Effect of rotation in the $\gamma$-ray emission from 60 meV polarized neutron-induced fission of the ${}^{235}\mathrm{U}$ isotope

In the paper we present the results obtained for the effect of rotation (ROT) in the angular distribution of prompt $\gamma$ rays from the fission of ${}^{235}\mathrm{U}$ induced by monochromatic polarized “warm” neutrons ($E_n$ = 60 meV). The polarization vector direction is determined at the target position in order to correctly determine the sign of the measured effect. The asymmetry parameter was found to be equal to $R_{\gamma }=-(17.3\pm 2.8)\times {10}^{-5}$. Moreover, the rotation angle of the fission axis was determined to be equal to $0.{069}^{\circ }\pm 0.{008}^{\circ }$ taking into account the fit results of the angular distribution of the prompt fission $\gamma$ rays. The obtained value and sign of the ROT effect are consistent with the results of the ROT effect obtained for cold neutrons. We describe the experiment, methodology of sign determination, and implications of the results.

Figure 1

The scheme of the formation of the shift in the angular distribution (a) of $\alpha$ particles from ternary fission and (b) of $\gamma$ rays from fission fragments with respect to their measurement detection. $P_{LF}, P_{HF}$ are the initial directions of fragments; $P_{\alpha }$ is the $\alpha$-particle motion at the moment of scission. “$+$” and “$-$” label the final object directions for opposite directions of the neutron polarization.

Figure 2

Model of the ROT effect (a) for $\alpha$ particles and (b) for $\gamma$ quanta. The solid line shows the angular distribution of the $\alpha$ particles from ternary fission and of $\gamma$ quanta from binary fission relative to the fission axis. Dashed and dotted lines show this distribution for two polarization directions of the neutron beam.

Figure 3

SEOP polarizer with a roof mounted cooling unit and the control system.

Figure 4

Picture of the low-pressure position sensitive multiwire proportional counters. 1: fissile target; 2: start detector; 3: segmented stop detector.

Figure 5

The experimental layout for measuring the ROT effect in binary fission of ${}^{235}\mathrm{U}$ induced by polarized, “warm,” monochromatic neutrons.

Figure 6

The direction of the magnetic field and the neutron spin in the space between the polarizer and the analyzer. P: SEOP polarizer; Ch: fission chamber; A: analyzer; D: neutron detector; S1, S2: spin control coils. The blue arrow in the figure indicates the direction of the magnetic field, the red arrow direction of the neutron spin.

Figure 7

Angular distribution of $\gamma$ rays relative to the fission axis.

Figure 8

Asymmetry ratio $D$ as a function of angle for the $\gamma$ rays.