Rarest Decay Process: Xenon-124 Has Half-Life Measured To Be a Trillion Times Longer Than Age of the Universe

In nuclear physics, the idea of half-life plays a vital role in understanding the decay of radioactive substances. Experts use the half-life formula in various disciplines to predict the rate or decay of an isotope and measure the age of ancient artifacts.

What is Half Life?

Not all atoms are stable. Depending on their chemical makeup, some of them tend to stabilize themselves by emitting subatomic particles and transforming into an atom of a different element. This process is called radioactive decay. Most people are more familiar with radioactive elements such as uranium and plutonium which constantly hurl off particles.

In the context of radioactive decay and nuclear physics, half-life refers to the time it takes for half of a quantity of a substance undergoing decay to go through transformation. Starting from a certain amount of a radioactive substance, after one half-life, half of that substance will have decayed, and it will end up having half of the original amount. After two half-lives, three quarters will have decayed, and so on.

Radioactive substances have different half-lives. For instance, radon-222 has a half-life of only four days, while zinc-71 decays by half in 2.4 minutes. Of all naturally occurring isotopes, francium-223 has the shortest half-life at only 22 minutes. Some elements, however, decay very slowly.

Longest Half-Life Ever Measured

In 2019, researchers have directly observed for the first time a rare type of radioactive decay which they referred to as two-neutrino double electron capture. This decay was seen in xenon-124 atoms using the XENON1T dark matter detector.

The detector is a 1,300-kilogram (2,866-pound) vat of super-pure liquid xenon which is protected from cosmic rays in a cryostat submerged in deep 1.5 kilometers (0.93 mile) beneath the Gran Sasso mountains of Italy. The observed decay happens so sparingly that it would take 18 sextillion years for a sample of xenon-124 to shrink by half. This makes the decay process extremely difficult to detect.

Xenon-124 is an isotope, or a form of an element with the same number of protons but a different number of neutrons in the nucleus. It is considered as one of the few radioactive isotopes which decays via two-neutrino double electron capture. However, atoms undergo this decay so rarely that scientists needed to monitor a huge amount of xenon to stand a chance of seeing it.

In two-neutrino double electron capture, an atomic nucleus snags two electrons from the surrounding electron shells. As a result, two protons in the nucleus are transformed into neutrons and spit out two neutrinos. While the neutrinos themselves avoid being detected, the electron capture process releases X-rays and gives off other electrons from the atom which are detectable.

From February 2017 to February 2018, XENON1T picked up the telltale emissions of two-neutrino double electron capture about 126 times. Physicists believe that they can also use the newly measured half-life to test how well theoretical models of physics inside atomic nuclei describe such an observation. These models of nuclear physics can also inform the design of experiments which hunt for the neutrinoless version of the decay.