A solenoid coil was powered with DC current to produce a .4 T (check) magnetic field, to bend charged particles in the plane perpendicular to the beam. This made it possible to detect tracks in three dimensions, and measure charged particles, to determine if they originated from the interaction region of the beam pipe. Lead-Scintillator Shower Detectors Just outside of the magnet coil were 24 shower counter. Each counter was roughly 4 feet wide by 12 feet long. 10 plates of .25 inch lead were alternated with 10 plastic scintillators. Electrons or photons passing through this sandwich detector produced electromagnetic cascade showers. Light pulses from the scintillator plates were guided to a photomultiplier tube at each end, using plastic (lucite) light guides. These counters, plus one spare, were designed and constructed at LBL, and transported to SLAC. Iron Flux Return Eight 8 inch (25 cm) iron plates, plus two endcap steel pieces, completed the magnetic flux return path. The eight iron plates form an octagon. Muon Spark Chambers References Article on SPEAR history Particle experiments