Efficient detection of $2^3S_1 m=0$ states of atomic helium for improved precision measurements of helium $2^3P$ fine structure

Thermal helium $2^3S$ metastable atoms can be detected with near unit efficiency by the electron ejected when they strike a stainless-steel surface. However, the $2^1S$ atoms and UV photons created in generating the metastable beam also produce ejected electrons. We remove the $2^1S$ atoms from our beam using 2.06-micron photons from a dc discharge lamp to drive the $2^1S$ atoms to the $2^1P$ state (which subsequently decays to the ground state). A Stern-Gerlach magnet removes the m=-1 and m=+1 $2^3S$ atoms. Elastic collisions with argon gas scatters the $2^3S$ atoms out of the initial beam path, and thus away from the direction of the UV photons. The combination of these elements allows for high-efficiency detection of $2^3S_1$ m=0 atoms with very low background due to singlet atoms, UV photons or $2^3S_1$ m=$\pm$1 atoms, allowing for an improved signal-to-noise ratio for precision helium fine-structure measurements.