from excited states in Si-28 in the vicinity of 10 MeV. However, while the candidate 4(+) SD state at 10.944 MeV was populated, a 1148-keV transition to the candidate 2(+) SD state at 9.796 MeV was not observed, and only an upper limit for its transition strength of B(E2) < 43 W.u. could be established. This contradicts AMD predictions of approximate to 200 W.u. for such a transition. Conclusion: The present study strongly rejects the hypothesis that the candidate set of states identified in 28 Si represents an SD band, which demonstrates the potential of the methodology devised here.