A solenoidal spectrometer for positron-electron pairs produced in heavy-ion collisions

A new solenoidal spectrometer, designed to study the production mechanism of electrons and positrons in heavy-ion collisions, has been constructed at Argonne National Laboratory. The spectrometer uses a 300 G magnetic field to transport the leptons to two highly segmented silicon arrays that are centered on the solenoid axis, 1.2 m from the target. Positrons are identified by detecting their annihilation radiation with two arrays of position sensitive NaI(Tl) crystals that surround the silicon arrays. A novel design feature of the spectrometer is the ability to measure the angles of emission of the leptons relative to the solenoid axis. The measured response of the apparatus to electrons and positrons is in very good agreement with the calculated response obtained from Monte Carlo simulations.