[ad_1]
More than a kilometer under the surface of Italy, deep under the Gran Sasso mountain, lies a cylindrical reservoir. It is about a meter high, a little less than this wide, and contains an extraordinary substance: three and a half tons of ultra-pure xenon, kept liquid at a temperature of nearly one hundred degrees Celsius below zero.
The tank is part of an experiment called XENON1T, and a scientist built it in the hope of detecting an extremely rare event: the interaction of a dark matter particle with a xenon nucleus, provided if the dark matter is a very specific type of particle. If they attend such an event, it will determine what dark matter is and change the course of astronomy.
Unfortunately, they have not seen it yet. But instead, what they saw is much rarer: the disintegration of xenon 124 into tellurium-124. The conditions must be so perfect for this to happen inside the nucleus of a 124 xenon atom that the half-life* because this event is extremely rare: it is 1.8 x 1022 years.
Or, if you prefer, 18 sextillion years. Written, it's 18 billion years ago.
It's a long time. How long? It's all over a trillion times the current age of the universe.
So, yes, it's rather rare. In fact, it's the rarest event ever recorded. Already.
How did they do that?
Xenon is an element, like carbon or oxygen. It consists of a nucleus composed of 54 positively charged protons and a whole group of neutral neutrons. Around the nucleus is a cloud of negatively charged electrons, one for each proton.
The number of protons is what defines an element. Hydrogen has one proton, two helium, and so on in the periodic table. Xenon has 54. The number of neutrons in the nucleus can however be different from one atom to the other. Atoms like these are called isotopes; the majority of xenon atoms contain 75, 77 or 78 neutrons. But a small fraction of xenon atoms, about 0.1% (one in a thousand), contains only 70 neutrons. 54 + 70 = 124, so we call this isotope xenon-124.
Xenon-124 is extremely stable. If you have one of these atoms, it will stay in place for a very long time. But not forever! In some atoms, it is extremely rare for an electron cloud around the nucleus to suddenly find itself inside the nucleus.† where it can be absorbed by a proton. This process converts the pair into a neutron (and eventually emits a neutrino, another type of subatomic particle).
This electronic capture occurs very rarely. But the xenon-124 pushes this limit even further: it is very rare that two of these events occur at the same time. This is called double electron capture, and when two protons occur each absorbs an electron, transforms into a neutron and emits a neutrino each. So this event is ridiculously rare; you take a really rare event and get there twice at the same time.
It happens, however. And when that happens, the xenon-124 changes because two of its protons are now neutrons. It is no longer xenon, but an atom of 52 protons: tellurium. And this brand new tellurium atom has a problem: two of its lower energy electrons have also disappeared. It's like being kicked on the ground floor of a building. the higher energy electrons break down to the lower energy levels, and when they do this they emit very specific light colors (in fact, a similar process creates nebulae, gas clouds in space, glows, neon signs or even xenon signs).
Xenon shines blue when it is excited by an electric field. These glass tubes, in the form of a symbol element of xenon, are filled with xenon gas. It really has nothing to do with this article, but I thought it was fun and clever. Credit: Pslawinski / wikipedia
It is the emission of this very fine band of colored light that scientists have seen in their xenon tank! Although double electron capture is extremely rare, they have 3.5 tons xenon in the reservoir, which represents a lot of atoms (26 700 moles, or 1.6 x 1028 atoms, so yes a lot). When you have so many atoms, even super rare events will occur.
… not that they are easy to detect. Many other events can emit light that detracts from the detection of xenon lightning that decomposes into tellurium. But that's why the tank is 1.5 kilometers underground; the large amount of rock above them protects the tank from a whole quantum noise. It is also surrounded by a water reservoir that also absorbs subatomic particles from the rock. All this protects the xenon quite efficiently. So when decay occurs, scientists can detect it.
Over a year (2017 to 2018), they have detected about 126 of these lightning bolts (because of the methods they use to detect them, there is some uncertainty in this regard, so it's really 126 ± 29). Knowing how much xenon they have, their look time and the number of flashes, they could calculate the half-life of xenon-124: 1.8 x 1022 years.
Two scientists are examining the partially assembled XENON1T tank before installing it deep into the ground in Italy. Credit: Roberto Corrieri and Patrick De Perio / Gran Sasso National Laboratories
It turns out that the same process occurs in the isotopes of krypton and barium, and although these also have extremely long half-lives, they are slightly less than xenon-124. This makes the detected events in the XENON1t experience the rarest ever seen.
Think of it this way: if they have 3.5 tonnes of xenon in the tank and 0.1% of these atoms are xenon 124, that means they have 3,500 grams of that isotope in the tank. It will stay that way for a while, but if they wait long enough – 18 sextillions of years – they will only have 1,750 grams of xenon-124 left. The other half will have decomposed into tellurium.
It's so cool! But it is also important: it has been difficult to identify this number (you need extremely sensitive detectors in an extremely shielded environment), and it is important for nuclear physics to define other properties of the element. In addition, there is an ongoing debate about the nature of neutrinos – the technical details are, um, complicated – but if one side is good, the standard model of subatomic particles is correct, but if the other side is correct, the standard model is wrong. This debate will take some time before anyone can settle this debate, but knowing the half-life of xenon-124 (which emits neutrinos) is another step.
And, hey, I understand: it may sound strange and esoteric, but here is how the universe works. Some processes are obvious and spectacular, like starburst or banana peel (that's physics, everyone), but others are subtle and rare. But they have no less impact. a whisper in your ear can be as provocative as an orchestra causing the end of a symphony. Both have their impact.
At the moment, the XENON1T experience has been temporarily suspended for an upgrade; Once the equipment is completed, the XENONnT experiment will contain three times more xenon, which will allow scientists to further study this phenomenon.
And this is not even the main purpose of the experiment, which is to detect dark matter … but it will be necessary to wait another day to explain. In the meantime, if you want to know more about it, this guy can tell you about it:
*Half a life is a convenient way to measure the time needed to move from one substance to another. For example, the 238 uranium atoms disintegrate into thorium with a half-life of 4.47 billion years. So, if you start with a kilo of uranium-238, in 4.47 billion years, you will have half a kilo of uranium and thorium. You can not know when an individual atom is going to disintegrate, but when you have a lot of it, the stats become pretty solid.
† This is done through a process called quantum tunnel, which is exceptionally strange – just like all quantum mechanics, in reality – but it is a well-understood process, responsible for many of the effects we rely on, such as nuclear fission, Sun) , and even semiconductors in electronics.
[ad_2]
Source link