Possible parity-violating effects in ternary fission

Possible parity-violating effects are considered in ternary fission and estimated for different assumptions regarding the mechanism of the process. Here we shall consider differentP-violating effects in neutron-induced ternary fission. As shown in [1], all theP-violating effects in nuclear reactions proceeding via the compound-nucleus stage bear some features in common. All of them are characterized by the factors of dynamical enhancement (caused by the large density of compound-nucleus states), resonance enhancement defined by the ratio of the level spacing for the states of opposite of opposite parity to the width these states [1], and structural enhancement (or hindrance) factors in the exit or entrance channels. Since we believe that neutron-induced ternary fission proceeds via the compound-nucleus stage, one can apply here the basic methods and results [1] used for binary fission, with some modifications necessitated by the presence of 3 particles in the exit channel. Since, however, the analytic analysis of the 3-body problem is rather abstruse and difficult even in classical mechanics, we shall use some reasonable hypothesis about the mechanism of ternary fission in order to obtain the order of magnitude estimates forP-violating effects.     

Correlations in ternary fission

An experiment investigating correlations in ternary fission of²³³U induced by cold polarized neutrons was performed at the high flux reactor of the ILL in Grenoble (France). With the experimental setup, Left—Right and time reversal symmetry violating and conserving correlations between neutron spin $\hat \sigma $ , neutron momentum $\hat p_n $ , momenta of fission fragments $\hat p_f $ and ternary particles $\hat p_t $ could be investigated.     

Angular distribution in ternary fission

The angular distribution of long-range alpha particles emitted in keV-neutron induced fission of²³⁵U has been measured using a technique which employs only a particle telescope to derive the angular information. The neutron energy region investigated is 100 keV-1 MeV. The angular distribution oflras has been found to be peaked perpendicular to the neutron-direction with a substantial amount of anisotropy near 200 keV.     

Nuclear molecules in ternary fission

Experimental results on particle accompanied cold fission of ²⁵²Cf suggest the existence of a short-lived quasi-molecular state. Within our three-center phenomenological model, described in this paper, we found a possible explanation based on a new minimum in the deformation energy at a separation distance very close to the touching point. Half-lives of some quasimolecular states which could be formed in ¹⁰Be accompanied fission of ²³⁶U, ²³⁶Pu, ²⁴⁶Cm, ²⁵²Cf, ^252,256Fm, ^256,260No, and ²⁶²Rf are roughly estimated.     

T-odd correlation in ternary fission

A possible explanation is presented of the T-odd correlation observed in ternary fission induced by polarized cold neutrons.     

Unified quantum description of TRI and ROT effects in ternary fission

It is shown that consideration of the Coriolis interaction makes it possible to explain both T-odd effects observed in ternary fission induced by polarized neutrons. A possible reason for the difference in the effects observed for ²³³U and ²³⁵U targets is discussed.     

Angular distribution in ternary cold fission

We describe the spontaneous ternary cold fission of ²⁵²Cf, accompanied by ⁴He, ¹⁰Be and ¹⁴C. The light cluster decays from the first resonant eigenstate in the Coulomb potential plus a harmonic oscillator potential. We have shown that the angular distribution of the emitted light particle is strongly connected with its deformation and the equatorial distance.     

In-beam investigation of ternary fission

Nuclear fission accompanied by long-range alpha particles has been investigated. Fission was induced by irradiating a natural uranium target with 13.5 MeV deuterons. The alpha energy and angular distributions are similar to those observed in spontaneous or thermal neutron induced fission. The correlation angle between alpha particles and light fission fragments has a most probable value of \(\bar \vartheta _{l f - \alpha } = 82.1^\circ \pm 0.9^\circ \) and a dispersion (FWHM) of Δ?=18.4°±1.2°. The mean value and dispersion of the energy distribution are \(\bar {\rm E}_\alpha = 14.8 \pm 1.0 MeV\) and ΔE=9.1±1.2 MeV (FWHM), respectively. The peak-to-valley ratio of the ternary fission fragment mass distribution is found to increase with increasing alpha energy. For near-symmetric mass division, a strong broadening of the angular distribution is observed. The results are discussed in the frame of a multichannel-fission model.     

Charged particle induced ternary fission

Ternary fission has been investigated by irradiating a natural uranium target with 13.5 MeV deuterons. The energy and angular distributions of ternary alpha particles do not differ from those observed in spontaneous or thermal neutron induced fission. The angle between alpha particles and light fragments has a most probable value of \(\bar \vartheta _{\ell f - \alpha } = 82.1 \circ \pm 0.6 \circ \) with a dispersion (FWHM) of \(\Delta \vartheta = 18.4 \circ \pm 1.2 \circ \) . The corresponding values of the energy distribution are \(\bar E\alpha \) =(14.8 ±0.5)MeV and ΔE(FWHM)= (9.1±1.1)MeV. The peak-to-valley ratio of the ternary fission fragment mass distribution is found to increase with increasing alpha energy. For near-symmetric mass division a strong broadening of the angular distribution is observed.     

Dynamic treatment of ternary fission

The new dynamic model of light charged particle (LCP) formation in ternary fission is presented. The model is based on the assumption that light particles are formed as a result of two random neck ruptures during the time interval about one single-particle period. The connection of the final stage of ternary fission and of the saddle point descent stage was obtained in the framework of the density moments method. The analysis of LCP formation has shown that LCP mass and charge distributions are strongly governed by statistical nucleon exchange in the LCP-light fragment double system. New semiclassical expressions for the calculations of LCP yields and relative ternary fission probability are given. The results of calculations are in satisfactory agreement with the experimental data.     

Bimodal fission in binary and ternary spontaneous fission of 252Cf

The hot bimodal fission of ²⁵²Cf is reexamined with new high-statistics data. We constructed a γ-γ-γ coincidence cube for binary fission and LCP-gated γ-γ matrix for ternary fission. By identifying the secondary fission fragments from their γ-ray transitions, we measured the yields for various fission splits. The normal neutron yield distribution is found to be Gaussian for Xe-Ru. However, the binary fission split of Ba-Mo is found to exhibit a bimodal neutron distribution with the “hot mode” corresponding to ≈3.1% of the total yield. In α ternary fission, the first measurements of yields for specific fission splits are presented. The Te-α-Ru and Xe-α-Mo neutron yields fit well with a single mode, but the Ba-α-Zr split shows evidence for an enhanced hot mode with an intensity of ≈13.8% of the normal mode. The text was submitted by the authors in English.     

True ternary fission and the rotation effect

Path calculations for fragments of the spontaneous true ternary fission of a ²⁵²Cf nucleus are performed. The results are considered from the standpoint of symmetries underlying the generalized nuclear model, and substantiation of the physical picture of the coaxial emission of the fragments during true ternary fission. The calculations are performed with allowance for the ROT effect, which is regarded as a major disturbance in this scenario. The obtained results are in agreement with experimental data.     

Cluster aspects of binary and ternary fission

On the basis of the statistical approach and calculation of the potential energy of the scission configurations, binary and ternary fissions are described. The text was submitted by the authors in English     

A three-center shell model for ternary fission

We propose a generalization of the phenomenological shell model based on the harmonic oscillator potential with spin-orbit term andl ²-corrections to systems made up of three clusters. The centers of these may be in arbitrary geometrical configurations and the clusters may be of different masses. The method of determining the eigenstates of the single-particle Hamiltonian is sketched and results for the cluster structure of light nuclei and the ternary fission of a superheavy system are presented.     

Mechanisms of binary and ternary low-energy fission of nuclei with allowance for nonsphericity effects

Within the quantum-mechanical theory of fission, wave functions for fragments of binary nuclear fission and amplitudes of partial fission widths are constructed with allowance for a strong nonsphericity of fragment-interaction potentials. It is shown that, in the strong-coupling approximation, the symmetry axes of fission fragments are oriented along the symmetry axis of a fissile nucleus. The structure of the fragment-interaction potentials is analyzed, and the mechanism that is responsible for the alignment of the spins and relative orbital angular momenta of fission fragments and which explains the emergence of high fragment-spin values in experiments is substantiated. The mechanisms in question are generalized to the case of ternary nuclear fission. The fragment-interaction potentials and fragment wave functions are investigated, along with the partial fission widths with respect to the ternary fission of nuclei.     

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.     

Recent experimental studies on 252Cf ternary fission

The rare ternary fission (TF) process was hitherto studied mainly by inclusive measurements of the energies and fractional yields of the light charged particles (LCPs) from fission, or by experiments on the angular and energy correlation between LCPs and fission fragments (FF). The present article briefly describes a series of recent correlation measurements on ²⁵²Cf(sf) TF that include either the registration of neutrons and γ rays with LCPs and FFs, or the coincident registration of two LCPs. The population of excited states in LCPs has been identified, as well as the formation of neutron-unstable nuclei as short-lived intermediated LCPs, the sequential decay of particle-unstable LCP species into charged particle pairs, and “quaternary” fission with the emission of two charged particles right at scission.     

Cold and hot binary and ternary fission yields in the spontaneous fission of 252Cf

The spontaneous fission (SF) of ²⁵²Cf has been studied via γ-γ-γ coincidence and light charged particle—γ-γ coincidence with Gammasphere. The yields of correlated Mo-Ba pairs in binary fission with 0–10 neutron emission have been remeasured with an uncompressed cube. The previous hot fission mode with 8–10 neutron emission seen in the Mo-Ba split is found to be smaller than earlier results but still present. New 0n binary SF yields are reported. By gating on the light charged particles detected in ΔE-E detectors and γ-γ coincidence with Gammasphere, the relative yields of correlated pairs in alpha ternary fission with zero to 6n emission are observed for the first time. The peak occurs around the α2n channel. A number of correlated pairs are identified in ternary fission with ¹⁰Be as the light charged particle. We observed only cold, 0n ¹⁰Be and little, if any, hot, xn ¹⁰Be channels.     

Nuclear temperatures obtained from light-charged-particle yields in low-energy ternary fission

Nuclear temperatures were determined from yields of isotopes with 1 ?Z? 14 accompanying the spontaneous and neutron-induced fission of heavy elements. The mean temperature derived from the corresponding temperature distributions amounts to 1.10±0.15 MeV. Received: 10 August 2001 / Accepted: 5 October 2001