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

Journal Article · Tue Nov 30 00:00:00 EST 2021 · Physical Review. C

DOI:https://doi.org/10.1103/physrevc.104.054323· OSTI ID:1869043

Morris, L. ^[1]; Jenkins, D. G. ^[2]; Harakeh, M. N. ^[3]; Isaak, J. ^[4]; Kobayashi, N. ^[5]; Tamii, A. ^[5]; Adachi, S. ^[5]; Adsley, P. ^[6]; Aoi, N. ^[5]; Bracco, A. ^[7]; Brown, A. ^[1]; Carpenter, M. P. ^[8]; Carroll, J. J. ^[9]; Courtin, S. ^[10]; Crespi, F. L. ^[7]; Davies, P. J. ^[1]; Fruet, G. ^[11]; Fang, Y. D. ^[5]; Fujita, H. ^[5]; Gey, G. ^[5] ; Hoang, T. H. ^[5]; Ichige, N. ^[12]; Ideguchi, E. ^[5]; Inoue, A. ^[5]; Iwamoto, C. ^[13]; Koike, T. ^[12]; Raju, M. Kumar ^[5]; Liu, M. L. ^[14]; Montanari, D. ^[10]; von Neumann-Cosel, P. ^[15]; Noji, S. ^[16]; Ong, H. J. ^[17]; Savran, D. ^[18]; Schmitt, J. M. ^[16]; Sullivan, C. ^[16]; Wasilewska, B. ^[19]; Weinert, M. ^[20]; Werner, V. ^[15]; Yamamoto, Y. ^[5]; Zegers, R. T. ^[16]; Zhou, X. H. ^[14]; Zhu, S. ^[8]

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1. Univ. of York (United Kingdom)
2. Univ. of York (United Kingdom); Univ. of Strasbourg (France). USIAS
3. Osaka Univ., Ibaraki (Japan). Research Center for Nuclear Physics; Univ. of Groningen (Netherlands). ESRIG
4. Osaka Univ., Ibaraki (Japan). Research Center for Nuclear Physics; Technische Univ. Darmstadt (Germany). Inst. fur Kernphysik
5. Osaka Univ., Ibaraki (Japan). Research Center for Nuclear Physics
6. iThemba Lab. for Accelerator Based Sciences, Somerset West (South Africa)
7. Univ. degli Studi di Milano (Italy); Istituto Nazionale di Fisica Nucleare (INFN), Milano (Italy)
8. Argonne National Lab. (ANL), Argonne, IL (United States)
9. Army Research Lab., Adelphi, MD (United States)
10. Univ. of Strasbourg (France). USIAS; Univ. of Strasbourg (France). IPHC
11. Univ. of Strasbourg (France). IPHC
12. Tohoku Univ., Sendai (Japan)
13. Univ. of Tokyo (Japan). Center for Nuclear Study (CNS)
14. Chinese Academy of Sciences (CAS), Lanzhou (China). Inst. of Modern Physics
15. Technische Univ. Darmstadt (Germany). Inst. fur Kernphysik
16. Michigan State Univ., East Lansing, MI (United States). National Superconducting Cyclotron Lab.
17. Osaka Univ., Ibaraki (Japan). Research Center for Nuclear Physics; Chinese Academy of Sciences (CAS), Lanzhou (China). Inst. of Modern Physics
18. GSI-Helmholtzzentrum fur Schwerionenforschung, Darmstadt (Germany)
19. Polish Academy of Sciences (PAN), Kraków (Poland). Inst. of Nuclear Physics
20. Univ. of Cologne (Germany). Inst. for Nuclear Physics

Background: Superdeformed (SD) bands are suggested by theory around ${}_{40}\mathrm{Ca}$ and in lighter alpha-conjugate nuclei such as ${}_{24}\mathrm{Mg}$ , ${}_{28}\mathrm{Si}$ , and ${}_{32}\mathrm{S}$ . 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 ${}_{40}\mathrm{Ca}$ and its near neighbors, evidence of their existence in the lighter, midshell nuclei is circumstantial at best. Additionally, 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 ${}_{28}\mathrm{Si}$ 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 ${}_{28}\mathrm{Si}$ were populated in the ${}_{28}\mathrm{Si}(\alpha ,{\alpha }^{\text{'}})$ reaction using a 130-MeV ${}_4\mathrm{He}$ 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 $J\approx 4$ . Coincident gamma rays were detected with the CAGRA array of 12 HPGe clover detectors augmented by a set of four large ${\mathrm{LaBr}}_3$ 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 ${}_{28}\mathrm{Si}$ 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 $B\left(E2\right)<43$ W.u. could be established. This contradicts AMD predictions of $\approx 200$ W.u. for such a transition.

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

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: German Research Foundation (DFG); USDOE
• Grant/Contract Number: AC02-06CH11357
• OSTI ID: 1869043
• Journal Information: Physical Review. C, Journal Name: Physical Review. C Journal Issue: 5 Vol. 104; ISSN 2469-9985
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

73 NUCLEAR PHYSICS AND RADIATION PHYSICS
H &amp; He induced nuclear reactions
electromagnetic transitions
nuclear structure &amp; decays
spectrometers &amp; spectroscopic techniques