Neutrino 

The neutrino is an elementary particle. It has spin 1/2 and so it is a fermion. Its mass is very small, though recent experiments (see Super-Kamiokande) have shown it to be different from zero. It only interacts through the weak interaction and feels neither the strong nor the electromagnetic interaction.

Because the neutrino only interacts weakly, when moving through ordinary matter its chance of interacting with it is very small. It would take a light year of lead to block half the neutrinos flowing through it. Neutrino detectors therefore typically contain hundreds of tons of a material constructed so that a few atoms per day would interact with the incoming neutrinos. In collapsing supernova, the densities at the core become high enough (10¹⁴ grams / cc) that the produced neutrinos can be detected.

There are three different kinds, or flavors, of neutrinos: the electron neutrino ν_e, the muon neutrino ν_μ and the tau neutrino ν_τ, named after their partner lepton in the Standard Model.

The neutrino was first postulated by Wolfgang Pauli to explain the continuous spectrum of the beta decay.

Massive neutrinos can oscillate between the three flavors, in a phenomenon known as neutrino oscillation (which provides a solution to the solar neutrino problem and the atmospheric neutrino problem[?] at the same time).

Most of the energy of a collapsing supernova is radiated away on the form of neutrinos which are produced when protons and electrons in the core combine to form neutrons. This produces an inmense burst of neutrinos. The first experimental evidence came in the year 1987, when neutrinos coming from the supernova 1987a were detected.

Some years ago it was believed that massive neutrinos could account for the dark matter, though with the current knowledge of neutrino masses they don't contribute a significant fraction to it. Cosmological observations provide themselves limits on the properties of the neutrino.

Neutrino detectors

• Chlorine detectors were the first used and consist of a tank filled with dry cleaning fluid. In these detectors a neutrino would convert a chlorine atom into one of argon. The fluid would periodically be purged with helium gas which would remove the argon. The helium would then be cooled to separate out the argon. These detectors had the failing that it was impossible to determine the direction of the incoming neutron. It was the chlorine detector in Homestake, South Dakota, containing 520 tons of fluid, which first detected the deficit of neutrinos from the sun that led to the solar neutrino problem.
• Gallium detectors are similar to chlorine detectors but more sensitive to low-energy neutrinos. A neutrino would convert gallium to germanium which could then be chemically detected. Again, this type of detector provides no information on the direction of the neutrino.
• Pure water detectors such as Super-Kamiokande contain a large area of pure water surrounded by sensitive light detectors known as photomultiplier tubes. In this detector, the neutrino transfers its energy to an electron which then travels faster than the speed of light in the medium (though slower than the speed of light in a vacuum). This generates an "optical shockwave" known as Cherenkov radiation which can be detected by the photomultiplier tubes. This detector has the advantage that the neutrino is recorded as soon as it enters the detector, and information about the direction of the neutrino can be gathered. It was this type of detector that recorded the neutrino burst from Supernova 1987a.
• Heavy water detectors use three types of reactions to detect the neutrino. The first is the same reaction as pure water detectors. The second involves the neutrino striking the deuterium atom releasing an electron. The third involves the neutrino breaking the deuterium atom into two. The results of these reactions can be detected by photomultiplier tubes. This type of detector is in operation in the Sudbury Neutrino Observatory.

See also solar neutrino problem, particle physics.

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