Dark Matter Search Discovers A Spectacular Bonus: The Longest-Lived Unstable Element Ever

Xenon-124 isn’t stable, and the direct detection of its decay could lead us to an even greater prize.

Our Universe is old: 13.8 billion years old, to be precise. Many of the chemical elements that appear stable on short timescales will turn out to be fundamentally unstable, decaying away into other elements if we wait long enough. While many of these decays are easily observable, some elements and isotopes are so long-lived that their half-lives are greater than the age of the Universe.

In a spectacular discovery, the XENON collaboration has just publicly announced the discovery that xenon-124, an isotope of the element Xenon, is fundamentally unstable. Its half-life is a whopping 1.8 × 10²² years: more than one trillion times the present age of the Universe. It’s the longest half-life humanity has ever measured directly, and its implications for the nature of reality couldn’t be more profound.

The mass spectrum of the element xenon, obtained through photoionization mass spectrometry. Naturally occurring xenon is made of nine separate isotopes, with Xe-124 being the lightest, composing under 0.1% of the xenon, and Xe-136 being the heaviest, and the only one known to exhibit double beta decay. (Z. Y. ZHOU, Y. WANG, X. F. TANG, W. H. WU, AND F. QI, REV. SCI. INSTRUM. 84, 014101 (2013))

Every imaginable combination of protons and neutrons represents a possible isotope of an element on the periodic table. Some of these combinations are absolutely stable, such as carbon-12, which has six protons and six neutrons. Even if you waited an arbitrarily long time, the evidence thus far indicates that the carbon-12 nucleus will never decay.

But different combinations aren’t stable, and will spontaneously either emit or capture one or more particles, transforming into a different element or isotope in the process. Carbon-14, for example, contains six protons and eight neutrons. If we observe carbon-14 for long enough, we’ll find that it’s unstable: it will radioactively decay into nitrogen-14, emitting an electron and an antineutrino in the process.

Schematic illustration of nuclear beta decay in a massive atomic nucleus. Carbon-14, which has six protons and eight neutrons, undergoes beta decay with a half-life of around 5770 years. This decay converts it into a nitrogen-14 nucleus, with seven protons and seven neutrons, emitting an electron and an antielectron neutrino in the process. (WIKIMEDIA COMMONS USER INDUCTIVELOAD)

For those of us who learned about radioactivity prior to 2003, we were taught that each element containing more protons than bismuth (83) is fundamentally unstable. For elements like radium, thorium, radon, uranium and plutonium, every one of their isotopes undergoes radioactive decay.

In 2003, however, the world learned the truth about bismuth: it, too, is fundamentally unstable. There’s one isotope of bismuth, containing 83 protons and 127 neutrons, that was previously thought to be stable. But on timescales of 1.9 × 10¹⁹ years, it, too, will radioactively decay, emitting a helium nucleus and transmuting into thallium (the element before lead). If your periodic table is newer than that discovery, it indicates that lead — with 82 protons — is the heaviest stable element.

Although Bismuth is still considered to be ‘stable’ by many, it is fundamentally unstable and will undergo alpha decay on timescales of around ~1⁰¹⁹ years. Based on experiments conducted in 2002 and published in 2003, the periodic table has been revised to indicate that lead, not bismuth, is the heaviest stable element. (MICHAEL DAYAH / PTABLE.COM)

It sounds like a bizarre proposition: to measure a process that takes longer to occur than the age of the Universe. A single atom of bismuth will last, on average, more than one billion times longer than the Universe has been around for.

But we don’t measure radioactivity by watching a single atom; we take enormous collections of atoms and search for any telltale signature that even one of them decays. If we had a mole (6.022 × 10²³) of bismuth atoms, even with their massively long half-lives (the amount of time it takes half of the atoms to decay), we’d see tens of thousands of them decay away with every year that goes by.

This graph shows (in pink) the amount of a radioactive sample that remains after several half-lives have passed. After one half-life, half the sample is left; after two half-lives, one half of the remainder (or one quarter) is left; and after three half-lives, one half of that (or one eighth) is left. This applies to many types of natural processes, including any type of radioactive decay that results in the transmutation of elements.(ANDREW FRAKNOI, DAVID MORRISON, AND SIDNEY WOLFF / RICE UNIVERSITY, UNDER C.C.A.-4.0)

There are two extremely common ways for radioactive decay to occur:

alpha decay, where an atomic nucleus emits an alpha particle (a helium nucleus), containing two protons and two neutrons, producing a new nucleus that’s two elements earlier on the periodic table,
or beta decay, where an atomic nucleus emits an electron and an antineutrino, transforming one of its neutrons into a proton in the process, producing a new nucleus that’s one element higher on the periodic table.

Carbon-14 decays via beta decay; uranium-238 decays via alpha decay. So long as the combined masses of the decay products are lighter than the initial atomic nucleus, radioactive decay is possible.

*An alpha-decay is a process where a heavier atomic nucleus emits an alpha particle (helium nucleus), resulting in a more stable configuration and releasing energy.* (NUCLEAR PHYSICS LABORATORY, UNIVERSITY OF CYPRUS)

But there are even rarer decays that can occur, and they can be seen when the more common decay pathways are either suppressed or forbidden. Some nuclei undergo inverse beta decay: transforming a proton into a neutron by emitting a positron (the antimatter counterpart of the electron) and a neutrino, dropping down one element on the periodic table. Other nuclei drop down one element by capturing one of the innermost electrons orbiting it, turning a proton into a neutron and causing the emission of a neutrino.

Because there are subtle differences between oddly-charged and evenly-charged nuclei, sometimes double beta decay can occur where normal beta decay cannot, resulting in the emission of two electrons and two antineutrinos. And, in the rarest type of known decay of all, we can have double electron capture: where two electrons are simultaneously captured by the atomic nucleus.

A schmatic of the standard double electron capture process, which results in the emission of two neutrinos. The atomic relaxation that occurs results in the emission of photons and the ionization of electrons, both of which can be picked up by the XENON detector and used to reconstruct the processes that occurred. (XENON COLLABORATION, FIG. 2, NATURE (APRIL 25))

Up until now only two known isotopes in nature — krypton-78 and barium-130 — have been shown to transmute via double electron capture. In both cases, neither of the two emitted neutrinos can be detected, nor can the minute recoil of the nucleus. Instead, it’s the effects of the electrons that cascade down in energy that we can detect. As the electrons transition to lower energy levels to fill those gaps resulting from the earlier electron capture, they emit X-rays and also cause surrounding electrons to become free and unbound.

That’s where having an ultra-sensitive detector comes in. You want to be able to both detect the X-rays at the pinpoint location of their creation, and also to observe how the newly-freed electrons drift when you apply an external field. Through the detection of both secondary signatures, which is only possible in an extraordinarily pristine environment, we can reconstruct what happened inside the detector, as well as where and when.

The XENON1T detector, with its low-background cryostat, is installed in the centre of a large water shield to protect the instrument against cosmic ray backgrounds. This setup enables the scientists working on the XENON1T experiment to greatly reduce their background noise, and more confidently discover the signals from processes they’re attempting to study. (XENON1T COLLABORATION)

The XENON collaboration possesses exactly the type of environment that should be sensitive to rare processes like these. Designed to uncover the signature of any dark matter particle that might pass through the detector and collide with a xenon nucleus, the XENON collaboration has placed some of the strongest limits on dark matter’s interaction cross-sections with normal matter in history. In order to look for these detections, they have to understand and eliminate their backgrounds in a superior, never-before-achieved fashion.

According to postdoc Laura Manenti, a member of XENON’s public relations team:

it shows how low in background our detector is, which means we have the capability to build technology able to find the elusive dark matter.

Well, dark matter hasn’t yet been found by XENON, but something remarkable has.

The spin-independent WIMP/nucleon cross-section now gets its most stringent limits from the XENON1T experiment, which has improved over all prior experiments, including LUX. While many may be disappointed that XENON1T didn’t robustly find dark matter, we mustn’t forget about the other physical processes that XENON1T is sensitive to. (E. APRILE ET AL., PHYS. REV. LETT. 121, 111302 (2018))

You see, the way the XENON detector works is by arranging a large amount of xenon — the inert, non-interacting gas whose nucleus has 54 protons — inside one of the world’s most well-shielded, sophisticated detectors. Although it’s called the XENON1T detector, there are actually 3,200 kg of xenon inside. Many of xenon’s most sensitive interactions can be revealed, including the possibility of finding processes and decays that have never been seen before. The ultimate goal of this quest, though, is to reveal the presence (or constrain the properties) of dark matter.

Xenon, naturally, comes in not one but nine different isotopes, with the lightest being xenon-124 (with 70 neutrons) and the heaviest being the long-lived but unstable xenon-136, which undergoes double beta decay after around 2 × 10²¹ years. Of the other eight isotopes, they have always been observed to be stable, but three of them are theoretically expected to undergo double electron capture. It’s just never been observed.

*The XENON experiment located underground in the Italian LNGS laboratory. The detector is installed inside a large water shield; the building next to it accommodates its various auxiliary subsystems*. (XENON1T COLLABORATION)

Until, that is, the latest run of the experiment! From 2016 until 2018, the XENON collaboration monitored and collected observations involving everything that occurred inside the detector. One of the surprising signals they found were of x-rays emitted from a particular point, followed by free electrons drifting up and triggering the detector with a slight delay. There were a total of 126 events that correspond to this process, with the energy matching the theoretical predictions of the double electron capture of one of xenon’s isotopes: xenon-124.

With a paper accepted by the prestigious journal Nature (to be published on April 25), the XENON collaboration has now broken the record for measuring the longest-lived decay in history. With a half-life of 1.8 × 10²² years, the double electron capture process of xenon-124 has both revealed the incredible sensitivity of the detector, and demonstrated the importance of looking past the known frontiers of science.

It’s also a testament to the contributions of the collaboration’s members who add a wide variety of skills and specialties. “Observing such a rare process would not have been possible without the joint work of analysers as well as of the people who have built and operated the detector,” according to scientist Christian Wittweg, a coauthor on the discovery paper. “It is a big collaborative effort!”

Here, the signatures of various energetic processes that show up in the XENON1T detector over a particular energy range. The shaded area, with red arrows added by E. Siegel for emphasis, shows where the new 126 events that indicate the double electron capture of Xe-124 occurred. (XENON COLLABORATION, FIG. 2, NATURE (APRIL 25))

Whenever you build an experiment that can take you beyond your previous sensitivity limits, you open yourself up to the possibility of discovery. In robustly detecting this extraordinarily rare decay with a longer lifetime than any other we’ve ever seen, the XENON collaboration has demonstrated how capable their apparatus is. Although it was designed to search for dark matter, it’s also sensitive to a number of other possibilities which might herald rare or even entirely new physics.

While the direct detection of the longest-lived unstable decay is an incredible feat, its implications go far beyond a simple discovery. It’s a demonstration of XENON’s sensitivity, and its ability to tease out even a tiny signal against a well-understood, low-magnitude background. It gives us every reason to be hopeful that, if nature is kind, XENON may reveal some of its even more profound secrets.

When you collide any two particles together, you probe the internal structure of the particles colliding. If one of them isn’t fundamental, but is rather a composite particle, these experiments can reveal its internal structure. Here, an experiment is designed to measure the dark matter/nucleon scattering signal. However, there are many mundane, background contributions that could give a similar result. This particular signal will show up in Germanium, liquid XENON and liquid ARGON detectors. (DARK MATTER OVERVIEW: COLLIDER, DIRECT AND INDIRECT DETECTION SEARCHES — QUEIROZ, FARINALDO S. ARXIV:1605.08788)

With the rarest double electron capture decay under their belt, the XENON collaboration is now looking ahead to other possibilities, such as neutrinoless double electron capture or neutrinoless double beta decay, which could both occur if the neutrino has certain special properties that make it its own antiparticle: that of a Majorana fermion.

The XENON detector is currently being upgraded to even greater precision, where perhaps new decays and properties of nature will be revealed. Will other isotopes of xenon be discovered to exhibit double electron capture? Will neutrinoless double electron capture or neutrinoless double beta decay show up? Will the direct signatures of dark matter be revealed at long last?

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

With this latest discovery, there’s every reason to believe that whatever the natural truths of our reality are, the XENON collaboration will help reveal them.

The author credits Nature and scientist Laura Manenti as essential sources of information used in putting this story together.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.