Right-left asymmetry of radiation from fission induced by polarised neutrons
F. F. Karpeshin
备注：
12 pages, 1 figure. Delivered as an invited talk at the XLI PNPI Winter School on the Nuclear and Particle Physics, St. Petersburg, Repino, 19-24 February 2007; ISINN-2008, June 11-14, 2008, Dubna, Russia
The effect of the right-left asymmetry is considered in the angular distribution of gamma quanta from fission of U-235 by polarised thermal neutrons, which depends on the polarisation of the neutrons with respect to the gamma-fission plane. Electric dipole radiation from fission fragments arising due to the Strutinsky-Denisov induced polarisation mechanism may give rise to such an effect. Earlier, this mechanism was shown to fit the non-statistical part observed in the gamma spectrum from spontaneous fission of Cf-252. The calculated value of the magnitude of the asymmetry parameter is on the level of 10$^{-4}$. That is in agreement with the current experimental data. A crucial experiment to give a more definite picture of the concrete mechanism would be determination of the energy of the quanta responsible for the asymmetry. Detection of the quanta with the energy of ~5 MeV approaching the giant dipole resonance is needed in order to identify prompt gamma rays emitted at the stage of fissioning.
The effect of the right-left asymmetry is considered in the predicted earlier electric dipole radiation from fission fragments arising due to the Strutinski-Denisov induced polarisation mechanism. The magnitude of the asymmetry parameter is on the level of 10^-3. That is in agreement with the recent experimental data on the radiative ROT effect in ^{235}U fission induced by cold polarised neutrons.
The large body of experimental data on the fission fragments anisotropies are analyzed in several heavy-ion induced fission reaction systems. The entrance channel mass asymmetry parameters of these systems put on the both sides of the Businaro-Gallone mass asymmetry parameters. The role of the mass numbers of the projectile and the target in the prediction of a normal or an anomalous behavior in angular anisotropy, as well as the validity of standard saddle-point statistical model are considered. The average contribution of non compound nucleus fission for the systems with an anomalous behavio…
It is well known that most actinides fission into fragments of unequal size. The first attempt to understand this difference suggested that division leading to one of the fragments being near doubly magic $^{132}$Sn is favored by gain in binding energy. After the Strutinsky shell-correction method was developed an alternative idea that gained popularity was that the fission saddle might be lower for mass-asymmetric shapes and that this asymmetry was preserved until scission. Recently it was observed [Phys. Rev. Lett. {\bf 105} (2010) 252502] that $^{180}$Hg preferentially fissions asymmetrical…
We give a semiclassical interpretation of the mass asymmetry in the fission of heavy nuclei. Using only a few classical periodic orbits and a cavity model for the nuclear mean field, we reproduce the onset of left-right asymmetric shapes at the fission isomer minimum and the correct topology of the deformation energy surface in the region of the outer fission barrier. We point at the correspondence of the single-particle quantum states responsible for the asymmetry with the leading classical orbits, both lying in similar equatorial planes perpendicular to the symmetry axis of the system.
Cluster radioactivity in $^{114}$Ba is described as a spontaneous fission with a large mass asymmetry within the self-consistent HFB theory. A new fission valley with large octupole deformation is found in the potential energy surface. The fragment mass asymmetry of this fission mode corresponds to the expected one in cluster radioactivity with the emission of $^{16}$O predicted with a very long half-life.
The decimal logarithm of spontaneous fission half-life of the superheavy nucleus $^{286}$Fl experimentally determined is $\log_{10} T_f^{exp} (s) = -0.632$. We present a method to calculate the half-life based on the cranking inertia and the deformation energy, functions of two independent surface coordinates, using the best asymmetric two center shell model. In the first stage we study the statics. At a given mass asymmetry up to about $η=0.5$ the potential barrier has a two hump shape, but for larger $η$ it has only one hump. The touching point deformation energy versus mass asymmetry shows …
A correlation is established between the neutron multiplicity and the neutrons number in the fission state of Curium and Californium isotopes within a microscopic study using relativistic mean field formalism. The study includes the isotopes of Cm and Cf nuclei near the valley of stability, and hence is likely to play an important role in the artificial synthesis of superheavy nuclei. The static fission path, the neutron$-$proton asymmetry, the evolution of the neck and their composition in terms of nucleon numbers are also estimated. We find a maximum ratio for average neutron to proton densi…
A model for ternary fission is discussed in which a third particle (alpha-particle) is emitted due to non-adiabatic change of the nuclear potential at neck rapture. An expression for energy and angular distribution of alpha-particles is proposed. It is shown that an interaction between spin of fissioning system and orbital momentum of alpha-particle (spin-orbit interaction in the final state) results in recently observed asymmetry of alpha-particle emission, which can be formally related to T-odd correlation. No strong dependence of the asymmetry on the angle of alpha-particle emission with re…
The characteristics of the odd-even effect in fission-fragment Z distributions are compared to a model based on statistical mechanics. Special care is taken for using a consistent description for the influence of pairing correlations on the nuclear level density. The variation of the odd-even effect with the mass of the fissioning nucleus and with fission asymmetry is explained by the important statistical weight of configurations where the light nascent fission fragment populates the lowest energy state of an even-even nucleus. This implies that entropy drives excitation energy and unpaired n…
Fission of $^{180}$Hg produces mass asymmetric fragments which are expected to be influenced by deformed shell-effects at N=56 in the heavy fragment and Z=34 in the light fragment [G. Scamps and C. Simenel, arXiv:1904.01275 (2019)]. To investigate both shell-effects and to determine which one has the main influence on the asymmetry in the region of the $^{180}$Hg, we produce a systematic of Constraint-Hartree-Fock calculations in nuclei with similar N/Z ratio than the $^{178}$Pt. It is found that N=56 determines the asymmetry of systems in this region of the nuclear chart.
The origin of the asymmetry in the fragment mass distribution of low-energy nuclear fission is considered from the semiclassical point of view. Using the semiclassical periodic-orbit theory, one can define and quantify the shell effect associated with spatially localized nascent-fragment (prefragment) part of the potential. We investigate the roles of prefragments in the deformed shell effect using a simple cavity potential model but with realistic shape degrees of freedom for describing the fission processes. The results suggest that the prefragment magic numbers play essential roles in deter…
We propose a method to calculate the two-dimensional (2D) fission-fragment yield $Y(Z,N)$ versus both proton and neutron number, with inclusion of odd-even staggering effects in both variables. The approach is to use Brownian shape-motion on a macroscopic-microscopic potential-energy surface which, for a particular compound system is calculated versus four shape variables: elongation (quadrupole moment $Q_2$), neck $d$, left nascent fragment spheroidal deformation $ε_{\rm f1}$, right nascent fragment deformation $ε_{\rm f2}$ and two asymmetry variables, namely proton and neutron numbers in eac…
Recently at JLab using polarised electron beam on unpolarised deuteron target measurements have been performed for the parity violating asymmetry($A_{PV}$) and there are future plans to measure this asymmetry using various nuclear targets. In this paper, we study $A_{PV}$ in nuclear targets like $^{12}C$, $^{56}Fe$ and $^{208}Pb$, in a local density approximation using spectral function which takes into account Fermi motion, binding energy correction and nucleon correlations. Furthermore, the pion and rho cloud contributions have also been taken into account. The present model has been used ea…
Fission of atomic nuclei often produces mass asymmetric fragments. However, the origin of this asymmetry was believed to be different in actinides and in the sub-lead region [A. Andreyev {\it et al.}, Phys. Rev. Lett. {\bf 105}, 252502 (2010)]. It has recently been argued that quantum shell effects stabilising pear shapes of the fission fragments could explain the observed asymmetries in fission of actinides[G. Scamps and C. Simenel, Nature {\bf 564}, 382 (2018)]. This interpretation is tested in the sub-lead region using microscopic mean-field calculations of fission based on the Hartree-Fock…
The Coulomb effects hypothesis is used to interpret even-odd effects of maximum total kinetic energy as a function of mass and charge of fragments from thermal neutron induced fission of 235U. Assuming spherical fragments at scission, the Coulomb interaction energy between fragments (C_sph) is higher than the Q-value, the available energy. Therefore at scission the fragments must be deformed, so that the Coulomb interaction energy does not exceed the Q-value. The fact that the even-odd effects in the maximum total kinetic energy as a function of the charge and mass, respectively, are lower tha…
To disentangle the role of shell effects and dynamics, fission fragment mass distributions of $^{191}$Au, a nucleus in the newly identified island of mass asymmetric fission in the sub-lead region, have been measured down to excitation energy of $\approx$20 MeV above the fission barrier via two different entrance channels, viz. $^{16}$O+$^{175}$Lu and $^{37}$Cl+$^{154}$Sm reactions. Apart from having signature of the shell effects in both the cases, clear experimental evidence of quasifission has been observed in the mass distributions of the Cl induced reaction, that has also been substantiat…
We report observables for elastic Compton scattering from $^3$He in Chiral Effective Field Theory with an explicit $Δ(1232)$ degree of freedom ($χ$EFT) for energies between 50 and 120 MeV. The $γ\,{}^3$He amplitude is complete at N3LO, $\mathcal{O}(e^2δ^3)$, and in general converges well order by order. It includes the dominant pion-loop and two-body currents, as well as the Delta excitation in the single-nucleon amplitude. Since the cross section is two to three times that for deuterium and the spin of polarised $^3$He is predominantly carried by its constituent neutron, elastic Compton scatt…
We discuss the localization of wavefunctions along planes containing the shortest periodic orbits in a three-dimensional billiard system with axial symmetry. This model mimicks the self-consistent mean field of a heavy nucleus at deformations that occur characteristically during the fission process [1,2]. Many actinide nuclei become unstable against left-right asymmetric deformations, which results in asymmetric fragment mass distributions. Recently we have shown [3,4] that the onset of this asymmetry can be explained in the semiclassical periodic orbit theory by a few short periodic orbits ly…
