Neutron Beam Effects on Spin-Exchange-Polarized He3

We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal spin-exchange-polarized He3 cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable He3 polarization and was attributed to a decrease of alkali-metal polarization, which led us to directly measure alkali-metal polarization and spin relaxation over a range of neutron fluxes at Los Alamos Neutron Science Center and Institute Laue-Langevin. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as ϕ_n, where ϕ_n is the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the concentration of electron-ion pairs but is much larger than expected from earlier work.