Potassium-40

Potassium-40 is a long lived and the main naturally occurring radioactive isotope of potassium, with a half-life of 1.248 billion years. It makes up about 117 of natural potassium, making that mixture very weakly radioactive; the short life means this was significantly larger earlier in Earth's history.
Potassium-40 undergoes four different paths of radioactive decay, including all three main types of beta decay:

Electron emission to Ca with a decay energy of 1.31 MeV at 89.6% probability
Electron capture to Ar followed by a gamma decay emitting a photon with an energy of 1.46 MeV at 10.3% probability
Direct electron capture to the ground state of Ar at 0.1% probability
Positron emission to Ar at 0.001% probability

Both forms of the electron capture decay release further photons, when electrons from the outer shells fall into the inner shells to replace the electron taken from there.
The EC decay of K explains the large abundance of argon in the Earth's atmosphere, as well as prevalence of Ar over other isotopes.

Potassium–argon dating

Potassium-40 is especially important in potassium–argon dating. Argon is a gas that does not ordinarily combine with other elements. So, when a mineral forms – whether from molten rock, or from substances dissolved in water – it will be initially argon-free, even if there is some argon in the liquid. However, if the mineral contains traces of potassium, then decay of the K isotope present will create fresh argon-40 that will remain locked up in the mineral. Since the rate at which this conversion occurs is known, it is possible to determine the elapsed time since the mineral formed by measuring the ratio of K and Ar atoms contained in it.
The argon in Earth's atmosphere is 99.6% Ar, but the argon in the Sun – and presumably in the primordial material that condensed into the planets – is mostly Ar, with less than 15% of Ar. It follows that most of Earth's argon derives from potassium-40 that decayed into argon-40, which eventually escaped to the atmosphere.

Contribution to natural radioactivity

The decay of K in Earth's mantle ranks third, after Th and U, in the list of sources of radiogenic heat. Less is known about the amount of radiogenic sources in Earth's outer and inner core, which lie below the mantle. It has been proposed, though, that significant core radioactivity may be caused by high levels of U, Th and K.
Potassium-40 is the largest source of natural radioactivity in animals including humans. A 70 kg human body contains about 140 g of potassium, hence about of K; whose decay produces about 3850 to 4300 disintegrations per second continuously throughout the life of an adult person.

Banana equivalent dose

Potassium-40 is famous for its usage in the banana equivalent dose, an informal unit of measure, primarily used in general educational settings, to compare radioactive dosages to the amount received by eating one banana. The radioactive dosage from eating one banana is around 10 sievert, or 0.1 microsievert, under the assumptions that all of the radiation produced by potassium-40 is absorbed in the body and that the biological half-life of potassium-40 is around 30 days. At the estimated 0.1 μSv, one banana equivalent dose is around 1% of the average American's daily exposure to radiation.