Research Highlights

Atomic surprise from a search in the dark

Published online 24 April 2019

Xenon atoms used in search of dark matter have revealed an exceptionally rare form of radioactive decay.

Andrew Scott

View of the XENON1T detector under construction
View of the XENON1T detector under construction
The Xenon Collaboration, 2019
The search for particles that might be part of the universe’s elusive dark matter has delivered a bonus discovery unrelated to the dark matter quest. It has instead confirmed that an isotope of xenon undergoes a rare form of radioactive decay – ‘two-neutrino double electron capture’ – with a half-life that is a trillion times longer than the age of our universe.

“This is a first step toward answering another open question in physics, namely: what is the nature of neutrinos?” says researcher Alexander Fieguth of the University of Münster in Germany. Fieguth belongs to an international team, including several researchers at New York University Abu Dhabi, working with the XENON1T detector in the Gran Sasso underground laboratory in Italy.

The detector contains several thousand kilograms of liquid xenon, intended to capture interactions with proposed components of dark matter called weakly interacting massive particles (WIMPS). As a side project, it has been able to spot xenon atoms with a mass of 124 (the 124Xe isotope) capturing two electrons and then releasing two neutrinos. This converts the xenon into tellurium, and is such an unlikely event that the time taken for half of the atoms in a sample of 124Xe to decay – its ‘half-life’ – is estimated at around 18 sextillion years (18 followed by 21 zeroes).

Besides its potential to assist understanding neutrinos, Fieguth points out that detecting atoms with such an exceptionally long half-life demonstrates the utility of the XENON1T detector as “a powerful tool to look into unexplored physics.”

He hopes that other discoveries may follow, including insights into the nature of dark matter.


Xenon collaboration. Observation of two-neutrino double electron capture in 124Xe with XENON1T. Nature (2019).