Radiocarbon dating is abundantly used and offers very high precision dates, but we often want to date an event that is either too far in the past, or without the right type of organic matter, to be dated by C.
While there is a slew of other dating techniques to choose from, cosmogenic nuclide exposure dating is useful for relatively young (~100 to 10 million years old) samples.
Super high energy particles—mostly protons— are produced by our Sun, supernovae, and probably other extraterrestrial sources.
These particles continuously enter the Earth system at incredible rates and are often, but misleadingly, called cosmic rays.
The intensity at which cosmic rays collide with the Earth’s atmosphere varies.
The first interaction is when the high energy particles collide with nuclei in the upper atmosphere. A spallation reaction is a nuclear reaction where a highly energetic nucleon (usually a secondary cosmic-ray neutron of energy) collides with a target nucleus.
If we are particularly interested in the timing of the uncovering of a surface—say, bedrock that had been covered by ice, or sediments that had been revealed by the incision of a stream—we can employ cosmogenic nuclide surface exposure dating to study that uncovering process.
This is different from techniques (like Ar, or U/Th) that date the formation of a rock itself.
A muon is about 2/3rds of the size of a proton or a neutron.
They are unstable, lasting only a few hundredths of a microsecond.