A Measurement of the ^(10)Be/^9Be ratio above 1.0 GeV/nucleon: Results from the 1998 Flight of ISOMAX

The Isotope Magnet Experiment, ISOMAX, was a balloon-borne instrument designed to measure the isotopic composition of the light elements in cosmic rays (3 ≤ Z ≤ 8), with a particular emphasis on the measurement of the radioactive isotope 10 Be. ISOMAX, flown in August 1998 from Lynn Lake, Manitoba, Canada, measured isotope mass with excellent resolution by combining velocity measurements from a time-of-flight (TOF) system and two Cherenkov detectors with magnetic rigidity (charge/momentum) measurements from the magnetic spectrometer. Velocity from the TOF can be used to resolve isotopes of beryllium from ˜0.2 GeV/nucleon to just above 1 GeV/nucleon. The Cherenkov counters employed silica-aerogel radiators with indices of refraction n=1.14, corresponding to an energy threshold of ˜1 GeV/nucleon. Thus, the velocity measurement from the Cherenkov counters complements and extends the energy range covered by the TOF. We discuss improvements to the mass resolution above Cherenkov threshold and present results for the 10 Be/9 Be and 7 Be/Be ratios in the energy range covered by the Cherenkov counters (1.1-2.0 GeV/nucleon)