Neutrino
The
neutrino is an
elementary particle. It has
spin 1/2 and so it is a
fermion. Its
mass is very small, although recent experiments (see
Super-Kamiokande) have shown it to be above zero. It feels neither the
strong nor the
electromagnetic force, so it only interacts through the
weak force and
gravitation.
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.
Types of neutrinos
\n
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 (see table at right). In a phenomenon known as
neutrino oscillation neutrinos spontaneously mutate between the three flavors.
History
\nThe neutrino was first postulated in 1931 by
Wolfgang Pauli to explain the continuous spectrum of
beta decay, the decay of a neutron into a
proton and an electron. Pauli theorized that an undetected particle was carrying away the observed difference between the
energy and angular momentum of the initial and final particles. Because of their "ghostly" properties, the first experimental detection of neutrinos had to wait until about 25 years after they were first discussed. In 1956
Clyde Cowan,
Frederick Reines, F. B. Harrison, H. W. Kruse, and A. D. McGuire published the article "Detection of the Free Neutrino: a Confirmation" in
Science (see
neutrino experiment), a result that was rewarded with the
1995 Nobel Prize. The name
neutrino was coined by
Enrico Fermi as a word play on
neutrone, the
Italian name of the
neutron particle. (
Neutrone in Italian also means
big and neutral, and
neutrino means
small and neutral.)\nIn 1962 Leon Max Lederman, Melvin Schwartz and
Jack Steinberger find out, that not only one types of neutrino exists.
Mass
The basic standard model of particle physics assumes that the neutrino is massless, although adding massive\nneutrinos to the basic framework is not difficult, and\nrecent experiments\nsuggest that the neutrino has a small although non-zero\nmass.
The strongest upper limits on the mass of the neutrino\ncome from
cosmology. The
big bang model predicts\nthat there is a fixed ratio between the number of neutrinos\nand the number of
photons in the
cosmic microwave background. If the total mass of all three types of neutrinos exceeded 50 electron volts, there would be so \nmuch mass in the universe that it would collapse. This limit can be circumvented by assuming that the neutrino is unstable, however there are limits within the standard model that make this difficult.
However, it is now widely believed that the mass of the\nneutrino is non-zero. When one extends the \n
standard model to include neutrino masses, one finds\nthat the prediction that massive neutrinos can change\ntype whereas massless neutrinos cannot. This phenonemnon\nknown as
neutrino oscillation explains why there\nare many fewer electron neutrinos observed from the\n
sun and the upper atmosphere than expected, and has\nalso been directly observed.
\n
Neutrino Sources
\nHuman generated
\nNuclear power stations are the major source of human generated neutrinos. An average plant may generate over 50,000 neutrinos per second.
Particle accelerators are another source.
The Earth
\nNeutrinos are produced as a result of the natural background radiation from radioactive atomic nuclei within the Earth.
Atmospheric neutrinos
\nAtmospheric neutrinos result from the interaction of cosmic rays with atoms within
Earth's atmosphere, creating showers of particles including neutrinos.
Solar neutrinos
\nSolar neutrinos originate from the nuclear fusion powering the
Sun and other stars.
\nRaymond Davis Jr and
Masatoshi Koshiba were jointly awarded the
2002 Nobel Prize in Physics for their work in the detection of cosmic neutrinos.
Cosmological phemomena
\nNeutrinos are an important product of supernovas. Most of the energy produced in supernovas is radiated away in the form of an inmense burst of neutrinos, which are produced when protons and
electrons in the core combine to form
neutrons. The first experimental evidence of this phenomenon came in the year
1987, when neutrinos coming from the
supernova 1987a were detected. In such events, the
densities at the core becomes so high (10
14 gram/cm
3) that interaction between the produced neutrinos and surrounding stellar matter becomes significant. It's thought that neutrinos would also be produced from other events such as the collision of neutron stars.
Cosmic background radiation
\nIt is thought that the cosmic background radiation left over from the
Big Bang includes a background of low energy neutrinos. In the
1980s it was proposed that these may be the explanation for the
dark matter thought to exist in the universe. Neutrinos have one important advantage over most other dark matter candidates: we know they exist. However, they also have serious problems. From particle experiments, it is known that neutrinos tend to be
hot, i.e. move at speeds close to the
speed of light—hence this scenario was also known as
hot dark matter. The problem is that being hot and fast moving, the neutrinos would tend to spread out evenly in the
universe. This would tend to cause matter to be smeared out and prevent the large
galactic structures that we see.
Neutrino detectors
\nThere are several types of neutrino detectors. Those used to detect stellar neutrinos consist of a large amount of material in an underground cave designed to shield it from cosmic radiation.
- In 1953 the first neutrino detection device was used to detect neutrinos near a nuclear reactor. Reines and Cowan used two targets containing a solution of cadmium chloride in water. Two scintillation detectors were placed next to the cadmium targets. Neutrino interactions with protons of the water produced positrons. The resulting positron annihilations with electrons created photons with an energy of about 0.5 MeV. Pairs of photons in coincidence could be detected by the two scintillation detectors above and below the target. The neutrons were captured by cadmium nuclei resulting in gamma rays of about 8 MeV that were detected a few microseconds after the photons from a positron annihilation event.\n* Chlorine detectors consist of a tank filled with carbon tetrachloride. 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 neutrino. 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. This type of detector is only sensitive to νe.\n* 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.\n* 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. This type of detector is sensitive to νe and νμ.\n* 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 (SNO). This type of detector is sensitive to all three neutrino flavors.
See also
\n*solar neutrino problem\n*
particle physics\n*
list of particles
External link
\n* Astronomy Magazine article on neutrino detectors
Category:Leptons
\n\n\n\n\n\n\n\n\n\n\n
