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 iso- topic 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 iso- tope mass with excellent resolution by combining velocity measurements from a time-of-flight (TOF) system and two Cherenkov detectors with magnetic rigidity (charge/momen- tum) measurements from the magnetic spectrometer. Veloc- ity from the TOF can be used to resolve isotopes of beryl- lium 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 en- ergy threshold of 1 GeV/nucleon. Thus, the velocity mea- surement from the Cherenkov counters complements and ex- tends the energy range covered by the TOF. We discuss im- provements to the mass resolution above Cherenkov thresh- old 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).