CRIS determines the nuclear charge, mass, and kinetic energy of incident cosmic rays that stop in one of four identical stacks of large-area silicon solid-state detectors. Particle trajectories are measured with scintillating optical fibers arranged in six layers above the four detector stacks. When a charged particle passes through one of the 7000 plastic fibers, it sparkles, and a port ion of this light is piped to the ends of the fiber. This is where it is amplified and read by two CCD (charge-coupled device) cameras that record the pattern of "hits" in each fiber layer. An onboard microprocessor then uses these data to determine the position coordinates of the incident particles. A primary objective for CRIS will be to measure the abundances of various radioactive isotopes, such as cobalt-578 and nickel-59, that can be used to measure the elapsed time since cosmic ray material was synthesized in supernovae; and isotopes such as beryllium-10, aluminum-26, and chlorine-36 that measure how long cosmic rays are confined by galactic magnetic fields.