neutrino astronomy

(astronomy) The observation of neutrinos from the sun and from extrasolar astronomical sources.

The detection and study of neutrinos to learn about astronomical objects and the universe. These neutral, weakly interacting particles come almost without any disruption straight from their sources, traveling at very close to the speed of light. A low-energy neutrino in flight would not notice a barrier of lead 50 light-years thick. Neutrino light would provide a wondrous new view of the universe.

Neutrinos in the universe

Neutrinos were made in huge numbers at the time of the big bang. Like the cosmic background radiation, they now possess little kinetic energy as a result of the expansion of the universe. The problem with observing these relic neutrinos is that probability of a neutrino interacting within a detector decreases with the square of the neutrino's energy, for low energies. Nobody has been able to detect these lowest-energy neutrinos, and prospects are not good for doing so. See also Big bang theory; Cosmic background radiation.

Stellar neutrinos

Neutrinos also originate in the nuclear fusion in stars. The Sun close by produces a huge flux of neutrinos, which have been detected in five experiments. However, observations of stellar neutrinos are limited to the Sun. Just as the sky is dark at night despite all the stars, the Sun far outshines all the rest of the cosmos in numbers of detectable neutrinos. See also Solar neutrinos.

Supernovae

On February 23, 1987, two detectors in deep mines in the United States (the IMB experiment) and Japan (the Kamiokande experiment) recorded a total of 19 neutrino interactions over a span of 13 seconds. Two and a half hours later, astronomers in the Southern Hemisphere saw the first supernova to be visible with the unaided eye since 1604. Many deductions followed about the nature of neutrinos, such as limits on mass, charge, gravitational attraction, and magnetic moment.

Supernovae of the gravitational-collapse type occur when elderly massive stars run out of nuclear fusion energy and can no longer resist the force of gravity. The neutrinos carry off most of the in-fall energy. Much can be learned from the final stages of stellar evolution, not only about the process of stellar collapse to a neutron star or black hole (the latter if the progenitor is very massive) but also about properties of neutrinos. Four underground detectors have significant capability for supernova detection from the Milky Way Galaxy. From historical records and from observations of distant spiral galaxies, the rate of supernovae in the Milky Way Galaxy is expected to be between one and five per century. Thus experimentalists may have to wait a long time before the next observation, and there is no way of predicting when it will occur. See also Supernova.

High-energy cosmic neutrinos

Higher-energy neutrinos must be made in many of the most luminous and energetic objects in the universe, such as active galactic nuclei and gamma-ray bursters. Two things make prospects brighter in the near future for higher-energy neutrino astronomy than for lower energies: (1) the interaction probability for neutrinos goes up with energy, and (2) the consequences of neutrino interaction with a target (Earth or detector) become more detectable as the energy release is greater. The favored method is to detect muons produced by neutrinos. These charged particles produce Cerenkov radiation, a short flash of light detectable at tens of meters distance by photomultipliers in clear water or ice. See also Cerenkov radiation; Photomultiplier.

High-energy neutrino telescopes

Neutrino detectors must be placed deep underground or underwater to escape the backgrounds caused by the rain of cosmic rays upon the atmosphere. The lead project, DUMAND (Deep Underwater Muon and Neutrino Detector), was canceled in 1995, but made great headway in pioneering techniques, studying backgrounds, exploring detector designs, and stimulating interest in astrophysical neutrinos.

Two projects similar to DUMAND are under way in the Mediterranean, the more developed NESTOR (Neutrino Experimental Submarine Telescope with Oceanographic Research) Project, and the ANTARES (Astronomy with a Neutrino Telescope and Abyss Environmental Research) Project (see illustration). These projects employ basically the same method of bottom-anchored cables, with photomultipliers protected in spherical glass pressure housings, as developed for DUMAND. A different type of neutrino telescope, the AMANDA (Antarctic Muon and Neutrino Detector Array) Project, is under construction in ice at the South Pole.

View of the ANTARES Project from the ocean bottom. The optical detectors consist of modules of a cluster of photomultipliers and electronics, spaced along vertical buoy strings. Spherical floats at the top of each detector string keep the string close to vertical; anchors and releases are at the bottom. Fiber-optic cables go from each string to a junction box, which is serviced by the submarine at the far side of the array. A cable descends the slope from shore in the background. Scales are exaggerated. (ANTARES Collaboration)

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