Search for <Emphasis Type="Italic">P</Emphasis>-odd asymmetry of prompt neutron emission in <Superscript>235</Superscript>U fission induced by cold polarized neutrons

The preliminary result of the P-odd asymmetry of prompt neutron emission in ²³⁵U fission induced by polarized cold neutrons is a=(2.7±0.8)×10^?5. Only scission neutrons can show such asymmetry, whereas neutrons emitted by excited fragments are the unavoidable background, which suppress the sought asymmetry. The P-odd asymmetry of light fragment emission for ²³⁵U is equal to (8.4±0.6)×10^?5. Assuming that the last figure defines the parity mixture of the fissile nucleus, then the suppression factor is equal approximately to 3.     

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.     

On Ternary Fission Induced by Neutrons

Experiments on measuring the rotational effect of the ²³⁴U fissile nucleus at the scission point showed that the fissile nucleus rotates as a right screw with respect to the longitudinally polarized neutron beam direction in the ternary fission of the ²³³U target nucleus induced by polarized s-neutrons; in the binary fission of the same nuclei it rotates in the opposite direction. Moreover, it was found that ternary fission “prefers” the spin state of J = I +1/2. This phenomenon cannot be explained within the existing concepts of ternary fission as one of the two “final” states after neck rupture. The same “parent” ²³⁴U nucleus cannot rotate in opposite directions in the two different final states. It should be assumed that ternary fission is a special branch of descent from the saddle point to the point of neck rupture. It can also be assumed that this branch is formed at the saddle point in a configuration favorable for cluster formation. Why does it prefer the spin state of J = I + 1/2? This is an interesting question for further studies.

     

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.     

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.     

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.     

Searches for <Emphasis Type="Italic">T</Emphasis>-odd correlations in the emission of prompt neutrons in the polarized-neutron-induced fission of <Superscript>235</Superscript>U nuclei

The results of an experiment aimed at searches for formally T-odd correlations in the angular distribution of prompt neutrons from the fission of ²³⁵U nuclei are presented. The experiment was performed in the MEPHISTO polarized cold-neutron beam from the Munich FRMII reactor. The correlation coefficient proved to be (−3.5 ± 3.4) × 10⁻⁵ for a three-vector correlation (TRI effect) and (−5.0 ± 3.4) × 10⁻⁵ for a five-vector correlation (ROT effect). This means that no significant effects were discovered within the measurement errors. A comparison with the analogous effects in the ternary fission of ²³⁵U nuclei was performed. The values of the corresponding correlations in the angular distribution of prompt fission gamma rays were refined.     

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)