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Extremely Rare and Incredibly Old

A part of the EXO-200 underground detecter. (Photo credit: EXO-200 Collaboration)

Neutrinoless double beta decay is a type of radioactive decay in which two neutrons are simultaneously transformed into two protons inside an atomic nucleus. In ordinary double beta decay, two electrons and two electron antineutrinos are emitted from the decaying nucleus. However, in neutrinoless double beta decay, only electrons are emitted. It is an extremely rare process.

How rare is neutrinoless double beta decay?

It would take more than 35 trillion trillion (that’s 24 zeros) years for half of the nuclei in a given volume of the noble gas xenon to decay in this way, according to the international EXO-200 collaboration which includes Michelle Dolinski, PhD, an associate professor of physics and dean of Graduate Education in Drexel University’s College of Arts and Sciences.

For comparison, the age of the universe is “only” around 14 billion years old.

“This is really fundamental physics research. We have set one of the best limits in the world on the half-life for an extremely rare decay,” said Dolinski, co-spokesperson for EXO-200. “The observation of this decay would help us understand the existence of matter in the universe.”

Neutrinoless double beta decay could prove neutrinos, which are a highly abundant elementary particles with an extremely small mass, are their own antiparticles. This could help researchers to determine neutrinos’ mass and how they acquire it.

The EXO-200 experiment, which concluded in late 2018, published the analysis of its complete data set in Physical Review Letters in October 2019. The experiment did not observe the decay but did set one of the best limits in the world for the decay’s half-life and for the mass neutrinos may have.

“We will continue looking for it with a larger future experiment called nEXO,” explained Dolinski. “EXO-200 proved that the technique that we will use for nEXO is a powerful way to look for this decay.”

“We went to extreme measures to eliminate as much residual radioactivity in the detector materials as we possibly could,” said Dolinski.

To eliminate backgrounds from cosmic rays, EXO-200 conducted the experiment at the Waste Isolation Pilot Plant (WIPP) in New Mexico, 2,100 feet below the surface in a salt deposit. The experiment was cooled to -100 degrees C in order to liquefy almost 200 kilograms of xenon, a noble gas.

In addition to Dolinski, Erin Hansen and Prakash Gautam, current graduate students; Yung-Ruey Yen, former postdoctoral researcher; Yi-Hsuan Lin, former graduate student; and Mike Jewell, former undergraduate, all in the College of Arts and Sciences, contributed to this research.

Drexel’s participation in EXO-200 and nEXO has been funded by the Department of Energy Office of Science. 

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