Optical Pumping for Beta-decay Neutrino Helicity Tests

We seek to improve our measurements of beta and neutrino asymmetry of direction with respect to the nuclear spin, similar to the first measurements that demonstrated that parity was broken by the weak interaction. Our decay measurements depend on achieving high atomic (hence nuclear) spin polarization of laser-cooled potassium atoms. This poster summarizes long-term improvements of circularly polarized light quality for optical pumping, magnetic field switching from MOT configuration to constant field, and measurements of the resulting spin polarization of stable laser-cooled ⁴¹K atoms.

We've improved diagnostics by replacing an absorptive polarizer with a plate beamsplitter polarizer. Characterization of the plate beamsplitter polarizer showed similar behavior in both directions on the transmitted beam: this allows us to extract the light from two counterpropagating beams after they optically pump the atoms.

We also characterized and implemented fiber-coupled electro-optic modulators to add frequency sidebands onto the optical pumping lasers. This improves reliability over direct RF pumping of the laser diode, which creates frequency mode instability in an external cavity and complicates the number of Doppler-free peaks in saturation spectroscopy.

For the analysis of the nuclear polarization of the trapped atoms, we adapted a scheme for estimating the polarization [D.G. Roberge, ``Polarization diagnostics for rubidium-80...'' M.Sc. thesis, U. British Columbia, 2006 https://open.library.ubc.ca/soa/cIRcle/collections/ubctheses/831/items/1.0099864] of ⁴¹K based on time dependence of atomic excitation after the start of optical pumping.