V. I. Petrova

New observation of space-parity violation in neutron-induced fission of 229Th, 241Pu and 241Am

Abstract The space-parity violating effect of the asymmetry of light fragment angular distributions relative to the polarization of cold neutrons inducing 229 Th, 241 Pu and 241 Am fission has been observed for the first time with the following values for the P -odd asymmetry coefficients: −(5.6 ± 0.2) × 10 −4 , −(0.89 ± 0.04) × 10 −4 and −(1.0 ± 0.25) × 10 −4 , respectively. In addition the following P -odd asymmetry coefficients have been obtained for 233 U (used as a standard) and for the subbarier fission of 237 Np: (3.67 ± 0.06) × 10 −4 and −(3.3 ± 3.7) × 10 −4 , respectively. For all fissile targets studied, except 237 Np, P -odd asymmetry coefficients for three separate intervals of the heavy fragment kinetic energy have been evaluated and found to be equal to the average values for each target.

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.

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.

Parity nonconservation in nuclear fission

The aim of this work is to determine the size αnf of the global PNC asymmetry coefficient for the isotopes 237Np, 245Cm and 249Cf, to search for a mass- and energy dependence αnf(mLF,EFF) in the case of 233U(n,f) and to test the predicted angular distribution of fragments W(Θ). Analysis of the new results together with those obtained before for six other fissile isotopes [1] shows an evident decrease of the PNC effect when going from 229Th to 249Cf. A tentative interpretation of this tendency as a result of saddle point properties of the fission-prone nucleus is given. For the mass- and energy analysis the principle of measurement is presented and first results for the test of the angular dependence are given.

Investigation of parity violation and interference effects in the angular distributions of fragments originating from 233U fission induced by resonance neutrons

Three interference asymmetry effects in the angular distributions of fragments originating from 233U fission induced by resonance neutrons were measured. The energy dependences of the asymmetry factors being studied show sizable irregularities that are associated, according to modern theory, with the interference of s and p resonances at the stage of a compound nucleus. The basic features of weak p-wave resonances in the low-energy region were obtained from a global theoretical analysis of the asymmetry factors as functions of energy. The first estimates of nuclear matrix elements of weak interaction were derived for a few p-wave resonances.

Triple correlation in the 10B(n, αγ)7Li reaction

The formally T-odd triple correlation between the directions of the momenta of α particle and γ quantum and the polarization pseudovector of a thermal neutron was examined for the 10B+n=7Li+4He+γ reaction. Such T-odd correlations can be directly used for checking time reversal invariance in the elastic scattering of particles. In more complex reactions, this correlation can occur as a result of particle interactions in the entrance and exit channels of the reaction and, being a background effect, requires correct theoretical or direct experimental estimation. Our experiments gave an upper limit of 3.2×10−4 (90% confidence level) for the possible T-odd asymmetry parameter in the reaction under study.