Precision measurement of the nuclear polarization of laser-cooled, optically pumped 37K

We have spin-polarized laser cooled $^{37}$K by direct optical pumping and measured the polarization to $<$ 0.1\% accuracy [B. Fenker arXiv:1602.04526]. Our polarization method naturally monitors the polarization of the nuclei as they decay. The atoms absorb circularly polarized light directed along the quantization axis near-resonant with the atomic S$_{1/2}$ to P$_{1/2}$ transition. Once the atoms are polarized, they stop absorbing light, so the ratio between the final P$_{1/2}$ population and its initial maximum probes the degree of polarization. We monitor the P$_{1/2}$ population using UV photons energetic enough to photoionize the P$_{1/2}$ state but not the S$_{1/2}$ state. Since the final P$_{1/2}$ population nearly vanishes, 5\% precision on the final/maximum ratio determines the polarization to 0.1\%. We eliminate a nonclassical effect, coherent population trapping, which could produce poorly polarized unexcited atoms. We show planned upgrades. Our result for the nuclear vector polarization during our A$_\beta$ measurement [B. Fenker, this conference] was 99.13(9)\%, not the dominant systematic.