Measurement of the Abundance of Radioactive ¹⁰Be and Other Light Isotopes in Cosmic Radiation up to 2 GeV Nucleon^–1 with the Balloon-borne Instrument ISOMAX

The Isotope Magnet Experiment (ISOMAX), a balloon-borne superconducting magnet spectrometer, was designed to measure the isotopic composition of the light isotopes (3 ≤ Z ≤ 8) of cosmic radiation up to 4 GeV nucleon^-1 with a mass resolution of better than 0.25 amu by using the velocity versus rigidity technique. To achieve this stringent mass resolution, ISOMAX was composed of three major detector systems: a magnetic rigidity spectrometer with a precision drift chamber tracker in conjunction with a three-layer time-of-flight system, and two silica-aerogel Cerenkov counters for velocity determination. A special emphasis of the ISOMAX program was the accurate measurement of radioactive ¹⁰Be with respect to its stable neighbor isotope ⁹Be, which provides important constraints on the age of cosmic rays in the Galaxy. ISOMAX had its first balloon flight on 1998 August 4-5 from Lynn Lake, Manitoba, Canada. Thirteen hours of data were recorded during this flight at a residual atmosphere of less than 5 g cm^-2. The isotopic ratio at the top of the atmosphere for ¹⁰Be/⁹Be was measured to be 0.195 ± 0.036 (statistical) ± 0.039 (systematic) between 0.26 and 1.03 GeV nucleon^-1 and 0.317 ± 0.109 (statistical) ± 0.042 (systematic) between 1.13 and 2.03 GeV nucleon^-1. This is the first measurement of its kind above 1 GeV nucleon^-1. ISOMAX results tend to be higher than predictions from current propagation models. In addition to the beryllium results, we report the isotopic ratios of neighboring lithium and boron in the energy range of the time-of-flight system (up to ~1 GeV nucleon^-1). The lithium and boron ratios agree well with existing data and model predictions at similar energies.