Dennis R. Rich, Neutron Interactions and Dosimetry (Div. 846)
The development of polarized 3He neutron spin filters is
of interest for a
diverse array of experiments. The large angular acceptance of
3He
based
neutron spin analyzers opens the possibility for entirely new classes
of
materials science experiments involving expanding beams, as in small
angle
neutron scattering (SANS) and diffuse reflectometry. Neutron
polarizers
based on 3He are of interest in symmetry tests involving
the electro-weak
interaction, because they are broadband in energy, contribute negligibly
to
gamma backgrounds, and (by traditional NMR techniques) provide a simple
and
convenient way of flipping the neutron spin without otherwise changing
the
conditions of the experiment.
At NIST, we are developing 3He neutron spin filters using
the techniques of
spin exchange optical pumping (SEOP) and metastability exchange optical
pumping (MEOP). Both techniques have been used to provide spin
analyzers
for experiments on the NG-3 SANS beamline at the NIST Center for Neutron
Research (NCNR). A recent test involved the use of a SEOP-based
spin
analyzer in separating magnetic from structural scattering in ErNi2B2C.
Cell development is underway to provide a neutron polarizer based on
SEOP
for an experiment to determine N-N weak coupling constants by measuring
the
parity violating asymmetry A(gamma) in polarized neutron capture on
protons
at Los Alamos. Recent developments in the narrowing of laser
diode arrays
(LDAs) inspired us to pursue SEOP at lower pressures (around 1 bar,
versus
the 3 bar and greater operating pressures more traditionally pursued
when
using unnarrowed LDAs), with encouraging results. We have recently
produced a SEOP cell at just under 1 bar, with a polarization preserving
lifetime of around 800 hours and have polarized it to 50% with our
unnarrowed laser. The lifetime of this cell approaches the fundamental
limit at this pressure due to dipole-dipole interactions, and is more
than
twice the lifetime of the previously documented world best. The
polarization achieved is comparable to those achieved at higher pressures,
in cells with shorter lifetimes.