The energy of neutrinos

As if having neutrinos of various flavors, masses, and stuff (antimatter and ordinary matter) wasn’t perplexing enough, neutrinos also occur in a wide range of energies.

A neutrino’s energy is determined by the mechanism that created it. Because neutrinos have no charge, scientists can’t employ electric fields to accelerate and give them additional energy, like they can with protons. There will be more energetic neutrinos as a result of higher intense reactions. These are advantageous to scientists because higher-energy particles are more likely to interact and leave traces. Regular matter is more likely to stop them, transferring the energy to something else (other particles) that detectors can detect.

Low-energy neutrinos, such as those left over from the Big Bang, are difficult to locate because they are weakly interacting (as are all neutrinos) and have little energy to pass on to other visible particles. Even if they do, the signal will most likely be weak and difficult to distinguish from all the other noise.

The energy of neutrinos is usually measured in electronvolts. However, neutrino energy span a wide range. Some have a millionth of an electronvolt, while others have a quintillion (a 1 followed by 18 zeros). That implies there will be plenty of neutrinos to investigate, as well as valuable information about the processes that created them.

Because neutrinos have such a wide range of energy, a wide range of techniques must be utilized to detect them.

The lowest-energy neutrinos are thought to have barely a fraction of an electronvolt of energy and emanate from just a few seconds after the Big Bang. This is less energy than it takes to knock an electron out of a hydrogen atom, making them extremely difficult to detect because an even lower threshold is required. Because materials vibrate with thermal energies much higher than those of these Big Bang neutrinos at room temperature, one way to see these lowest-energy neutrinos is to use stiller, colder materials (at cryogenic temperatures) and look for nuclei that receive a small amount of energy seemingly out of nowhere. Another approach for detecting these neutrinos is to drive a beta decay using these low-energy neutrinos, then look for an outgoing electron with slightly more energy than expected. The key is to create a detector that can detect even minute variations in electron energy.

The sun produces neutrinos with energy ranging from tens to millions of electronvolts as a result of the numerous distinct fusion processes that occur there at the same time. Most neutrinos from the sun are now known to be in the tens to hundreds of electronvolt range, according to scientists. Only by constructing a massive scintillator detector and ensuring that there were no radioactive pollutants in the area were researchers able to identify these.

Scientists were able to witness the first neutrinos from the sun when they were intense enough to transform a chlorine atom into an excited argon atom (by changing a neutron inside a nucleus to a proton). At the Homestake Gold Mine, Ray Davis conducted an experiment. Scientists produced the first measurements of neutrinos from the sun by monitoring the radioactive decay of that stimulated argon atom.

When a neutrino is powerful enough to knock an electron out of its orbit, it can be detected by detectors that are sensitive to electric charges. The electron gets thrown out of its orbital at the exact same angle as the arriving neutrino. You can genuinely “see” the sun with neutrinos if a detector can measure the outgoing electron angle and take into consideration the detector’s relationship to the sun. The detector will view the sun at all times of the day and night, whether it is on the surface or underneath.

Because nuclear reactor neutrinos have a million times the energy of Big Bang neutrinos, their interactions with atoms may be measured. One significant difference is that reactor neutrinos are actually antineutrinos, which means that instead of converting neutrons to protons, they convert protons to neutrons, making the neutrons considerably harder to detect. Certain particles can trap neutrons, which subsequently decay and create photons, light particles that can signify the presence of a neutrino.

As the energy of neutrinos increases from a million to a billion electronvolts, they can begin to transmit more energy to the particles in a detector. The same action of a neutrino colliding with a nucleus at a billion electronvolts can produce an electron that can go through dozens of centimeters of plastic or a muon that can travel through meters of steel. The neutrinos have enough energy to totally break up a nucleus at 10 billion electronvolts.

Finally, if a meter of steel is required to see a 1-GeV muon, a kilometer of steel is required to see a 1-TeV muon. The detectors with a cubic kilometer of detector material have seen the highest-energy neutrinos. The difficulty is, how can you finance a detector that is a cubic kilometer in size? You must figure out how to make a signal out of some material that is readily available in huge quantities. To see these highest-energy neutrinos, people have built detectors out of both ocean water and Antarctic ice.