L. V. Rodionova

Bio: L. V. Rodionova is an academic researcher from Voronezh State University. The author has contributed to research in topics: Fission & Fission products. The author has an hindex of 3, co-authored 3 publications receiving 22 citations.

Angular distributions, relative orbital angular momenta, and spins of fragments originating from the binary fission of polarized nuclei

Abstract: The angular distributions of fragments originating from the binary fission of odd and odd-odd nuclei capable of undergoing spontaneous fission that are polarized by a strong magnetic field at ultralow temperatures and from the low-energy photofission of even-even nuclei that is induced by dipole and quadrupole photons are investigated. It is shown that the deviations of these angular distributions from those that are obtained on the basis of the A. Bohr formula make it possible to estimate the maximum relative orbital angular momentum of fission fragments, this estimate providing important information about the relative orientation of the fragment spins. The angular distributions of fragments originating from subthreshold fission are analyzed for the case of the 238U nucleus. A comparison of the resulting angular distributions with their experimental counterparts leads to the conclusion that the maximum relative orbital angular momentum of binary-fission fragments exceeds 20, the fragment spins having predominantly a parallel orientation. The possibility is considered for performing an experiment aimed at measuring the angular distributions of fragments of the spontaneous fission of polarized nuclei in order to determine both the spins of such nuclei and the maximum values of the relative orbital angular momenta of fission fragments.

Angular Distributions of Fragments Originating from the Spontaneous Fission of Oriented Nuclei and Problem of the Conservation of the Spin Projection onto the Symmetry Axis of a Fissile Nucleus

Abstract: The concept of transition fission states, which was successfully used to describe the angular distributions of fragments for the spontaneous and low-energy induced fission of axisymmetric nuclei, proves to be correct if the spin projection onto the symmetry axis of a fissile nucleus is an integral of the motion for the external region from the descent of the fissile nucleus from the external fission barrier to the scission point. Upon heating a fissile nucleus in this region to temperatures of T ≈ 1 MeV (this is predicted by many theoretical models of the fission process), the Coriolis interaction uniformly mixes the possible projections of the fissile-nucleus spin for the case of low spin values, this leading to the loss of memory about transition fission states in the asymptotic region where the angular distributions of fragments are formed. Within quantum-mechanical fission theory, which takes into account deviations from A. Bohr’s formula, the angular distributions of fragments are calculated for spontaneously fissile nuclei aligned by an external magnetic field at ultralow temperatures, and it is shown that an analysis of experimental angular distributions of fragments would make it possible to solve the problem of spin-projection conservation for fissile nuclei in the external region.

Subthreshold Photofission of Even-Even Nuclei

Abstract: Within quantum-mechanical fission theory, the angular distributions of fragments originating from the subthreshold photofission of the even-even nuclei 232Th, 234U, 236U, 238U, 238Pu, 240Pu, and 242Pu are analyzed for photon energies below 7 MeV. Special features of various fission channels are assessed under the assumption that the fission barrier has a two-humped shape. It is shown that the maximum value of the relative orbital angular momentum Lm of fission fragments can be found upon taking into account deviations from the predictions of A. Bohr’s formula for the angular distributions of fission fragments. The result is Lm ≈ 30. The existence of an “isomeric shelf” for the angular distributions of fragments from 236U and 238U photofission in the low-energy region is confirmed.

Angular momenta of near-spherical fission fragments

Abstract: Angular momenta of fission fragments are sometimes exceeding 10 ℏ for thermal neutron induced and spontaneous fission. This is surprising since in the latter case the mother nucleus may have zero spin (e.g. 252Cf). In theory fragment spins are explained as a quantum mechanical effect. It is argued that they are due to zero-point oscillations of fragments being deformed at scission. In contrast to current theory it is shown that, for the specific case of near-magic 132Te, a large spin is generated by thermal excitation of single-particle states.

Effect of transverse vibrations of fissile nuclei on the angular and spin distributions of low-energy fission fragments

Abstract: It is shown that A. Bohr’s classic theory of angular distributions of fragments originating from low-energy fission should be supplemented with quantum corrections based on the involvement of a superposition of a very large number of angular momenta Lm in the description of the relative motion of fragments flying apart along the straight line coincidentwith the symmetry axis. It is revealed that quantum zero-point wriggling-type vibrations of the fissile system in the vicinity of its scission point are a source of these angular momenta and of high fragment spins observed experimentally.

Decisive role of wriggling vibrations in the formation of angular and spin distributions of products originating from binary and ternary fission of oriented nuclei

Abstract: It is shown that the multiplicities and angular and energy distributions of neutrons and photons evaporated from thermalized fragments originating from the spontaneous and low-energy induced fission of nuclei, the relative yields of ground and isomeric states of final fragments, and the features of delayed neutrons emitted upon the beta decay of the above fragments can successfully be described by employing nonequilibrium distributions of spins and relative orbital angular momenta of fission fragments formed in the vicinity of the scission point for the fissile nucleus being studied. It is also shown that these distributions, which are characterized by large mean values of the spins and orbital angular momenta directed orthogonally to the symmetry axis of the fissioning nucleus are successfully constructed upon simultaneously taking into account zero-mode transverse wriggling and bending vibrations of a fissile compound nucleus in the vicinity of its scission point, the wriggling vibrations being dominant. It is confirmed that the zero-mode wriggling vibrations considered immediately above are directly involved in the formation of the angular distributions of fragments originating from the spontaneous and low-energy fission of nuclei. This makes it possible to describe successfully such distributions for photofission fragments.

Role of wriggling vibrations of fissile nuclei in the formation of angular and spin distributions of neutrons and gamma-quanta emitted by fission fragments

Abstract: Possible orientations of the compound fissile nuclei associated with the polarization and alignment of these nuclei for the spontaneous and low-energy induced binary and ternary fission of nuclei by cold neutrons and their effect on the emergence of anisotropies and different kinds of asymmetry in the angular distributions of fission fragments and light prescission and evaporated third particles are considered. On the basis of quantum fission theory, the determining role of wriggling vibrations of the compound fissile nucleus in the vicinity of the point of its scission into fission fragments in the formation of the angular and spin distributions of evaporated neutrons and gamma quanta emitted by these fission fragments is demonstrated. It is shown that the wriggling vibrations resulting in high values of fission fragments spins are responsible for the retention in the spin tensors of these fragments of only the even-order orientations determining the alignment of these fragments and lead to anisotropy in the angular distributions of evaporated neutrons and gamma quanta. It is demonstrated that even with appreciable polarization of a compound nucleus having the low spin values associated with the presence of odd values of orientation orders, allowing for wriggling vibrations leads only to low polarization values of fission fragments and thus to low values of the polarization of evaporated neutrons and the circular polarization of evaporated gamma-quanta.

Photofission of nuclei with allowance for wriggling vibrations of fissioning nuclei

Abstract: It is confirmed that one source of the large relative orbital momenta L of fragments in spontaneous and stimulated low-energy nuclear fission is quantum transverse zero-point wriggling vibrations of the fissioning system near its scission point. The angular distributions of fragments of low-energy photofission of actinide nuclei, calculated using the quantum theory of fission, are compared. Vibrations are allowed for by using parameter C w determined by Nix and Swiatecki. Agreement between the experimental and theoretical angular distributions for 234U, 236U, 238U, 238Pu, 240Pu, 242Pu nuclei is observed. The strong sensitivity of the theoretical angular distributions for 238Pu, 240Pu, 242Pu nuclei toward the choice of parameters of transient fissioning states at the external and internal fission barriers is demonstrated.