An apparatus to measure the structure of the pion

We discuss the design and performance of a large-acceptance double-magnet spectrometer used in Fermilab Experiment 615. The apparatus was used to measure the properties of high-invariant-mass muon pairs produced in collisions of a pion beam with a tungsten target in order to infer the distribution of quarks within the pion and to study the production mechanism in detail. The emphasis was on interactions in which a single quark, and subsequently the muon pair, carried a large fraction of the pion's momentum. A hardware trigger processor enabled the detector to extract the relatively weak signal while operating in a high-rate environment.