Radioactivity detected from a half-life of once per billion of the universe

One of the means used to measure the age of the Earth is to use the half-life of uranium. With a half-life of about four billion years, your typical uranium atom has only equal chances of being disintegrated over the course of a whole year. history of the Earth. But it only takes a few hundred atoms to see enough decays to be able to accurately measure the age of something, even though disintegration itself may be rare. In fact, with enough atoms, it is possible to measure the radioactive decays of events having a half – life longer than the age of the Universe.

Today, researchers have used a tank filled with two tons of liquid xenon, set up to detect dark matter, to identify the rarest caries ever detected. The XENON1T detector detected some xenon atoms transformed into tellurium, an event with a half-life measured at 1.8 x 10²² years, about one trillion times the age of the Universe.

Tons of xenon

What is the point of having two tons of liquid xenon in the first place? XENON1T was designed to detect a different, but also extremely rare event: a dark matter particle hitting on one of the xenon atoms. This would give enough energy to the atom to allow the event to be detected by detectors monitoring the xenon reservoir. For this to work, however, the tank had to be protected from any event that could also create a signal in the surveillance system. As a result, it was installed deep in the Italian facilities at Gran Sasso and all potentially radioactive contaminants were removed from liquid xenon.

These precautions were not enough to allow a dark matter signal to emerge (although XENON1T has managed to place extremely strict limits on dark matter properties). But the same precautions can also detect extremely rare nuclear transformations occurring among the xenon atoms.

We tend to consider nuclear transformations as disintegrations, an element losing some parts and appearing somewhere lower in the periodic table. But there is also a relatively common process where the atom wins something: the capture of electrons. Here, an atomic nucleus will tear one of the electrons of a lower orbital and melt it with a proton. The conservation of the charge requires that the resulting object be electrically neutral, and the result is a neutron. This leaves the atom at the bottom of a place in the periodic table, leaving it an annoying gap in its collection of electrons.

This is solved by pulling the electron from an outer orbital down, a process that releases photons. A neutrino is also released in the process, but these are extremely difficult to detect.

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

Almost discovered

As usual in this type of experiment, the researchers calculated the expected background noise based on their knowledge of the material, and then sought to determine if there was a signal diverging from the background. In this case, the signal corresponded to 4.4 standard deviations from the background noise, a decisive confirmation of the detection of this event. This is a data-taking year, so it's safe to say that the accuracy of these measurements will increase dramatically.

There were enough detections to calculate the half-life of this transformation, which gave a value of 1.8 x 10²², or about a trillion times the life of the universe. This was the first direct measure of this particular transformation and one of only two cases in which we confirmed double electron capture.

But there is value here beyond simply finding an extremely rare event. We have a number of models (the XENON1T researchers call it a "plethora") that describe the behavior of particles within an atomic nucleus. These should all predict the half-life of this transformation. A measure helps us determine which of these models is on the right track.

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