Isotopes of argon

Argon (Ar) has 24 known isotopes, from ³⁰Ar to ⁵³Ar and 1 isomer (^32mAr), three of which are stable, ³⁶Ar, ³⁸Ar, and ⁴⁰Ar. On the Earth, ⁴⁰Ar makes up 99.6% of natural argon. The longest-lived radioactive isotopes are ³⁹Ar with a half-life of 269 years, ⁴²Ar with a half-life of 32.9 years, and ³⁷Ar with a half-life of 35.04 days. All other isotopes have half-lives less than two hours, and most less than one minute. The least stable is ³⁰Ar with a half-life shorter than 20 nanoseconds.

The naturally occurring ⁴⁰K, with a half-life of 1.248×10⁹ years, decays to stable ⁴⁰Ar (10.72%) by electron capture and by positron emission, and also transforms to stable ⁴⁰Ca (89.28%) via beta decay. These properties and ratios are used to determine the age of rocks through potassium-argon dating.

Despite the trapping of ⁴⁰Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all of the argon in the Earth's atmosphere is the product of potassium-40 decay, since 99.6% of Earth atmospheric argon is ⁴⁰Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of < 15% ³⁸Ar and mostly (85%) ³⁶Ar. Similarly, the ratio of the three isotopes ³⁶Ar: ³⁸Ar: ⁴⁰Ar in the atmospheres of the outer planets is measured to be 8400: 1600: 1

In the Earth's atmosphere, radioactive ³⁹Ar (half-life 269 years) is made by cosmic ray activity, primarily from ⁴⁰Ar. In the subsurface environment, it is also produced through neutron capture by ³⁹K or alpha emission by calcium. The content of ³⁹Ar in natural argon is measured to be of (8.0±0.6)×10⁻¹⁶ g/g, or (1.01±0.08) Bq/kg of ^{36, 38, 40}Ar. The content of ⁴²Ar (half-life 33 years) in the Earth's atmosphere is lower than 6×10⁻²¹ parts per part of ^{36, 38, 40}Ar. Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon. In December 2013, ³⁶Argon, in the form of argon hydride, was found in cosmic dust associated with the Crab nebula supernova. This was the first time a noble molecule was detected in outer space.

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