The SMILI experiment measured the isotopic abundances of the elements He through Boron in the cosmic rays from 100-2000 MeV/nucleon. The experiment consisted of a superconducting magnet surrounded above and below by several electronic particle detectors; two planes of plastic scintillator to measure the particle charge and time-of-flight, 512 thin-walled drift tubes to measure the particle trajectory, a liquid water Cerenkov detector to measure particle velocity at the highest energies, and a third plane of scintillator to measure particle charge to detect particle spallation within the instrument.
This information is useful for investigating cosmic ray sources and to study the acceleration and propagation history of cosmic rays. The helium-4 isotope is considered a primary article which originates directly from the source, whereas the helium-3 isotope is a secondary resulting from collisions of helium-4 nuclei on the interstellar medium. The isotopes of lithium, beryllium, and boron are all secondary nuclei produced as by-products of the collisions between heavier nuclei (mostly carbon, nitrogen, and oxygen) and interstellar material. In addition, beryllium-10 is radioactive, with a half-life similar to the propagation time of cosmic rays (millions of years).
The experiment flew from a high altitude balloon to minimize the atmospheric overburden. The Helium results from the first flight, which took place in 1989, are published in ApJ, 413, 268 (1993). Analysis of the Helium, Lithium, Beryllium, and Boron data from the second flight, which took place in 1991, is also now published.