[8] These are relatively low energies; the maximum distance traveled is estimated to be 22 cm in air and 0.27 mm in body tissue.

Libby estimated that the radioactivity of exchangeable 14C would be about 14 decays per minute (dpm) per gram of carbon, and this is still used as the activity of the modern radiocarbon standard.

[14][15] Cosmogenic nuclides are also used as proxy data to characterize cosmic particle and solar activity of the distant past.

The resulting neutrons (n) participate in the following n-p reaction (p is proton): The highest rate of carbon-14 production takes place at altitudes of 9 to 15 kilometres (30,000 to 49,000 ft) and at high geomagnetic latitudes.

[28] The most notable routes for 14C production by thermal neutron irradiation of targets (e.g., in a nuclear reactor) are summarized in the table.

One side-effect of the change in atmospheric 14C is that this has enabled some options (e.g. bomb-pulse dating[33]) for determining the birth year of an individual, in particular, the amount of 14C in tooth enamel,[34][35] or the carbon-14 concentration in the lens of the eye.

[36] In 2019, Scientific American reported that carbon-14 from nuclear testing has been found in animals from one of the most inaccessible regions on Earth, the Mariana Trench in the Pacific Ocean.

[37] The concentration of 14C in atmospheric CO2, reported as the 14C/12C ratio with respect to a standard, has (since about 2022) declined to levels similar to those prior to the above-ground nuclear tests of the 1950s and 1960s.

[38][39] Though the extra 14C generated by those nuclear tests has not disappeared from the atmosphere, oceans and biosphere,[40] it is diluted due to the Suess effect.

[29] In 2009 the activity of 14C was 238 Bq per kg carbon of fresh terrestrial biomatter, close to the values before atmospheric nuclear testing (226 Bq/kg C; 1950).

These amounts can vary significantly between samples, ranging up to 1% of the ratio found in living organisms (an apparent age of about 40,000 years).

[49] This may indicate contamination by small amounts of bacteria, underground sources of radiation causing a 14N(n,p)14C reaction, direct uranium decay (though reported measured ratios of 14C/U in uranium-bearing ores[50] would imply roughly 1 uranium atom for every two carbon atoms in order to cause the 14C/12C ratio, measured to be on the order of 10−15), or other unknown secondary sources of 14C production.

In connection with building the Borexino solar neutrino observatory, petroleum feedstock (for synthesizing the primary scintillant) was obtained with low 14C content.

In the Borexino Counting Test Facility, a 14C/12C ratio of 1.94×10−18 was determined;[51] probable reactions responsible for varied levels of 14C in different petroleum reservoirs, and the lower 14C levels in methane, have been discussed by Bonvicini et al.[52] Since many sources of human food are ultimately derived from terrestrial plants, the relative concentration of 14C in human bodies is nearly identical to the relative concentration in the atmosphere.

In the event of a H. pylori infection, the bacterial urease enzyme breaks down the urea into ammonia and radioactively-labeled carbon dioxide, which can be detected by low-level counting of the patient's breath.