Stress Induced Birefringence in Atom Trap Viewports

At TRIUMF Neutral Atom Trap (TRINAT), the current goal is a measurement of the angular asymmetry of beta particles with respect to the nuclear spin, $A_{\beta }$, from the beta decay of $^{\mathrm{37}}$K nuclei. Trapped atoms are spin-polarized by optically pumping with circularly polarized light. We characterize the degree of circular polarization of the light with the Stokes parameter S$_{\mathrm{3}}$; with an S$_{\mathrm{3}}$ value of 0.999 required to spin polarize the atoms to a proportional degree of accuracy of 10$^{\mathrm{-3}}$. One major difficulty we have encountered is stress induced birefringence on the viewports of the atom trap, which alters the S$_{\mathrm{3}}$ value. Fully annealed copper gaskets consistently achieved S$_{\mathrm{3}}$ values of over 0.999 with reproducibility down to 0.9986 and a strong dependence on the torque applied to the bolts sealing the CF flange. Using elastomer o-rings to seal the viewports, we achieved S$_{\mathrm{3}}$ values of over 0.9999, corresponding to a birefringence of $\Delta $n $=$ 3x10$^{\mathrm{-6}}$ [see Solmeyer, Rev. Sci. Instrum$.$ 82 (2011) 066105]. The drawbacks of this method are outgassing and permeation. We achieved 3x10$^{\mathrm{-8}}$ Torr with Viton and Kalrez o-rings, and we are testing Neoflon. In this talk methods of producing and quantifying circularly polarized light will be discussed, as well as techniques for reducing stress-induced birefringence.