Left-right asymmetry of the angular distribution of prompt neutrons from 235U fission induced by polarized thermal neutrons

Left-right asymmetry of the angular distribution of prompt neutrons from ²³⁵U fission induced by polarized thermal neutrons was measured. This asymmetry is caused by the interference of the s and p waves in the input reaction channel and was found to be equal to b=(?5.8±1.4)×10^?5.     

On the left-right asymmetry of the angular distribution of α particles from ternary nuclear fission by polarized neutrons

Some experimental possibilities of establishing the nature of the recently observed T-odd angular correlation in ternary fission of ²³³U by polarized neutrons are discussed. Specifically, investigation of the neutron energy dependence of the correlation coefficient in the ternary fission of ²³⁹Pu and ²³⁵U could shed light on the problem under discussion. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 9, 561–564 (10 November 1999)     

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.     

Search for scission neutrons using specific angular correlations in 235U fission induced by slow polarized neutrons

The experimental data concerning scission (or prescission) neutrons are very contradictory—the relative part of these neutrons in the prompt fission neutrons varies from 1 to 35% owing to arbitrary assumptions made in different analyses. To solve this problem, we have used a new alternative method to search for the scission neutrons. We have found the left-right asymmetry of prompt-fission-neutron (PFN) emission caused by sp-wave interference in the entrance channel of the reaction and the P-odd asymmetry of the PFN emission caused by parity nonconservation at the exit channel of the fission process. Both effects cannot reside in PFN evaporated by excited fission fragments. The scission (or prescission) neutrons are responsible for these effects. The text was submitted by the authors in English.     

Search for sp interference in the emission of prompt neutrons from 235U fission induced by polarized thermal neutrons

It is shown that the softening of the acoustic mode and the ensuing ferroelastic phase transition are due to the linear-quadratic interaction between the symmetric and antisymmetric deformations, which is presently neglected in the literature. An expression is obtained which can be used to predict the phase transition pressure if the initial elastic moduli are known.     

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.     

Mass and kinetic energy distribution in cold fission of233U,235U and239Pu induced by thermal neutrons

The time-of-flight technique was used to measure the mass and kinetic energy distribution of fragments from fission of²³³U,²³⁵U and²³⁹Pu, induced by thermal neutrons at the Grenoble High Flux Reactor. The data array is presented as equal probability lines in the high kinetic energy regions. The fluctuations observed in those experimental lines are explained by a static scission configuration model, in which the most important influence comes from the Coulomb interaction energy between the two fragments. The highest values of total kinetic energy are obtained for fragmentations with heavy fragmentsZ=50–52,N=80–82 and light fragmentsZ=40–42,N=60–64.     

Rotational effect of fissile nucleus in binary fission of U induced by cold polarized neutrons

We have found a small inclination of the symmetry axis of the prompt fission gamma-quanta angular distribution relative to the “fission axis” in binary fission of ²³⁵U induced by cold polarized neutrons using the neutron beam line V13 at Berlin Neutron Scattering Center (BENSC). The sign of the shift depends on the direction of the cold neutron beam polarization and it can only be explained by the quasi-classical rotation of the fissile nucleus at the scission point.     

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.     

Angular correlations in emission of prescission neutrons from <Superscript>235</Superscript>U fission induced by slow polarized neutrons

A new approach to searching for and studying scission neutrons, which is based on the analysis of specific angular correlations in nuclear fission induced by polarized neutrons, is described and used to evaluate the fraction of scission neutrons in the total number of prompt neutrons of ²³⁵U fission emitted perpendicularly to the fission axis.     

Mechanisms of formation of P-odd,P-even and T-odd asymmetries in the angular distributions of products of binary and ternary fission induced by cold polarized neutrons

The experimentally measured P-odd, T-even; P-even, T-even; and P-even, T-odd asymmetries in the angular distributions of products of binary and ternary fission induced by cold polarized neutrons are classified, and the mechanisms of their appearance are studied. Basic asymmetries in the angular distributions of binary-fission fragments are found theoretically and are used to construct induced asymmetries in the angular distributions of prescission and evaporated third particles emitted in true and delayed ternary fission. If effects associated with the collective rotation of the polarized fissile system are disregarded, P-odd, T-even; P-even, T-even; and P-even, T-odd asymmetries in the angular distributions of prescission and evaporated photons are proven to be absent, and the coefficients of the analogous asymmetries in the angular distributions of prescission and evaporated neutrons are calculated for this case. The features of the coefficients of induced P-even, T-odd asymmetries are studied for evaporated photons and neutrons associated with taking into account the quantum rotation of the polarized fissile system.     

Experimental study of ternary fission of U235 with thermal and resonance neutrons

The alpha particle spectra from ternary fission of U²³⁵ have been investigated in the thermal and the resonance neutron energy regions. The resonance neutrons have been obtained by passing the reactor neutron flux through a boron filter. A silicon surface barrier detector telescope technique for alpha particle detection has been used. In the resonance neutron energy region the ratio _f / _f has been found to be 1·02 ± 0·2 times the value of _f / _f in the thermal region. The non-gaussian tail of the LRA energy distribution has been observed.The authors are indebted to Dr. I.Wilhelm, Mrs. M.Pospíilová, Mr. J.vanda for their kindly help in the experimental data handling and to Mr. V.Kvítek for his assistance during the experiment.     

Long-range alpha emission inP-wave neutron induced fission of235U

The yield and energy distribution of long-range alpha-particles (lra) emitted from neutron-induced fission of²³⁵U have been measured at neutron energies; thermal, 125±12, 155±11, 185±10, 210±9, 240±9, 365±50 and 480±45 keV. The long-range alpha-particles were detected in cellulose nitrate track detector foils. Results showed an increase of about 50% in the yield at neutron energies in the region 150 keV≤E _n≤220 keV as compared to that of thermal neutrons. A calculation has been carried out to extract thelra to binary fission ratio forp-wave neutron induced fission.     

Systematics on fission fragment mass distribution of neutron induced 235U fission

Based on the neutron induced fission fragment mass distribution data up to     

Systematics on fission fragment mass distribution of neutron induced 235U fission

Based on the neutron induced fission fragment mass distribution data up to neutron energy 20 MeV measured with the double kinetic energy method (KEM) and the radio active method (RAM), the systematics of fission fragment mass distribution was investigated by using 5 Gaussian model and the systematics parameters were obtained by fitting the experimental data. With the systematics, the yields of any mass A and at any energy in the region from 0 to 20 MeV of neutron energy can be calculated. The calculated results could well reproduce the experimental data measured with KEM, but show some systematical deviation from the data measured by RAM, which reflects some systematical deviations between the two kinds of measured data.The error of systematics yield was calculated in an exact error transformation way, including from the error of the experimental yield data to the error of the discrete parameters, then to the systematics parameters,and at last to the yield calculated with systematics.     

Reflection asymmetry of the fission transition nucleus 235U

An analysis of the energy variation of the fission cross section and fragment angular distributions for the ²³⁴U (n,f) reaction reveals that the nuclear is reflection assymetric at the inner barrier.     

P-odd asymmetries for the binary fission of oriented target nuclei, induced by cold polarized neutrons

P-odd asymmetries in binary fission of oriented target nuclei induced by cold polarized neutrons are described for the first time using methods of the quantum theory of fission. A spin matrix of the compound nucleus density is constructed with allowance for the interference of the fission amplitudes of various pairs of neutron resonances excited in the given nucleus during the capture of a cold polarized neutron by the oriented target nucleus. In the differential cross sections for this reaction we obtain not only P-odd T-even correlations with the angular dependence different from that of the earlier investigated correlations in the fission of unoriented target nuclei by polarized neutrons but also P-odd T-odd correlations that do not occur in the fission of unoriented target nuclei by polarized neutrons and oriented target nuclei by unpolarized neutrons.     

Influence of pre-saddle neutrons on the fission fragment angular distribution

In the present note it is shown that distinguishing between pre-saddle and saddle-to-scission neutrons removes a discrepancy previously found [2–4] between data for the fission fragment anisotropy and an analysis in the framework of the transition state theory in which it has been assumed that all pre-scission neutrons influence the temperature at the saddle point. The analysis of the present paper requires a fairly weak (=2 * 10²¹ s ^–1) friction at compact shapes, in agreement with a previous [9,10] analysis of prescission neutron multiplicities and fission probabilities.