High resolution Cherenkov and range detectors for balloon-borne cosmic-ray experiment

Abstract

We have proposed a novel combination of an active Cherenkov detector and passive range detectors for the high resolution measurement of isotopic composition in the neighborhood of iron in the galactic cosmic rays. Here we describe tests both of a large area (4300 cm 2) Cherenkov counter, and of passive range detectors, built for an experiment based on this principle, the University of California IRIS experiment. Tests with heavy ions (2.1 GeV/amu 12C, 289 MeV/amu 40Ar, and 594 MeV/amu 20Ne) at the Lawrence Berkeley Bevalac have shown the spatial uniformity of response of the Cherenkov counter to be better than 1% peak-to-peak. Light collection efficiency is independent of projectile energy and incidence angle to within at least 0.5%. We believe this measured uniformity of response as a function of position, energy and angle to be the best reported to date. The counter, a 1.27 cm slab of Pilot 425, produces 35.3 Z 2 sin 2 θ cphotoelectrons/cm of Cherenkov signal from the primary ion (i.e., scintillation by the primary or by delta rays, and Cherenkov emission by delta rays are excluded) where Z is the projectile charge number, and θ c the Cherenkov angle. In addition, using 594 MeV/amu 20Ne, we tested the capability of passive Lexan track recorders to measure range in the presence of the nuclear interaction background which results from stopping particles through 0.9 interaction lengths of matter. We find that nuclear interactions produce an effective range straggling distribution only ∼75% wider than that expected from range straggling alone. The combination of these tested techniques makes possible high mass resolution in the neighborhood of iron. We discuss the implications of these measurements on the expected mass resolution of the IRIS experiment.