Measurement of the transverse asymmetry of $\gamma$ rays in the ${}^{117}\mathrm{Sn}(n,\gamma ){}^{118}\mathrm{Sn}$ reaction

Largely enhanced parity-violating effects observed in compound resonances induced by epithermal neutrons are currently attributed to the mixing of parity-unfavored partial amplitudes in the entrance channel of the compound states. Furthermore, it is proposed that the same mechanism that enhances the parity violation also enhances the breaking of time-reversal invariance in the compound nucleus. The entrance-channel mixing induces energy-dependent spin-angular correlations of individual $\gamma$ rays emitted from the compound nuclear state. For a detailed study of the mixing model, the $\gamma$-ray yield in the reaction of ${}^{117}\mathrm{Sn}(n,\gamma ){}^{118}\mathrm{Sn}$ was measured using a pulsed beam of polarized epithermal neutrons and Ge detectors. An angular dependence of asymmetric $\gamma$-ray yields for the orientation of the neutron polarization was observed.

Figure 1

Ge detector arrangement and detector numbers. A hexagon and rectangle indicate Ge crystals, and the number in the hexagon or rectangle is the detector number.

Figure 2

Definition of the angles and axes. ${\vec{k}}_{\gamma }$ and ${\vec{k}}_n$ represent the directions of the emitted $\gamma$ ray and incident neutron. The angle ${\theta }_{\gamma }$ is defined as the angle between the $\gamma$ ray and neutron directions in the plane perpendicular to the direction of neutron polarization ($0<{\theta }_{\gamma }<\pi$). $\phi$ is the angle between the $x$ axis and $\gamma$-ray direction ($0<\phi <2\pi$).

Figure 3

Ratio obtained by dividing the transmission through the polarized ${}^3\mathrm{He}$ cell by that through the unpolarized one.

Figure 4

Time dependence of the ${}^3\mathrm{He}$ polarization ratio.

Figure 5

Capture reaction rate $n_{\text{res}}$ (top) and correction factor $n_{\text{sct}}/n_{\text{res}}$ (bottom) obtained by phits simulation. The reaction rate corresponds to the capture cross section considering effects of self-shielding and multiple scattering in the sample.

Figure 6

$\gamma$-ray deposit energy spectrum of detector 1 in Sn measurement.

Figure 7

Gated histogram of $\gamma$-ray counts as a function of $t^m$. The open circle represents the histogram of detector 14 in Sn measurement using down-polarized neutrons, and the closed circle represents that after background subtraction. The solid line is the fitting result of the background.

Figure 8

$\gamma$-ray counts for each polarization direction after background subtraction and $A_{\text{LR}}$. The closed and open circles indicate for the up- and down-polarization measurements in the upper side figure. The sign of $A_{\text{LR}}$ is reversed for the up-side detector (detector 1) and down-side detector (detector 14).

Figure 9

Left-right asymmetry defined same as in the previous study. The closed and open circles represent the present and previous results [8], respectively.

Figure 10

Angular dependence of the transverse asymmetry. The solid line represents the fitting results. Each point plots the weighted average of $A_{\text{LR}}$ for detector 14 for $sin{\theta }_{\gamma }sin\phi =-1$, detectors 8, 10, 11, and 13 for $-0.925$, detectors 9 and 12 for $-0.916$, detectors 15 through 21 for 0, detectors 2 and 5 for 0.916, and detectors 1, 3, 4, and 6 for 0.925.