Cosmic ray flux

Stuiver, M. and Quay, P. D. (1981). Atmospheric C changes resulting from fossil fuel CO2 release and cosmic ray flux variability. Earth Planet. Sci. Lett. 53, 349-362. 

The method assumes that the average cosmic ray flux has been constant over the mean-life of 81Kr (t81 + 3.07 x 105y) and over the overall exposure time of the sample cosmic rays. Furthermore, it is assumed that the exposure geometry has remained fixed over the period of irradiation, but no other assumptions are made regarding shielding. 

Nishiizumi, K., Regnier, S., and Marti, K., "Cosmic ray exposure ages of chondrites, pre-irradiation and constancy of cosmic ray flux in the past, 1980, Earth Planet. Sci. Lett., 50, 156-170. 

Yaniv, A., Marti, K., and Reedy, R. C., "The solar cosmic-ray flux during the last two million years", Lunar and Planet. Sci. XI., Lunar and Planet. Inst., Houston, 1980, 1291-1293. 

A fundamental assumption made for most dating with atmospheric radionuclides is that the cosmic radiation flux and hence, the natural production of the radionuclides has been constant with time. Various studies of this problem using 14C and tree-ring calibration have been made. Isotopic studies of meteorites have also been useful [17]. Considering the probable lack of basic accuracy of dating water, the problem of changes in cosmic ray flux is not serious. 

About 80 per cent of the cosmic-ray flux is at energies above 150 MeV per nucleon where the cross-sections are more or less constant (Fig. 9.4). 

Nevertheless, as has been emphasized by Ramaty et al. (2000), the kind of boost to the cosmic-ray flux per supernova implied by Eq. (9.57) is untenable on energetic grounds. From present-day abundances, one can estimate the quantity Q/W, the number of Be atoms per erg of cosmic-ray energy. Given an iron yield of 0.2 M per average supernova (of both types) today, and a Be/Fe ratio of 10-6, one finds a yield of 4 x 1048 Be atoms per supernova. If the typical supernova explosion energy is 1051 erg and the cosmic-ray acceleration efficiency is 10 per cent, this... 

The above models are all rather unsatisfactory, because they involve somewhat arbitrary assumptions about the time-dependence of the cosmic-ray flux and spectrum and because they predict a secondary-like behaviour for Be and B abundances, whereas the overall trend indicated by the data is more like a primary one and there are the energetic difficulties pointed out above. In the case of nB, there is a possible primary mechanism for stellar production in supemovae by neutrino spallation processes (Woosley et al. 1990 Woosley Weaver 1995), but the primary-like behaviour of beryllium in metal-poor stars, combined with a constant B/Be ratio of about 20 fully consistent with cosmic-ray spallation (Garcia Lopez et al. 1998) in the absence of any known similar process for Be, indicates that this does not solve the problem unless a primary process can be found for Be as well. Indeed,... 

Production rates increase with elevation of a target because cosmic rays at higher elevations have traveled through less mass in the atmosphere. Several methods for scaling production with elevation have been developed (e.g., Lai and Peters 1967 Lai 1991 Dunai 2000 Stone 2000). In the simplest case, changes in production of a nuclide with elevation can be approximated by an exponential function that mimics the exponential pattern of secondary cosmic ray flux for both neutrons and muons (Gosse and Phillips 2001) ... 

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. 

C-14 dating was discovered by Libby11 and co-worker [2], The cosmic ray flux has been fairly constant over prehistoric and current time and provides a small but almost constant supply of 6C14, at a rate averaged over the whole atmosphere of about 2.2 atoms cm-2 s 1. The radioactive 6C14 will bind to oxygen in the atmosphere to form radioactive carbon dioxide, but will decay, with a half-life fi/2 = 5730 years, by emitting an electron (or "fj ray") and an electron antineutrino ... 

According to the ion-molecule scheme, the chemistry in dense molecular clouds is driven by the assumed cosmic ray ionization of the most abundant species i.e. H, H2 and He. The cosmic ray ionization rate should be 10 sec tmsed upon the cosmic ray flux measured at earth. 

The related unresolved issue concerns the prevalence of multiple exposures. For example, Lavielle et al. (1999) identify Bendego as an iron that may have undergone multiple periods of exposure, presumably initiated by collisions, and there are others such as Canyon Diablo (Michlovich et al., 1994). In general, we would expect collisions to reduce the sizes of meteoroids and thereby increase production rates, i.e., to have the same qualitative effect as would an increase in the cosmic-ray flux. Such changes, however, should change production rates at random times and by random increments. The relative importance of multiple exposures in irons is difficult to assess from the available data. 

The situation changed with the publication of a paper by Bond et al. (2001) in which it was shown that a correlation exists between the cosmic ray flux changes required to generate the tree-ring-based reconstruction of atmospheric ratio... 

Figure 35 Reconstructed variations in mean temperature of shallow low-latimde seawater during the Phanerozoic based on the data in Figure 34. Note the good agreement of the cooling episodes with the extended latitudinal dispersion of ice rafted dehris (shaded histograms). In the subsequent publication, Shaviv and Veizer (2003) showed that the proposed temperamre variations correlated with the intensity of cosmic-ray flux reaching the Earth. The pco2 (PAL— present-day atmospheric level) is that for the Geocarb model of Berner (1994).

Although I will not discuss the subject here, The most important secondary cosmic-ray flux is the atmospheric neutrino beam because of the discovery of neutrino oscillations by Super-Kamiokande (Fukuda el al., 1998). The experimental situation is reviewed by Kajita Totsuka, 2001 and Jung el al., 2001, and the calculations by Gaisser Honda, 2002. 

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