Physicists around the globe affiliated with the IceCube Neutrino Observatory project received an early Christmas present last weekend, when the final strand of particle detectors was lowered into the Arctic ice, capping six long years of painstaking construction in one of the harshest research environments on Earth.
It’s the culmination of a dream dating back to the 1970s, when physicists first conceived of building a massive neutrino detector spanning a full cubic kilometer.
Neutrinos are tiny subatomic particles that travel very near the speed of light. The two most defining features of neutrinos are that they have no charge and, until quite recently, physicists believed they had no mass.
They’re extremely difficult to detect, because they very rarely interact with any type of matter, even though they’re the most abundant type of particle in the known universe.
Only one out of every 1,000 billion solar neutrinos would collide with an atom on its journey through the Earth. Isaac Asimov dubbed them “ghost particles.”
There are three different kinds of neutrinos (electron, muon and tau). The most common are the solar neutrinos that come from our own sun — specifically, the nuclear processes taking place at its core.
When a neutron inside an atom decays, it produces a proton, an electron and a neutrino. This occurs hundreds of billions of times every second in the core of stars like ours, as hydrogen is converted into heavier elements like helium, releasing huge amounts of energy in the process. Trillions of neutrinos are produced by the sun every day.
IceCube consists of a 86 strings, buried nearly two and a half kilometers in the ice. Each of those strings contains 60 detectors, the size of basketballs, called Digital Optical Modules (DOMs), designed to detect and amplify the faint flashes of blue light known as Cherenkov radiation — the result of a “shock wave” that occurs when neutrinos collide with oxygen atoms in the ice and turn into fast-moving muons.
Those collisions are extremely rare: Even though trillions of neutrinos course through the ice all the time, IceCube will only be able to detect a few hundred per day.
These are not ideal work conditions for scientists: average temperatures are always below freezing, and during the winter temperatures can drop to 100 degrees below zero Fahrenheit (-73 degrees Celsius). Most aircraft can’t land on the ice at those temperatures, so the IceCube teams can only work during the Antarctic summer, which runs from November through February.
Air pressure is low, so it can be difficult to breathe until the body acclimates. Also, at those lower pressures, water evaporates more quickly, so it’s easy for scientists to become dehydrated. Here’s how they get their drinking water: “A heated water loop driven into an ice cavern melts a bulb into the ice. The melted water is then siphoned out for drinking and other water uses.” (The IceCube scientists maintain a blog, so you can follow along with their Antarctic adventures.)
Getting all those sensors into the ice was no picnic, either. The researchers used an Enhanced Water Drill, which can bore cleanly through more than two kilometers of ice in less than two days using hot water. Once a hole was drilled, they could attach the optical sensors to cables and lower them into the hole. Then the ice froze back over the strings. (Fortunately, fully 98% of the more than 5000 DOMs are working perfectly, since they can’t be accessed for repairs or service once installed.)
Given the short “season” at the South Pole, the drill and deployment teams worked around the clock in shifts to make the most of their limited time on location.
Why go to all this trouble? Well, the polar ice is very pure, optically transparent, and free of radioactivity, which can add a lot of extra background “noise” to the data. These qualities make the ice of Antarctica the ideal medium for neutrino detection.
IceCube is designed to look for neutrinos through the Earth, using the planet as a filter to disregard muons from cosmic rays — which make up most of the muons picked up by the detector. But unlike cosmic rays, neutrinos can pass through the Earth unhindered. It’s only when those rare collisions occur that muons from neutrinos are detected. And those muons would be traveling upward. (See image at right for a simulated track of a high-energy neutrino moving upward through IceCube.)
Originally, IceCube was designed to have only 80 DOM strings, but in 2009, the collaboration decided to add six more strings right smack in the middle of the array, at shorter intervals (seven meters apart). Called DeepCore, this smaller, denser array should be able to detect very low-energy neutrinos as well as neutralinos (candidates for dark matter particles), and to study neutrino oscillations — how neutrinos change “flavors” as they travel through the Earth.
IceCube started recording data in 2005, when the first sensor strings were deployed, and has continued to do so as subsequent strings have been deployed. But with the project finally complete, scientists will be able to collect data from the fully operational telescope, with much greater sensitivity.
“Already, IceCube has extended the measurements of the atmospheric neutrino beam to energies,” principal investigator Frances Halzen of the University of Wisconsin, Madison, said in the NSF press release. “With the completion of IceCube, we are on our way to reaching a level of sensitivity that may allow us to see neutrinos from sources beyond the sun.”
Image credit: NSF/B. Gudbjartsson