So by measuring carbon 14 levels in an organism that died long ago, researchers can figure out when it died.
The procedure of radiocarbon dating can be used for remains that are up to 50,000 years old.
This scintillator produces a flash of light when it interacts with a beta particle.
A vial with a sample is passed between two photomultipliers, and only when both devices register the flash of light that a count is made.
Over the years, carbon 14 dating has also found applications in geology, hydrology, geophysics, atmospheric science, oceanography, paleoclimatology and even biomedicine.
The impact of the radiocarbon dating technique on modern man has made it one of the most significant discoveries of the 20th century.
In living organisms, which are always taking in carbon, the levels of carbon 14 likewise stay constant.
But in a dead organism, no new carbon is coming in, and its carbon 14 gradually begins to decay.
Back in the 1940s, the American chemist Willard Libby used this fact to determine the ages of organisms long dead.
Most carbon atoms have six protons and six neutrons in their nuclei and are called carbon 12. But a tiny percentage of carbon is made of carbon 14, or radiocarbon, which has six protons and eight neutrons and is not stable: half of any sample of it decays into other atoms after 5,700 years.