I. S. Guseva

Neutron-neutron angular correlations in spontaneous fission of 252Cf

An experiment has been carried out to study neutron-neutron angular correlations in spontaneous fission of 252Cf. Angular dependences of the number of neutron-neutron coincidences obtained in the experiment were compared with the results of the Monte Carlo calculations for various neutron detection thresholds in the range 425–1600 keV. It was inferred that 10–11% of the total number of prompt neutrons from 252Cf (s.f.) in the laboratory system were emitted isotropically and may probably be interpreted as neutrons directly associated with the instant of scission of the nucleus. The analysis allowed their energy distribution to be determined as well. A similar method was also used to describe the angular correlation of prompt neutrons that accompanied the reaction 235U(nth,f).

Shift in the angular distributions of γ quanta accompanying 235U fission by polarized thermal neutrons

The angular dependence of the γ-ray asymmetry relative to the plane formed by the directions of fission-fragment separation and longitudinal polarization of the thermal neutrons inducing 235U(n, f) fission was investigated. The results obtained confirm the existence of γ-ray emission asymmetry and the dependence of its coefficient on the angle between the axes of the fission-fragment and γ-ray detectors, revealed for the first time by the ITEP group at the FRM-II reactor in Munich. The observed T-odd effect of around ∼2 × 10−4 can be explained by the angular anisotropy of the γ-ray emission from fission fragments with large angular momenta oriented relative to the fission axis.

Investigations of parity violation and interference effects in 235U fission induced by resonance neutrons

Abstract These investigations were performed to test the modern theory and to obtain first estimates of nuclear matrix elements of weak interaction and the main parameters of p -resonances. Combined investigations of the space parity non-conservation effect of 235 U fission fragment emission and the interference effects of the forward-backward and left-right asymmetries were carried out over the range of neutron energies from 0.02 eV to 90 eV. The experimental results are found to be in good agreement with the theory. The first estimates of the nuclear matrix element of the weak interaction for three p -resonances and the main parameters of some low energy p -wave resonances are obtained from a combined theoretical description of all experimental data.

Search for Scission Neutrons Emitted in Low Energy Fission of Heavy Nucleus

In accordance with the PNPI Program of the fast neutron emission mechanism investigations in low excitation energy fission of heavy nucleus, the energy and angular distributions of neutron emission and angular dependence of (n‐n)‐ coincidences for different thresholds of neutron energy recording have been investigated in 252Cf(sf), 233,235U(n,f), and 239Pu(nf) fission. As a result of experimental data analysis performed on the assumption that the main part of neutrons are evaporated from the fully accelerated fragments with a big oriented angular momenta, the following figures for the yields of neutrons emitted before appearance of the fragments had been obtained for 252Cf(sf) and thermal neutron induced fission of 233,235U, and 239Pu: (10±2)%, (5±2)%, (7±2)%, and (10±2)% respectively.

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.

Experimental Studies of Nuclear Fission Dynamics Near the Scission Point

Conditions for formation of angular and energy distributions of light particles emitted in ternary fission of 233, 235U, 239Pu, and 245Cm nuclei induced by cold polarized neutrons have been studied in the course of investigating T-odd asymmetry in emission of these particles with respect to the plane formed by the fission axis and the polarization axis of the fissioning nucleus. The results obtained lead to the conclusion that in ternary fission charged particles are emitted by the fissioning nuclear system rotating around the polarization direction.

Prospects for further studies of effects of T-odd asymmetry in the emission of light particles in the polarized-neutron-induced ternary fission of heavy nuclei

Prospects for further studies of TRI and ROT effects of T-odd asymmetry in the emission of light particles in the ternary and binary fission of heavy nuclei that is induced by slow polarized neutrons are considered with a view to studying the mechanism for the formation of these effects and using them to get new information about fission dynamics. It is planned to investigate the dependence of the corresponding T-odd-asymmetry coefficients on the main characteristics of the fission reaction.

T-odd angular asymmetry in nuclear reactions involving sequential particle emission

A triple T-odd angular correlation is considered in the kinematically similar reactions 10B(n, αγ) and 233U(n, αf) induced by cold polarized neutrons. It is shown that, in the former reaction, this correlation is suppressed by the double parity-conservation selection rule due to the two-step character of the process; however, T invariance does not impose any specific constraints on this correlation. The mechanism through which the T-odd correlation found in ternary-fission reactions is formed seems to be closely related to a nearly simultaneous disintegration of the nucleus involved into two fission fragments and an alpha particle.

Search for instantaneous radiation near the instant of break momentum of various fissioning nuclear systems at low excitation energies

The main results of studying the properties of “instantaneous” neutrons and γ photons during the fission of 233, 235U(nth, f) and 239Pu(nth, f) nuclei and spontaneous fission of 252Cf, which were performed on the WWR-M reactor at the St. Petersburg Nuclear Physics Institute, Russian Academy of Sciences, are presented. Along with obtaining the main characteristics of the instantaneous radiation from fission fragments, these studies were also aimed at gaining deeper insight into such exotic processes as the emission of break neutrons and γ photons from a fissioning nucleus near the break point. These investigations were performed on different experimental setups using different analytical methods. This approach allowed us not only to find but also to reduce to minimum possible systematic effects. The yields of break neutrons were found to be about (5–7) × 10−2 of the total number of neutrons per 233, 235U(n, f) fission event and approximately twice as much for 239Pu(n, f) and 252Cf. The coefficient of T-odd asymmetry for γ photons is in agreement with the estimate obtained on the assumption that the observed effect is mainly related to the γ photons emitted by excited fragments with highly oriented angular momenta. This fact gave grounds to conclude that the desired break γ photons cannot be reliably selected (within the obtained experimental accuracy) against the much larger background of γ photons from fission fragments.

Tri And Rot Effects In Ternary Fission: What Can Be Learned?

Inducing fission by polarized neutrons allows studying subtle effects of the dynamics of the process. In the present experiments ternary fission of 235U and 239Pu was investigated with cold neutrons in the (n,f) reaction at the Institut Laue‐Langevin, Grenoble. Asymmetries in the emission of ternary particles were discovered by making use of the neutron spin flipping. It was found that two effects are interfering. There is first an asymmetry in the total yields of ternary particles having been called the TRI‐effect. Second, it was observed that the angular distributions of ternary particles are shifted back and forth when flipping the neutron spin. This shift was named ROT effect. Guided by trajectory calculations of the three‐body decay, the signs and sizes of the ROT effect are interpreted in terms of the K‐numbers of the transition states at the saddle point of fission.