The Gran Sasso National Laboratory complex, in Italy, stands at the foot of the massif that gives it its name, about 120 kilometers from Rome, between the towns of L’Aquila and Teramo. At first glance, it is difficult to imagine that in these simple installations, which are easy to overlook in the beauty of the surrounding landscape, one of the most intriguing and strange physical phenomena known has been measured for the first time , a process so slow that it develops over much longer than the age of the universe itself; in fact, it is estimated that it takes about a trillion times longer to complete.
In Gran Sasso, where more than a thousand researchers from almost thirty countries work, much of the action takes place many meters underground. Precisely, one of its technological jewels, the XENON1T detector, lies a kilometer and a half deep. In essence, it is a container approximately one meter long that contains 3,200 kilos of liquid xenon at a temperature of -95 ° C. And it is not there by chance.
In search of dark matter
XENON1T’s main objective is the search for the elusive dark matter. Different hypotheses consider that this constitutes 85% of all that is in the cosmos and, nevertheless, nobody knows for sure what it is, not even if it really exists, although its presence is intuited in certain gravitational effects that could not explain without it. Finding its trace is extraordinarily complex, among other things because the cosmic radiation that constantly reaches us from space makes it difficult to detect the particles that supposedly make it up.
Well, the location of the XENON1T keeps it safe from its influence. The idea, in theory, is this: when the dark matter particles pass through the detector – it is suspected that they do not interact with practically anything, so that they could even pass the entire planet without more – they could run into a xenon atom, which would end up raising a very weak signal that, even if, they would be able to detect the sensors that the cylinder has at both ends.
Now, a new study has shown that, although they have not yet found a trace of the aforementioned dark matter, they have managed to measure the radioactive decay of xenon-124, an isotope of this element – the isotopes of an atom are also atoms that have the same number of protons, but different from neutrons – which have a half-life of 1.8 X 10 raised to 22 years. The number is so huge that it is unimaginable. “It has been amazing to witness it. It is the longest and slowest process that has been directly observed to date, but the detector is sensitive enough to pick it up; it is the strangest thing that has been registered ”, indicates in a statement Ethan Brown, a professor of Physics and Astronomy at the Rensselaer Polytechnic Institute, in New York, who has participated in this initiative.
A rarity among rarities
In an article published in the journal Nature , the scientists explain that decay happened when a proton inside the nucleus of a xenon atom turned into a neutron. In most elements in which this process occurs, this takes place when an electron enters the nucleus. But with xenon something else happens. In his case, to become a neutron, a proton needs to absorb two electrons – instead of one -, which is known as double electron capture. What’s more, this only happens if two of the electrons are next to the nucleus at the right time. Brown has been quite restrictive in this regard: ” It is a very rare thing multiplied by another rare thing, which makes all of it something ultra-rare .”
However, the double capture occurred. And the moment such a thing occurred, the instruments picked up the signal emitted by the electrons as they resettled to fill in the gap left, so to speak, by the two that had been absorbed into the nucleus.
Referencia: Observation of two-neutrino double electron capture in 124Xe with XENON1T. XENON Collaboration. Nature (2019).
Image: XENON Collaboration
