Spin-dependent absorption in ³He

Polarized ³He gas can be used to polarize neutrons because of the tremendous spin dependence of the ³He neutron absorption cross sections. As shown in the plot, a neutron beam propagating through polarized ³He will become more and more polarized as the "wrong" spin state is absorbed. However, imperfect polarization of the ³He causes some of the "right" spin state to also be absorbed, so there is an optimum thickness for the polarizer that depends on the experiment. The behavior of a common figure of merit (neutron polarization squared times transmission) is shown in the plot.

The circles and squares on the plot show the actual performance of the metastability exchange and spin exchange filters tested in July 1998. These do not show any limitations of either method, but only what was recently tested.

Inquiries or comments: david.gilliam@nist.gov.
Online: November 1998