The Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE) spacecraft is intended to be a major step in ascertaining the isotopic composition of the cosmic rays and hence a major step in determining their origin. The GCRs consist, by number, primarily of hydrogen nuclei (~92%) and He nuclei (~7%). The heavier nuclei (1%) provide most of the information about cosmic-ray origin through their elemental and isotopic composition. The intensities of these heavy cosmic rays are very low and progress in the past has been impeded by limited particle collection power, particularly regarding individual isotopes. CRIS is designed to have far greater collection power (~250 cm²-sr) than previous satellite instruments (< 10 cm²-sr) while still maintaining excellent isotopic resolution up through Z=30 (Zinc) and beyond.
The elemental and isotopic composition of the Sun, which will be studied by SIS and other instruments on ACE, will provide one of the essential yardsticks by which to interpret the cosmic ray composition.
Many of the isotopes that will be collected by CRIS provide other information about the mechanisms of cosmic ray propagation (the study of the processes cosmic rays undergo between the time they are produced and the time of their arrival in the heliosphere). Radioactive "clock" nuclei provide information about the time between the production of cosmic rays and their acceleration to high energies. "Propagation clock" nuclei such as 10Be, 14C, 26Al, 36Cl, and 54Mn can tell us about the amount of time cosmic rays spend between acceleration and arrival in the heliosphere; that is, how long they spent wandering through the Galaxy before arriving here. K-capture secondary nuclei provide information about average densities of interstellar material encountered during propagation.
One of the most significant processes cosmic ray nuclei undergo during the time they spend in the Galaxy is spallation. This is when a nucleus interacts with another nucleus (say, a proton) and "breaks apart". Some GCR nuclei we observe are almost entirely produced in this way; these are called "secondary" nuclei. "Primary" nuclei, those for which most of the nuclei seen are survivors from the original time of their production, mixed-population nuclei (those which, upon arrival in the heliosphere, contain a substantial fraction of source nuclei) and secondary nuclei provide checks on propagation and specific source models, as well as providing information to those workers in the field of galactic chemical evolution.
In CRIS, energy information about the cosmic ray particles is collected by the 4 stacks of thick silicon detectors (shown as A-D), while the trajectory of the particles are determined by the Scintillating Optical Fiber Trajectory (SOFT) system. The 3 x- and y-layers of SOFT fibers are present for trajectory redundancy and are viewed by two camera assemblies; which camera is actually used at any given time is ground-commandable.
More information about the CRIS hardware is available.