To detect neutrinos, very large and very sensitive detectors are required. Typically, a low-energy neutrino will travel through many light-years of normal matter before interacting with anything. Consequently, all terrestrial neutrino experiments rely on measuring the tiny fraction of neutrinos that interact in reasonably sized detectors.
For example, in the Sudbury Neutrino Observatory, a ton heavy water solar-neutrino detector picks up about 10 12 neutrinos each second. About 30 neutrinos per day are detected. In this solar neutrino event, 75 of the 9, light sensors in the detector observed a photon of light.
Lines trace the path from the neutrino's impact with heavy water to the light sensors. Wolfgang Pauli first postulated the existance of the neutrino in At that time, a problem arose because it seemed that both energy and angular momentum were not conserved in beta-decay. But Pauli pointed out that if a non-interacting, neutral particle--a neutrino--were emitted, one could recover the conservation laws. The first detection of neutrinos did not occur until , when Clyde Cowan and Frederick Reines recorded anti-neutrinos emitted by a nuclear reactor.
Natural sources of neutrinos include the radioactive decay of primordial elements within the earth, which generate a large flux of low-energy electron-anti-neutrinos.
Calculations show that about 2 percent of the sun's energy is carried away by neutrinos produced in fusion reactions there. Supernovae too are predominantly a neutrino phenomenon, because neutrinos are the only particles that can penetrate the very dense material produced in a collapsing star; only a small fraction of the available energy is converted to light.
It is possible that a large fraction of the dark matter of the universe consists of primordial, Big Bang neutrinos. The fields related to neutrino particles and astrophysics are rich, diverse and developing rapidly. So it is impossible to try to summarize all of the activities in the field in a short note. That said, current questions attracting a large amount of experimental and theoretical effort include the following: What are the masses of the various neutrinos?
How do they affect Big Bang cosmology? IceCube trapping polar phantoms. But apparently super-hasty motion is not the only strange thing about neutrinos. What exactly are they? With a neutral charge and nearly zero mass, neutrinos are the shadiest of particles, rarely interacting with ordinary matter and slipping through our bodies, buildings and the Earth at a rate of trillions per second.
First predicted in by Wolfgang Pauli, who won a Nobel prize for this work in , they are produced in various nuclear reactions: fusion, which powers the sun; fission, harnessed by humans to make weapons and energy; and during natural radioactive decay inside the Earth.
If they are so stealthy, how do we know they are there at all? Wily neutrinos usually avoid contact with matter, but every so often, they crash into an atom to produce a signal that allows us to observe them. Fredrick Reines first detected them in , garnering himself a Nobel prize in Most commonly, experiments use large pools of water or oil.
When neutrinos interact with electrons or nuclei of those water or oil molecules, they give off a flash of light that sensors can detect. A neutrino is a particle! But while electrons have a negative charge, neutrinos have no charge at all. Neutrinos are also incredibly small and light. They have some mass, but not much. They are the lightest of all the subatomic particles that have mass. These little particles have an interesting history.
While we keep learning more about neutrinos, with new answers come new mysteries. Neutrinos are also tricky to study. The only ways they interact is through gravity and the weak force, which is, well, weak.
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