Cosmogenic noble gases

The interaction of cosmic ray particles within the Earth s atmosphere produces a cascade of secondary particles, many of which have enough energy to reach the 

Target elements in parentheses are for the Earth s atmosphere. Data are from Lai (1988). 

There are three principal types of nuclear reactions due to the interactions of terrestrial materials with cosmic rays (i) by high-energy spallation of nucleons (E 40MeV), principally by neutrons, (ii) by thermal neutron capture, and (iii) muon-induced nuclear disintegrations. Muon reactions become important only at depths below sea level. The estimation of the production ratio is difficult because of lack of knowledge of the probabilities of formation of nuclides in the different reactions. 

The theoretical estimation of the production rates of 3He, Ne, and Ar (Table 5.4) on various target elements has been made by several authors (e.g., Lai, 1991 Masarik Reedy, 1995). The values shown in Table 5.4, however, should be regarded to be tentative and may be subject to uncertainties 25 30% (Lai, 1991). 

With the use of the production rates, we can then estimate production rates at sea level and at latitudes higher than 60° for any material composed of the elements given in Table 5.4. After the production rate of nuclear interactions at sea level is known, the total rate of nuclear interactions in any target, for example in rocks, can be obtained by scaling the values by the A2 3 factor, where A is the atomic mass number of the target (Lai, 1991). 

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Table 5.4. Production rate of cosmogenic noble gases from selected targets...

Trapped and cosmogenic noble gases in chondrules offer clues to their irradiation history and formation but not definitive constraints (see Chapter 1.14). Trapped noble gases are generally absent or rare in chondrules, but significant amounts of trapped Ar, Kr, and Xe from a... 

The Cl and CM chondrites retain some cosmogenic noble gases from early, regolith irradiations. 

Garrison D. H. and Bogard D. D. (1998) Isotopic composition of trapped and cosmogenic noble gases in several Martian meteorites. Meteorit. Planet. Sci. 33, 721-726. 

Hohenberg C. M., Marti K., Podosek F. A., Reedy R. C., and Shirck J. R. (1978) Comparisons between observed and predicted cosmogenic noble gases in lunar samples. Proc. 9th Lunar Planet. Sci. Conf. 2311-2344. 

Shielding parameters. The most widely used shielding parameter for stony meteorites is the ratio Ne/ Ne of the cosmogenic component (note that we will always talk here about the cosmic-ray-produced fraction of a nuclide, which often has been obtained from measured values after substantial correction for other contributions, such as primordial noble gases see Isotopic abundances of cosmogenic noble gases... 

Cosmogenic noble gases produced by capture of low-euergy ueutrous... 

COSMOGENIC NOBLE GASES PRODUCED BY SOLAR COSMIC RAYS (SCR)... 

Noble gases are produced by spallation reactions when high energy particles, produced by interactions between cosmic rays and the Earth s atmosphere, interact with nuclei at the Earth s surface (Niedermann 2002, this volume). The purpose of this section is to evaluate how production and preservation of cosmogenic noble gases affect the crustal noble gas budget. 

For many applications a combined study of cosmogenic noble gases and radionuclides will be useful, for example in order to identify and evaluate complex exposure histories. Clearly, both techniques should be further developed without concentrating too much on just one of them. 

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