Production mechanisms of cosmogenic nuclides in terrestrial rocks

THE FUNDAMENTALS OF SURFACE EXPOSURE DATING Production mechanisms of cosmogenic nuclides in terrestrial rocks 

Spallation. The cosmic ray energy spectrum below - 12 km altitude in the atmosphere is invariant despite the roughly exponential decrease of its flux (Lai and 

Target element Spallation Thermal neutron capture Negative muon capture 

2000) data for the noble gas isotopes are not eurrently available, but would look at least qualitatively similar. Depth is given in meters of water equivalent, where 1 m.w.e. eorresponds to a shielding depth of 100 gfcnP. [Used by permission of the editor of Nuel. Instr. and Meth. in Phys. Res. B, from Heisinger and Nolle (2000), Fig. l,p. 791.] 

No and N are the numbers of particles at the top of the atmosphere and at the location of the observer, respectively, d is the atmospheric depth (expressed in units of g/cm ), and A is the attenuation length or mean free path (in the same units). A is not a constant but changes with latitude, due to the modification of the cosmic ray energy spectrum depending on the cutoff rigidity, and is also slightly variable with altitude (e.g., Lai 1991). 

---