The IceCube neutrino detector was an audacious design. The Super Kamiokande detector had shown that a huge mass of water could act as an effective particle detector. But that involved a giant tank built in a deep mine. IceCube would rely on a massive volume of water, but one that was put in place by nature: the Antarctic ice cap.
Its location poses a large collection of challenges, from how to find hardware that can hold up to being buried in the ice, to how to get the data back out and someplace useful.
The success of the detector—it has given us insight into some of the highest-energy events in the Universe—indicates that these challenges have been overcome. But now IceCube is expanding, with an upgrade in the works and a much larger next-generation detector being planned, posing a new set of challenges. To get a perspective on this one-of-a-kind detector, we talked with John Kelly, IceCube’s detector operations manager and self-proclaimed Ars reader.
Lighting up the ice
While neutrinos only rarely interact with other matter—it’s estimated that you need about a lightyear of lead to stop them—there are a lot of neutrinos around, and IceCube monitors a lot of matter. During construction, teams melted holes deep into the ice, allowing them to run long strings of photodetectors more than two kilometers deep into the ice. Collectively, they monitor over a cubic kilometer of it.
Energetic particles that interact with the ice within that volume give off flashes of light that are picked up by these photodetectors. Those signals are brought up to the surface and transferred to a South Pole datacenter so they can be reconstructed into an event. With careful timing information on when and where flashes of light were detected, it’s possible to work backward to determine if they came from a single source and, if so, figure out that source’s path through the ice.
“IceCube’s main mission was to find high-energy astrophysical neutrinos,” Kelly told Ars. “And we first found these five or six years ago now.” Those high energies are generated by some of the most violent events in the Universe, which accelerate particles to enormous speeds. Some of those particles decay into neutrinos, which can travel between galaxies at essentially the speed of light. If they’re within the right energy range, their collision with Earth’s ice will be picked up by the photodetectors of IceCube.
(Kelly said that there’s a test detector set up to look into the prospects of picking up even higher energy neutrinos using radio radiation, but there’s not enough ice instrumented yet to have picked up any.)
The upgrade, Kelly said, is to focus on lower energy neutrinos. It’s placed inside the existing detector, forming an area with a higher density of detector strings. “Spacing [between detectors] sets the energy scale,” he told Ars. With the new detectors, IceCube will become sensitive to neutrinos produced within our own atmosphere as cosmic rays slam into it. This will help us with the study of neutrino oscillations, the process by which neutrinos shift among their three identities (electron, muon, and tau).
Timing is everything
The other challenge being addressed in the upgrade is timing. To accurately reconstruct a particle’s track through the ice, you need extremely precise measurements of when different detectors picked up the light it produced. This process is critical, because the particle’s trajectory tells us where it came from. That information can be used to direct traditional observatories to do follow-up observations, providing optical images of the same object or event.
Another aspect of the upgrade is trying to put in place hardware that will help scientists calibrate IceCube. “The big uncertainty is the ice itself,” Kelly told Ars. “The ice scatters differently along the direction of the ice flow.” Other complications include the fact that the ice includes layers of dust from volcanic eruptions, and the fact that the area immediately next to the detectors has been melted and refrozen. All of these can influence the travel of photons through the ice.
While you can’t control the ice, you can minimize other sources of uncertainty in the timing. Right now, all the wiring for the system is copper, which Kelly described as “not great” in terms of power use (which is more of an issue at the South Pole). Copper also causes some signal dispersion, so the team is in the process of replacing the surface-level connections with optical ones. “Having something refreeze into a two kilometer deep hole is a harsh environment—we haven’t figured out how to get optical fibers to survive in that,” Kelly said.
Even at the surface, though, the environment is far from ideal. “Cables and fiber need to survive down to -70°C on the surface,” Kelly said. “A PVC coated cable will disintegrate within a few minutes.” In addition, actual humans need to be able to install and maintain the hardware, so you need connectors that can be manipulated with gloved hands. Plus all of it has to get to the South Pole in the first place. “Any time we think of building something new, we have to have a conversation with the National Science Foundation and its contractor,” Kelly said. “How many cargo flights [will this take] etc.”
“We have a lot of freezers” Kelly laughed when asked about how they made sure everything would work once it got to the South Pole. He’s been working with a company called Clearfield, which told Ars that its hardware is designed to handle conditions in places like Northern Minnesota. There, the temperatures can hit -35°C in the winter and shoot up to 30°C in the summer.
IceCube’s discoveries came at an interesting time for astronomy. After having been limited to detecting photons for its entire existence, IceCube had provided the field with a completely new way of detecting events; it was followed shortly after by the detection of gravitational waves. While the proximity in time was purely a matter of chance, it signaled the arrival of what has been called “multimessenger” astronomy: the combination of optical signals with some information on the physical processes that generate them.
The signals from photons are critical, Kelly said, because “neither non-optical option has the resolution needed to identify a source.” But that data doesn’t necessarily tell you what’s producing a photon. A gravitational wave signal can identify the massive objects that are behind a given event (though it can’t link their collision directly to photon production). While Kelly said high-energy neutrinos can arise “anywhere you have high energy particle acceleration,” the list of possible sources is long: “shock waves, supernova remnant shells, flares, active galactic nucleus jets—any of those could potentially produce cosmic rays and neutrinos.”
We’re still trying to figure out which of these is producing most of the neutrinos we see, but IceCube has already determined that the optical signals associated with gamma ray bursts aren’t accompanied by neutrinos, suggesting they’re not a major source.
And plans are underway for a greatly expanded IceCube, far beyond the upgrade that’s in the works. Referring to IceCube Gen 2, Kelly said, “it’s where we go big.” Ten times the size, more sensitive optics, and multiple sensors will make sure we get more out of every neutrino that interacts with the ice.