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SUMMARY:The Role of Rotational Dynamics of the Dinuclear System in the For
 mation of Fission Fragment Spins
DTSTART;VALUE=DATE-TIME:20261028T135000Z
DTEND;VALUE=DATE-TIME:20261028T140500Z
DTSTAMP;VALUE=DATE-TIME:20261026T130535Z
UID:indico-contribution-6348@cern.ch
DESCRIPTION:Speakers: Julia Shcherbina ()\nThe angular momenta acquired by
  fission fragments are formed already at the stage of the compact dinuclea
 r system. One of the sources of their excitation is collective rotational 
 degrees of freedom associated with deviations of the fragments from the co
 axial configuration. However\, the transition from the collective motion o
 f the system to the individual angular momenta of the fragments is not una
 mbiguous: the result depends on the choice of coordinates used to describe
  the rotational motion and on which quantities are assumed to be conserved
  during such motion.\nThis issue is of particular importance for transvers
 e oscillations\, which are commonly identified with the bending degree of 
 freedom. Geometrically\, the corresponding motion can be represented as a 
 change in the mutual orientation of the fragments with respect to the fiss
 ion axis. However\, such a description alone does not determine the distri
 bution of angular momentum between the intrinsic rotation of the fragments
  and the rotation of the dinuclear system as a whole. This ambiguity leads
  to different effective moments of inertia used in fission models [1–4].
 \nIf the transverse motion is treated as an independent relative degree of
  freedom\, conservation of the total angular momentum implies opposite dir
 ections of rotation of the fragments. The energy of such motion is charact
 erized by the reduced moment of inertia constructed from the moments of in
 ertia of the fragments themselves [1\,2]. In this case\, no additional orb
 ital rotation of the dinuclear system is introduced\, and the bending exci
 tation is directly associated with oppositely directed angular momenta of 
 the fragments.\nAn alternative approach to constructing the rotational deg
 ree of freedom arises when oscillations about the equilibrium rotational s
 tate of the dinuclear system are considered [3\,4]. In this case\, a chang
 e in the orientation of one of the fragments is accompanied by a rearrange
 ment of the motion of the entire system. Consequently\, part of the angula
 r momentum associated with the excitation of the transverse motion is orbi
 tal. Depending on the scheme adopted\, the rotation of either the light or
  the heavy fragment may be considered\, compensated by a rotation of the r
 emaining system.\nFrom a physical point of view\, this means that the same
  term\, bending mode\, may be used for motions having different angular-mo
 mentum structures. In the first case\, the motion corresponds to pure rela
 tive rotation of the fragments\, whereas in the second case\, the excited 
 degree of freedom contains an additional collective component. Therefore\,
  a direct comparison of the results of different models based solely on th
 e magnitude of the effective moment of inertia may be inappropriate. For t
 he schemes considered in [3\,4]\, it appears appropriate to use the term p
 seudo-bending motion\, since the corresponding description includes a chan
 ge in the orbital angular momentum.\nThis distinction manifests itself dir
 ectly in the transition from collective dynamics to the observable spins o
 f the fission fragments. In the statistical description of rotational degr
 ees of freedom\, the mean-square angular momentum is determined by the eff
 ective moment of inertia and the excitation temperature. Consequently\, a 
 change in the mechanical model of the bending motion\, while keeping the o
 ther parameters of the system unchanged\, leads to a change in the predict
 ed fragment spins. At temperatures of the order of 1–2 MeV\, the contrib
 ution of the considered degree of freedom may reach several units of ℏ\,
  making it comparable with characteristic fragment-spin values.\nIn the pr
 esent work\, a systematic comparison of the effects of different descripti
 ons of the rotational dynamics on the angular characteristics of fission p
 roducts is performed. Within the framework of the cold-fission model\, the
  reactions 238U(n\,f) and 232Th(n\,f)\, as well as the spontaneous fission
  of 252Cf\, are considered. Different prescriptions for the moments of ine
 rtia of the fragments\, corresponding to the hydrodynamic and superfluid l
 imits\, are employed. For each case\, the quantities determining the forma
 tion of fragment spins and orbital angular momenta\, as well as their corr
 elations\, are calculated.\nThe obtained results are compared with the cal
 culations and experimental data presented in [5]. The analysis makes it po
 ssible to assess the extent to which the predictions of spin characteristi
 cs are determined by the properties of the fragments themselves and the ex
 tent to which they depend on the adopted description of the collective rot
 ational degree of freedom. It is shown that the choice of the effective mo
 ment of inertia of the bending motion is not merely a formal element of th
 e model and can lead to significant changes in the quantitative characteri
 stics of the angular momenta of the fission fragments.\nReferences\n	J. Ra
 ndrup\, R. Vogt\, Phys. Rev. C 89\, 044601 (2014).\n	J. R. Nix\, W. J. Swi
 atecki\, Nucl. Phys. 71\, 1 (1965).\n	J. Randrup\, T. Døssing\, R. Vogt\,
  Phys. Rev. C 106\, 014609 (2022).\n	T. M. Shneidman et al.\, Phys. Rev. C
  65\, 064302 (2002).\n	J. N. Wilson et al.\, Nature 590\, 566 (2021).\n\nh
 ttps://indico.particle.mephi.ru/event/626/contributions/6348/
LOCATION: Conference Hall 1
URL:https://indico.particle.mephi.ru/event/626/contributions/6348/
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