Scientists Record the Rarest Event Ever

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Scientists have made an extraordinary observation in the quest to understand dark matter. Using a detector called XENON1T, based in Italy, they recorded the radioactive decay of a xenon-124 atom.

This event is incredibly rare, as xenon-124 has a half-life of 18 billion trillion years—over a trillion times longer than the age of the universe.

This remarkable discovery is detailed in the journal Nature.

How It Happened

Ethan Brown, a physics professor at Rensselaer Polytechnic Institute, explains: “We saw this decay happen. It’s the longest, slowest process ever directly observed, and our detector was sensitive enough to measure it.”

A half-life is the time it takes for half of a sample’s atoms to decay.

In other words, for xenon-124, it would take 18 billion trillion years for half of the xenon-124 atoms in a sample to decay.

Despite this incredibly long half-life, the event was observable because the detector contained 3,500 kilograms (7,716 pounds) of xenon, which equates to about 17 billion billion billion atoms.

Out of all these atoms, just one decayed during the observation period, and the detector, impressively, caught this rare event.

The Process

This rare decay occurred through a process called two-neutrino double electron capture. Previously, this type of decay had only been observed in two other elements: krypton and barium.

During this process, the xenon nucleus captures two electrons from its surrounding electron shell.

These captured electrons interact with two protons in the nucleus, converting them into neutrons and releasing two neutrinos.

Brown elaborates: “Electrons in double capture are removed from the innermost shell around the nucleus, creating room in that shell. The remaining electrons then collapse to the ground state, and we observed this collapse process in our detector.

This first direct detection of xenon-124 decay allowed researchers to refine the half-life measurement for this particular isotope, enhancing our understanding of nuclear physics and the behavior of rare isotopes.

The XENON Collaboration

The XENON Collaboration is a large, international team comprising over 160 scientists from Europe, the United States, and the Middle East.

The XENON1T detector is strategically located deep beneath the Italian Apennine Mountains at the Gran Sasso National Laboratory.

This location is crucial because it shields the detector from cosmic rays and other sources of background radiation that could interfere with the detection of rare events.

Both the location and the substance used in the detector are essential for the potential discovery of dark matter, a mysterious form of matter that is thought to make up most of the mass in the universe but does not emit or interact with light in a way that makes it directly observable.

Researchers hope that dark matter particles might occasionally interact with the xenon atoms in the detector, producing detectable flashes of light.

Xenon is chosen for these experiments because of its extreme stability, as evidenced by the incredibly long half-life of xenon-124.

The underground location of the detector further ensures that it remains free from cosmic radiation, making it possible to detect even the rarest of events.

The XENON Collaboration is currently upgrading the system to XENONnT, which will include 8 additional tons of xenon, expanding the detector’s capabilities.

This upgrade will allow for even more extensive and sensitive experiments, increasing the likelihood of detecting interactions involving dark matter or other rare processes.

The ongoing efforts of the XENON Collaboration represent a significant advancement in the field of particle physics and our understanding of the universe.


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