Measurement of the parity-violating triton emission asymmetry in the reaction ${}^6\mathrm{Li}$$(n,\alpha )$${}^3\mathrm{H}$ with polarized cold neutrons

We describe measurements of the parity-violating ($P\text{−}\mathrm{odd}$) triton emission asymmetry coefficient $a_{P\text{−}\mathrm{odd}}$ in the ${}^6\mathrm{Li}$$(n,\alpha )$${}^3\mathrm{H}$ reaction with polarized cold neutrons. Experiments were carried out at the Petersburg Nuclear Physics Institute (Gatchina, Russia) and at the Institut Laue-Langevin (Grenoble, France). We employed an ionisation chamber in a configuration allowing us to suppress the left-right asymmetry well below ${10}^{-8}$. An additional test for a false asymmetry due to eventual target impurities (“zero test”) resulted in $a_{0\text{−}\mathrm{test}}=(0.0\pm 0.5)\times {10}^{-8}$. As final result of this series of experiments we obtained $a_{P\text{−}\mathrm{odd}}=(-8.8\pm 2.1)\times {10}^{-8}$.

Figure 1

Sketch of the ionisation chamber placed in the neutron beam: 1-ionization chamber; $2$-${}^6\mathrm{Li}$F beam-stop for absorption of neutrons at the chamber exit; 3-solenoid for guiding the magnetic field; 4-grids and ${}^6\mathrm{Li}$F targets; 5-base plate of the ionization chamber.

Figure 2

Signals at the preamplifier output are shown as a function of the voltage at the high-voltage detector electrodes.

Figure 3

A typical variable component of the electrical signal at the entrance of the integrator as a function of time for two detectors.

Figure 4

The time intervals in the experiment.

Figure 5

The constant components $U_f$ (“forward”), $U_b$ (“backward”) and (b) the value of the left-right asymmetry coefficient $a_{\mathrm{LR}}$ are shown as a function of the angle $\beta$ between the neutron momentum and the triton momentum. These measurements were performed with a vertical guiding magnetic field.

Figure 6

Results of the $P\text{−}\mathrm{odd}$ effect measurements at the ILL for two opposite directions of the guiding magnetic field: “$\to$” (A), and “$\leftarrow$” (C). The corresponding histograms for the statistical distribution of these values and the fits by Gaussian distribution are given in B and D including the values of ${\chi }^2$ and the numbers of degrees of freedom.

Figure 7

Results of the “0”-test are shown for two opposite directions of the guiding magnetic field: “$\to$” (A), and “$\leftarrow$” (C). The corresponding histograms for statistical distribution of these values and fits by Gaussian distributions are given in B and D including the values of ${\chi }^2$ and the numbers of degrees of freedom.