P-Peven T-Odd Asymmetries in Differential Cross sections of Fission Reactions for Unoriented Nuclei by Cold Polarized Neutrons with Emission of Prescission and Evaporative Light Particles

Abstract

Differential cross sections ${d\sigma_{nf,p}(\theta)\mathord{\left/{\vphantom{d\sigma_{nf,p}(\theta)d\Omega}}\right.\kern-1.2pt}d\Omega}$ of the fission of unoriented target nuclei by cold polarized neutrons with the emission of light particles ($p$), which include prescission $\alpha$-particles and evaporative neutrons and $\gamma$-quanta, can be represented as the sum of the cross section ${d\sigma_{nf,p}^{0}(\theta)/d\Omega}$ corresponding to unpolarized neutrons, and the cross section ${d\sigma_{nf,p}^{1}(\theta)/d\Omega}$, which is linearly related to the neutron polarization vector $\boldsymbol{\sigma}_{n}$. The latter cross section is expressed as the sum of Peven odd ternary $A\left(\boldsymbol{\sigma}_{n}\left[\mathbf{k}_{LF},\mathbf{k}_{p}\right]\right)$ and quinary $B\left(\boldsymbol{\sigma}_{n}\left[\mathbf{k}_{LF},\mathbf{k}_{p}\right]\right)\left(\mathbf{k}_{LF},\mathbf{k}_{p}\right)$ scalar correlators, whose signs change and do not change, respectively, in the transformation $\theta\to\pi-\theta$. The experimental cross sections ${d\sigma_{nf,p}(\theta)/d\Omega}$ and ${d\sigma_{nf,p}^{0}(\theta)/d\Omega}$ were used to find the experimental values of the above correlators. In this study, in the framework of the quantum fission theory with account for the Coriolis interaction of the total spin of the compound fissile nuclei with orbital angular momenta of fission fragments and prescission $\alpha$-particles, it was possible to satisfactorily describe the characteristics of the above experimental correlators in the case of emission of these $\alpha$-particles and prompt neutrons and $\gamma$-quanta for most considered target nuclei. A noticeable discrepancy between theoretical and experimental characteristics was observed for the ternary correlator only in the case of $\alpha$-particles for the ${}^{233}$U target nucleus. This mismatch is probably related to taking the additional mechanism of such correlators formation determined by the influence of transverse bending and wriggling oscillations of the compound fissile nucleus in the neighborhood of its scission point with $\alpha$-particle escaping from the neck of the nucleus into account.