Potassium-40

Some things in nature are special.

A type of potassium is very rare. It stays around for a long time. This part of potassium makes a gas. This gas stays inside rocks. Scientists use it to learn about Earth. It can also make heat deep inside our world. This special potassium is even in bananas! It is the main thing that makes our bodies a tiny bit radioactive. We can even use a banana to measure it.

Potassium-40 is a special part of potassium. It is radioactive. This means it is unstable. It slowly changes into other things over a long time. This change is called decay.

One decay path makes a gas called argon. This gas gets trapped inside rocks. Scientists study these rocks to find their age. They look at the amount of argon inside. This is called potassium-argon dating. This potassium also helps make heat deep inside the Earth. It is the third largest source of this heat.

You can even find it in your own body! It is the main source of natural radiation in humans. A 70 kg person has about 140 g of potassium. This makes the body slightly radioactive. You can even find it in food. Bananas have this potassium in them. People use a "banana equivalent dose" to talk about radiation. It is a way to compare small amounts of radiation to eating one banana. This helps people understand how much radiation they get.

Potassium-40 is a special kind of potassium. It is a radioactive isotope, which means it is unstable. Most natural potassium is not radioactive. However, potassium-40 makes up about 1/117 of all natural potassium. This makes the mixture very weakly radioactive. It has a very long half-life of 1.248 billion years. This is the time it takes for half of it to change. This slow change is why it is so important to our world.

This substance changes in four different ways. This thing that happens is called decay. One way is called electron emission. This happens 89.6% of the time. It turns potassium into calcium. Another way is called electron capture. This happens to argon. This path can release a photon, which is a gamma ray. This happens 10.3% of the time. A tiny 0.1% of the time, it goes straight to argon. A very small 0.001% of the time, it uses positron emission.

Scientists use this decay to learn about Earth's history. This is called potassium-argon dating. When a mineral forms from rock or water, it starts with no argon. This is because argon is a gas. If the mineral has potassium, the potassium-40 will decay into argon-40. This new gas gets trapped inside the mineral. Scientists measure the ratio of potassium to argon. This helps them find the age of the mineral. Most of the argon in our air comes from this process.

Potassium-40 also plays a big role deep underground. It is the third largest source of radiogenic heat in the Earth's mantle. This heat comes from the decay of elements. Some people think it also helps heat the Earth's core. The core is the deep center of our planet. This heat is very important for how the Earth works. It helps move things around inside the planet over long periods of time.

You even have potassium-40 inside you right now! It is the largest source of natural radiation in humans. A 70 kg person has about 140 g of potassium. This causes about 3850 to 4300 disintegrations every second. You can find this in food like bananas. People use a funny name called the banana equivalent dose. This helps people understand small amounts of radiation. Eating one banana is about 0.1 microsievert. This is only 1% of what an average American gets every day.

Potassium-40 (K) is a long-lived, naturally occurring radioactive isotope of potassium. It is an unstable version of the element that undergoes radioactive decay over vast periods of time. This isotope is a vital part of our planet's chemistry and history. It makes up about 1/117 of all natural potassium found on Earth. Because this fraction is small, natural potassium is only very weakly radioactive. However, potassium-40 has a massive half-life of 1.248 billion years. A half-life is the time required for half of a radioactive sample to decay.

This isotope follows four distinct paths of radioactive decay. The most common path is electron emission, also called beta decay. This process occurs with an 89.6% probability and turns potassium into calcium. During this decay, it releases an energy of 1.31 MeV. Another major path is electron capture (EC). In this process, the nucleus captures an electron and turns into argon. This happens at a 10.3% probability and produces a photon, or gamma ray, with an energy of 1.46 MeV.

There are two other ways potassium-40 decays into argon. One path is direct electron capture to the ground state of argon. This occurs at a very low probability of 0.1%. The final path is positron emission, which also results in argon. This is the rarest path, occurring only 0.001% of the time. Both types of electron capture decay also release photons known as X-rays. These X-rays are emitted when electrons from outer shells fall into inner shells to replace a missing electron.

Scientists use these decay paths for a method called potassium-argon (K–Ar) dating. This technique helps determine the age of minerals. Argon is a gas that does not usually combine with other elements. When a mineral forms from molten rock or dissolved substances, it starts with no argon. If the mineral contains potassium, the potassium-40 will decay into argon-40. This new argon gas becomes trapped inside the mineral structure. By measuring the ratio of potassium to argon atoms, scientists can calculate how much time has passed since the mineral formed.

This decay process also explains the composition of our atmosphere. Most of the argon in Earth's atmosphere is argon-40. Specifically, the atmosphere is 99.6% argon. In contrast, the Sun and the original materials that formed the planets contain much less argon. In those environments, argon makes up less than 15% of the total. This suggests that most of Earth's argon comes from the decay of potassium-40. Eventually, this gas escaped from the ground into the atmosphere.

Potassium-40 also contributes to the heat of our planet. It is the third largest source of radiogenic heat in the Earth's mantle. It follows thorium (Th) and uranium (U) in this ranking. Some scientists believe it also contributes to the heat of the Earth's core. They propose that high levels of uranium, thorium, and potassium could cause 1–2 TW of radioactivity in the core. This internal heat is a major part of the Earth's energy system.

Finally, potassium-40 is a significant part of human biology. It is the largest source of natural radioactivity in animals, including humans. An average 70 kg human body contains about 140 g of potassium. This results in about 3,850 to 4,300 disintegrations per second throughout an adult's life. Because of this, people use the "banana equivalent dose" as an informal educational tool. Eating one banana provides a dose of about 0.1 microsievert. This amount is only about 1% of the average American's daily radiation exposure.