## Half Life

*Measure of rate of *__exponential decay__, especially of __radioactive__ isotopes.

The half life of a radioactive isotopes is the time it takes for half of the isotopes to __radioactively decay__. The longer this takes then the greater the half life and therefore the more stable the isotopes.

Rates of spontaneous radioactive dacay are fundamental characteristics of isotopes such that they occur independently of environmental variation. As a consequence, half lives can and are employed in the absolute dating of fossils and other __geological__ features.

The half life of ^{14}C, pronounced, "carbon 14", is 5,730 years. As a consequence, relatively recent __remains__ can be dated using __radiocarbon dating__, up to approximately 60,000 years old. Note that it is only formerly living material that can be dated via carbon dating since the technique is based upon ratios of ^{14}C to ^{12}C at the point of organism death in combination with how that ratio changes over time as a consequence of the radioactive dacay of ^{14}C (along with lack of decay by ^{12}C).

The half life of ^{40}K, pronounced, "__potassium__ 40", is over one-billion years (10^{9}). As a consequence, very old samples can be dated using __potassium-argon dating__, with an upper limit well beyond the age of __planet__ __Earth__. Alternatively, materials that are younger than 100,000 years old are not readily dated using this technique. In addition, potassium-argon dating requires that the clocks be set, literally via the purging of __argon__ __gas__ from __rocks__ via __melting__, which is a phenomenon associated particularly with __volcanic__ processes. As __volcanic ash__ tends to widely disseminate, it is typically layers of volcanic ash that are subject to potassium-argon dating.

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