A Polarized $^3$He Neutron Spin Filter for $n+p\rightarrow d+\gamma$

The $n + p \rightarrow d + \gamma$ experiment uses a pulsed cold neutron beam to measure $A_\gamma$, the parity-violating correlation of neutron spin and the direction of gamma--ray emission upon capture of polarized neutrons by protons. A large area polarized $^3$He neutron spin filter has been constructed and used for measurements on the FP12 beam line at the Los Alamos Neutron Scattering Center. The spin--filter consists of roughly cylindrical cells 5 cm thick with 10--12.5 cm inside diameter, which covers most of the available neutron beam area. The cells, made at NIST, use boron free, alumino--silicate glass (GE-180) and are filled with about 1 atmosphere (at room temp.) of $^3$He, natural rubidium and N$_2$. Two broadband, 30 W laser diode arrays coupled to optical fibers irradiate the cell and polarize rubidium vapor. Polarization is transferred to $^3$He nuclei though spin-exchange collisions. The transmission of neutrons through the spin filter is used to measure the neutron polarization, and the $^3$He polarization is extracted from the wavelength dependence. Continuous $^3$He polarization greater than 50\% ($\pm$ 2\%) has been maintained over the course of several weeks. A $^3$He analyzer cell, polarized off-line can be positioned down stream from the experiment to monitor neutron polarization and spin flipping. Cell construction, polarizer design, performance, and polarimetry will be described in this talk.