The E771 open geometry, large acceptance spectrometer, consisted of 22 planes of multiwire proportional chambers and 9 planes of drift chambers (DC) positioned upstream of the analysis magnet, and of a set of 12 DC planes and 6 drift/pad chambers (4 planes per chamber) located downstream of the magnet. The chambers were deadended in a region around the beam axis, resulting in a minimum acceptance angle of about 25 mrad. Six silicon microstrip planes, located upstream of the target, were used for beam counting and as beam position detectors. The spectrometer included a 12 plane silicon microstrip vertex detector and an electromagnetic calorimeter. A 185 X 95 cm2 aperture dipole analysis magnet provided, in the horizontal plane, a pt kick of 0.821 GeV/c. In order to identify and distinguish muons from charged hadrons, a muon detector was located downstream of the tracking chambers. It consisted of three planes of Resistive Plate Counters (RPCs) providing two-dimentional track information and three planes of scintillator hodoscopes interspersed among the steel and concrete absorbing walls. Their thickness imposed a lower bound of approximately 6 GeV/c (10 GeV/c in central region) on the muon momentum.
The average beam intensity during the E771 data run was approximately 4 x 107 protons/s resulting in an integrated number of live protons on target of (1.313+/-0.065) x 1013 (the beam count error is dominanted by the systematic error in the measurement of the efficiency of the beam silicon planes).
The Target consisted of 12 silicon foils (each foil 2 mm thick) spaced 4 mm apart, representing a total of 5.3% of an interaction length. Using the muon detector, selective single and dimuon triggers were implemented, with a muon defined by the triple coincidence of signals in an approximately projective set of pads of the three PRC planes. The presence of the scintillator planes allowed us to extract both RPC and Trigger efficiency by analyzing a few special runs taken with a dedicated scintillator trigger.
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