Excitation energy dependence of fragment-mass distributions from fission of 180,190Hg formed in fusion reactions of 36Ar + 144,154Sm

K Nishio,H Ikezoe,K Hirose,I Nishinaka,M Venhart,I Tsekhanovich,T Ohtsuki,J Khuyagbaatar,H Makii,L Ghys,R Chapman,B Lommel,B Kindler,X Derkx,Y Wakabayashi,S D Pain ,R N Sagaidak ,F P Heßberger ,Ch E Düllmann ,A N Andreyev ,S Q Yan

doi:10.1016/j.physletb.2015.06.068

Abstract

Mass distributions of fission fragments from the compound nuclei 180Hg and 190Hg formed in fusion reactions 36Ar + 144Sm and 36Ar + 154Sm, respectively, were measured at initial excitation energies of E⁎(Hg180)=33–66 MeV and E⁎(Hg190)=48–71 MeV. In the fission of 180Hg, the mass spectra were well reproduced by assuming only an asymmetric-mass division, with most probable light and heavy fragment masses A¯L/A¯H=79/101. The mass asymmetry for 180Hg agrees well with that obtained in the low-energy β+/EC-delayed fission of 180Tl, from our earlier ISOLDE(CERN) experiment. Fission of 190Hg is found to proceed in a similar way, delivering the mass asymmetry of A¯L/A¯H=83/107, throughout the measured excitation energy range. The persistence as a function of excitation energy of the mass-asymmetric fission for both proton-rich Hg isotopes gives strong evidence for the survival of microscopic effects up to effective excitation energies of compound nuclei as high as 40 MeV. This behavior is different from fission of actinide nuclei and heavier mercury isotope 198Hg.

Highlights

A predominantly asymmetric mass distribution (MD) of fission fragments (FFs) observed in spontaneous fission or in low-energy induced fission of actinide nuclei is usually attributed to the effects of shell structure of the fissioning parent nucleus or final FFs
We report on the experimental study of the FFs mass distributions, and their dependence on excitation energy, for the compound nuclei of 180Hg and 190Hg populated in fusion reactions of 36Ar + 144Sm and 36Ar + 154Sm, respectively
Fission-fragment mass and total kinetic-energy distributions were determined for the excited 180,190Hg nuclei formed in fusion reactions of 36Ar + 144Sm and 36Ar + 154Sm

Summary

Introduction

A predominantly asymmetric mass distribution (MD) of fission fragments (FFs) observed in spontaneous fission or in low-energy induced fission of actinide nuclei is usually attributed to the effects of shell structure of the fissioning parent nucleus or final FFs. Some nuclides between the lead and actinide regions are known to have intermediate properties, which is reflected in a triple-humped structure of the mass distribution, arising from contributions of both symmetric and asymmetric mass splits [3,4]. This was further confirmed for some of the neutron-deficient At– Ac (85 ≤ Z ≤ 89) isotopes using Coulomb excitation of relativistic radioactive beams in inverse kinematics at GSI [5]. Nishio et al / Physics Letters B 748 (2015) 89–94

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Discussion

Conclusion