Neutrinos are some of the most elusive particles that are well-known to science. These tiny subatomic particles have no electric charge and an extremely small mass, making them incredibly difficult to detect. They are produced in abundance by the sun, as well as by nuclear reactions on Earth and in supernovae. Despite their elusive nature, scientists are keen to detect neutrinos as they can provide valuable information about the processes that produce them.
Neutrinos interact with matter so rarely that it takes a very special kind of detector to catch them in the act. These detectors come in a few different flavors, each employing its unique method to spot these elusive particles. In this article, we’ll take a closer look at how these detectors work and some of the most notable examples of neutrino detectors in the world today.
Patience and Scale
Modern physics tells us that around 100 trillion neutrinos pass through your body every second. You’d think being so common would make these particles easy to find, but it’s anything but the case. These ultralight uncharged particles interact with matter so rarely that detecting them requires a rather specialized experimental setup.
The first successful neutrino detection was achieved in 1956 by Frederick Reines and Clyde Cowan. Two targets were created, using a solution of cadmium chloride in water, with scintillation detectors placed next to the targets. Antineutrinos from a nuclear reactor underwent an “inverse beta decay” with protons in the water. This reaction saw the proton turn into a neutron, and the antineutrino forming a positron. The positron quickly annihilated with an electron, releasing a gamma ray, while the neutron was captured by a cadmium nuclei, itself releasing a gamma ray a few microseconds later. By capturing the gamma ray signature of these events, the duo proved a successful detection of an antineutrino, which would later see them awarded the Nobel Prize in 1995.
This method was useful for detecting neutrinos, but little more than that. To learn more about the universe, physicists needed to study neutrinos in greater detail, determining their natural sources, their interactions, and their behaviour. Thus, a variety of more advanced detectors have been built over the years. Many of these are at a grand scale, involving hundreds of tons of this, or thousands of tons of that. The sheer scale is often required to capture a rare interaction with a neutrino, given their propensity to pass through great expanses of material without any interaction whatsoever.
A more modern and popular method of neutrino detection is via Cherenkov radiation, which has netted scientists richer information on neutrinos and their origins. When a neutrino moves faster than the speed of light in a given material, like water, Cherenkov radiation is produced in a sort of optical shockwave, analogous to an airplane breaking the speed of sound in air. The ring of light released can be detected with simple photomultiplier tubes. With an appropriate array of photodetectors, it can be possible to determine the direction and energy levels of incident neutrinos.
These detectors use large tanks filled with water, heavy water, or oil, and are equipped with sensors that can detect the faint flashes of Cherenkov radiation produced when a neutrino interacts with matter. A prime example of a water Cherenkov detector is the Super-Kamiokande in Japan, a massive underground tank holding 50,000 tons of ultra-pure water, lined with 11,000 photomultiplier tubes. The whole experiment is buried a kilometer underground, helping to shield it from other natural phenomena like cosmic rays. It’s set to be superceded by the Hyper-Kamiokande in coming years. Another example is the Sudbury Neutrino Observatory, with 1,000 tons of heavy water surrounded by a cylinder of regular pure water. The experiment is able to capture the gamma rays released when a neutrino breaks up a deuterium atom in the heavy water.
The MiniBooNE detector works on the same scintillation principal, but uses oil as its medium instead of water. It was designed to investigate the concept of neutrino oscillation, where neutrinos change between several “flavors” over time. The experiment hunted for the signature of an electron neutrino hitting a neutron—which would generate an electron plus a slow-moving proton—which occurs rarely. This would be contrasted against the signature of more common events where muon neutrinos struck protons, creating a muon and a proton. These different events can be determined by patterns of light detected by the experiment’s photomultipliers, as different particles at different speeds create their own telltale patterns of Cherenkov radiation.
Other Cherenkov detectors eschew the use of a giant purpose-built container, instead electing to take advantage of naturally-existing bodies of water or ice. The Antares experiment sits at the bottom of the Mediterranean Sea, 2.5 km below the surface. As a result, it has to filter out light caused by things like radioactive decays from potassium-40 in sea salt, and bioluminescent organisms. Meanwhile, at the South Pole, the AMANDA and IceCUBE experiments use photodetectors in holes drilled deep into the ice.
Time Projection Chambers
Time-projection chambers represent another method of neutrino detection, where a neutrino interaction ionizes atoms in a gas or liquid, and the resulting trail of charged particles is then detected. The DUNE experiment in South Dakota, USA, currently under construction, is an example of a project employing this method. With its four massive detectors, each holding thousands of tons of liquid argon, DUNE aims to study neutrinos with unprecedented precision. The density of the liquid argon helps increase the chances of an interaction with a neutrino.
Time-projection chambers use light detection just like scintillator experiments, but also go further. The chambers use a cathode plane to create an electric field across the chamber. On the opposite side of the chamber, there are multiple planes of parallel anode wires. Inner planes are typically termed as induction grids, which allow so-called drift electrons from neutrino-particle interactions to pass by. If a drift electron passes by a wire in the induction plane, it produces a bump in the current in the wire which can be picked up. Beyond the one or more induction grids, a collection grid then picks up electrons directly, outputting a signal for collection. The benefit of having multiple planes of anode grids is that it allows a two-dimensional reconstruction of an ionization event to be made as an electron is picked up moving by the multiple grids.
If you’re less interested in directionality and more interested in quanitities of neutrino interactions, liquid metal can be a useful tool. In a gallium detector, neutrinos pass through a tank filled with, wait for it, gallium. Thanks to the neutrino-induced inverse beta decay reaction, the neutrino impacting the gallium atom sees one of the atom’s neutrons become a proton, turning gallium into germanium and releasing an electron. The produced germanium can be chemically extracted and due to its instability, its decay can be detected with proportional counters.
The GALLEX experiment in Italy is one of the notable examples of gallium-based neutrino detectors. Located deep underground to shield it from cosmic rays, the GALLEX detector consisted of a tank holding 30 tons of liquid gallium, and ran from 1991 to 1997. This experiment played a crucial role in studying solar neutrinos, and its results have significantly contributed to our understanding of the Sun and its processes. It’s successor was the Gallium Neutrino Observatory, which was in operation from 1998 to 2003. The Soviet-American Gallium Experiment, or SAGE, was another long-running gallium-based neutrino detector. These experiments were prized for their ability to detect low-energy neutrinos, though they were expensive due to their requirement for many tons of gallium, either in liquid metal form or as a gallium trichloride-hydrochloric acid solution.
The nature of particle physics today is that a wide variety of large-scale experiments are needed to investigate all manner of phenomenon. This is by no means an exhaustive list of neutrino detection methods, but instead a guide as to the many methods that can be used to hunt down these elusive particles. New and more exciting detectors will be built, and hopefully reveal to us more secrets about the sub-atomic particles beyond the basic ones we learn about in high-school physics. If you’re currently studying particle physics at the university level, you may yet find yourself working on one of these advanced neutrino projects!