Polarized 3He target
Overview
The polarized 3He target, to be used during the measurement of GEn, comprises a high-pressure sample
of nuclear-polarized 3He contained in a sealed, mostly glass, cell, which is part of a larger target system.
The 3He will be polarized using the technique of alkali-hybrid spin-exchange optical pumping, a two-step
process in which 1) an alkali vapor is polarized by optical pumping, and 2) the 3He is polarized through
spin-exchange collisions. The largely-glass target cell will be the central component of a target system
that will provide the necessary hardware for polarizing the 3He, monitoring its polarization and density,
and providing alternative unpolarized targets as needed.
Function
The polarized 3He target will provide what is effectively a polarized neutron target for the SBS program.
Approved experiments include the GEn experiment, which is part of the core SBS experiments, as well
the SIDIS experiment that will measure single-spin asymmetries using a transversely polarized target. A
critical feature of the polarized target is the ability to provide good performance at high luminosities
Performance Requirements
Polarization of 60% at a beam current of 60A
Target length of 60cm as seen by the electron beam
Target density of 10 amagats (ten atmospheres at 0 C).
(The density, length and beam current above correspond to a luminosity of 6x1036cm-2s-1)
Capable of tolerating running conditions for roughly four weeks before needing replacement.
Polarimetry capable of providing a relative uncertainty <3.3%.
Non-3He material in the electron beam not to exceed approximately 0.14 grams/cm2 (twice the
thickness represented by the 3He gas itself).
Calibration
The online polarization monitoring will be accomplished using pulse NMR. The absolute calibration of
the NMR will be accomplished by measuring the frequency shift in the EPR spectra of the Rb and K
atoms that results from the effective magnetic field created by the polarized 3He. While this “frequency
shift” method has been used since early experiments at SLAC to measure the neutron’s spin structure
using a polarized 3He target, we note that separate studies of these frequency shifts are being carried
out at UVa under the specific operating conditions that will be used during the SBS program.
Physical Characteristics
The glass cells containing the 3He have two chambers: a pumping chamber, in which the 3He is polarized,
and a long cylindrical target chamber, through which the electron beam passes. Experiments at JLab to
date have used cells in which these two chambers were connected by a single glass “transfer tube”.
Movement of the 3He between the two chambers was thus largely due to diffusion. With the higher
luminosities of the more recent polarized 3He experiments, it has become increasingly desirable to
speed up the rate at which 3He moves between the pumping and target chambers. For SBS, the target
cells will use what we call a “convection design” in which there are two tubes connecting the pumping
and target chambers. One of the tubes is heated, which causes a well-controlled flow of gas through the
target chamber through convection (PRC 84, 065201 (2011)). Another difference in the SBS target cells
will be the use of metal end windows on each end of the target chamber to better insure integrity when
using a high-current electron beam.
Of great importance is the target system within which the polarized 3He cells are operated. Several
important subsystems include the lasers and optics, components related to polarimetry, the magnetic
holding field (roughly 20 Gauss, but fairly homogeneous) and a target ladder system that provides access
to a variety of targets other than the polarized 3He. With the choice of a target-cell design (target
milestone #1), it is clear that much of the existing hardware that was used for the “Transversity”
experiments (the last polarized 3He experiments conducted at JLab) can also be used for SBS. An
advanced conceptual design exists for the magnetic field, utilizing Helmholtz coils within a doublewalled box-shaped iron box that provides shielding from the fringe fields of the SBS magnet. Some
enhancements of the laser system will also be needed as the volume of 3He that will be polarized is
roughly twice the volume that has been used previously.
Electronics
The target electronics include a computer running Lab View for control and data acquisition related to
polarimetry. There are also analog electronics related to NMR (online polarization monitoring) and EPR
(absolute polarization calibration).