We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal spin-exchange-polarized ${}^3\mathrm{He}$ cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable ${}^3\mathrm{He}$ 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 $\sqrt{{\varphi }_n}$, where ${\varphi }_n$ 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.

• Received 19 February 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.083002