Experience with the UNC Polarized $^3$He Target

We discuss our experience with the UNC polarized $^3$He target\footnote{T. Katabuchi \textit{et al.}, Rev. Sci. Instrum. \textbf{76}, 033503 (2005)} to collect for p+$^3$He spin-correlation coefficients below 6 MeV. The use of a compact, enclosed external polarizer was convenient in a general-purpose accelerator target room, but made optimization and improvement difficult. The target initially used spin-exchange with Rb to achieve $\sim$30\% polarization in 24hr with a T1 of 36hr, before the original optical pumping cell was destroyed. The wide variation in performance, both individually and over time, of the 10 replacement cells we fabricated included the deterioration of T1 in individual cases after optical pumping and reversing the magnetic holding field, but no clear difference between GE-180 and Pyrex cells. The best replacement cell yielded $\sim$20\% polarization with a 15hr T1. Cells using a mixture of Rb and K gave similar results,\footnote{A. Couture \textit{et al.}, Bull. Am. Phys. Soc.,\textbf{51} No. 8, 20 (2006)} though they reached saturation polarization in $\sim$12hrs. A frequency-narrowed 30W diode laser\footnote{C. W. Arnold \textit{et al.}, Bull. Am. Phys. Soc. \textbf{51}, no 8, 20 (2006)} produced similar results to those of the 60W broadband diode laser.