Neutrinos are extremely tiny particles with a neutral electrical charge, a half-integral spin that rarely reacts with matter, and a mass close to zero. Once we have the means to study them, they may unlock many of the mysteries of physics around the universe.
The problem with measuring these tiny particles is that they do not participate in the strong force, making their gravitational interaction very weak, so that they typically pass through normal matter unimpeded and undetected. In fact, most neutrinos pass through our entire planet without ever bumping into another molecule. This poses some interesting issues in detecting and measuring neutrinos and their interactions. It also means that observing neutrinos requires some high tech equipment, which is now being put in place at neutrino observatories.
Neutrinos are generally created through radioactive decay of nuclei, nuclear reactions in stars, from supernovas, or when cosmic rays strike atoms. The ways that neutrinos are created mean that observing neutrinos can allow also us to learn things about distant stars and distant nuclear reactions. We can also learn things about our own sun, the star closest to us, as that's a significant source of neutrinos that pass through the earth.
Another important thing to note about neutrinos is that for each of them, a corresponding antiparticle exists. These particles are called antineutrinos, which have some different characteristics from their corresponding neutrino (lepton number, opposite chirality, etc.), but we won't get into those specifics here – back to figuring out how to observe and measure these incredibly tiny neutral particles.
What are neutrino observatories?
Neutrino observatories consist of a bunch of neutrino detectors, which are apparatuses designed specifically to study and detect neutrinos. Drawing back to the earlier problem that we discussed – that neutrinos don't usually interact with other particles – neutrino detectors have to be incredibly large to detect anything significant. They also have to be built in locations with low background noise, such as underground, underwater, or under the ice, in order to isolate the detectors from other cosmic rays and radiation.
All of this is more practical than you think though, as neutrino observatories and the detectors therein lend themselves to a field which still finds itself in its infancy: neutrino astronomy. Through the study of neutrinos, we can learn a great deal about our universe. The study of neutrinos is on the cutting edge of new physical discovery.
To date, neutrino detectors have really only been able to confirm two extraterrestrial sources of neutrinos, the sun and a supernova that goes by the name of 1987A, but this could quickly change as more neutrino observatories are built and are operational for longer periods of time.
Many objects we can observe visually throughout the universe can appear to be distorted. Or rather, the data we receive is not telling the full story. This is because photons generated by steller events, such as supernovae, are absorbed on their journey. However, since neutrinos don't interact with other matter, and can penetrate gas and dust as they travel, studying neutrinos could allow astronomers to identify and study the phenomena that generate them.
In fact, it is estimated that roughly 20% of the universe is missed when just measuring through existing means. Neutrino observatories could unlock that missing 20%.
How do neutrino observatories work?
Neutrino observatories have one main function, to detect and measure neutrinos. With that goal in mind, there are actually many different paths for these observatories to take to get there.
One observation technique is utilizing a piece of equipment known as a scintillator. Scintillator detectors use materials that show scintillation - a type of luminescence that occurs when particles are excited by ionizing radiation.
These detectors can be put in place, usually underwater, and when antineutrinos — the associated anti-particles to neutrinos — pass through them with a high enough energy, they can spark a series of interactions, resulting in the release of coincident photons that are measurable.
The scintillation technique is generally used to study neutrinos generated by nuclear reactors, as only a very small number of neutrinos carry enough energy to be detected on this equipment, meaning that realistically terrestrial sources like nuclear reactors are the only strong-enough neutrino sources.
In addition to scintillation, researchers can also use chlorine detectors. Tanks filled with chlorine-containing fluid will be occasionally affected by neutrinos, meaning that some of the chlorine atoms will be turned into argon-37. This argon can be periodically filtered out and the states and quantity of the isotope can be measured.
Cherenkov detectors are also another way of detecting neutrinos. These utilize the principle of Cherenkov light, named after Nobel prize-winning physicist Pavel A. Cherenkov.
Cherenkov detectors incorporate a large volume of clear material, like water or ice, which is then surrounded by light-sensitive, photomultiplier tubes. As neutrinos move through the clear medium at speeds greater than the speed of light, a shockwave of Cherenkov radiation is produced. This radiation can be picked up by the photomultiplier tubes, the data from which can then be interpreted to determine the direction, energy, and other characteristics of neutrinos.
Radio detectors also utilize clear mediums like ice to detect Cherenkov radiation from neutrinos. In this case, however, a detector known as an impulse transient antenna is flown over large ice sheets, typically in Antarctica, in order to measure ambient radiation from high-energy neutrinos interacting with the ice below.
What is the largest neutrino observatory?
Currently, the largest neutrino telescope is found on — or in — the South Pole, and is essentially made out of a giant cube of ice. This observatory, known as the IceCube Neutrino Observatory, was recently completed and marks a massive joint project between the National Science Foundation and many educational institutions.
This giant telescope is made up of 86 drilled holes and 5,160 optical sensors placed in the south pole ice to form the main parts of the detector.
The detectors themselves watch out for muon-neutrino and other types of charged neutrino (leptons), which are created from collisions between neutrinos and water molecules in the ice. If these charged particles are energetic enough, they will emit Cherenkov radiation. This happens when the charged particle travels through the ice faster than the speed of light in the ice. The light can then be detected by the sensors in the digital optical modules making up IceCube.
Muon-neutrinos maintain the direction of the original neutrino, meaning that by observing and tracking these particles, the observatory can map out the "path" of the neutrino throughout the universe.
The observatory consists of around a kilometer of ice and a number of surface buildings for the crew. The large area and volume of ice increase the chance for the researchers to collect data.
Where are other neutrino observatories located?
Because neutrinos are some of the hardest cosmic particles to detect, their detectors have to be built in the best possible locations. This means very elaborate and expensive detector arrays, and you certainly don't want to build these arrays in the wrong location.
To get a grasp of all of the different neutrino detectors that have been built, let's take a look at the most prominent ones currently in operation.
IceCube - South Pole
We've already spent some time discussing the IceCube observatory in Antarctica, so we won't go into too much detail in this section. Let's just mention the last notable thing about the observatory: its cost — $271 million. This amount was funded through university grants from around the world, along with grants from the National Science Foundation. In 2013, IceCube researchers reported intercepting the first extragalactic neutrinos.
NOvA - Ash River, Minnesota, USA
NOvA is a neutrino detector that's located in Minnesota. Specifically in Ash River, this long-range detector monitors neutrinos that are produced all the way over in Illinois. Specifically, neutrinos that are produced from the Fermilab particle accelerator. Rather than functioning as a cosmic detector of neutrinos to study the universe, the NOvA detector was designed to study the neutrinos themselves and to further our knowledge of neutrino observation.
Another aspect of the NOvA that is notable is the cost, roughly $267 million, which corresponds fairly closely in cost with that of the IceCube detector. As you can tell, neutrino observatories aren't cheap.
Super-Kamiokande - Hida, Gifu Prefecture, Japan
The Super-Kamiokande detector, also known as T2K, is a long-distance neutrino detector, which measures the particles from the J-PARC lab 183 miles (300 km) away. Specifically, the T2K measures antineutrinos using photomultiplier tubes placed in water. Coming in at a total cost of $100 million, this detector is on the cheaper side of neutrino detectors.
OPERA - Gran Sasso underground labs
Then, we have the OPERA detector, which has detected some of the rarest neutrinos to ever have been studied. This detector array specifically looks at the oscillation of the neutrinos from CERN in Switzerland. The neutrinos it picks up travel roughly 450 miles (725 km) until they are detected and studied.
Coming in at a cost of $160 million for initial construction. this detector array finds itself in the middle ground of affordability. You know, in case you were in the market for a new neutrino detector array to call your own.
Coming soon - the P-ONE
Finally, astrophysicists are planning to build a neutrino telescope even larger than IceCube, on the seafloor off the coast of Canada. The Pacific Ocean Neutrino Experiment (P-ONE) will consist of seven groups of 10 detector strings and a depth of around 1.6 miles (2.6 km), and covering a volume of around 106k cubic ft (3 km3). Its aim is to detect rare, higher-energy neutrinos. After initial exploration, two initial strings of light emitters and sensors were deployed in 2018, and the first part of the observatory is planned to be installed around the end of 2023. If that is successful, the researchers hope to raise the estimated $200 million USD needed and complete the project sometime around the end of the decade.