The optical properties of the BigBite spectrometer at NIKHEF

The optical properties of the BigBite spectrometer currently in use at the Internal Target Facility of the AmPS ring at NIKHEF have been determined. The spectrometer, which consists of a single dipole magnet, combines a large solid angle with a large momentum acceptance. The track of a particle is determined from the information of two sets of drift chambers behind the magnet. Tracing this track through the magnetic field to the target yields the position of the scattering vertex and the size and direction of the momentum vector of the scattered particle at the target position. These quantities are calculated using an analytical approximation of the spectrometer, followed by a refinement with the matrix method. The σ-resolutions of the reconstruction for 600 MeV electrons are 3 mrad for the angles, 3.2 mm for the vertex position, and 8.4×10−3 for δp/p.