Brilliant arrays of golden photomultiplier tubes amount to astrophysical ghost detectors.
Image: Super-Kamiokande Photo Gallery/The University of Tokyo
Neutrino detectors don't even look real—or they at least don't look like utilitarian instruments for doing fundamental physics research. They look like elegant sci-fi art installations that one might expect to find in the lobby of a five-star hotel built into the ice encasing a future human settlement on Europa. But their appearance is all utility, which is amazing. It turns out that this is just the best way to go about hunting one of the most unhuntable particles in existence.
First of all, there are in reality a variety of different neutrino detection methods. These mostly vary by what sort of liquid is being used to facilitate neutrino collisions with other particles: water, heavy water, cadmium, germanium, mineral oil. The basic principle is that an incoming neutrino will enter a bath of one of these liquids and will collide with one of the particles making up that liquid, ideally a charged, very lightweight particle like an electron.
Thanks to the added energy of the neutrino, the electron will be accelerated to a velocity higher than the phase velocity of light for the particular medium (water, germanium, whatever). We mostly think of the speed of light in terms of light moving through a vacuum, but in reality different materials have their own speeds of light; the speed of light through water is about three-quarters that of the speed of light in a vacuum, for example. And, unlike light moving through a vacuum, it becomes possible to exceed these localized light speeds if, say, a particle is imbued with some outside energy.
One visible result of this scenario is known as Cherenkov radiation. This is the eerie blue glow most often associated with nuclear reactors. The light is the result of a sort of shock wave analogous to the shock wave caused by a supersonic aircraft—the accelerated electron is moving faster than the surrounding particles can get out of the way, so the particle winds up accumulating a wave in front of it, which is made up of high-intensity particles that we see as blue light.
This is where all of the glass orbs of a neutrino detector come in. These are photomultiplier tubes. Their purpose, as one might guess, is to catch these small flickers and to amplify them. It's possible to get information about a neutrino's energy, momentum, and, sometimes, "flavor" using this method.
If this seems like an extreme way to bag single particle events from what amounts to a continuous neutrino stream passing through Earth, it's because neutrinos are all but particle ghosts. Because they're so light and carry no electrical charge, they interact only weakly with other matter. For the most part, they just cruise through the universe and its residents as though it/we don't exist.
This is the reason for a second crucial feature of neutrino detectors—usually, they're located far underground. Projects like the Sudbury Neutrino Observatory in Ontario and Super-Kamiokande in Japan are located deep within former mines, while the ANTARES telescope is located 2.5 kilometers underneath the Mediterranean Sea. The sensors (above) of the IceCube Neutrino Observatory are located 1,450 to 2,450 meters deep in Antarctic ice, where they're suspended by strings.
Given all of the effort it takes to bag individual neutrino events, why do we even bother? Well, for one thing, because neutrinos are so disinterested in interacting with the universe, they make good candidates for observing phenomena that might otherwise be "cloaked" by its effects on other particles, like photons. For example, it may take 40,000 years for a photon to propagate from the center of the Sun to the surface, but a neutrino could make the trip virtually unimpeded.
And then there's the whole matter of existence itself. It turns out that neutrinos have a habit of oscillating into other sorts of neutrinos and this may be linked to the very difficult question in cosmology of why all of the universe's matter didn't annihilate with all of its antimatter and disappear in a hot blink. Watching neutrinos do their thing is hoped to offer some answers. Seems like a question worthy of lining a giant tank of heavy water with thousands of golden glass eyes.