How do you detect a tiny little particle, much smaller than an electron, that has barely any mass, and no charge? With an ice cube, of course!
Matsu exploring an ice cube
We decided to use the IceCube project as our contribution to the project-sharing that will take place at Escudaro Station in Antarctica, so I thought I could write a summary here on this amazing project. I suggest you visit the following websites if you are interested on learning more about IceCube. Check out IceCube's website, and also the following PolarTREC teachers' websites that have had the opportunity to work with IceCube: Lizz Ratliff, and Katey Shirey.
The JASE team chose to share IceCube with students and teachers from Chile because it is an Antarctic project based in Madison, Wisconsin; next door to our high school. IceCube is actually an international project with 41 institutions participating from 12 countries. It is huge! We have had the luck to meet Dr. Jim Madsen, and his team, who generously have spent time working with us.
Luke, Claire and Juan meeting with Dr. Jim Madsen at Ice Cube offices in Madison, WI
The idea behind IceCube is that there is this extremely small particle lurking the Universe, with barely any mass and no electrical charge, called neutrino. This particle is so close to being nothing that scientist call it "the ghost particle". Neutrinos are tiny, but quite common; trillions of them pass through your extended hand. The neutrino (from the Italian "small neutral charge") can help us explore many important questions in physics, if we are able to detect it. Scientists believe, for example, that neutrinos could help us understand why at the beginning of the Universe matter became more abundant than antimatter. We also believe neutrinos could help us elucidate what dark matter is. At the moment, we do not even know for sure where very high energy neutrinos originate in the Universe. Lower energy neutrinos are produced during fusion reactions that power stars, like our Sun, or more exotic stars like the neutron stars. Bursts of neutrinos come from the explosions of stars at the end of their life cycle, called Supernovae, and others from very violent explosions called Gamma Bursts. The challenge is that neutrinos are very elusive; with their extremely small mass and no charge, they barely interact with anything. If we build a sensor, the neutrino would just go through!
But when there is a will there is a way, so scientists figured out a cool way to detect them. Einstein showed with relativity that nothing can travel faster than light on empty space (vacuum). We say empty space because light slows down when it travels through a transparent medium, like water or ice. Since neutrinos do not interact much, they go through water and ice without changing their speed. This means that if they are close to the speed of light as they enter a block of ice, they will travel in the ice faster than light itself! It turns out that when charged particles travel faster than light in a transparent medium, that charged particle gives of a tiny bit of light. This is called Cherenkov radiation.
If you want to detect charged particles that are traveling faster than the speed of light in ice, all yo have to do is to place a light sensor inside the ice that is kept in complete darkness. The sensor will detect the Cherenkov radiation from that speedy particle. But this light sensor is not an ordinary light sensor like the ones you can buy at Radio Shack. You need a much more sensitive one. IceCube calls its detectors Digital Optical Modules, or DOMs. If you want to know the direction from which the particle came from, you need more than one DOM in the ice so you can compute the time delay between the detection by both sensors.
A Digital Optical Module being lowered into the ice on a string. Courtesy of IceCube/NSF.
"Wait a second!" you might say. "Didn't you say that neutrinos have no charge, and that Cherenkov radiation is given by charged particles traveling faster than light on a transparent medium?" You are quite right, neutrino detection is not that easy. Neutrinos do not give Cherenkov radiation even when they travel faster than light inside the ice because they have no charge, but they can generate other charged particles that travel at the same speed when they interact with matter. Remember we said neutrinos barely interact with matter? The key is in the barely. Out of those trillions of neutrinos that just went through your hand, about every hundred years or so, one will hit the nucleus of an atom. A bunch of other subatomic particles come out of this high speed collision, muons among them. The benefit is that the muon has a negative charge and thus will emit Cherenkov radiation in the ice; the DOM's are be able to detect the light and track the muon. Bottom line, scientists detect neutrinos from outer space by detecting the debris that forms when they crash into bigger particles inside the ice. Since neutrinos do not crash often, we better put a lot of DOMs in a very large chunk of ice if we want to detect those crashes.
Here is a question for you, where would you build a neutrino detector? Let's see; we need a very large chunk of very clear ice, one that has no bubbles, that stays in complete darkness so the DOMs detect only the Cherenkov light. We need to keep the ice for many years without melting. A large freezer anyone? Mmmm, how about the South Pole? Yes!, the South Pole is the ideal location! Even in the South Pole, in order to have very clear ice and very dark, we need to put the DOMs about 1450 m below the ice surface. That is just short of a mile!
The neutrino detector at South Pole is a cube of ice of one square kilometer of area and one kilometer of depth (1 cubic km) that starts, as we said before, 1450m below the surface. It has 5160 DOMs in 86 vertical strings. The question is, how did they embed all those sensors in the ice more than two kilometers below the surface? Easy --- just the way you would poke holes on a large chunk of ice --- with hot water. They used hot water drills that poke more than two kilometers deep vertical holes on the ice. A string with DOMs was then lowered into position. Can you imagine drilling in ice 86 holes of more than two kilometers deep, just with hot water, in one of the remotest and coldest part of the world? What an achievement of technology! How I wish I could visit the observatory.
Diagram of IceCUbe observatory, showing the distribution of strings and DOMs. Courtesy of IceCube/NSF.
In summary, IceCube does not study Antarctic processes, but uses the Antarctic environment for studying the cosmos. The observatory is functioning well, and scientists from all over the world are happily working on the data to uncover more mysteries of our Universe. But don't you worry, there will be plenty mysteries waiting for you to uncover if you decide to become a scientist!