Cosmic ray composition

Secondary cosmic ray flux and cosmic ray composition at the Earth s surface are complex quantities to evaluate, and in practice assumptions about the constancy of cosmic rays over timescales relevant to paleoaltimetry research. Short time scale variations in production rates, such as might result from the 11-year cyclicity in the cosmic ray flux due to solar flares (Raisbeck et al. 1990), will average out of the data over million-year timescales. Likewise, assumptions about the constancy of atmospheric density must be made so that atmospheric depth can be converted to elevation. 

Cosmic rays The 6Li/7Li ratio near 0.6 observed in the cosmic rays is much larger than in the solar system because in cosmic rays both isotopes are spallation fragments of carbon and oxygen created by nuclear reactions between the cosmic rays and the interstellar atoms with which they collide. (See Nucleosynthesis origin, Cosmic rays above, and also see 7Li, below). Thus the Li isotope anomaly in the cosmic rays is understood in terms of the nuclear physics that alters the cosmic-ray composition from what it was originally when the cosmic rays began their high-speed journey. The 6Li/ 7Li ratio observed in the cosmic rays does not, therefore, represent the isotopic composition of Li in any bulk sample of stellar or planetary matter. 

The correction of observed at the Earth cosmic ray composition for nuclear fragmentation in the interstellar medium makes it possible to determine the initial elemental and isotopic composition of accelerated particles, to clarify the process of cosmic ray acceleration and the nature of cosmic ray sources. 

Meyer, J.P., L. O C. Drury L., and D.C. Ellison. (1997). Galactic cosmic rays from supernova remnants. I. A cosmic-ray composition controlled by volatility and mass-to-charge Ratio11, ApJ 487, 182. 

In this lecture, we have tried to summarize the importance of accurate measurements of the cosmic-ray composition in order to understand the nature of the cosmic-ray source. We recall that the discovery of the decrease of the relative abundance of secondary cosmic rays at high energies, i.e., the energy dependence of the propagation pathlength A(E), came as a complete surprise in the early 1970 s. However, this discovery provided major observational support to supernova shock acceleration models which, a few years later, predicted that the cosmic-ray sources should be characterized by relatively hard energy spectra. 

The identification of actinides in the GCRs has been made possible quite recently by the use of the Trek detector [55]. An accurate measurement of their abundances relative to each other and to the Pt group is within the reach of the planned Extremely Heavy Cosmic Ray Composition Observer (ECCO) [56]. Such data would in particular help discriminating between various GCR sources that have been proposed, including fresh supernova ejecta, supper-bubble material, or old galactic material (see [24] for some details and references). 

Cosmic ray composition at low energies is often quoted at a fixed energy per nucleon. When presented in this way, protons constitute roughly 90% of the flux, helium nuclei about 10% and the remainder sum to a total of about 1%,... 

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

The choice of chemical networks is complicated and even for simple clouds such as TMC the species list is 218 species, with 2747 chemical reactions linking them. Network reduction mechanisms have been employed to reduce the number of reactions but preserve the chemical composition of at least the major species. All models must include simple ion-molecule chemistry with UV and cosmic ray ionisation initiation reactions, as shown in Figure 5.20. 

Rauscher T, Heger A, Hoffman RD, Woosley SE (2002) Nucleosynthesis in massive stars with improved nuclear and stellar physics. Astrophys J 576 323-348 Rayet M (1995) The p-process in type II supemovae. Astron Astrophys 298 517-532 Rayet M, Prantzos N, Amould M (1990) The p-process revisited. Astron Astrophys 227 271-281 Reedy RC, Arnold JR, Lai D (1983) Cosmic-ray record in solar system matter. Science 219 127-135 Rehkamper M, Halliday AN (1999) The precise measurement of T1 isotopic compositions by MC-ICPMS application to the analysis of geological materials and meteorites. Geochim Cosmochim Acta 63 935-944... 

The study of galactic cosmic rays is perhaps more an exercise in taste than in visual appreciation. In fact we determine their composition without ever really seeing them. However, they constitute the only sample of matter in our possession that comes from outside the Solar System. The chemical and isotopic composition of this sample is measured using balloon- or satellite-borne particle detectors, since the Earth s atmosphere is fatal to them. When they slam into nuclei in the air, they fragment into tiny particles, thereby losing their original identity. 

The Mn- Cr system can be studied by TIMS, ICPMS, and SIMS techniques. For TIMS and ICPMS work, bulk samples or mineral separates are dissolved and the solutions are passed through ion-exchange columns to produce clean solutions of manganese and chromium. For minerals with high Mn/Cr ratios SIMS can obtain isotopic data while retaining the petrographic context of the measurements. The chromium isotopic compositions may have to be corrected for small additions of chromium from spallation reactions induced by cosmic rays. This is particularly important in iron-rich meteorites. 

CHEMICAL COMPOSITION OF HIGH-ENERGY COSMIC-RAY NUCLEI AND ITS POSSIBLE ORIGIN IN TYPE-II SUPERNOVA... 

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