L. V. Titova

Problem of the conservation of the fissile-nucleus-spin projection onto the fissile-nucleus symmetry axis and quantum dynamics of the low-energy fission process

Within quantum-mechanical fission theory, it is shown that, in the low-energy fission of actinide nuclei, the projection K of the spin of a fissile nucleus onto its symmetry axis is conserved as this nucleus traverses the second well of the deformation potential and the region of descent from the external fission barrier to the scission point, this spin projection being an integral of motion despite the effect of Coriolis interaction. This leads to the conclusion that the fission probability is determined by the concerted effect of the internal- and external-fission-barrier penetration factors. A global analysis of information that can be extracted from experimental investigations of (n, f) and (γ, f) reactions and which can be employed in theoretically describing fission mechanisms is performed.

Properties of true quaternary fission of nuclei with allowance for its multistep and sequential character

An analysis of basicmechanisms of binary and ternary fission of nuclei led to the conclusion that true ternary and quaternary fission of nuclei has a sequential two-step (three-step) character, where, at the first step, a fissile nucleus emits a third light particle (third and fourth light particles) under shakeup effects associated with a nonadiabatic character of its collective deformation motion, whereupon the residual nucleus undergoes fission to two fission fragments. Owing to this, the formulas derived earlier for the widths with respect to sequential two- and three-step decays of nuclei in constructing the theory of two-step twoproton decays and multistep decays in chains of genetically related nuclei could be used to describe the relative yields and angular and energy distributions of third and fourth light particles emitted in (α, α), (t, t), and (α, t) pairs upon the true quaternary spontaneous fission of 252Cf and thermal-neutron-induced fission of 235U and 233U target nuclei. Mechanisms that explain a sharp decrease in the yield of particles appearing second in time and entering into the composition of light-particle pairs that originate from true quaternary fission of nuclei in relation to the yields of analogous particles in true ternary fission of nuclei are proposed.

Probabilities for the emission of light particles and their energy and angular distributions for true quaternary nuclear fission

On the basis of quantum-mechanical fission theory, the features of true quaternary nuclear fission are studied by treating this fission process as a sequence of three processes following one another in the course of time. The first two processes are the escape of the first and then the second of the two light particles emitted from the neck of a fissioning nucleus because of a nonadiabatic character of the collective deformation motion of this nucleus. Finally, the third process is the separation of the fissioning nucleus into two rather heavy fission fragments. The differences that arise in the emission probabilities and in the angular and energy distributions upon going over from the first emitted to the second emitted prescission third and fourth particles are analyzed by invoking experimental data on the spontaneous and thermalneutron-induced fission of nuclei, and it is shown that these differences are caused by the changes both in the geometric configuration of the fissioning nucleus and in the shell structure of its neck after the first prescission particle is emitted from it.

Role of dynamical effects in the formation of T-Odd asymmetries for products of polarized-neutron-induced ternary fission of nuclei

Basic dynamical effects that accompany the cold-polarized-neutron-induced binary and ternary fission of actinide nuclei and which determine the properties of T -odd asymmetries in angular distributions of various prescission and evaporated light third particles emitted in true and delayed ternary fission are analyzed on the basis of quantum-mechanical fission theory. It is emphasized that effects associated with the conservation of axial symmetry of the fissioning system under study at all stages of its evolution from the formation of neutron resonance states of the fissile compound nucleus to the separation of its fission fragments, including the appearance of zero wriggling vibrations of the cold compound nucleus in the vicinity of its scission point, are of particular importance, the influence of quantum collective rotation of the polarized fissile system on the asymmetry of the angular distribution of both fission fragments and third particles being taken into account. It is shown that the difference in the behavior of the coefficients characterizing the T -odd asymmetries under analysis for the target nuclei being studied can be explained, upon taking into account the interference between the fission amplitudes for the neutron resonance states of fissile compound nuclei, by the difference in the contributions of even and odd components of the amplitudes of angular distributions of third particles to the coefficients in question.

P-odd and P-even asymmetries for reactions of double and triple fission of polarized target nuclei by cold neutrons

The problem of describing of P-odd and P-even asymmetries in reactions of the double and triple fission of oriented nuclei by unpolarized cold neutrons is solved on the basis of the quantum theory of fission. P-odd and P-even asymmetries in differential cross sections of the studied reactions having a character of angular dependence other than the fission reaction of unoriented nuclei by polarized neutrons are obtained, using a spin matrix constructed for the density of a compound nucleus and allowing for interference from the fission amplitudes of various pairs of neutron resonances excited in a nucleus upon the capture of cold unpolarized neutrons by an oriented target nucleus. It is demonstrated that observation of the investigated P-odd asymmetries is possible only upon polarization of the target nucleus, while P-even asymmetries can be observed for both the polarization and alignment of the target nucleus.

Development of the quantum theory of T-odd asymmetries for prescission and evaporated third particles in ternary nuclear fission induced by cold polarized neutrons

A comparative analysis of the results obtained by experimentally and theoretically studying T-odd asymmetries for various third particles in the true and delayed ternary nuclear fission induced by cold polarized neutrons was performed. It was confirmed that the appearance of these asymmetries was associated with the effect of rotation of a polarized system undergoing fission on the angular distributions of prescission and evaporated third particles with respect to the direction along which the emerging fission fragments flew apart, this effect being determined by the Coriolis interaction of the rotational and the internalmotion of the fissioning system. A quantum-mechanical description of particle motion in a rotating coordinate system was generalized to the case where gamma-ray emission was present. It was shown that the separation of the motions of an axially symmetric fissile system into a rotational and an internal motion was valid in the external region as well, where ternary-fission products had already been formed, if it was considered that the motion of fission fragments was tightly connected with the system symmetry axis, which rotated in the laboratory frame. It was found that the dependence of the fissile-system moment of inertia appearing in the Coriolis interaction Hamiltonian on the distance between the fission fragments flying apart generated an additional phase in the amplitude of the radial distribution of fission fragments. It was shown that this phase might change sizably the contribution of the interference between fission amplitudes of neutron resonances excited in a fissile compound nucleus to the absolute values of T -odd asymmetries, especially for third particles such as neutrons and photons, which interacted only slightly with fission 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

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.

P-odd, T-even symmetries for products of the spontaneous fissioning of oriented nuclei

P-Odd, T-even asymmetries in angular distributions of products from the binary and ternary spontaneous fissioning of nuclei oriented in strong magnetic fields at ultralow temperature are described for the first time by means of the quantum theory of fission. Using the spin density matrix of a fissioning nucleus and considering the notable octupole deformations of the nucleus that appear in the vicinity of its scission point, we show that coefficients of the asymmetries in question are governed by the orientation parameters of a fissioning nucleus with Q = 1 and Q = 3. The coefficients obtained in this work are compared to the analogous coefficients in the fissioning of unoriented target nuclei by polarized neutrons and oriented target nuclei by unpolarized neutrons.

T-odd asymmetries for products from the spontaneous fission of oriented nuclei

The problem of describing T-odd asymmetries in ternary fission reactions of oriented nuclei is solved for the first time on the basis of quantum theory. Estimates of the T-odd asymmetry coefficients in the angular distributions of the reaction products are obtained using the spin density matrix of the oriented fissioning nucleus. It is demonstrated that experiments on observing T-odd asymmetries in the spontaneous fission of oriented nuclei are of interest because the T-odd asymmetry coefficients can be around an order of magnitude greater than similar coefficients in the fission of unoriented target nuclei induced by polarized neutrons.

P -odd asymmetries for the binary fission of oriented target nuclei, induced by cold polarized neutrons

P-odd asymmetries in binary fission of oriented target nuclei induced by cold polarized neutrons are described for the first time using methods of the quantum theory of fission. A spin matrix of the compound nucleus density is constructed with allowance for the interference of the fission amplitudes of various pairs of neutron resonances excited in the given nucleus during the capture of a cold polarized neutron by the oriented target nucleus. In the differential cross sections for this reaction we obtain not only P-odd T-even correlations with the angular dependence different from that of the earlier investigated correlations in the fission of unoriented target nuclei by polarized neutrons but also P-odd T-odd correlations that do not occur in the fission of unoriented target nuclei by polarized neutrons and oriented target nuclei by unpolarized neutrons.