Improving Optical Pumping Methods for Nuclear Beta Decay

To improve our nuclear beta decay asymmetry experiment [B. Fenker et al. Phys. Rev. Lett. 120 062502 (2018)]), we are trying to improve the vector polarization of our laser-cooled atoms from our present 99.1 +- 0.1% [B. Fenker et al. New J. Phys 18 073028 (2016)]. We cycle on and off a MOT, and optically pump ³⁷K atoms with trap off. We use circularly polarized light on the 4S_1/2 → 4P_1/2 transition, using RF sidebands on a diode laser to excite transitions from both F=1 and F=2 ground states. We test techniques with stable ⁴¹K atoms, which have very similar hyperfine splitting to ³⁷K. Upgrades to improve our systematic uncertainties include: use of twisted nematic liquid crystal rotators for better spin flips; replacing in-vacuum 0.25 mm thick SiC substrate mirrors in front of the beta detectors with 0.004 mm unprotected Au-covered kapton to minimize beta straggling; higher-power laser diode closer to saturation intensity to better overcome Larmor precession around stray B fields. Diagnostics of the polarization include the time dependence of the excited state population after optical pumping light is applied, probed by measuring fluorescence and by nonresonant photoionization. The improvements project to vector polarization 99.6%.