Last week, scientists announced that they had made an incredible physical observation with a pool of liquid xenon. It is officially the rarest nuclear disaster – and really the rarest event ever directly measured.
How rare? As I wrote in my previous article about the result: "The average time it takes for one half of the xenon atoms in a sample to undergo this reaction is 1.8 × 10 22 years. That's about a trillion times the age of the universe. "My ears, in the figurative sense, began to skip brain juice and tried to fathom it. I thought I would try to recapitulate how scientists can measure such a rare event.
On Wednesday, researchers from the XENON1
Physicists have theorized and are most interested in the existence of this nuclear decay since 1955, as this is a springboard for more interesting physical results. Another type of event, electron capture without neutrinos (when a pair of the neutrons of the atom spontaneously emit electrons and neutrinos but the neutrinos disappear and destroy each other to gamma rays), could have deep truths about the nature of the mysterious neutrino , the second most abundant particle in the universe.
But let's come back to the time of 1.8 × 10 22 . How is it possible to directly measure an event that you seem to have to wait for after all the stars have burned out? Good, luckily, half-lives do not really work. Essentially, the concept of half-life implies that for a group of atoms, the half-life is the time it takes for half of the atoms to go through the nuclear decay you are studying. The more atoms you have, the more likely you are to see the nuclear decay you are looking for.
The XENON1T experiment involves 3,500 kg of liquid xenon buried deep in a mountain in Italy. Its main objective is to measure particles of dark matter that interact with the atoms (something that has not been done yet). But of the 3,500 kilograms, about 2 kilograms is a strain of xenon that can undergo this two-neutrino double-electron-capture event, an isotope called xenon-124 (because the number of protons and neutrons in its nucleus is 124), studied author Christian Wittweg, a doctoral student at the University of Münster in Germany, told Gizmodo.
A quick estimate using chemistry at high school would say that this corresponds to approximately 10 25 xenon-124 atoms. If it takes 1.8 × 10 22 years for half of these xenon atoms to decay, then a few hundred or so will pass in a year (thanks Wolfram Alpha).
And indeed the scientists reported seeing 126 of the two-neutrino double-electron capture events in the tub of liquid xenon.
This is the rarest directly measured decay, although scientists have seen indirect evidence of other nuclear decays with even longer half-lives. But you can imagine where a physicist would like to go. Neutronless double beta decay and neutrino-less double electron trapping would both have longer half-lives, so a reasonable number of these events – which physicists want to do to prove that they exist in the real world and not just in theory – would be even larger wells of atoms in a timely manner require. In fact, physicists are already working on such experiments and hope to continue to increase the existing detectors to look for these rare decays.
The measurement made by the XENON1T experiment was one of the rarest nuclear decay ever observed. However, if you see a sufficiently large heap of atoms, you have the chance to see a one-in-a-sextillion event.