ISOMAX: a balloon-borne instrument to measure cosmic ray isotopes

The Isotope Magnet Experiment (ISOMAX) is a new balloon-borne instrument de veloped to measure the isotopic composition of the light elements in the co smic radiation, in particular to obtain the ratio of the radioactive Be-10 to stable Be-9. ISOMAX was first flown in August 4-5, 1998, from Lynn Lake, Manitoba, Canada ISOMAX has a geometry factor of 450 cm(2) sr and was conf igured for this flight with a large, Helmholtz-like, superconducting magnet in combination with a drift-chamber tracking system, a state-of-the-art ti me-of-flight system and two aerogel Cherenkov detectors to measure light is otopes with a mass resolution of better than 0.25 amu. In the 1998 flight t he obtained maximum detectable rigidity of the magnetic spectrometer was 97 0 GV/c for helium at 60% of the full magnetic held. ISOMAX took data for mo re than 16 h at float altitudes above 36 km. We here present the performanc e of the individual detectors and initial isotopic results of the instrumen t. (C) 2000 Elsevier Science B.V. All rights reserved.