Enhancement ofT-noninvariant effects in neutron-induced nuclear reactions

The previously developed approach to P-nonconservation in nuclear reactions is applied to the investigation of P- and T-noninvariant effects in neutron-induced reactions. Dynamical and resonance enhancement effects for T-noninvariant quantities are considered. The estimate is given of the expected effect in the compound-nucleus p-resonance.     

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.     

P-odd,T-odd asymmetries in the binary and ternary fissioning of oriented target nuclei by cold polarized neutrons

Since P-odd and T-odd asymmetries are generally not observed during the fissioning of unoriented target nuclei by polarized neutrons and oriented target nuclei by unpolarized neutrons, analogous asymmetries in angular distributions of products from binary and ternary fissioning of oriented target nuclei by cold polarized neutrons are thoroughly analyzed within the quantum theory of fission for finding these correlations. It is demonstrated that these correlations occur only when interference of the compound fissioning nucleus states with different spins is considered. Contributions from target nucleus orientations of different orders to coefficients of the asymmetries in question for fission fragments are estimated. Characteristics of analogous asymmetries for prescission and evaporation third particles are analyzed.     

<Emphasis Type="Italic">P</Emphasis>-even correlations in binary and ternary nuclear fission induced by polarized neutrons

The evolution of a fissile nucleus from transition fission states specified at the saddle point of the deformation potential to fission states associated with prescission configurations of this nucleus and characterized by a pearlike shape of the nucleus is studied within the quantum-mechanical theory of fission processes that is based on the time-independent formalism. The coefficients of P-even asymmetries in the angular distributions of a light fragment and a third particle are calculated on the basis of the idea of the one-step mechanism of the production of a third particle and two fragments from the ternary fission of nuclei that is induced by polarized thermal neutrons. In order to confirm the developed concepts, it is proposed to repeat, at a higher level of statistical accuracy, experiments devoted to observing left-right asymmetries in the angular distributions of alpha particles from the ternary fission of nuclei.     

A consistent study of precompound and compound-nucleus emission mechanisms in neutron-induced reactions

The exciton model in its closed form and the Weisskopf-Ewing model have been consistently applied to neutron-induced reactions. The analysis has been performed for 12 nuclei in the rangeA=45–209. The primary particle-emission is considered to be governed by an admixture of precompound and compound-nucleus mechanisms, while all consecutive particles are considered to undergo pure compound-nucleus emission. The calculated primary-neutron spectra and excitation functions for (n, p), (n, 2n) and (n, 3n) reactions atE _n=4–24 MeV have been compared with experimental data. The average best-fit values of the exciton-model free parameterK have been found to be ¯K=700 MeV³. Departures form this value appear to cancel out structure effects introduced into transition rates through the structure-sensitive single-particle level density.     

Search for electric dipole moments of light ions in storage rings

The Standard Model (SM) of Particle Physics is not capable to account for the apparent matterantimatter asymmetry of our Universe. Physics beyond the SM is required and is searched for by (i) employing highest energies (e.g., at LHC), and (ii) striving for ultimate precision and sensitivity (e.g., in the search for electric dipole moments (EDMs)). Permanent EDMs of particles violate both time reversal (T) and parity (P) invariance, and are via the CPT-theorem also CP-violating. Finding an EDM would be a strong indication for physics beyond the SM, and pushing upper limits further provides crucial tests for any corresponding theoretical model, e.g., SUSY. Direct searches of proton and deuteron EDMs bear the potential to reach sensitivities beyond 10^?29 e cm. For an all-electric proton storage ring, this goal is pursued by the US-based srEDM collaboration [2], while the newly found Julich-based JEDI collaboration [1] is pursuing an approach using a combined electric-magnetic lattice which shall provide access to the EDMs of protons, deuterons, and ³He ions in the same machine. In addition, JEDI has recently proposed to perform a direct measurement of the proton and/or deuteron EDM at COSY using resonant techniques involving Wien filters.     

Unified Mechanism behind the Appearance of <Emphasis Type="Italic">T</Emphasis>-Odd TRI and ROT Asymmetries in Actinide Fission Induced by Cold Polarized Neutrons

Some shortcomings of the approaches that are used to describe T-odd ROT and TRI asymmetries in true ternary fission via reactions involving the emission of prescission alpha particles and which are based on employing the classical method of trajectory calculations are analyzed. These shortcomings are caused by the disregard of the interference between the fission widths of different sJ_s neutron resonance states formed in the first well of the deformation potential of fissile compound nuclei. It is shown that the method used in some studies to determine T-odd TRI-asymmetries for prescission alpha particles is at odds with basic concepts of the generalizedmodel of the nucleus and approaches to constructing collective (for example, bending) vibrations of a fissile compound nucleus. Quantum-mechanical fission theory is generalized via employing a unified mechanism of formation of T-odd TRI and ROT asymmetries for prescission alpha particles and evaporated photons (neutrons). The proposed mechanism takes correctly into account the effect of quantum rotation of a fissile compound nucleus on the angular distributions of fission fragments and alpha particles for true ternary fission, as well as on the angular distribution of prompt photons (neutrons) emitted by fragments originating from the delayed fission of the aforementioned nuclei.     

Levels in 109Rh

The levels in ¹⁰⁹Rh have been investigated via the γ-rays following the β⁻ decay of 34.5 s ¹⁰⁹Ru. The ruthenium activity was produced in the thermal neutron-induced fission of ¹⁴⁹Cf and separated chemically from the fission product mixture with the on-line centrifuge system SISAK. The emitted γ-rays were studied by γ singles and γγ(t) coincidence measurements. In addition, the ¹¹⁰Pd(d, ³He) reaction was investigated at E_d = 50 MeV to study the proton-hole states in ¹⁰⁹Rh. From the comparison of the measured angular distributions with DWBA calculations l-transfers and spectroscopic factors were deduced. The level scheme of ¹⁰⁹Rh that was obtained is discussed and compared with neighbouring nuclei and with recent calculations in the framework of the interacting bosonfermion model (IBFM-1). An interpretation of the observed candidates for intruder states as a rotational band built upon the [431] Nilsson configuration is proposed.     

Parity nonconservation in nuclear physics

We present an overview of the parity-nonconservation effects in nuclear physics. In the processes of polarized neutron scattering by nuclei, apart from the ordinary dynamical enhancement, we also consider the additional resonant enhancement in the entrance channel due to the proximity of the compound-nucleus p-wave resonance. We discuss the problem of extracting information on the electroweak interaction of nucleons from nuclear data.     

Determination of neutron-induced fission cross-sections of unstable nuclei via surrogate reaction method

Heavy ion reaction studies around Coulomb barrier energies have been generally used to investigate the effect of the structure of projectile/target on reaction dynamics. Other than providing an understanding of basic physics of the reaction dynamics, some of these reactions have been used as tools to serve as surrogates of neutron-induced compound nuclear fission cross-sections involving unstable targets. In this paper, we report some of the recent results on the determination of neutron-induced fission cross-sections of unstable actinides present in Th–U and U–Pu fuel cycles by surrogate reaction method by employing transfer-induced fission studies with ^6,7Li beams.     

On Ternary Fission Induced by Neutrons

Experiments on measuring the rotational effect of the ²³⁴U fissile nucleus at the scission point showed that the fissile nucleus rotates as a right screw with respect to the longitudinally polarized neutron beam direction in the ternary fission of the ²³³U target nucleus induced by polarized s-neutrons; in the binary fission of the same nuclei it rotates in the opposite direction. Moreover, it was found that ternary fission “prefers” the spin state of J = I +1/2. This phenomenon cannot be explained within the existing concepts of ternary fission as one of the two “final” states after neck rupture. The same “parent” ²³⁴U nucleus cannot rotate in opposite directions in the two different final states. It should be assumed that ternary fission is a special branch of descent from the saddle point to the point of neck rupture. It can also be assumed that this branch is formed at the saddle point in a configuration favorable for cluster formation. Why does it prefer the spin state of J = I + 1/2? This is an interesting question for further studies.

     

P-odd effects observed in neutron-induced reactions and isospin structure of weak nucleon-nucleon interaction

It is shown that, by introducing a resonance phase for two opposite-parity quasistationary states of the same spin, the observed sign dependence of P-odd effects in neutron-induced reactions can be matched with theoretical predictions. The proposed approach makes it possible to deduce information about the isospin structure of weak nucleon-nucleon interaction.     

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.     

Fast fission phenomenon,deep inelastic reactions and compound nucleus formation described within a dynamical macroscopic model

We present a dynamical model to describe dissipative heavy ion reactions. It treats explicitly the relative motion of the two ions, the mass asymmetry of the system and the projection of the isospin of each ion. The deformations, which are induced during the collision, are simulated with a time-dependent interaction potential. This is done by a time-dependent transition between a sudden interaction potential in the entrance channel and an adiabatic potential in the exit channel. The model allows us to compute the compound-nucleus cross section and multidifferential cross sections for deep inelastic reactions. In addition, for some systems, and under certain conditions which are discussed in detail, a new dissipative heavy ion collision appears: fast-fission phenomenon which has intermediate properties between deep inelastic and compound nucleus reactions. The calculated properties concerning fast fission are compared with experimental results and reproduce some of those which could not be understood as belonging to deep inelastic or compound-nucleus reactions.     

The quantum and thermodynamical characteristics of fission taking into account adiabatic and nonadiabatic modes of motion

In the framework of the quantum theory of spontaneous and low-energy induced fission, the nature of quantum and thermodynamical properties of a fissioning system is analyzed taking into account adiabatic and nonadiabatic modes of motion for different fission stages. It is shown that, owing to the influence of the Coriolis interaction, the states of the fissile nucleus and of primary fission products are cold and strongly nonequilibrium. The important role of superfluid and pairing nucleon-nucleon correlations for binary and ternary fission is demonstrated. The mechanism of pumping of high values of relative orbital momenta and spins of fission fragments for binary and ternary fission and the nonevaporation mechanism of formation of third particles for ternary fission are investigated. The anisotropies and P-odd, P-even, and T-odd asymmetries for angular distributions of fission products are analyzed. The text was submitted by the author in English.     

Neutron-induced fission cross sections of short-lived actinides with the surrogate reaction method

Neutron-induced fission cross sections for ^242,243Cm and ²⁴¹Am have been obtained with the surrogate reaction method. Recent results for the neutron-induced cross section of ²⁴³Cm are questioned by the present data. For the first time, the ²⁴²Cm cross section has been determined up to the onset of second-chance fission. The good agreement at the lowest excitation energies between the present results and the existing neutron-induced data indicates that the distributions in spin and parity of states populated with both techniques are similar.     

Investigations of fission characteristics and correlation effects

We review the experimental results on the P-even and P-odd angular correlations of fission fragments in the fission of the ²³⁵U and ²³⁹Pu nuclei induced by unpolarized and polarized resonance neutrons, and on the TRI and ROT effects in the ternary and binary fission of actinides induced by polarized thermal neutrons. Also reported are the measured yields of prompt and delayed neutrons per fission event. The experimental data are analyzed within a novel theoretical framework developed by the JINR—RNC KI Collaboration, whereby the reduction of the multidimensional phase space of fission fragments to the JπK-channel space is consistently validated and the role of resonance interference in the observed correlation effects is revealed.     

A scenario for the 220-MeV40Ar+238U reaction

Singles and coincidence charge distributions are combined to illustrate the mechanism for the 220-MeV⁴⁰Ar+²³⁸U reaction. It is found that the apparent peak in the coincidence fragment distribution corresponding toZ=82 (A=208) can be explained in terms of the fissionability of the target-like fragments produced in deep-inelastic collisions rather than as a manifestation of shell effects in compound-nucleus fission, as has been postulated for a similar system.     

Inelastic deuteron breakup followed by fission at intermediate energy

The results of experimental and theoretical studies of the double-differential proton and neutron spectra measured in coincidence with fission fragments in the deuteron-induced reaction on a ²³⁸U target at E _d =65 MeV are presented. These spectra measured in the forward direction are analyzed in the plane-wave Born approximation by using the modified model of stripping into a continuum. The pre-neutron emission fission fragment mass distributions were measured for the (d, f), (d, pf), and (d, nf) reaction channels. The enhancement of highly asymmetric mass division in the (d, pf) channel for the low-energy part of the breakup proton spectrum was observed. The (d, pf) channel can be used to imitate the neutron-induced fission at intermediate energy. The fission characteristics were analyzed in the model taking into account nuclear friction and relevant fission modes.     

On the Relative Signs of “ROT-Effects” in Ternary and Binary Fission of <Superscript>233</Superscript>U and <Superscript>235</Superscript>U Nuclei Induced by Polarized Cold Neutrons

Signs of the ROT-effects in ternary fission of ²³³U and ²³⁵U experimentally defined by PNPI group are the same, whereas in binary fission defined by ITEP group are opposite. This contradiction cannot be explained by the errors in the experiments of both groups, since such instrumental effects would be too large not to be noticed. Therefore, it is necessary to find the answer to this problem in the differences of the ternary and binary fission mechanisms.