Experimental determinations of production rates

PRODUCTION RATES OF COSMOGENIC NUCLIDES Experimental determinations of production rates 

When the early studies of cosmogenic nuclides in terrestrial surface rocks were published, production rates were (quite poorly known. Davis and Schaeffer (1955) and Phillips et al. (1986) estimated the C1 production rate from the cosmic ray neutron flux and the thermal neutron capture cross section of Cl. Srinivasan (1976) scaled the Xe production rate from the lunar surface to the Earth s surface by considering the attenuation of cosmic rays all through the atmosphere. Kurz (1986a,b), Craig and Poreda (1986), and Porcelli et al. (1987) used the production rate as calculated by Yokoyama et al. (1977) to estimate the He production rate. The problem with such estimates was a 

1988) because He (and H) nuclei are sputtered off any target nucleus, the production rates of most other nuclides depend critically on nuclear excitation functions which may vary substantially for different target nuclides (cf Production mechanisms of cosmogenic nuclides in terrestrial rocks section). For example, the Ne production rate from Mg is substantially higher than that from Al or Si, because of the large cross sections of the Mg(n,a) Sle and Mg(n,a) Ne reactions (e.g., Hohenberg et al. 1978 Masarik and Reedy 1996 Leya et al. 2000). 

Whereas all the above production rate determinations were performed on rocks 20 ka in age, Dunai and Wijbrans (2000) studied much older lava flows from Lanzarote, Canary Islands. In olivines from two flows with °Ar- Ar ages of 152 and 281 ka, they derived a normalized He production rate of 118 11 atoms g a whereas a 

Volcanic rocks dated by the C and Ar- Ar methods have thus provided quite an extensive data set for the He production rate (predominantly determined in olivine separates), along with a few Ne data. In addition, the Cl production rate was determined in basalt lavas from the western USA (Zreda et al. 1991 Stone et al. 1996), in dependence of the K and Ca content. However, to my knowledge there have not been any studies of the °Be and Al production rates in basalt, probably because of experimental difficulties to detect these radionuclides in such material. 

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No experimental determinations of production rates by muons for other cosmogenic nuclides are available in the current literature. Therefore production models (e.g., Heisinger et al. 1997 Heisinger and Nolle 2000) should be used in all cases where production by muons might be relevant. 

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