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As part of a series of experiments to search for antimatter in cosmic rays, the New Mexico State University balloon-borne magnet spectrometer was configured for a flight to study positrons. Two completely new instruments, a transition radiation detector and a silicon-tungsten imaging calorimeter, were added to the magnet spectrometer. These two detectors provided a proton rejection factor better than 33 104. This instrument was flown from Fort Sumner, New Mexico, at an average depth of 4.5 g cm22 of residual atmosphere for a period o