Some cosmic ray particles reach the surface of the earth and contribute to the natural background radiation environment. It was discovered about a decade ago that cosmic ray interaction with silica and oxygen in quartz produced measurable amounts of the isotopes Beryllium and Aluminium Researchers suggested that the accumulation of these isotopes within a rock surface could be used to establish how long that surface was exposed to the atmosphere. Assuming a constant rate of production, the number of atoms of Be and Al that accumulate in a rock surface will be proportional to the length of time the rocks were exposed to cosmic ray bombardment and the respective rates of radioactive decay for each isotope. An age determined by measurement of the amount of each nuclide would be an estimate of the minimum time that the particular surface had been exposed, but would not date the maximum age of the surface exposure, that is, the surface could have been exposed for much longer than the minimum calculated age. Theoretically, exposures of surfaces from between a few thousand to about 10 million years old can be dated by the measurement of the Be and Al isotopes.
Surface exposure dating
Cosmogenic Dating | main
Figure: Quartz band on sliding surface bombarded by a cosmic ray and producing here the nuclide 10Be. Earth is constantly bombarded with cosmic rays that are high-energy charged particles. These particles interact with atoms in atmospheric gases and thereby producing northern lights and the surface of Earth. In rock and other materials of similar density, most of the cosmic ray flux is absorbed within the first meter of exposed material in reactions that produce new isotopes called cosmogenic nuclides.
Cosmogenic nuclide dating
The Earth is constantly bombarded by galactic cosmic rays, which primarily consist of protons. This secondary cosmic ray shower is rapidly attenuated as it travels down into the atmosphere. Only a very small fraction of the secondary cosmic rays, which mostly consist of neutrons, reach the surface of the Earth. These neutrons then collide with the elements that are found in rocks and soils, such as silicon, oxygen, calcium etc. But some of the spallation products are very rare yet sufficiently long lived to accumulate in measurable quantities in terrestrial rocks.
Lewis A. Owen, Marc W. Caffee, Kelly R. Bovard, Robert C. Finkel, Milap C.