The Cosmic-Ray Isotope Spectrometer for the Advanced Composition Explorer

The Cosmic-Ray Isotope Spectrometer is designed to cover the highest decade of the Advanced Composition Explorer's energy interval, from similar to 50 to similar to 500 MeV nucl(-1), with isotopic resolution for elements from Z similar or equal to 2 to Z similar or equal to 30. The nuclei detected i n this energy interval are predominantly cosmic rays originating in our Gal axy. This sample of galactic matter can be used to investigate the nucleosy nthesis of the parent material. as well as fractionation, acceleration, and transport processes that these particles undergo in the Galaxy and in the interplanetary medium. Charge and mass identification with CRIS is based on multiple measurements of dE/dx and total energy in stacks of silicon detectors, and trajectory me asurements in a scintillating optical fiber trajectory (SOFT) hodoscope. Th e instrument has a geometrical factor of similar or equal to r250 cm(2) sr for isotope measurements, and should accumulate similar to 5 x 10(6) stoppi ng heavy nuclei (Z > 2) in two years of data collection under solar minimum conditions.