Morris, L. ; Jenkins, D. G. ; Harakeh, M. N. ; Isaak, J. ; Kobayashi, N. ; Tamii, A. ; Adachi, S. ; Adsley, P. ; Aoi, N. ; Bracco, A. ; Brown, A. ; Carpenter, M. P. ; Carroll, J. J. ; Courtin, S. ; Crespi, F. C. L. ; Davies, P. J. ; Fruet, G. ; Fang, Y. D. ; Fujita, H. ; Gey, G. ; Hoang, T. H. ; Ichige, N. ; Ideguchi, E. ; Inoue, A. ; Iwamoto, C. ; Koike, T. ; Raju, M. Kumar ; Liu, M. L. ; Montanari, D. ; von Neumann-Cosel, P. ; Noji, S. ; Ong, H. J. ; Savran, D. ; Schmitt, J. M. ; Sullivan, C. ; Wasilewska, B. ; Weinert, M. ; Werner, V. ; Yamamoto, Y. ; Zegers, R. G. T. ; Zhou, X. H. ; Zhu, S. (2021)
Search for in-band transitions in the candidate superdeformed band in Si-28.
In: Physical Review C, 104 (5)
doi: 10.1103/PhysRevC.104.054323
Artikel, Bibliographie

Kurzbeschreibung (Abstract)

Background: Superdeformed (SD) bands are suggested by theory around 40Ca and in lighter alpha-conjugate nuclei such as 24Mg, 28Si, and 32S. 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 40Ca 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 28Si 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 28Si were populated in the 28Si(α,α′) reaction using a 130-MeV 4He 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∘ to favor the population of states with J≈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 28Si 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 ≈200 W.u. for such a transition.

Conclusion: The present study strongly rejects the hypothesis that the candidate set of states identified in 28Si represents an SD band, which demonstrates the potential of the methodology devised here.

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