How the XENON1T experiment for Dark Matter calculated the half-life of Xenon-124

“A key point here is that we have so many atoms, so if any decays, we’ll see it. We have a (literal) ton of material.”

Christopher Tunnell, assistant professor of physics and astronomy and of computer science at Rice University on the calculation of the half-life of Xenon-124

Finally, we now know the half-life of Xenon-124, an isotope of Xenon. Located in Group VIII of the Periodic Table, its half-life has eluded scientists...until now.

An international team of physicists at Rice University reported the half-life of Xenon-124 is close to 18 sextillion years as reported in the article “Half-Life of Xenon-124 is about 18 sextillion years”, 24 April 2019 by By Mike Williams of Rice University, Universal Sci.

Their calculation, as laid out in the journal Nature, was the result of observing the decay process in a very large amount of Xenon in an experiment called the XENON1T experiment.

Support came from the following bodies:

§  The National Science Foundation

§  The Swiss National Science Foundation

§  The German Ministry for Education and Research

§  Max Planck Gesellschaft

§  Deutsche Forschungsgemeinschaft

§  The Netherlands Organization for Scientific Research

§  NLeSC

§  The Weizmann Institute of Science

§  I-CORE

§  Pazy-Vatat

§  Initial Training Network Invisibles

§  Fundacao para a Ciencia e a Tecnologia

§  Region des Pays de la Loire

§  The Knut and Alice Wallenberg Foundation

§  The Kavli Foundation

§  The Abeloe Graduate Fellowship

§  Istituto Nazionale di Fisica Nucleare

So what exactly is half-life of a radioactive sample anyway?

Radioactivity and half-life - Xenon-124 may be older than the Known Universe

The half-life of radioactive sample is the time it takes for half of that sample to decay to more stable forms. Most Xenon isotopes have half-lives of less than 12 days. Researchers, however, have suspected for some time that a few long-lived and stable isotopes exist.

Xenon-124 is one of those, with a half-life estimated to be around 160 trillion years as it decays via beta particle emission into Tellurium-124. For comparison, cosmologists have presumed that the Universe is a mere 13 to 14 billion years old. This means that Xenon-124 could have been present even before the Universe even existed.

Still, the chance of seeing such a decay incident for Xenon-124 is vanishingly small—unless one gathers enough Xenon atoms and puts them in the “most radio-pure place on Earth”, as stated by Christopher Tunnell, assistant professor of physics and astronomy and of computer science at Rice University.

So what exactly is the XENON1T experiment? And how did the scientists calculate its half-life?

Half-life of Xenon-124 - How the XENON1T experiment Works

The XENON1T experiment is designed to find the first direct evidence of Dark Matter. Dark Matter is the mysterious substance thought to account for most of the matter in the universe.

The XENON1T experiment is set deep inside a mountain in Italy. It is an EM (Electro-magnetic) and radioactivity-decay shielded chamber that contains a ton of highly purified liquid Xenon.

The XENON1T experiment can detect the following signs that indicate when Xenon-124 decays:

§  Photons being emitted by the liquid Xenon

§  Electrons emitted by beta particle decay at the top layer of charged Xenon gas

To quote Christopher Tunnell, assistant professor of physics and astronomy and of computer science at Rice University: “We can see single neutrons, single photons, single electrons,” he says. “Everything that enters into this detector will deposit energy in some way, and it’s measurable.”

Please note that the half-life is, on average, the time for 50% of the radioactive Xenon to be converted to other stable forms. So the scientists had detectors monitor basically every single atom of Xenon for signs of decay.

Most particle detector work on this principle of detecting the remnant of the decay of a radioactive nuclei instead of direct observation. The results of the decay of a radioactive nuclei are not stable and may exist for thousands or millionths of a fraction of a second.

So what exactly did the XENON1T experiment detect?

Xenon-124 decay - Electron capture and neutrinos released

There are three (3) ways a radioactive isotope can decay:

§  Alpha Decay - Helium nuclei with a 2+ positive charge splits from the nucleus , which is the alpha particle

§  Beta Decay - Neutron decays into a proton and an electron, with the high energy electron being the beta particle

§  Gamma Ray Emission - Electron falls from a higher energy level to a lower energy levels, emitting Gamma rays

§  Electron Capture - An electron goes into the nucleus and turns a proton into a neutron. This is basically the reverse of Beta Decay

Electron Capture was the method used to determine the half-life of Xenon-124. It also generates a sub-atomic particle known as a neutrino.  Yes I know, dear reader, that you were not taught this in High School Chemistry or Physics, but it does happen. 

In the case of the electron capture, high level electrons fall to the empty shells below, due to the reduced negative charge repulsion. This process of electron falling from higher levels to fill lower level shells until they fall into the nucleus generates photons and an emission spectra equivalent to the difference in energy levels or orbitals.

“Normally, you have one electron come in and one neutrino come out,” Tunnell says. “That neutrino has a fixed energy, which is how the nucleus expels its mass. This is a process we see often in nuclear particle physics, and it’s quite well understood”.

However, scientists have seen two electrons come into the nucleus at the same time and give off two neutrinos. This event, called two-neutrino double electron capture, if possible, is detected as a spike in the emission spectra that can only be interpreted as multiple two-neutrino double electron capture.

So unique is the event and given that background radiation is excluded, it has only one explanation as Tunnell explains, “It can’t be explained with any other background sources that we know of”.

From this release of neutrinos in a  two-neutrino double electron capture, which was spotted some 126 times as reported in the article “This is the slowest radioactive decay ever spotted”, published April 24 2019 by Maria Temming, Science News, scientists can deduce a count rate of Xenon-124.

Then using algorithms to do a reverse calculation based on basic high-school Physics principles on half-life, they can extrapolate the length of each half-like i.e. how long before the release of neutrinos and radiation drops to half of its value and thus its half-life.

From XENON1T to XENONnT - How to build a bigger, better Dark Matter Detector

 

As the search for Dark Matter continues unsuccessfully, scientists are using these expensive detectors to do other important scientific work.

“It gets tricky, because while we have the science we’re trying to do, we also have to think about what else we can do with the experiment,” Tunnell says. “We have a lot of students looking for thesis projects, so we make a list of 10 or 20 other measurements—but they’re a shot in the dark, and we almost always come up with nothing, as is typical of curiosity-driven science.

XENON1T experiment can theoretically detect WIMPs which stands for "weakly interactive massive particles". The hypothetical particles believed to constitute Dark Matter and may also be the result of the decay of Dark Matter; detecting it would prove that Dark Matter exists. It can also be used to detect neutrino-less double electron capture, in which no neutrinos are released.

So although the researchers who coauthored the current paper have not detected detect Dark Matter, work is ongoing to build a larger instrument, XENONnT, to further the search.