Neutrino astronomy

From Academic Kids

Neutrino astronomy is the science of observing astronomical phenomena by detecting neutrinos, a product of thermonuclear reactions going on inside every star. It is still very much in its infancy - the only confirmed extra-terrestrial sources detected so far are the sun and supernova SN1987A.

Contents

Observation challenges

Neutrinos interact only very rarely with matter. The enormous flux of solar neutrinos racing through our earth is sufficient to produce only 1 interaction for 10^36 target atoms, and each interaction produces only a few photons or one transmuted element. To observe neutrino interactions a large detector mass is required, along with a sensitive amplification system.

Given the very weak signal, sources of background noise must be reduced as much as possible. The major sources of detector noise are the showers of exotic particles produced by cosmic rays striking the atmosphere, and particles produced by radioactive decay. To avoid cosmic rays, the detectors must be shielded by a large shield mass, and so are constructed deep underground, or underwater. Sources of radioactive isotopes must also be controlled as they produce energetic particles when they decay.

In order to produce any kind of image, the detector must provide information not only about the flux of neutrinos, but also their direction of travel. While several methods of detecting neutrinos exist, most do not provide directional information, and the ones that do have poor angular resolution. To improve the angular resolution, a large array of neutrino detectors may be used

Detector design

The detector design used generally consists of a large mass of water or ice, surrounded by an array of sensitive light detectors known as photomultiplier tubes. This design takes advantage of the fact that particles produced in the interaction of the incoming neutrino with an atomic nucleus typically travel faster than the speed of light in the detector medium (though of course 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.

The Super-Kamiokande neutrino detector uses a 50,000 tons of pure water surrounded by 11,000 photomultiplier tubes burried 1 km underground. It is able to detect the incident direction of incomming neutrinos by detecting which photomultipliers fire. Super-Kamiokande was able to detect the burst of neutrinos associated with supernova 1987A, and in 1988 it was used to directly confirm the production of solar neutrinos.

The Antarctic Muon And Neutrino Detector Array (AMANDA) operated from 1996 to 2004. This detector used photomultiplier tubes mounted on strings, buried deep (1.5-2km) inside the glacial ice at the South Pole in Antarctica. The ice itself is used as the detector mass. The direction of incident neutrinos is determined by recording the arrival time of individual photons using a three-dimensional array of detector modules containing one photomultiplier tube each. This method allows detection of neutrinos above 50GeV with a spatial resolution of approximately 2 degrees. AMAMDA has been used to generate neutrino maps of the northern sky in order to search for extraterrestrial neutrino sources and in searches for dark matter. AMANDA is currently in the process of being upgraded to the IceCube observatory, eventually increasing the volume of the detector array to one cubic kilometer.

See also

External links

ja:ニュートリノ天文学

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