The primary current monitor for the experiment will be an ion chamber based on a successful design from LAMPF. It will provide absolute current measurements of several percent accuracy, and relative measurements of the two spin state currents to better than .01% accuracy. The active collection region of the ion chamber is 10 cm long. With a working gas of Helium at atmospheric pressure, the chamber gain will be of order . A large signal to noise ratio is achieved by using exceedingly high resistivity fused quartz standoffs. Leakage currents less than 100 pA are expected.
In order to avoid significant ion pair recombination due to space charge effects, the beam current density at our operating field of 1 kV/cm must be kept below 1 nA/cm . This is achieved in this experiment by placing the ion chamber downstream of the target, at the entrance to the beam dump tunnel, where the multiply scattered beam spot is very large.
The ion chamber is currently under construction by members of the collaboration at CEBAF, and will be installed as a permanent element in the Hall C beamline before the start of physics. The materials used in the construction have been carefully selected to avoid problems with activation and radiation damage. Whenever possible, aluminum has been used in place of stainless steel, and no organic materials will be used in the construction.