One of the Universe’s rarest events captured by Dark Matter detector

The radioactive decay of Xenon-124 — a process that takes more than one trillion times longer than the age of the universe — has been observed using an instrument designed to find dark matter.

The XENON1T dark matter collaboration has observed the radioactive decay of xenon-124, which has a half-life of 1.8 X 10²² years ( XENON1T)

The XENON Collaboration research team made the discovery with an instrument built to find the most elusive particle in the universe — dark matter. The announcement of the observation, the radioactive decay of the isotope xenon-124 — which has a half-life of 1.8 X 10²² years — is made in a paper to be published in the journal Nature.

The XENON Collaboration and Rensselaer Polytechnic Institute XENON1T — a 1,300-kilogram vat of super-pure liquid xenon shielded from cosmic rays in a cryostat submerged in water deep 1,500 meters beneath the Gran Sasso mountains of Italy.

Ethan Brown, an assistant professor of physics at Rensselaer and co-author of the study, says: “We actually saw this decay happen. 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.”

The XENON Experiment underground. Left the water tank with a poster showing what’s inside. Right the three-story service building (Xenon1T)

The XENON1T enables researchers to search for dark matter by recording tiny flashes of light created when particles interact with xenon inside the detector. Whilst XENON1T may have been built to capture the interaction between a dark matter particle and the nucleus of a xenon atom, the detector is capable of picking up signals from any interactions with the xenon.

The team captured evidence of a proton inside the nucleus of a xenon atom being converted into a neutron. In most elements that are subject to this kind of decay, this happens when one electron is pulled into the nucleus. But a proton in a xenon atom must absorb two electrons to convert into a neutron — an event known as “double-electron capture.”

The heart of the project: The XENON1T Time Projection Chamber TPC after assembly in a clean room (Xenon1T)

Brown points out that double-electron capture only happens when two of the electrons are right next to the nucleus at a very precise time, stating that this is“a rare thing multiplied by another rare thing, making it ultra-rare.” Or rare² perhaps?

When this rare² event happened, and a double-electron capture occurred inside the detector, instruments picked up the signal of electrons in the atom re-arranging to fill in for the two that were absorbed into the nucleus.

double electron capture can be responsible for the conversion of a proton to a neutron in certain isotopes kicking off the process of radioactive decay — but the event is extremely rare

Brown explains: “Electrons in double-capture are removed from the innermost shell around the nucleus, and that creates room in that shell.

“The remaining electrons collapse to the ground state, and we saw this collapse process in our detector.”

The achievement is the first time scientists have measured the half-life of this xenon isotope based on direct observation of its radioactive decay.

Curt Breneman, dean of the School of Science, explains the significance of the finding: “This is a fascinating finding that advances the frontiers of knowledge about the most fundamental characteristics of matter.

“Dr Brown’s work in calibrating the detector and ensuring that the xenon is scrubbed to the highest possible standard of purity was critical to making this important observation.”

The XENON Collaboration includes more than 160 scientists from Europe, the United States, and the Middle East. Since 2002, it has operated three liquid xenon detectors in the Gran Sasso National Laboratory in Italy, each more sensitive than the last.

XENON1T is the largest detector of its type ever built operating to gather data from 2016 until December 2018, when it was switched off. Scientists are currently upgrading the experiment for the new XENONnT phase, which will feature an active detector mass three times larger than XENON1T.

Together with a reduced background level, these upgrades will boost the detector’s sensitivity by another order of magnitude.