Energy dependence of $$\textit{p}$$ + ²³²Th fission mass distributions: Mass-asymmetric standard I and standard II modes, and multichance fission

Journal Article · 22 June 2022 · Physical Review. C

DOI: https://doi.org/10.1103/physrevc.105.064614 · OSTI ID:1882272

Berriman, A. C. ^[1]; Hinde, D. J.  ^[1]; Jeung, D. Y.  ^[1]; Dasgupta, M. ^[1]; Haba, H. ^[2]; Tanaka, T.  ^[1]; Banerjee, K.  ^[1]; Banerjee, T. ^[1]; Bezzina, L. T. ^[1]; Buete, J.  ^[1]; Cook, K. J. ^[3]; Parker-Steele, S. ^[1]; Sengupta, C. ^[1]; Simenel, C. ^[1]; Simpson, E. C. ^[1]; Stoyer, M. A. ^[4]; Swinton-Bland, B. A. ^[1]; Williams, E.  ^[1]

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1. Australian National Univ., Canberra, ACT (Australia)
2. RIKEN, Saitama (Japan)
3. Australian National Univ., Canberra, ACT (Australia); Michigan State Univ., East Lansing, MI (United States)
4. Australian National Univ., Canberra, ACT (Australia); Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Background: The predominant mass-asymmetric fission of actinide nuclides occurs mainly through the so-called standard I and standard II modes. Though understood to be caused by shape-dependent shell structures encountered between the fission barrier deformation and scission, the most relevant shell gaps are still not firmly established. The standard I mode had been associated with the spherical doubly magic ¹³²Sn, and thus the $$\textit{Z}$$ = 50 proton shell, but recently it has been proposed that standard I and standard II are associated with quadrupole and octupole deformed gaps at $$\textit{Z}$$ = 52 and 56, respectively. Purpose: In this work, we investigate how the relative probabilities of the standard I and standard II modes vary with excitation energy near threshold, probing where the two modes bifurcate. Methods: The Australian National University Heavy Ion Accelerator Facility and CUBE fission spectrometer have been used to measure fission mass distributions for the $$\textit{p}$$ +²³²Th reaction (forming ²³³Pa) at closely spaced bombarding energy intervals from 6.5 to 28 MeV. Results: A model-independent analysis of the energy dependence of the shape of the mass-asymmetric peak shows a strong dependence of the standard I and standard II relative probability on excitation energy near threshold. Conclusions: The results are consistent with the standard II mode having a lower fission barrier than standard I in ²³³Pa, with the latter increasing continually in relative probability above its barrier energy. It is concluded that multichance fission, in particular last chance fission, plays a strong role in determining the observed energy dependence of all fission modes.

• Research Organization: Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
• Sponsoring Organization: Australian Federal Government; Australian Research Council (ARC); Japan Society for the Promotion of Science (JSPS); USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: AC52-07NA27344
• OSTI ID: 1882272
• Report Number(s): LLNL-JRNL-838679; 1059218
• Journal Information: Physical Review. C, Journal Name: Physical Review. C Journal Issue: 6 Vol. 105; ISSN 2469-9985
• Publisher: American Physical Society (APS)Copyright Statement
• Country of Publication: United States
• Language: English