kálium40

Kálium-40, often written as 40K, is a naturally occurring radioactive isotope of potassium. It has a mass number of 40 and makes up a small fraction of natural potassium, with an abundance of about 0.012% in potassium. Its long half-life of 1.248×10^9 years (roughly 1.25 billion years) means it has persisted since the formation of the solar system and remains present in minerals that contain potassium.

40K decays via two main pathways. About 89% decays by beta minus decay to calcium-40 (40Ca), while about 11% decays by electron capture to argon-40 (40Ar). A small fraction of decays via other minor channels is negligible for most purposes. The decay of 40K releases energy that contributes to natural terrestrial heat, in addition to producing radiogenic argon.

Because of its long half-life and presence in common minerals, 40K is a primordial radionuclide and a key component in radiometric dating. The radiogenic production of 40Ar from 40K is exploited in potassium-argon dating, a method used to determine the age of rocks and minerals, particularly volcanic rocks. In practice, the dating technique compares the amount of remaining 40K with the amount of accumulated 40Ar to estimate elapsed time since the rock formed. The technique is widely used for ages ranging from thousands to billions of years.

In biological and environmental contexts, 40K occurs together with the stable isotopes of potassium. It contributes a small portion of natural radioactivity within living organisms and their surroundings.