NMR calibration of optical measurement of nuclear polarization inHe3

We have performed precise NMR measurements in order to calibrate a standard optical technique for measuring the polarization of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei. This optical technique requires knowledge of the pressure-dependent relationship between the nuclear polarization and the degree of circular polarization of the 668-nm light emitted from a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ discharge. We determined the nuclear polarization by comparing the adiabatic fast-passage NMR signal from samples of polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at pressures between 0.1 and 5 torr to an identical sample of water. For water, the polarization is known from statistical mechanics. The ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ is polarized using the metastability exchange technique for optical pumping of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. The accuracy of the calibration is ±2%, which is required for applications in nuclear physics.