The CHAOS polarized proton target

The CHAOS polarized proton target (CPPT) is a new dynamically polarized target for the recently commissioned Canadian High Acceptance Orbit Spectrometer at TRIUMF. The target must travel 2 m through a field-free region between the polarizing magnet and the spectrometer magnet. Therefore, a superconducting magnet, giving a field of 0.13 T on the target, is incorporated in the cryostat shell surrounding the dilution refrigerator. The magnetic field in the spectrometer will be as low as 0.5 T. These conditions require operation in frozen spin mode. Particle access and exit is by two windows spanning 320° in the horizontal plane. Vertically the opening angle is ±20°. The energy of the incident particles will be as low as 30 MeV. This requires very thin windows. A major goal is the determination of the πN σ-term from low energy pion scattering. Then the strange sea quark content of the proton can be derived.