The Hall-C Moller Polarimeter measures the polarization of the electron beam arriving in Hall-C. It does so by observing the rate of production of Moller electrons at 90 degrees in the center of mass when the beam strikes a thin iron target. The outer shell electrons in the iron are polarized parallel (or anti-parallel) to the beam direction by a 4 Tesla magnetic field. The Moller electron production rate differs when the beam and target electron spins are aligned parallel or anti-parallel to one-another. Measurement of this rate difference provides a measure of the beam polarization.
The Hall-C device is designed to provide an absolute polarization measurement with accuracy better than 0.5%. A system of movable collimators and a pair of quadrupole magnets allows the system to be tuned to operate at any beam momentum between about 0.9 and 6 GeV/c. The figure below shows the layout of the Polarimeter, which is placed in the beam alcove upstream of Hall-C.