Neutron Beam Effects on Spin Exchange Polarized He-3

TL;DR

This study investigates how high-intensity cold neutron beams cause depolarization in polarized He-3 cells, revealing a new alkali-metal spin-relaxation mechanism that scales with neutron flux.

Contribution

It identifies a novel spin-relaxation process in alkali-metal-spin-exchange polarized He-3 cells induced by neutron beams, with detailed measurements across different fluxes.

Findings

Depolarization reduces maximum He-3 polarization.

A new relaxation mechanism scales with the square root of neutron flux.

The observed effect exceeds previous theoretical expectations.

Abstract

We have observed depolarization effects when high intensity cold neutron beams are incident on alkali-metal-spin-exchange polarized He-3 cells used as neutron spin filters. This was first observed as a reduction of the maximum attainable He-3 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 LANSCE and ILL. The data reveal a new alkali-metal spin-relaxation mechanism that approximately scales as the square root of the neutron capture-flux density incident on the cell. This is consistent with an effect proportional to the recombination-limited ion concentration, but is much larger than expected from earlier work.