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.     

“T-odd effects” in unpolarized Drell–Yan scattering

We consider the leading twist “T-odd” contributions as the dominant source of the cos2 azimuthal asymmetry in unpolarized di-lepton production in Drell–Yan scattering. This asymmetry contains information on the distribution of quark transverse spin in an unpolarized proton. In a parton-spectator framework we estimate these asymmetries at 50 GeV center of mass energy. This azimuthal asymmetry is interesting in light of proposed experiments at GSI, where an anti-proton beam is ideal for studying the transversity properties of quarks due to the dominance of valence quark effects.     

Parity nonconservation in binary and ternary nuclear fission induced by polarized neutrons

Within the quantum-mechanical theory of the nuclear-fission process, the conditions of the emergence of coherent effects in the angular distributions of fragments originating from the binary and ternary fission of polarized nuclei are analyzed with allowance for the properties of transition fission states. In the case of ternary fission, the coefficients of P-odd asymmetry in the angular distributions of a light particle and a third particle, which is taken here to be an alpha particle, are calculated under the assumption that the third particle and two fragments are produced through a one-step mechanism. In order to confirm the ideas developed here, it is proposed to repeat, at a higher level of statistical accuracy, experiments devoted to seeking P-odd asymmetries for alpha particles in the ternary fission of nuclei.     

Comparison of semiclassical and quantum mechanical angular distributions for nuclear fission and evaporation

There is quite a variety of statistical-model equations in the literature that have been derived with different assumptions and for various classes of nuclear reactions. Some have been obtained by consideration of the quantum mechanics and some by semiclassical approximations thereto. Often it is not clear whether or not the results of certain analyses are clouded by these semiclassical approximations. We consider three statistical models that have been heavily employed in recent years: (1) fission decay from a rigid rotor (2) particle evaporation from a spherical system and (3) fission decay from two uncoupled spheres. Comparisons are made of numerical results from semiclassical and from quantum mechanical equations for certain specific reactions. In addition, some general features are displayed and discussed for the differences between these formalisms. For average spins that are often encountered in heavy ion reactions, the results are very similar. We conclude that major differences in current analyses are not due to the methodology; instead they must center on assumptions concerning the physical nature of the transition-state (or decision-point) configurations and their associated level densities.     

Mechanism of the appearance of angular distribution anisotropy for evaporation neutrons in center-of-mass systems emitting their thermalized fission fragments

Anisotropy in the angular distributions of cascade-evaporation neutrons in center-of-mass systems emitting their fission fragments is analyzed in the context of the quantum theory of fission. It is emphasized that such anisotropy is caused not by bending but by wriggling oscillations of the fissioning nucleus in the vicinity of its point of scission; these lead to the appearance of high-value spins of primary fission fragments [(J)\vec]₁\vec J_1 and [(J)\vec]₂\vec J_2 oriented in a plane perpendicular to direction [(n)\vec]₀\vec n_0 of the axis of symmetry of the fissioning nucleus at the instant of scission. This direction coincides with the asymptotic direction of the emission of fission fragments with a high degree of accuracy. The analytical dependences of the anisotropy coefficient on the orbital momentum l and total spin j in angular distributions of cascade-evaporation neutrons are calculated using the methods developed in analyzing angular distributions of cascade-evaporation gamma quanta. The proper spin of a neutron is shown to have almost no effect on the aforesaid anisotropy coefficient due to the weak dependence of the neutron transmission coefficient T _lj ([`(e)]\bar \varepsilon ) on the values of j.     

Basic results of investigations of scission neutrons in nuclear fission at low excitation energies

To estimate the main characteristics of neutrons emitted shortly before the scission of a fissioning nucleus, various experiments sensitive to the presence of these scission neutrons in thermal-neutron-induced fission of ²³⁵U and spontaneous fission of ²⁵²Cf were performed. The results of the experiments were analyzed within theoretical calculations allowing for various possible neutron-emission mechanisms, including the possibility of the emergence of neutrons from the scission of a nucleus.     

Ternary-fission dynamics and asymmetries in reactions with polarized neutrons

We consider the results of experimental measurements of P-odd and P-even (left-right) and T-odd asymmetries of charged-particle emission in binary and ternary fission induced by polarized neutrons. We show what information on the ternary-fission mechanism can be obtained from a theoretical analysis of these data.     

Scission neutrons in the general systematics of the yields of light particles from ternary nuclear fission

An interpolation formula that was previously obtained on the basis of a statistical approach is applied to experimental data quoted in the literature over the past few years for light-particle yields from the ternary fission of nuclei. Arguments are adduced in favor of this approach, and an analysis of errors inherent in it is given. Tables are presented in which experimental light-particle yields for eight cases of fission are given along with those that were obtained from the interpolation formula. The applicability of this formula to estimating the yields of so-called scission neutrons is analyzed.     

Possible explanation of the difference in nuclear fission induced by the intermediate energy protons and neutrons

A possible explanation is given of the experimentally observed differences between the the fission cross-sections in the reactions induced by the intermediate energy (hundreds MeV) neutrons and protons. Received: 13 June 2000 / Accepted: 6 July 2000     

Viscosity coefficients for nuclear fission

An expression for the energy dissipation during nuclear fission (or heavy-ion) processes has been derived in the first-order viscosity approximation. Irrotational flow and incompressibility have been assumed in order to calculate a quadratic form for the heat production, microscopically interpreted as particle-hole excitations. A modified algebraic parameterization of the nuclear shape has been found, which is more suitable than the usual ones to describe shapes close to spheroids (or spheres). Furthermore, it allows for analytical solutions of the fluid equations for one degree of freedom (and in special eases of the nuclear shapes, for all degrees).     

Search for scission neutrons using specific angular correlations in 235U fission induced by slow polarized neutrons

The experimental data concerning scission (or prescission) neutrons are very contradictory—the relative part of these neutrons in the prompt fission neutrons varies from 1 to 35% owing to arbitrary assumptions made in different analyses. To solve this problem, we have used a new alternative method to search for the scission neutrons. We have found the left-right asymmetry of prompt-fission-neutron (PFN) emission caused by sp-wave interference in the entrance channel of the reaction and the P-odd asymmetry of the PFN emission caused by parity nonconservation at the exit channel of the fission process. Both effects cannot reside in PFN evaporated by excited fission fragments. The scission (or prescission) neutrons are responsible for these effects. The text was submitted by the authors in English.     

Classification of <Emphasis Type="Italic">T</Emphasis>-odd asymmetries for prescission and evaporated light particles in ternary and quaternary nuclear fission induced by cold polarized neutrons

A unified mechanism of the emergence of T-odd ROT- and TRI-asymmetries is proposed for describing experimental T-odd asymmetry coefficients D(θ) in the angular distributions of prescission alphaparticles that are emitted in true ternary and quaternary nuclear fission reactions induced by cold polarized neutrons. The mechanism is related to the different ways in which the Coriolis interaction of the total spin of a polarized compound fissile nucleus with the orbital moment of alpha-particles affects even (for ROT-asymmetries) and odd (for TRI-asymmetries) components of the amplitude of an undisturbed angular distribution of emitted alpha-particles. Coefficients D_ROT(θ) and D_TRI(θ) derived with this mechanism for T-odd ROT- and TRI-asymmetries successfully describe the dependences of corresponding experimental coefficients for ²³⁵U and ²³⁹Pu nuclei over the range of angles θ, and for the ²³³U nucleus in the angular range of 60° < θ < 110°. It is explained why only ROT-type T-odd asymmetries emerge for evaporated neutrons and γ-quanta emitted by fission fragments in similar reactions if we allows for the Coriolis interaction of the total spin of the compound fissile nucleus with the orbital moments of the fission fragments and the wriggling vibrations of the above nucleus near its scission point.     

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.     

Angular correlations in ternary fission induced by polarized neutrons

Ternary fission induced by cold polarized neutrons was studied for the two isotopes ²³³U and ²³⁵U at the Institut Laue-Langevin in Grenoble, France. In particular two types of angular correlations between the spin of the incoming neutrons and the emission directions of both, the fission fragments (FF) and the ternary particles (TP), were investigated. For FF and TP detectors facing the target at right angles to the neutron beam, first, for longitudinally polarized neutrons a triple correlation between spin and the emission of outgoing particles was explored and, second, for transversally polarized neutrons parity violating asymmetries in the emission of FFs and TPs were analyzed. Nonzero expectation values for the triple correlation were oberserved in the present experiments for the first time.     

Interference effects in nuclear fission

Various interference effects governing the character of angular distributions of binary and ternary nuclear fission products and P-odd, P-even, and T-odd asymmetries in these angular distributions have been studied within the quantum theory of spontaneous and low-energy induced nuclear fission.     

Search for heavy neutron clusters in nuclear fission

An experiment on search for heavy neutron clusters in fission of ²³⁵U nuclei has been carried out on a nuclear reactor. The method of searching for neutron clusters is based on irradiation of tellurium dioxide and lead oxide targets in reactor channels near the active zone and far from it. The method of γ-spectrometric analysis of irradiated targets was used to search for nuclei whose occurrence in the targets cannot be explained by other nuclear reactions, except for the reactions of original target nuclei with neutron clusters. The reactions of tellurium and lead nuclear fission by neutron clusters (nuclei) and direct capture of neutron clusters have been revealed.     

Energy partition in nuclear fission

A scission point model (two spheroid model TSM) including semi-empirical, temperature-dependent shell correction energies for deformed fragments at scission is presented. It has been used to describe the mass-asymmetry-dependent partition of the total energy release on both fragments from spontaneous and induced fission. Characteristic trends of experimental fragment energy and neutron multiplicity data as function of incidence energy in the Th — Cf region of fissioning nuclei are well reproduced. Based on model applications, information on the energy dissipated during the descent from second saddle of fission barrier to scission point have been deduced.