M. L. Lijowski

THE ORIGIN OF PRIMARY COSMIC RAYS: CONSTRAINTS FROM ACE ELEMENTAL AND ISOTOPIC COMPOSITION OBSERVATIONS

Cosmic-ray isotope observations from NASA’s Advanced Composition Explorer (ACE) mission have been used to investigate the composition of cosmic-ray source material. Source abundances relative to 56Fe are reported for eleven isotopes of Ca, Fe, Co, and Ni, including the very rare isotopes 48Ca and 64Ni. Although the source abundances range over a factor ∼104, most of the ratios to 56Fe are consistent with solar-system values to within ∼20%. However, there are some notable differences, the most significant being an excess of ∼(70±30)% relative to the solar system for the cosmic-ray source ratio 58Fe/56Fe. The possible association of such an excess with a contribution to the cosmic-ray source from Wolf–Rayet star ejecta is discussed.

Cosmic ray source abundances and the acceleration of cosmic rays

The galactic cosmic ray elemental source abundances display a fractionation that is possibly based on first ionization potential (FIP) or volatility. A few elements break the general correlation of FIP and volatility and the abundances of these may help to distinguish between models for the origin of the cosmic ray source material. Data from the Cosmic Ray Isotope Spectrometer instrument on NASA’s Advanced Composition Explorer spacecraft were used to derive source abundances for several of these elements (Na, Cu, Zn, Ga, Ge). Three (Na, Cu, Ge) show depletions which could be consistent with a volatility-based source fractionation model.

Constraints on cosmic-ray acceleration and transport from isotope observations

Observations from the Cosmic Ray Isotope Spectrometer (CRIS) on ACE have been used to derive constraints on the locations, physical conditions, and time scales for cosmic-ray acceleration and transport. The isotopic composition of Fe, Co, and Ni is very similar to that of solar system material, indicating that cosmic rays contain contributions from supernovae of both Type II and Type Ia. The electron-capture primary ^(59)Ni produced in supernovae has decayed, demonstrating that a time ≳10^5 yr elapses before acceleration of the bulk of the cosmic rays and showing that most of the accelerated material is derived from old stellar or interstellar material rather than from fresh supernova ejecta.

Abundances of the cosmic ray β-decay secondaries and implications for cosmic ray transport

Galactic cosmic rays (GCRs) pass through the interstellar medium (ISM) and undergo nuclear interactions that produce secondary fragments. The abundances of radioactive secondary species can be used to derive a galactic confinement time for cosmic rays using the amount of ISM material traversed by the cosmic rays inferred from stable GCR secondary abundances. Abundance measurements of long-lived species such as ^(10)Be, ^(26)Al, ^(36)Cl, and ^(54)Mn allow a comparison of propagation histories for different parent nuclei. Abundances for these species, measured in the energy range ~ 50 - 500 MeV/nuc using the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE) spacecraft, indicate a confinement time τ(esc) 16.2±0.8 Myr. We have modeled the production and propagation of the radioactive secondaries and discuss the implications for GCR transport.

Scintillating fibers and their use in the Cosmic Ray Isotope Spectrometer (CRIS) on the Advanced Composition Explorer (ACE)

The Cosmic Ray Isotope Spectrometer (CRIS) experiment was launched aboard the NASA Advanced Composition Explorer satellite on August 25, 1997. The experimental objective of CRIS is to measure the isotopic composition of galactic cosmic ray nuclei for elements with charge 3<Z<28 over the energy range ∼50–500 MeV/nuc. The instrument consists of a scintillating fiber hodoscope to determine particle trajectory, and four stacks of silicon wafers for multiple dE/dx and Etot measurements. This instrument is the first to use scintillating fibers in space. The CRIS instrument has a large geometrical factor of ∼250 cm2 sr. The spatial resolution obtained by the fiber hodoscope is ∼100 μm. The mass resolution achieved is ∼0.12 amu for Carbon and 0.30 amu for the heaviest isotopes measured. Mass histograms of selected isotopes are presented.