Sure, our atmosphere protects us from a horrible Universe that’s trying to kill us, but sometimes it prevents us from learning stuff too. Case in point, the atmosphere blocks highly energetic particles from reaching our detectors. But there’s a way astronomers can still detect their influence: Cherenkov Radiation; the cascade of radiation that blasts out as a high-energy particle makes its way through the atmosphere, like a radioactive rainshower.
Astronomy Cast episode 289 for Monday, January 14, 2013 – Cherenkov Radiation
Welcome to Astronomy Cast, our weekly facts based journey through the cosmos. Where we help you understand not only what we know, but how we know what we know.
My name is Fraser Cain, I’m the publisher of Universe Today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville.
Hi Pamela, how are you doing?
Pamela: I’m doing well and I’m love that this is the one and only time I can pronounce something that you mispronounced.
Fraser: Shhhhhhhhhhhhhh! You can do Russian- Sherrainkoff? How do you say it?
Pamela: Cherenkov (chuh-reng-kawf)
Fraser: Cherenkov, see I had wrong
Fraser: In case Preston wants to reuse this: Astronomy Cast episode 289 for Monday, January 14, 2013 – Cheeeerenkov Radiation
Pamela: (Still laughing)
Fraser: Well thank you very much I’m really glad you were able to bring your Russian training…
Pamela: It’s the ONLY time!
Fraser: I know, I know. You learned Russian, all of your training, for this very moment. So we’re going to be at Science Online, by the time you receive this we will have already been to Science Online which was a really cool conference in North Carolina and that’s going to cause a little bit of weirdness. We’re going to try to record some shows live while we’re there, at least hang out from a restaurant or something and do a couple episodes to catch up. Results have been a whole bunch of catch up episodes coming into the feed now; I hope you have been noticing. We apologize for episode 283 we lost the audio.
Pamela: If you want to know how to most effectively destroy a Macintosh with iPhoto streams. I Google plussed to death my computer.
Fraser: So you broadcast its death? Or you Google plussed it to death?
Pamela: No, no I rage posted against the machine.
Fraser: …and it rage quit. Right so we apologize, fortunately we had our backup which was a YouTube video so we were able to extract the audio from there and that’s what you’re hearing. The other thing is I hope you have noticed we have the weekly space hangout coming back into the feed which is fantastic. That’s the show of the weekly hangout of all of the space and astronomy journalists.
Pamela: We really have major kudos for noisy astronomer Nicole Gugliucci for being the force of nature behind hurting the cats to make that happen.
Fraser: That’s what threw me off the rails in the first place and I’m really glad that Nicole has stepped up; it’s fantastic.
Fraser: Sure, our atmosphere protects us from a horrible Universe that’s trying to kill us, but sometimes it prevents us from learning stuff too. Case in point, the atmosphere blocks highly energetic particles from reaching our detectors. But there’s a way astronomers can still detect their influence: Cherenkov radiation; the cascade of radiation that blasts out as a high-energy particles make their way through the atmosphere, like a radioactive rain shower. So Pamela lets go. First: Science. What is Cherenkov radiation?
Pamela: It’s radiation that is generated when a particle passes through a medium at faster than the speed of light in that medium which is a really interesting thing to wrap your head around. As the particle goes through, at faster than the speed of light in that medium, it causes all of the particles around it, depending on what they’re made of, this is specific to dielectric materials to get aligned, then when they collapse back down to their normal state of chaos they give off photons which are organized and you are able to see the distribution of this color of light and detect it. What is really awesome is as you have this particle barreling through some material; it doesn’t have to be just our atmosphere, this could happen with a neutrino passing through special fluids. This is how we detect treatments as well. The shockwave created by its motion through a medium will create a beautiful ring of light and depending on the crispness of the edges of that light, it tells us a lot about what was traveling through the medium.
Fraser: Now I’m going to need you to back up for one second. You said when something moves faster than the speed of light…
Pamela: Through the medium
Fraser: Through the medium… so can you go back. Obviously Einstein would not appreciate something moving faster than the speed of light…
Pamela: In a vacuum
Fraser: In a vacuum, no I understand. So what exactly is going on here? We’ve got this particle moving like a cosmic ray or something right? It’s moving through space in a vacuum, very quickly…
Pamela: Quite happily
Fraser: Quite happily, and it hits the atmosphere and what does it do?
Pamela: It breaks.
Fraser: It can’t go faster than the speed of light.
Pamela: Well that’s the crazy thing. Particles can quite happily go faster than the speed of light through a medium. There’s actually rubidium gas that you can, in a fancy set-up, make light travel at about human walking speed.
Fraser: So you could send a pulse of light, one side through the rubidium, run around to the other side and catch it on the other side before it makes it out?
Pamela: It’s hard to catch light but yes. Because the propagation speed of the phase of the wavelength through the medium is so slow in some cases I could actually, if I wanted to be in the rubidium gas, I could walk faster than the particle of light through the gas. That’s cool. Now in everyday reality, light traveling in our atmosphere travels slower than it does when it passes through a vacuum and in fact a particle traveling at relativistic speeds or even at the speed of light, in the case of gamma rays through a vacuum; when it starts passing through our atmosphere it starts undergoing breaking processes. These are energetic particles, these are charged particles and that charge influences the area around them. Moving charged particles generate electric and magnetic field effects and there are a bunch of different types of materials. We normally talk about conductors. The wires in the wall conduct electricity, they conduct telephone signals, they conduct a lot of different things. Then we talk about insulators. The wood of my desk is not going to allow a random sparking “something” to electrocute me. The plastic coating over the wires is going to protect you from being electrocuted. In between the conductors and the insulators is what’s called dielectric material. This is material that doesn’t so much transmit the electricity but unlike an insulator which just doesn’t care about the electromagnetic fields, a dielectric material is actually going to have all of its little charges happily flipped to coordinate; they are going to polarize. This is how they respond to the charge. It’s a higher energy state to have all of the particles flipped and line up. Normally they are nice and chaotic and everything balances out to neutral. When you have this high speed charged particle moving through, it causes all of the stuff and dielectric material to line up. That happens even when it’s going slow but what’s awesome is, when it’s going really fast the reemission of the energy of the lining up of all of the particles in the dielectric is coordinated and you get this Cherenkov radiation.
Fraser: Got it. Ok. So then what kind of events, what kind of particles stuff, is going to be causing this radiation?
Pamela: There are various different types of places that we observe this. With air detectors, detectors out in the open often in big observatories and tops of mountains, you are looking for the cascades of light created by cosmic rays entering our atmosphere, gamma rays entering our atmosphere interacting with the atmosphere. In underwater situations you are looking for neutrinos, you’re looking for (??? 9:36)
Fraser: We did a whole show on cosmic rays and neutrinos. The difference between a gamma ray and a cosmic thing sound like the same thing but they’re different right?
Pamela: So a gamma ray is just a photon, a very energetic photon, a very, very… we don’t have a word for higher energy light than this so gamma rays are just particles of light that are extraordinarily high energy. Cosmic rays can actually be particles so this isn’t a photon. This is something that has mass and it’s traveling at relativistic speeds so it’s not actually going at the speed of light but it’s going fast. They’re often generated in things like super nova explosions or high energy jets in a variety of different things like jets coming off of AGN, active galactic nuclei. When these high energy charged particles hit our atmosphere they start breaking but they also, while they are moving so fast, cause this coordinated emission of radiation that we detect as Cherenkov radiation.
Fraser: I think the big key here is that they are the most energetic events that we see in the universe. It’s the same thing with x-rays, with gamma rays, and with cosmic rays. By every measurement you are at eleven with them.
Pamela: (Laughs) Yes
Fraser: Normally if we didn’t have our atmosphere these events would be killing us with radiation… so thanks atmosphere. We need spacecrafts to view them.
Pamela: Right, and even with spacecrafts we can’t detect the Cherenkov radiation that comes from things like neutrinos passing through the various heavy water detectors we have on the surface of the planet. Cherenkov radiation is one of those amazing things that can be used both in astrophysics, in particle physics and in a variety of other subjects that are beyond what we can talk about in just these 30 minutes. Just focusing on those two different applications: Cherenkov radiation is how the Super Kamiokande reactor in Japan is able to detect neutrinos from our sun and neutrinos being emitted in a variety of nuclear reactors. What’s really actually kind of awesome, to me at least, it can differentiate between the different types of neutrinos based on how crisp the doughnuts of emitted light are as these neutrinos create the Cherenkov radiation with a neutron neutrino it creates this beautiful crisp doughnut of light that can get detected, whereas an electron neutrino because it creates multiple propagating cones where it triggers things that trigger things that trigger things that creates fuzzy doughnuts of light. Just looking at the type of radiation that’s created starts to tell us a story of what created it. This isn’t just a way to detect high energy stuff, it’s a way to detect and differentiate between different types of high energy stuff.
Fraser: So then there are a few Cherenkov radiation detector facilities set up around the world right? There’s the Pierre Auger…
Pamela: There’s the Pierre Auger observatory but that’s only one of many different types. It’s actually a really weird hybrid facility. It’s located down in the Andes Mountains and they use a variety of different detectors that look for fluorescent materials that are specifically detecting neurons. They’re trying to figure out how they can detect this in radio waves. It’s a kind of neat R&D facility that’s looking for a variety of different types of events. More classically people have used what are called air Cherenkov detectors. Whipple telescope was one of these and it’s one of the neatest looking telescopes. It’s this outdoor, fully-exposed facility that has a million, not literally a million, but a bunch of little tiny mirrors that are all mostly lined up with one another. What’s awesome about trying to detect this is you don’t need to have a perfect surface; you’re just trying to detect the full blob of light propagating through our atmosphere so you don’t need to focus it or anything. They have these big outdoor light collecting surfaces made of multiple mirrors that focus the light up to detectors that look at the distribution of wave lengths of light. By looking at all of the different colors that are given detected and all of the different timings of the detection and how the mirror is getting hit they’re able to figure out where in the sky this new cascade is coming from and tell various characteristics about it. You can see the cascades that are caused by gamma ray bursts and the cascades that are related to various other events. It’s kind of neat that there is a future for the badly focused telescope and it’s called detecting high energy particles.
Fraser: So you’ve got all of these different detectors set up across the landscape and as you mentioned you’ve got these cones of radiation coming down and these detectors are then letting them backtrack where the event came from right?
Pamela: Exactly. One of the frustrations with a lot of these detectors is you have this event that takes place at high, high up in the atmosphere that causes secondary particles to get generated that cause a Cherenkov light degenerated by all of these different cascading particles so you end up with a lot of these little different cones we use in air detectors. All of these cones from all of these different reactions end up creating this vast… it’s often referred to as a pool of light in the atmosphere that then only part of this pool is getting captured by the air Cherenkov detector like Whipple. It’s a much less precise science when you compare it to, say, the vast array of photomultiplier tubes are used to very precisely look at the doughnuts of light coming out of a single particle reaction within one of the Super Kamiokande tanks. When you’re looking at atmospheric things it’s just a mess but it’s a mess we can turn into science. Then when we were looking at single particles in the swimming pool detectors, essentually, we have these beautiful precise reactions of particles that in some cases have traveled all the way up through the earth. It’s neat to combine all of these different thing to try to learn about… it’s the same process in every case it’s just the same process getting triggered in a variety of different ways.
Fraser: We mentioned the Whipple observatory and how it’s sitting out on the landscape and there are all of these detectors. What do these water tank detectors look like?
Pamela: Basically, take an old mine underground, create a large spherical pool within it, line the walls of the sphere with photomultiplier tubes, fill the whole thing up, close the hatch and hope you never have to go back inside because if you do, something broke. They’re basically giant tanks underground waiting quietly for something to interact inside of them.
Fraser: We did a whole show on neutrinos again and talked a bit more about those tanks, but you’ve got this same situation where you’ve got a neutrino passing through this medium, this water, and you’re hoping that it’s going to interact?
Pamela: It’s again that Cherenkov radiation that we’re looking for. We have so many of these detectors scattered all over the planet. We have the ice cube neutrino observatory down in Antarctica, there’s super Kamiokande in Japan, and then we have the air observatories also scattered about the planet. One of the frustrating things, at a certain level, is with some of these things you’re detecting events where the particles traveled all the way through the planet so you can actually see the various detectors scattered through the world lighting up as these events take place. You can use the speed of light and the variation in time between when the different detectors light up to pin point, vaguely, where in the sky the event came from.
Fraser: I love the fact that you can put your detector on the other side of the earth where the event happened and still detect it and still see the particles making their way through the earth and catch them in your…
Pamela: Neutrinos are annoying that way
Fraser: Right, but you could have a planet Earth a light year across and still detect particles because they’ll go through a light years worth of lead.
Pamela: Neutrinos just don’t like the electromagnetic force. They don’t interact very strongly with anything.
Fraser: So our friend Nicole the noisy astronomer always says that she likes radio astronomy because you can go out and observe in the day and in bad weather, it doesn’t really matter. I think neutrino observers have it taken to the next level.
Pamela: But they’ve had a whole new level of frustration.
Fraser: I want to talk about the big sighs. What are some of the big questions that astronomers have been working on? I think one is just this concept of cosmic rays and what is causing them.
Pamela: Well cosmic rays is one of the issues. We know from taking digital images that there are these random bright explosions in our images where five or six or more pixels will get completely saturated as a cosmic ray hits the detector and overloads those pixels on the detector. We know over time that detectors in space gradually get burnt out by getting hit over and over again with these highly energetic particles. Trying to understand where these suckers are coming from has been a challenge that looking at this allowed us to take on. Looking at this has allowed us to pin point these high energy places in our universe that are accelerating particles generally using magnetic fields. That starts to tell us: Let’s just accelerate more things with magnetic fields. That starts to lead us to concepts like ion drives. Not entirely appropriate but one reasonable analogy to look at is an ion drive is just generating cosmic rays flying out its rear end. The cosmic rays fly in one direction and the spacecraft accelerates in the other.
Fraser: In the case of the cosmic rays that are hitting the atmosphere or these detectors, you’re looking at something that has amounts of energy that baffle the imagination, I mean, there are giga-electron volts.
Pamela: Yeah, nothing like a helium atom that has had its electrons stripped away hitting with enough energy to cause vast arrays of atmosphere to cascade with light and its cool.
Fraser: The challenge is, as you said, these things are hitting the CCD’s, they’re going through the back side of the camera and smashing into the CCD’s so it’ really hard to get a fix on where they’re coming from.
Pamela: Yeah, and not all cosmic rays come from space just to be clear. One of the problems I ran into in graduate school observing at McDonald observatory was the 30 inch telescope I was using, it’s dome was kind of cut into the side of the mountain and we had a radiation from granite issue going on so there was a high energy background of cosmic rays being generated by radioactive decay processes right there under foot. Cosmic rays from space, cosmic rays from ground it’s not the same energy and it’s not the same origin, clearly. Equally annoying on the detector and you really need to have, if everything’s getting saturated, you have to keep building more and more sensitive to higher energy detectors to start differentiating all these things that my little optical detector was getting blown out by.
Fraser: I really think that this is one of the great advances with the Cherenkov detector rays is that you finally can get a fix on these things. We don’t want to ruin the story here, you should definitely go back and listen to our cosmic ray episode, but what turned out to be generating these particles is really neat.
Pamela: Right, so there are things like high energy magnetic fields with active galactic nuclei so you have a black hole busily consuming material and as it’s busily consuming material you end up with the discs spiraling around it because conservation of the angular momentum prevents things from falling straight into a black hole except under very specific special alignments that don’t generally happen in reality. This disc is spitting non-neutral material, generates a very powerful magnetic field and that magnetic field can basically act like an ion drive and fling particles at high velocities. We see these also coming from the discs around different stellar events: white dwarfs, stellar mass black holes, neutron stars. They can all have varying degrees, these different types of jets. We also see this coming out of supernovae from hyper novae and from gamma ray bursts. Our high energy universe that our previous generation of astronomers never would have imagined.
Fraser: We always note how if you ever watch Cosmos, in the first couple of episodes Carl Sagan mentions that we have these quasars but we don’t really know what they are. He offers a few suggestions but now we know it is black holes with millions of times the mass of our sun with these incredible warped up accretion disks around them with these huge magnetic fields and that’s what capable of firing out these particles at such high energy levels.
Pamela: This is very new knowledge. As recently as when I began graduate school, faculty were still drawing small monsters squatting in the centers of galaxies as the cause of AGN’s and it was only towards the end of when I was in graduate school, the beginning of this century, the beginning of the 2000’s, that we have finally nailed down, “Yes there are black holes in the center of galaxies.”
Fraser: I really love this idea of being able to use: you can’t look at the phenomenon directly but you can look at it by some other effect like a reflection or an echo. It tells you as much as you need to know, or you can know about the original event. I think this is a fantastic example of how scientists get super clever about “We gotta figure this out, we can’t look at it directly but maybe there is something else we can see”.
Pamela: Unlike gamma ray telescopes in orbit or x-ray telescopes in orbit, using Cherenkov radiation we start to get additional information because there is this cascade of particles that is getting created and there is this cascade of radiation that’s getting created. In some detectors, specifically the ones used for particle physics we’re able to start getting at both the mass and the energy of what’s creating this Cherenkov radiation. Its starts get to us actual information about the particles involved as well as the direction, the source and the light involved.
Fraser: Well thank you very much Pamela that was fantastic. We’ll see you in a couple of days.
Pamela: Ok we’ll see you in a couple of days.
This transcript is not an exact match to the audio file. It has been edited for clarity.