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Fermilab is embarking on a multi-decade effort to understand the behavior of neutrinos. The detectors used in this research need tens of thousands of tons of liquid argon. How does someone get that much argon? That sounds like an interesting question. Let's look into it. (intro music) Argon is one of the many chemicals found on the periodic table. It's a noble gas, which means that it doesn't interact with other elements. In many ways, it's like helium's heavier cousin, although it doesn't make your voice funny. This video isn't about how we use argon
to study neutrinos, but perhaps a super quick description might help. Neutrinos interact very, very, rarely- indeed, most neutrinos originating from the sun can pass through the entire Earth without interacting. While neutrinos interact rarely, rarely doesn't mean never. A tiny few will bump into the atoms they pass, and that's true in any neutrino detector as well. Many modern neutrino detectors use liquid argon as the medium of choice. Argon is fairly heavy as atoms go, which makes it a good neutrino target, especially when it's liquified. Liquid argon looks a lot
like water. It's clear, just super cold. When neutrinos smash into argon atoms, they make a bunch of particles that then fly through the detector. As they do, they knock electrons off of other argon atoms. Using strong electric fields, scientists collect those electrons and use them to determine the path of the particles created in the collision. They can then use those paths to study the details of the interaction and thereby learn something about the neutrino and how it interacted. There are other reasons why liquid argon is used to study neutrinos,
but I don't want to talk about that technology in this video. Maybe a future one. However, it turns out that liquid argon is such a good material to study neutrinos that Fermilab scientists have used it in many of their detectors. There is the MicroBooNE detector, which used 170 tons of liquid argon. And then there's SBND and ICARUS, which use 270 and 760 tons, respectively. But those detectors are small potatoes compared to the DUNE detector, which is currently under construction. When completed, the DUNE detector is expected to use a staggering 70,000 tons of liquid argon.
70,000 tons! That's 50,000 cubic meters, or one point six million cubic feet for our American viewers. To give a sense of scale, that's the volume of about 110 typical American 3-bedroom homes. So where do scientists get that much liquid argon? Well, first the good news. Liquid argon is a commercially available product, commonly used in industry for welding and the manufacture of sensitive electronics. The bad news is that even the purest liquid argon commercially available isn't pure enough to use in neutrino detectors. So let's look into these two questions- first,
how do companies get argon, and second, what needs to be done to make that argon useful to neutrino hunters? It turns out that argon is relatively common here on Earth. Indeed, argon makes up about 1% of the air you breathe every day. That's where companies harvest argon. Air is mostly nitrogen- 78%, mixed with oxygen at 21%. Argon is most of the rest, followed by trace amounts of carbon dioxide, water, and smog and such. To make liquid argon, companies first take air and run it through filters to scrub out the water, carbon dioxide, dust, and other
contaminants. They then cool air cold enough that it liquifies. Liquid air looks just like water, although, of course, it's very cold. Companies then use the properties of nitrogen, oxygen, and argon to separate them out- specifically the fact that each of these materials boils at different temperatures. Oxygen boils at 90 Kelvin, argon at 87 Kelvin, and nitrogen at 77 Kelvin. So, what companies do is take very cold air and raise the temperature to just above 77 Kelvin. The nitrogen boils off, leaving the liquid argon and oxygen. Once the nitrogen has been harvested,
they raise the temperature to just above 87 Kelvin and the argon boils off, leaving just liquid oxygen behind. The argon is collected and then cooled again to liquefy it. It's a beautiful example of distillation- exactly the same principle as how both hard liquor and petroleum products are made. The amount of air that is needed to extract 70,000 tons of argon is simply staggering. Companies need to process about a cubic mile of air to collect that much argon. And, for our non-American viewers, that's a cube about one point six kilometers on a side. So that's a
lot of air. Let's talk about purity. If you buy the good stuff, commercially available argon is extraordinarily pure, with contamination as low as 0.1 parts per million. That means in 70,000 tons of liquid argon, the total impurities would be about 7 kilograms. Is that pure enough? First, let's talk about why purity matters. In order for liquid argon to be a good material to use in a facility as big as the DUNE detector, it has to be able to transport electrons several meters. That's because the location a neutrino interacts can be several meters away from the electronics
that detect the signal. Electrons moving through liquid argon are similar to light moving through glass. If the glass is even slightly cloudy, you won't be able to see very far. You want the glass to be transparent. To be a good neutrino detector, liquid argon needs to be transparent to electrons moving through it. However, this turns out to be difficult. The most common contaminant in liquid argon is oxygen, and oxygen loves to interact with electrons. Just a little bit of oxygen can make it impossible to transport electrons. If you built a detector using commercially available liquid
argon, the electrons could only travel something like two millimeters. And remember that neutrino scientists need something more like several meters. In order to build a working detector, scientists need liquid argon with a purity of more like ten parts per trillion- or at least several thousand times purer than you can buy. Getting to this next level of purity is tricky, but it's something that can be done. It relies on the fact that oxygen is very reactive. If you flow the argon through a very fine sieve of a variety of metals, the metal grabs the oxygen and leaves the
argon alone. The DUNE experiment uses copper, but other choices are possible. Using this filtering, DUNE scientists have been able to achieve the necessary level of purity. Oh, and by the way, fun fact- a purity of ten parts per trillion means that in 70 thousand tons of argon, which is 70 million kilograms, there is about a single gram of impurities. That's a single paper clip's worth of mass in a volume containing 70 thousand tons. That's just crazy when you think about it. I make this all sound easy, but it really isn't. One thing I didn't mention is that the DUNE detectors
will be located about a mile underground. So somehow, we need to transport 70,000 tons of argon from the surface to the detectors. That will be done by transporting the argon to the surface facility in liquid form, then converting it to gas, pumping the gas a mile underground, and then running refrigerators to reliquify it before it's put in the detectors. And all of that completely ignores the real-world issues of a plant this big, with all of the possibilities of leaks and contaminants in all of the pipes, pumps, and refrigerators that are involved. It's
a constant battle to keep the argon pure and I tip my hat to the engineers and technicians who make it all possible. (phasing sound) Okay, so that's the story of how one goes about getting enough pure liquid argon to fill more than a hundred houses. In a future video, I'll talk about how argon does the detection, which is its own interesting story. If you liked learning about the engineering challenges needed to do frontier physics, please like the video and subscribe to the channel. Today's story was a practical one, about a topic that makes the physics possible. And that means it's totally worth knowing because, as you know, physics is everything.
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