An Atomic Beam Source of Polarized ³He for the nEDM@SNS Experiment

The neutron Electric Dipole Moment at the Oak Ridge National Laboratory Spallation Neutron Source (nEDM@SNS) experiment requires a source of highly (>95%) polarized ³He to be used as both a comagnetometer and a neutron spin state analyzer. This latter use is possible because the neutron ³He capture cross-section is minimized, near zero, when their spins are parallel and is maximized when the spins are antiparallel. This will cause any difference between the two Larmor precession frequencies to generate a change in the capture rate. A second magnetic field can be used to critically dress the spins such that any change in capture rate is the direct result of the neutron electric dipole moment coupling to an electric field. Spin exchange and meta-stability exchange optical pumping techniques are unlikely to produce a ³He sample with high enough polarization. Therefore, an Atomic Beam Source (ABS) is being developed to produce the polarized ³He. This device uses a quadrupole magnet system to preferentially select a spin state in a cold ³He gas (at or below 1.3 K). The operating principles and requirements for the ABS and the significant progress that has been made on its development will be described.