The Cosmic‐Ray 3 He/ 4 He Ratio from 200 MeV per Nucleon −1 to 3.7 GeV per Nucleon −1

Abstract

The abundances of cosmic-ray helium isotopes between 0.2 and 3.7 GeV nucleon-1were measured by the Isotope Matter Antimatter Experiment (IMAX) during a flight from Lynn Lake, Manitoba, Canada on 1992 July 16-17. The IMAX balloon-borne magnetic spectrometer realized a direct measurement of the charge, the velocity, and the rigidity of cosmic rays using plastic scintillators, a high-resolution timeof-flight system, and two silica-aerogel Cerenkov counters in conjunction with a drift chamber/multiwire proportional chamber tracking system. About 75,000 helium isotopes are identified by their mass using the velocity versus magnetic rigidity technique. The measured3He/4He ratios are corrected to the top of the atmosphere, and a comparison with previous data is given. The observed isotopic composition is found to be generally consistent with the predictions of a standard leaky box model of cosmic-ray transport in the Galaxy. © 1998. The American Astronomical Society. All rights reserved.