Radioactivity was detected in xenon-124 — an extremely rare occurrence because the decay of this particular isotope is amazingly slow: its half-life is 1.8 x 1,000,000,000,000,000,000,000,000 years, or about one trillion times greater than the age of the universe.

Photodetectors. Image credits: Xenon Collaboration.

Radioactivity, the process by which an atomic nucleus loses energy, is useful for many things. Not only does it help to keep the Earth warm, but we can use it to generate electricity, and even study the Earth. For instance, the half-life of Uranium is very useful to study the age of the Earth or very old rocks. Uranium is fairly common within our planet and with a half-life of around four billion years, a typical uranium atom has about a 50-50 chance of having decayed through Earth’s history. If you study, say, 1000 of these atoms, we gain enough statistical significance to accurately measure the age of something. With enough atoms, you can measure reactive decays from events comparable to the age of the Universe.


We’ve used “half-life” quite a bit. Half-life is the time required for exactly half of the entities to decay, on average — it’s particularly useful in a radioactive context. In other words, the probability of a radioactive atom decaying within its half-life is 50%.

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It’s a statistical measure. After a half-life there are not exactly one-half of the atoms remaining, only approximately, because of random variation in the process. Thankfully, there are always plenty of atoms around to add statistical weight to measurements.

Now, researchers at the XENON1T detector used two tonnes of liquid xenon atoms put together to study dark matter — the elusive and hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe. Using this xenon, they were able to pick up on some xenon atoms being decaying into tellurium, an event with a half-life measured at 1.8 x 1022 years.

“We actually saw this decay happen,” says one of the researchers, Ethan Brown from the Rensselaer Polytechnic Institute (RPI) in New York. “It’s the longest, slowest process that has ever been directly observed, and our dark matter detector was sensitive enough to measure it.”

“It’s amazing to have witnessed this process, and it says that our detector can measure the rarest thing ever recorded.”

It’s a testament to the accuracy of the scientific instrument, as well as its versatility. Researchers were extremely precise in calibrating the instrument and looked for any signals that diverged from the background radioactivity. In this case, the signal was 4.4 standard deviations from the background — just short of the decisive “5-sigma” confirmation of the detection of this event, but still high enough to count with high reliability.

It’s only the first time the radioactive decay of this xenon isotope has even been observed, although its half-life has been theorised about since 1955. Researchers will continue hunting for evidence of dark matter, and there’s a good chance they will also gather more precise decay measurements.