Neutrino . Particle emitted in beta decay where a proton reacts with an antineutrino turning into a neutron and a positron or proton interacts with an electron to produce a neutron and a neutrino.


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  • 1 History
  • 2 types
  • 3 Physical Classification
  • 4 Standard Model
  • 5 Sources of neutrinos
  • 6 Neutrino detectors
    • 1 Detectors based on radioactive processes
  • 7 Detectors based on the Cherenkov effect
  • 8 Sources


The neutrino was first proposed in 1930 by Wolfgang Pauli to compensate for the apparent loss of energy and linear momentum in the decay of neutrons.

Pauli interpreted that both mass and energy would be conserved if a hypothetical particle called “neutrino” participated in the decay by incorporating the lost quantities. Unfortunately, the predicted particle had to be very elusive, without mass, charge, or strong interaction, so it could not be detected with the means of the time. This was the result of a very small cross section. The idea was therefore parked for 25 years.

The chance of a neutrino interacting with matter is very small. It would take a block of lead a light-year length to stop half of the neutrinos that pass through it. (9.46 billion km)

In 1956 Clyde Lorrain Cowan and Frederick Reines experimentally demonstrated their existence. They did this by bombarding pure water with a beam of 1018 neutrons per second. They observed the subsequent photon emission and thus their existence was determined. See the neutrino experiment. In 1987 Leon Max Lederman , Melvin Schwartz and Jack Steinberger discovered the two remaining types of neutrinos: tauonic and muonic.


There are three types of neutrinos associated with each of the leptonic families (flavors): electronic neutrino (ne), muonic neutrino (nm) and tauonic neutrino (nt) plus their respective antiparticles.

Physical Classification

Elementary particle belonging to the same Electron family . As its own name indicates, it is a particle devoid of electric charge . As for the mass, it is either zero or, as recent studies would show, it is very small, at least ten thousand times less than that of the electron, it is believed that the mass of neutrinos is less than about 5, 5 eV / c2, which means less than a billionth of the mass of a hydrogen atom .

Neutrinos are particles produced in great quantity in the course of thermonuclear processes that take place inside the stars. It is calculated that, from the Sun alone, we receive a flow equivalent to ten billion square cm per second.

Determining the mass and other physical characteristics of neutrinos is relatively problematic, because these particles interact very little with matter and are therefore difficult to determine. Suffice it to think that, while we are reading, billions and billions of neutrinos cross our house, our body, the entire Earth, without being diverted by the elementary particles that constitute all these things.

The mass of the neutrino has important consequences in the standard model of particle physics since it would imply the possibility of transformations between the three types of neutrinos that exist in a phenomenon known as neutrino oscillation. In any case, neutrinos are not affected by strong electromagnetic or nuclear forces, but by the weak and gravitational nuclear force.

Standard Model

In the standard model, the neutrino was initially considered as a massless particle.

It can be considered of zero mass because it is, at least ten thousand times less than that of the electron. This implies that neutrinos travel at speeds very close to the speed of light . Therefore, in cosmological terms the neutrino is considered hot matter, or relativistic matter. In contrast, cold matter would be non-relativistic matter.

In 1998 , during the 0-mass neutrino conference, the first papers were presented that showed that these particles have a negligible mass. Previous to these works it had been considered that the hypothetical mass of neutrinos could have an important contribution within the dark matter of the Universe. However, it turned out that the mass of the neutrino was insufficient, too small to even be taken into account in the huge amount of dark matter that is calculated to be in the universe. On the other hand, the models of cosmological evolution did not match the observations if hot dark matter was introduced. In that case the structures were formed from largest to smallest scale. While the observations seemed to indicate that gas clusters first formed, then stars , then proto- galaxies , then clusters , clusters of clusters, etc. The observations, then, fit a cold dark matter model. For these two reasons, the idea that the neutrino contributed prominently to the total mass of the universe was discarded.

Neutrino sources


There are various sources of neutrino emissions among the most important are:

-The sun .

-Cosmic background radiation.

-Astrophysical phenomena.

-The Earth and the atmosphere .

-Human sources.

Neutrino detectors

Knowing exactly the nuclear reactions that occur in the Sun, it was calculated that an appreciable flux of solar neutrinos had to traverse the Earth at every moment. This flux is huge, but neutrinos barely interact with ordinary matter. Even the conditions inside the Sun are “transparent” to them. In fact, a human being is traversed by billions of these tiny particles per second without being aware of it. So it was difficult to conceive of any system that could detect them.

Detectors based on radioactive processes

However, in 1967 Raymond Davis managed to find a detection system. He observed that chlorine -37 was capable of absorbing a neutrino to become argon -37 as shown in the following equation:

Naturally, this was not the only reaction between neutrinos and ordinary matter. What was special about chlorine-37 was that it fulfilled certain requirements to be able to be used in a future detector.

  1. a) The effective section of the chlorine-37 interaction with a neutrino is quite large, which implies a greater probability of such a reaction taking place.
  2. b) Argon-37 is radioactive, making it possible to detect its presence due to its emissions.
  3. c) Chlorine-37, although it is not the most abundant chlorine isotope, is very easy to obtain.

Normally chlorine-37 appears mixed with other isotopes. Particularly with chlorine-35, the most abundant. In addition, it can be mixed with other atoms or molecules, always knowing its proportion. To avoid false measurements due to argon-37 already present in the mixture, the first step was to clean the product. After this, the chlorine-37 mixture should be left to stand for a few months until it reached a stationary situation. This is when the amount of argon that decays equals the amount that forms. The equilibrium moment will be determined by the half-life period.

To protect the detector from the background noise produced by cosmic radiation, tank1 of the chlorinated mixture was buried at a South Dakota gold mine at great depth. However, the first observations only gave higher levels, still compatible with zero2. The results were less than expected and were confused with noise. After repeated increases in the sensitivity of the instruments and in the purity of the chlorine-37 mixture, it was finally possible to calculate that we reached approximately a third of the expected flow3. These results were not taken very seriously at first, so we continued to experiment with better but also more expensive mixtures based on gallium or boron .

  1. The tank contained 380,000 liters of perchlorethylene, a liquid frequently used in dry cleaners.
  2. The initial sensitivity of the detector was intended to detect the expected flux of solar neutrinos. But since it was below the precision of the system, initially only a higher level was obtained.
  3. An average of one neutrino and one captured medium was expected each day. But the result was only half a neutrino a day.

Detectors based on the Cherenkov effect

Doubts about the methods used by Davis encouraged the search for alternatives for the detection of such elusive particles. Thus, a new line of detectors arose, based on the collision of neutrinos with electrons contained in an aqueous medium.

These detectors are based on the fact that the neutrino, when hitting an electron, transmits part of its moment, giving it a speed that is sometimes higher than the speed of light in the same aqueous medium. It is at this moment that a characteristic light emission occurs, known as Cherenkov radiation, which is captured by the photomultipliers that line the walls of the container. As what is observed is a linear momentum transmission, we can approximately infer their mass and the direction from which they come, whereas with the previous detection system we could only calculate the neutrino flux.

Instead of conventional water, heavy water is used because it is more likely to capture neutrinos. This is the case of the most famous neutrino detector. The Super-Kamiokande, which is named after the Japanese Kamioka mine. The first thing that was done with this enormous container, 40 meters in diameter by 40 meters high, equipped with some 11,000 photomultiplier tubes, was to detect neutrinos from the 1987A supernova. The flux of the solar neutrinos was then measured by corroborating the results of the Davis detector. Its greatest success has been the recent measurement of the neutrino mass. It was with the supernova experiment with which the laboratory became most famous by being able to determine that the mass of the neutrino was not zero, reaching its value by measuring the delay with which the neutrinos from the explosion arrived. If these had lacked mass they would have arrived next to the photons (the light of the supernova).


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