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Hg178 and asymmetric fission of neutron-deficient pre-actinides

A. Jhingan et al.
Phys. Rev. C 106, 044607 – Published 19 October 2022
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  • INTRODUCTION
  • EXPERIMENTAL APPROACH
  • RESULTS
  • DISCUSSION
  • SUMMARY AND CONCLUSIONS
  • ACKNOWLEDGMENTS
    • References

    Abstract

    Fission at low excitation energy is an ideal playground to probe the impact of nuclear structure on nuclear dynamics. While the importance of structural effects in the nascent fragments is well established in the (trans-)actinide region, the observation of asymmetric fission in several neutron-deficient pre-actinides can be explained by various mechanisms. To deepen our insight into that puzzle, an innovative approach based on inverse kinematics and an enhanced version of the VAMOS++ heavy-ion spectrometer was implemented at the GANIL facility, Caen. Fission of Hg178 was induced by fusion of Xe124 and Fe54. The two fragments were detected in coincidence using VAMOS++ supplemented with a new SEcond Detection arm. For the first time in the pre-actinide region, access to the pre-neutron mass and total kinetic energy distributions, and the simultaneous isotopic identification of one the fission fragment, was achieved. The present work describes the experimental approach, and discusses the pre-neutron observables in the context of an extended asymmetric-fission island located southwest of Pb208. A comparison with different models is performed, demonstrating the importance of this new asymmetric-fission island for elaborating on driving effects in fission.

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    • Received 13 July 2022
    • Accepted 4 October 2022

    DOI:https://doi.org/10.1103/PhysRevC.106.044607

    ©2022 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    FissionModels & methods for nuclear reactionsNuclear reactions
    1. Properties
    150 ≤ A ≤ 189
    Nuclear Physics

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    Vol. 106, Iss. 4 — October 2022

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    Images

    • Figure 1
      Figure 1

      Schematic of the experimental setup for detecting the coincident fission partners (Ff1 and Ff2) at the enhanced VAMOS++ detection system. The general layout is shown on the right. A zoom of the target area with the VAMOS++ dual PS-MWPC and the new SED is given on the left. An exploded view of the SED and a photograph of its implementation in the reaction chamber are shown as well.

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    • Figure 2
      Figure 2

      (a) Correlation between the energy loss ΔE (in the first three segments of the ionization chamber) and residual energy Er (in segments 2–6) measured at the focal plane of VAMOS++. The intense (green) lines are due elastic scattering events. The black contour delineates the region expected to be populated by the fission events of interest. (b) Correlation between the time-of-flight T1 of the fragment entering VAMOS++ and the difference in time-of-flight T1–T2 of the fragment detected by the SED and the fragment in VAMOS++, for those events satisfying the selection criterion of (a). The black contour delineates the area populated by fission. (c) Fission-fragment folding angle θfold distribution for those events satisfying the selection criteria of (a) and (b). Vertical lines delineate the peak due to fission. (d) (ΔE, Er) correlation for those events satisfying the selection criteria of (a), (b), and (c). Some Z lines are indicated for reference.

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    • Figure 3
      Figure 3

      Experimental post-neutron mass Apost distribution for the fragments detected in VAMOS++. Different colors refer to ions populating different regions of the (ΔE, Er) correlation as defined in the inset. This matrix is identical to Fig. 2.

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    • Figure 4
      Figure 4

      Experimental (Apre, TKE) matrix (a), and its projection on the Apre (b) and TKE (c) axis. The solid line in (a) represents the Viola systematics [65].

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    • Figure 5
      Figure 5

      Experimental Apre distribution for Hg178 from this work (black squares), Hg180 from Ref. [53] (light blue dots), and Pt178 from Ref. [55] (orange triangles). Error bars are of statistical nature. Experimental counts were normalized to 100%.

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    • Figure 6
      Figure 6

      Comparison between the experimental Apre distribution for Hg178 from this work (black dots) and various model calculations: BSM (violet), SPY2 (green), SPM (light blue), and GEF (orange). See the text for details. Experimental counts were normalized to 100%.

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    • Figure 7
      Figure 7

      Pre-neutron fission mass yield distributions for mercury isotopes with mass 174–184 at 34 MeV of excitation energy within the SPY2 model [71].

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