Carbon dating volcanic rock
When they strike ordinary atoms in the upper atmosphere, the cosmic rays smash them apart. Some of these neutrons then collide with nitrogen atoms.
This collision is less destructive than the initial collision that produced them.
However, because it has too many neutrons for the number of protons it contains, it is not a stable atom.
Every 5,730 years, approximately half of this radioactive carbon spontaneously converts itself back into nitrogen by emitting an electron from a neutron.
Their results were 'two to three times less accurate than implied by the range of error they stated.' They thought the variations might have been caused by poor laboratory standards allowing contamination of the samples.
Some scientists believe the problem runs far deeper than this, as the following quote shows: In the light of what is known about the radiocarbon method and the way it is used, it is truly astonishing that many authors will cite agreeable determinations as "proof" for their beliefs...
...[Some authors have said] they were "not aware of a single significant disagreement" on any sample that had been dated at different labs.
If we know what the original ratios of C14 to C12 were in the organism when it died, and if we know that the sample has not been contaminated by contact with other carbon since its death, we should be able to calculate when it died by its C14 to C12 ratio.
The nitrogen atom, which began with seven protons and seven neutrons, is left with only six protons and eight neutrons.
As the number of protons decides the chemical nature of an atom, the atom now behaves like a carbon atom.
But in actual practice, we know neither the original ratios nor if the specimen has been contaminated and are forced to make what we hope are reasonable assumptions.
The tiny initial amount of C14, the relatively rapid rate of decay (the half-life of C14 is currently about 5700 years) and the ease with which samples can become contaminated make radiocarbon dating results for samples "older" than about 50,000 years effectively meaningless.