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.     

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.     

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.     

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.     

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.     

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.     

Relativistic effects in ternary fission

Relativistic effects can explain the energy shift of - 6 keV reported by Ramayya et al. for the -ray de-exciting the first 2 ⁺ state, at 3368.03 keV, of the ¹⁰Be cluster emitted together with the fragments ¹⁴⁶Ba and ⁹⁶Sr in the ternary fission of ²⁵²Cf; the calculated shift is equal to -6.14 (0.16) keV. An explanation is presented for the apparent absence of Doppler broadening. For the configurations ¹³⁸Xe-¹⁰⁴Zr-¹⁰Be and ¹³⁶Te-¹⁰⁶Mo-¹⁰Be, recently reported by Hamilton et al. , observable shifts of the 3367.43 keV -line are predicted.Received: 2 June 2003, Revised: 14 January 2004, Published online: 10 August 2004PACS: 03.30. + p Special relativity - 23.20.-g Electromagnetic transitions - 25.85.-w Fission reactions     

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.     

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.     

Semiclassical description of TRI asymmetry in ternary fission induced by cold polarized neutrons

The possibility of semiclassically describing T-even TRI-type asymmetry in ternary fission induced by polarized neutrons is considered on the basis of employing Coriolis interaction that takes into account the coupling of a light charged particle to the collective rotation of a polarized fissile nucleus. It is shown that allowance for this interaction makes it possible to explain qualitatively the magnitudes of two asymmetry effects observed in light-charge-particle emission both within the semiclassical and within the quantum-mechanical approach. The difference in the relative magnitudes and signs of the effects between different target nuclei is associated with the interference contributions to the cross section from neighboring neutron resonances and therefore cannot be explained within the semiclassical approach.     

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.     

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.     

Charged particle spectra in16O induced nuclear collisions at the CERN SPS

Charged particle multiplicity distributions and pseudo-rapidity density distributions from¹⁶O induced nuclear collisions at 200 and 60A GeV are presented. The data were taken, using a minimumbias trigger, with the WA80 set-up at the CERN SPS. In this presentation we focus on how the yield of charged particles depends on the projectile energy, the mass number of the target nucleus and the energy measured in a uranium-plastic sampling calorimeter covering angles close to zero degrees. The data are compared to simulations from the event generator FRITIOF.     

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.     

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.     

Charged particle emission from transient plasmas

When a voltage pulse is applied to an interelectrode vacuum gap, bunches of transient plasma are formed on the cathode which are efficient emitters of charged particles. The present review gives the results of investigations of the cathode-plasma emissivity, the stability conditions of the high-voltage discharge phase, the mass-charge composition of the cathode plasma and its relationship to the threshold current of the cathode material, and the conditions for forming an anode plasma and its influence on the duration of the high-voltage discharge phase. Examples are given in which the results are used to create a wide range of electron and ion beams and current beakers.Institute of Power Electronics, Siberian Branch, Russian Academy of Sciences. G. P. Bazhenov Institute of Electrophysics, Urals Branch, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 83–99, March, 1994.     

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     

An analysis of the binary and ternary fission of uranium induced by 12.2 GeV protons

In this work the binary and ternary fission of uranium induced by 12.2 GeV protons has been investigated with a polycarbonate detector registering the tracks of particles with mass numbersA>16. The basic characteristics of binary and ternary fission are discussed and the corresponding cross-sections are given. The values of the cross-sections for binary and ternary fission are calculated to be σ _b =(915 ± 120) mb and σ _t -(12.8 ± 2.5) mb, respectively.