CRIS: The Cosmic Ray Isotope Spectrometer

Designed and developed by:

California Institute of Technology
Goddard Space Flight Center, NASA
Jet Propulsion Laboratory
Washington University, St. Louis

Introduction

Galactic cosmic rays (GCRs) consist of energetic electrons and nuclei which are a direct sample of material from far beyond the solar system. They are accelerated by the shock waves from supernova explosions. One of the objectives of CRIS is to determine exactly what material is accelerated by these shock waves. Energetic atoms passing through even minute amounts of matter are rapidly stripped of their electrons. Hence, unlike photons which propagate along straight lines from their sources, the cosmic rays are charged and spiral around galactic magnetic field lines: their arrival directions tell us nothing about their specific sources. However, one can use the measured composition of the GCRs to provide information about the "average" sources.

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.

GCR Composition

The GCR abundances reveal conditions in the sources themselves. For example, the relatively higher abundances of even-Z elements over odd-Z elements emphasizes the importance of helium burning in both galactic cosmic ray sources and in the sources of solar system material. Other stellar sources (such as massive red giant stars) produce a variety of nuclei, some of which get accelerated and become the galactic cosmic rays. Finally, supernova explosions produce much of the heaviest cosmic ray nuclei (in fact, all nuclei heavier than about nickel come from supernovas!)

[Onion-Skin Model of Star]

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.

[Neutron # vs Z]

CRIS Physical Description

[CRIS
Element/Isotopic Identification Regions]

If CRIS is to achieve the objectives of isotopic particle identification beyond previous experiments, it must satisfy several design requirements. It must achieve mass resolution of ~0.25 amu or better for elements from Be to Ni (Z=4 to Z=28). For an instrument that uses the multiple dE/dx-total energy approach the mass resolution is a sum of several contributions: those factors based on the physics of interactions between charged particles and matter, and those based on instrument design (e.g., electronics noise, trajectory determination). The CRIS design was also driven by the requirement for statistically accurate measurements of rare isotopes, especially in the Fe-Ni charge region (see figure in previous section). As implemented in the actual instrument, this has resulted in a geometry factor of up to 250 cm²-sr, many times larger than previous instruments of its kind. The energy and Z intervals for which charge, mass, and incident energy can be identified are indicated in the gray area of the figure. Elemental discrimination only and incident energy estimates can be achieved at higher particle energies, while integral fluxes of particles can be calculated based on events which completely penetrate a telescope.

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.

Back to CRIS/SIS homepage.
Back to Caltech SRL ACE homepage.
last modified 28 November 2007
URL: /ACE/CRIS_SIS/cris.html