Medium Energy (MEP) Group - Polarized 3He Target Experiement
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Polarized 3He Target:
Ideally, this experiment would use a dense, polarized pure neutron target. However, neutrons decay rapidly and are hard to contain, so a stable nuclear is a necessary substitute. While polarized deuterium targets are widely used, a polarized 3He target has many advantages. In polarized 3He the contribution of the protons to the total spin of the 3He is strongly suppressed by the Pauli exclusion principle (which makes the spins mostly point in opposite directions). Therefore 86% of the polarization of the 3 He nucleus comes from the neutron .
The 3He nuclei in our target is polarized by spin-exchange with optically polarized rubidium vapor. In the diagram above, a circularly polarized 795nm laser light (1) excites an electron from the 5S state to the 5P state (2). Since the rubidium vapor is in a magnetic field the states are split and only electrons from the m=-1/2 state are excited. The electron will eventually decay spontaneously and will emit a photon in the process (3). This photon can be of either polarization so the electron will end up in either the m=-1/2 state or the m=1/2 state (4). Since only electrons in the m=-1/2 state are excited the electrons accumulate in the m=1/2 state and the rubidium vapor is polarized . 3He nuclei are then polarized through a hyperfine interaction with the rubidium electrons.
The polarized 3He target is centered around an aluminosilicate glass cell containing approx. 8 atm of 3He gas. The cell has two chambers, a pumping chamber which contains the rubidium vapor and the lower target chamber where the target material interacts with the beam. The pumping chamber must be kept at 170 C to keep the rubidium polarization optimal.
The cell is centered in a 25 Gauss holding field created by large Helmholtz coils. There are two pairs of Helmholtz coils so that the field can be oriented in any direction in the scattering plane.
The polarization is measured using the nuclear magnetic resonance (NMR) of 3He. The nuclei are swept through resonance by changing the holding field while applying a perpendicular RF magnetic field. This causes the nuclei to flip direction and creates a transverse electromagnetic flux which is measured by a set of pick-up coils. This signal can be compared to measurements of the proton NMR signal, which is well known, and an absolute polarization of the 3 can be determined.
This target is currently being assembled and should be ready to be used by the summer of 2004.