One of the ways we measure the age of the Earth is using the half-life of uranium. With a half-life of around four billion years, your typical atom of uranium only has even odds of having decayed during Earth's entire history. But it only takes a few hundred atoms to up the odds for us to see enough decays to be able to accurately measure the age of something, even though the decay itself may be rare. In fact, with enough atoms, it's possible to measure radioactive decays from events that have a half-life longer than the Universe's age.

Now, researchers have used a tank full of two tonnes of liquid xenon, put together to detect dark matter, to identify the rarest decay ever detected. The XENON1T detector picked up some xenon atoms being transformed into tellurium, an event with a half-life measured at 1.8 x 1022 years—or about a trillion times the age of the Universe.

Tonnes of xenon

What's the point of having two tonnes of liquid xenon in the first place? XENON1T was set up to detect a different but also extremely rare event: a dark matter particle bumping into one of the xenon atoms. This would impart enough energy to the atom to allow the event to be picked up by detectors that monitored the xenon tank. For this to work, however, the tank had to be shielded from any events that could also create a signal in the monitoring system. As a result, it was set up deep underground at Italy's Gran Sasso facility, and any potentially radioactive contaminants were eliminated from the liquid xenon.

These precautions weren't enough to allow a dark matter signal to emerge from the data (although XENON1T succeeded in that it placed extremely tight limits on dark matter's properties). But the same precautions also make it possible to detect extremely rare nuclear transformations occurring among the xenon atoms.

We tend to think of nuclear transformations as decays, with an element losing some parts and emerging somewhere lower down on the periodic table. But there's also a relatively common process where the atom gains something: electron capture. Here, an atomic nucleus will rip one of the electrons out of a lower orbital and fuse it with a proton. Conservation of charge demands that the resulting object be electrically neutral, and the result is a neutron. This drops the atom down a spot on the periodic table, leaving it with an awkward gap in its electron collection.

That's solved by pulling the electron down from an outer orbital, a process that releases photons. A neutrino is also released in the process, but those are annoyingly difficult to detect.

About those neutrinos Because they're so hard to detect, we've ignored the two neutrinos produced by this reaction. But physicists are very interested in them. There's an ongoing debate over whether neutrinos and antineutrinos are different particles, or if they're the same particles with different spin. Because they're so hard to detect, we've ignored the two neutrinos produced by this reaction. But physicists are very interested in them. There's an ongoing debate over whether neutrinos and antineutrinos are different particles, or if they're the same particles with different spin. If the latter is true, then a collision between the two neutrinos would annihilate them—you'd see the same atomic transformation, but no neutrinos would be produced. The photon produced by their annihilation would also give us a precise measure of the neutrino's mass. So researchers are very interested in understanding nuclear reactions like this one as a window into neutrino physics.

These sorts of atomic transformations are relatively common. What's dramatically less common is a process where two electrons are absorbed at the same time, dropping the atom two slots on the periodic table. But one of the elements that can undergo this sort of transformation is xenon itself, which (at least in theory) could absorb two electrons to become tellurium. But this had never been detected before. Enter XENON1T and its two tonnes of liquid xenon.

Almost discovery

As is usual in these sorts of experiments, the researchers calculated the expected background based on what they knew about the hardware, and they then looked to see whether there was a signal that diverged from the background. In this case, the signal was 4.4 standard deviations from the background—just short of a decisive confirmation of the detection of this event. This was from one year of taking data, though, so it's safe to say that the precision of these measurements will go up dramatically.

There were enough detections to calculate the half-life of this transformation, which produced a value of 1.8 x 1022, or about a trillion times the life of the Universe. This was the first direct measurement of this particular transformation and one of only two cases where we've confirmed that a double electron capture takes place.

But there's value here beyond simply finding an extraordinarily rare event. We have a number of models (the XENON1T researchers call it a "plethora") that describe the behavior of particles inside an atomic nucleus. These should all predict the half-life of this transformation, so a measurement helps us understand which of these models are on the right track.

Nature, 2019. DOI: 10.1038/s41586-019-1124-4 (About DOIs).