As seen in the table above, there are a wide variety of useful cosmogenic nuclides which can be measured in soil, rocks, groundwater, and the atmosphere. These nuclides all share the common feature of being absent in the host material at the time of formation. These nuclides are chemically distinct and fall into two categories. The nuclides of interest are either
noble gases which due to their inert behavior are inherently not trapped in a crystallized mineral or has a short enough half-life such that it has decayed since
nucleosynthesis, but a long enough half-life such that it has built up measurable concentrations. The former includes measuring abundances of 81Kr and 39Ar whereas the latter includes measuring abundances of 10Be, 14C, and 26Al. Three types of cosmic-ray reactions can occur once a cosmic ray strikes matter which in turn produce the measured cosmogenic nuclides. •
cosmic ray spallation, which is the most common reaction on the near-surface (typically 0 to 60 cm below) the Earth and can create secondary particles which can cause additional reaction upon interaction with another nuclei called a
collision cascade. •
muon capture, which pervades at depths a few meters below the subsurface because muons are inherently less reactive; in some cases, high-energy muons can reach greater depths •
neutron capture, which due to the neutron's low energy are captured into a nucleus, most commonly by water, but this process is highly dependent on snow, soil moisture and trace element concentrations.
Corrections for cosmic-ray fluxes Since the Earth bulges at the equator and mountains and deep oceanic trenches allow for deviations of several kilometers relative to a uniformly smooth spheroid, cosmic rays bombard the Earth's surface unevenly based on the latitude and altitude. Thus, many geographic and geologic considerations must be understood in order for cosmic-ray flux to be accurately determined.
Atmospheric pressure, for example, which varies with altitude, can change the production rate of nuclides within minerals by a factor of 30 between sea level and the top of a 5 km high mountain. Even variations in the slope of the ground can affect how far high-energy muons can penetrate the subsurface. Geomagnetic field strength which varies over time affects the production rate of cosmogenic nuclides though some models assume variations of the field strength are averaged out over geologic time and are not always considered. == See also ==