How are Neutrinos Detected?
Updated: 2 hours ago
Recap from the first article
In the first article on neutrinos, the reasons that they are so difficult to detect were presented. There are only two interactions in which it is possible to be certain which type of neutrino caused it.
A neutrino interacts through the weak nuclear force with a DOWN-quark in a neutron, changing it to an UP-quark. The neutron becomes a proton, provided that the neutrino has enough kinetic energy to become the corresponding charged lepton – the electron.
An anti-neutrino interacts through the weak nuclear force with an UP-quark in a proton, changing it to a DOWN-quark. The proton becomes a neutron, provided that the anti-neutrino has enough kinetic energy to become the corresponding charged anti-lepton – the positron.
All three types of neutrino can interact with protons and neutrons as defined above and the interaction is mediated by a positively charged W boson. It's the high speed electron or positron that gives away the identity of the incoming neutrino.
Neutrinos can also engage in scattering events with electrons or nucleons, where energy is transferred from the neutrino to the target through a weak nuclear force interaction, mediated by a neutral Z boson. However, this interaction does not provide a mechanism for determining which neutrino type was scattered.
The weak nuclear interaction operates over an extremely short range, roughly 10-18 metres and that's why all three interactions are so rare.
1. How are Solar Electron-neutrinos Detected?
Billions upon billions of neutrinos pour out of the sun every second and theoretically, it was expected that 100% of them would reach earth and pass straight through it. As we'll see in a short while, this is not the case - there appears to be a massive deficit!
The odd one or two solar neutrinos might penetrate a nucleon and get close enough to invoke the weak nuclear force and change a quark's flavour, or engage in a scattering event.
Consequence 1: To detect neutrinos coming from the sun, the detectors must be huge structures to try and catch as many ghost-like neutrinos as possible as they pass through.
Consequence 2: The detectors need to be shielded from other particles created in cosmic ray collisions in the upper atmosphere. As a result, the detectors are built under mountains, underground or at the bottom of oceans. All the earth or water above and the earth itself below the detector, act as filters, removing many of the unwanted background particles.
Neutrino Charged Current (CC) Interactions
The electron produced in a CC event, ends up with substantial kinetic energy, which can be detected through Cherenkov radiation in water. This is known as a charged current interaction.
So, build a huge container and fill it with heavy-water, consisting of water molecules made up from deuterium, which contains one proton and one neutron. The deuterium's neutron is an excellent target for solar electron-neutrinos.
All you need to do next, is surround the huge heavy-water container with Cherenkov detectors to catch the characteristic blue light that results from electrons, created in the CC events, moving faster than light moves in water. These detectors are known as photomultipliers and thousands of them are required to surround the giant tank to detect the cones of Cherenkov radiation. The cone of light from a single CC event is depicted in the left-hand side of the image at the top of the article.
Solar muon and tau neutrinos do not have enough kinetic energy to allow the neutrino to transform into the corresponding charged lepton and so, cannot engage in CC interactions. This allows solar electron-neutrinos to be studied in laboratories like the Canadian Sudbury Neutrino Observatory (SNO), two kilometres underground in a nickel mine, in Sudbury Ontario.
When an electron-neutrino interacts with the neutron of a deuteron nucleus, the end result can be two protons and the deuteron nucleus is broken apart. Consequently, from a strictly nuclear point of view, the interaction can be written as;
νe + d → p+ + p+ + e-
Where “d” represents a deuteron nucleus, consisting of one proton and one neutron.
How are muon and tau neutrinos from the sun detected?
Muon and tau-neutrinos from the sun do not have sufficient kinetic energy to cause CC interactions. This makes the CC interaction in heavy water, specific to solar neutrinos. However, all three neutrino types do engage in scattering events, where energy is transferred from neutrino to the target, but none of the particles change their flavour.
To study the total flux of all three types of neutrinos coming in from the sun, salt (NaCl) is added to the heavy-water to improve the efficiency of scattering detections.
Neutrino Neutral Current (NC) Interactions
High-energy neutrino scattering can smash apart deuteron nuclei, releasing the proton and the neutron without changing their type. This is referred to as a neutral current interaction. After loosing kinetic energy, the neutron becomes thermal, and can be captured by a nucleus in the heavy-water.
The scattering event must occur within the weak nuclear force's range, and a Z0 boson mediates a transfer of energy from the neutrino to the deuteron nucleus, releasing the proton and the neutron. This requires a minimum neutrino energy equivalent to about 2.2 MeV.
This is completely separate to the CC interaction, but has the effect of producing free neutrons and all three neutrino types engage in this type of scattering event.
Adding salt (NaCl) to the heavy-water, results in lots of chorine nuclei being present and chlorine nuclei have an affinity for absorbing thermal neutrons. This increases the efficiency with which NC interactions can be detected.
The neutrons captured by the chlorine nucleus of the salt solution, excite the nucleus, which produces, a cascade of gamma rays with a total energy of about 6 to 8 MeV. The gamma rays then scatter electrons in the heavy water and accelerate them to velocities greater than the speed at which light travels in the heavy water, producing Cherenkov radiation in all directions as rings. So, the Cherenkov signature of an NC event (see depiction below) is quite different to the single cones of the CC interaction, shown in the left-hand image at the top of the article.
The photomultipliers around the tank pick up a ring of Cherenkov light from each NC interaction and a cone of light from each CC interaction. From the Cherenkov rings, the total flux of all three types of neutrinos can be indirectly detected from NC interactions.
The Sudbury Neutrino Detector (SNO) in Canada
This is a real case-study of neutrino detection.
The observatory was built to solve a decades old mystery: early experiments showed that only about 35% of the expected number of electron-neutrinos coming from the sun, appeared to make it to earth. This begged the question; what happened to the 65% that didn't make it?
The SNO was able to detect the total flux of solar electron-neutrinos from CC events (the CC channel) and also the total flux of all three types of neutrinos coming from the sun from NC events (the NC channel).
The CC channel produced Cherenkov light cones and the NC channel produced Cherenkov rings. The almost 10,000 photomultipliers arranged around the detection tank picked up both types of blue light. Due to the different Cherenkov signatures of the resulting light, it was possible to separate the two sets of data.
Result 1: The electron-neutrinos reaching earth confirmed that it was about 35% of the theoretical amount that should have been detected.
Result 2: All three types of neutrino reaching earth agreed with the theoretical prediction of how many electron-neutrinos should be reaching earth.
Conclusion: This was conclusive proof that the solar neutrino deficit was due to the fact that 65% of electron-neutrinos had changed flavour in the journey from the sun to muon or even tau neutrinos.
The data was accumulated in three phases over a seven year period to reach the final conclusion that the missing electron-neutrinos had oscillated. This was further confirmed by a Japanese laboratory, the Super-Kamiokande experiment, that studied muon-anti-neutrinos produced in the upper atmosphere from cosmic-ray collisions. The experimental results showed that many upward-travelling muon-anti-neutrinos coming from the atmosphere on the other side of the planet, passing through the earth, vanished when compared to downward-travelling muon-anti-neutrinos produced immediately above the experiment. One again, the missing neutrinos in the upward-flux pointed to oscillation. This resulted in a joint Canadian/Japanese Nobel prize in 2015.
2. How are Electron-anti-neutrinos Detected?
When an electron-anti-neutrino creates a charged current (CC) interaction (also known as an inverse beta decay interaction) with a proton, it changes itself into a positron to preserve charge. Once the positron slows down and becomes thermal, it will soon find a nearby electron and the two annihilate each other, resulting in two characteristic gamma rays, each of 511 keV. If both particles were almost at rest, the gamma rays would be produced in opposite directions at 180 degrees. However, as the particles are rarely at rest when they collide, their net momentum creates a tiny deviation to the 180 degrees.
This interaction is depicted on the right hand side of the image at the top of this article. Detection is provided by scintillator-photomultiplier combinations arranged around the tank. A scintillator absorbs the energy of incoming gamma-rays, emitting flashes of light or scintillations, which are then picked up by the photomultiplier tube (PMT). The PMT produces a voltage pulse proportional to the amount of energy absorbed to create the scintillation. Flashes of light are also emitted when particles give up their energy in the scintillating medium and this includes the high-speed positron, before it annihilates with an electron.
Scintillators produce a lot of background flashes of light from spurious radiation and particles giving up their energy in the scintillator medium and this must be kept to an absolute minimum. Techniques are also used to create unique electronic signatures from the CC interactions to separate the event from spurious events produced by cosmic ray collision debris, as explained in a short while.
The First Ever Anti-neutrino Detection
This is a case study for anti-neutrino detection.
The original experiment, performed in 1956 by Cowan and Reines, to prove the existence of anti-neutrinos for the first time, involved two water targets containing dissolved cadmium chloride, sandwiched between three very large liquid-scintillator detectors. The total volume of the water solution amounted to about 400 litres. The water was used to detect anti-electron-neutrinos coming from a nearby nuclear reactor.
The liquid scintillators detected the energy deposited by the two gamma rays resulting from the annihilation of a positron and an electron, emitting flashes of light, but so too did the high-speed positrons before slowing down and becoming thermal. All light flashes were then directed into photomultipliers, which produced a voltage pulse proportional to the amount of energy deposited in the scintillator.
The experimental setup is as shown here;
Using two water tanks sandwiched between three scintillators turned a simple flash detector into a geometric camera, allowing the two physicists to determine in which tank a genuine CC event had occurred. If two flashes were detected in the top & middle detectors at exactly the same time and of the right energy, a CC event must have occurred in tank 1. If the event occurred in tank 2, then two flashes were detected in the middle and bottom detectors at exactly the same time and correct energy.
Using only a single tank and two detectors, any high-speed particles could produce flashes up and down that would appear to be possible CC events. So the ability to detect which tank created the event allowed many spurious scintillations to be ruled out. Remember, the two photomultiplier pulses had to be received at exactly the same time and be of the right energy, 511 keV.
To make absolutely certain that they had detected a neutrino event, the neutron from the CC interaction was used to help verify the interaction. As the neutron slowed down and became thermal, it was readily absorbed by a cadmium nucleus. This results in the excited cadmium nucleus releasing a cascade of gamma rays with a total energy of about 9 MeV, as it regains its ground state.
So, look for two separate gamma ray detections in two of the detectors sandwiching one of the water tanks, occurring at the same moment in time and of the right energy. Then, look for the gamma ray cascade released by the cadmium, shortly afterwards, at about 9 MeV into the detectors.
This sort of coincident experiment can be set up easily in electronics (or nowadays using computers) monitoring the signals produced by all three detectors. This precise signature is conclusive proof that an anti-neutrino event has been detected as the probability of some other spurious event(s) producing such a precise electronic fingerprint is very low.
In 1956, Cowan and Reines set up such an experiment, underground, not far from a nuclear reactor and by carefully checking the results, verified the existence of electron-anti-neutrinos for the first time, leading to a Nobel prize.
In fact, they only counted one to three anti-neutrino events each hour on average! To ensure that they were detecting them from the nearby reactor, it had to be shutdown to verify that the several events per hour were being generated by the reactor's anti-neutrinos and not from any other source. The rate dropped to a statistically significant level when the reactor was shut down, verifying that the reactor was the source.
Only Reines received the Nobel award in 1995 as it is not awarded posthumously and sadly, Cowan died in 1974.
Large Scale Detection of Anti-neutrinos
For large scale experiments, pure water is replaced with a scintillation fluid, which creates flashes of light when the energy of a gamma ray or particle is absorbed in the medium. Once again, the container must be shielded from cosmic ray debris and surrounded by photomultipliers to detect the flashes of light produced in the scintillation medium by the annihilation of the CC produced positron and a local electron. Liquid scintillators replacing water targets produce 100 times more light than the original method used to detect anti-neutrino events in pure water containing dissolved cadmium chloride. The scintillation medium is made up of hydrocarbons, which contain plenty of proton (hydrogen nuclei) targets.
As an example, the KamLAND (Kamioka Liquid Scintillator Antineutrino Detector) site in Japan, is about 180 Km from a ring of 53 industrial nuclear reactors and performs experiments on low energy anti-neutrinos produced by the nearby reactors. The detector uses 1,000 metric tons of liquid scintillator, which provides both proton targets and detection of the annihilation gamma-rays. The positron annihilation provides the two coincident detections of the 511 keV gamma rays and a delayed gamma ray is produced when the neutron from the CC event is absorbed by a hydrogen nucleus in the scintillator medium to form a deuteron nucleus.
Because the reactors are at a known distance from the kamLAND detector, physicists were able to calculate the number of electron-anti-neutrinos that should be reaching the detector from the reactor cores. They detected a massive discrepancy between expected number and actual number detected, just like the solar neutrino deficit detected by SNO.
However, the value detected by kamLAND, changed over time in a wave and indicated conclusively that electron-anti-neutrinos oscillate and therefore must have mass.
This was achieved by determining the total flux of electron-anti-neutrinos using what is referred to as the "inverse beta decay" channel (also termed the CC channel) over a 5 year period of time and statistically analysing the results to detect a wave pattern in the number of events detected, from a minimum to a maximum and back to a minimum over a lengthy period of time. This effect can be attributed to the phase difference of each neutrino type in the superposition of neutrino flavours.
How Come SNO didn't Detect Wave Variations in the results?
Good question, as SNO ran for 7 years and the kamLAND experiment for just 5, before detecting the wave nature of the measured flux.
High-energy solar neutrinos behave differently to the neutrinos produced in reactors due to the incredibly dense environment in which they are produced. This is explained by the Mikheyev-Smirnov-Wolfenstein effect inside the sun. If you want to understand this effect in more detail, just Google the bold text.
3. Final Takeaways
Because neutrinos are so difficult to detect, detectors need to be huge and buried under the earth or a mountain or an ocean to filter unwanted particles produced by cosmic ray collisions in the upper atmosphere.
Neutrino Charged Current Interaction: occurs when a neutrino interacts with a neutron changing it to a proton, the neutrino becomes its negatively charged lepton partner. The neutrino is not detected directly, but the high speed lepton is and can be used to verify a CC interaction has occurred.
Solar neutrinos can be detected indirectly from CC interactions in heavy-water from their Cherenkov, blue-light signature. Solar muon & tau neutrinos do not have sufficient energy to trigger CC interactions.
Neutrino Neutral Current Interaction: occurs when a neutrino of any type, smashes apart a deuteron nucleus releasing a proton and a neutron. The incoming neutrino and the freed proton and neutron are unchanged.
By adding salt to heavy-water, the NC interactions created by high energy neutrinos, of all three types coming in from the sun, can be detected more efficiently, when the neutron is absorbed by a chlorine nucleus. This allows the total neutrino flux (electron, muon and tau) to be monitored as the Cherenkov light is produced in random directions by the cascade of gamma rays scattering electrons in the medium in all directions. Once again, the actual neutrinos are not detected directly.
Electron-anti-neutrinos are not detected directly, but by monitoring the two gamma rays produced in opposite directions when the positron, produced in the CC event, and a neighbouring electron are annihilated. This can be achieved in pure water, surrounded by scintillator-photomultiplier detectors. However, water can be replaced by using a hydrocarbon based scintillator providing both target and detection as well as increased scintillation intensity.
Solar neutrino results from SNO indicated that the numbers reaching earth from the sun were only about 35% of the theoretical predictions. The number of all types of neutrinos reaching earth from the sun matched the theoretical prediction – indicating that electron-neutrinos underwent oscillation to muon or tau flavours on their journey.
A similar result was produced in Japan at the Super-Kamiokande experiment studying muon-anti-neutrinos produced in the upper atmosphere by cosmic-ray collisions. The experiment showed a difference in upper and lower flux detections, indicating oscillation had occurred.
The two discoveries from SNO (as it was called before it became SNOLAB) and Super-Kamiokande, indicated that neutrinos oscillated and therefore must have mass, which led to a shared Nobel prize in 2015 between the two laboratory directors Takaaki Kajita of Japan and Arthur B McDonald of Canada.
The kamLAND experiment in Japan, studying electron-anti-neutrinos from a ring of reactors, produced evidence that the number of detections varied over time from a maximum deficit to a predicted value and back to a maximum deficit. This confirmed that anti-electron-neutrinos behave in an identical fashion to electron-neutrinos and showed that oscillations occur due to the distance from the source.
SNO did not observe the wave nature of the results like kamLand, due to the fact that high-speed neutrinos are produced in a very dense environment in the sun's core, explained by the MSW effect.
There's no doubt that neutrinos are one of the most fascinating subjects in modern physics!
© 2026 Kieron Conway - All rights reserved.
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How Do you Detect Neutrino Oscillations?
&
What is the ICECUBE Experiment?
Soon to be published: These will be the subjects of the third and fourth article on neutrinos for the time being.
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