3He Spin-Filter Optically Pumped by Metastability-Exchange Method in High-Flux Neutron Beam

A.K. Petukhov, ILL, Grenoble, France

High pressure 3He gas polarized with metastability-exchange optical pumping (MEOP) has successful applications in many material science and fundamental neutron physics experiments as a neutron spin filter (NSF). For moderate neutron flux, this type of NSF does not show any dependence of its properties on neutron flux. In this paper we will present experiments with high pressure (~1 bar) MEOP polarized 3He exposed to a high flux density neutron beam (thermal capture flux Fn ~2.3 x 109 n cm-2 s-1) performed at the ILL. The experiment shows strong neutron-induced depolarization of the 3He. The rate of depolarization was found to be linearly proportional to the neutron flux density and strongly dependent on the 3He purity. It ranges from ~1 hour-1 for .pure. 3He up to a few thousand hours-1 for cells with ~0.1% of impurities. Both observations are found to be in qualitative agreement with theory [1,2] of spin-polarized 3He relaxation due to hyperfine coupling in 3He+ atomic ions and due to spin-rotation coupling in 3He2+ molecular ions. However, for low impurity concentrations theory predicts about two order of magnitude weaker depolarization than was observed in the experiment. We will argue that this discrepancy can be significantly reduced by extending the theory [1, 2] to include collisions with metastable 3He2m molecules. Finally, we conclude that both types of neutron-induced molecules can be easy quenched by adding a small amount of impurities (few 1 mbar of N2) which essentially eliminates neutron beam induced 3He relaxation.

1.K. D. Bonin, T. G. Walker, and W. Happer, Phys. Rev. A 37, 3270 (1988).

2.K. D. Bonin, D. P. Saltzberg, and W. Happer, Phys. Rev. A 38, 4483(1988).

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