Neutrino astronomy

Neutrino astronomy is the branch of astronomy that observes astronomical objects with neutrino detectors in special observatories. Nuclear reactions in stars and supernova explosions produce very large numbers of neutrinos, a very few of which may be detected by a neutrino telescope. Neutrino astronomy is motivated by the possibility of observing processes that are inaccessible to optical telescopes, such as the Sun's core.

Neutrino astronomy is still very much in its infancy: so far, the only confirmed extraterrestrial neutrino sources are the Sun and supernova SN1987A.

Applications

When astronomical bodies, such as the Sun, are studied using light, only the surface of the object can be directly observed. Any light produced in the core of a star will interact with gas particles in the outer layers of the star, making it impossible to observe the core directly. Neutrinos, however, very rarely interact with other particles. Neutrinos which are created in the cores of stars (as a result of stellar fusion) can be observed using neutrino astronomy.^[1]

Observatories

Observatory	Location	Detector volume	Angular resolution	Energy range	Years active	References
Super-Kamiokande	Nagoya, Japan	50,000 m³	26°	108–1012 eV	1996–Present	[1]
Pierre Auger Observatory	Mendoza, Argentina	50,000 m³	2°	1017–1021 eV	2004–Present	[1]
Antarctic Impulse Transient Antenna	McMurdo Station, Antarctica	1,000,000 km³	2°	1017–1021 eV	2006–2007, 2008–2009	[1]
ANTARES	Mediterranean Sea	0.05 km³	0.3°	1010–1016 eV	2008–present	[1]
IceCube Neutrino Observatory	South Pole	1 km³	1°	1011–1021 eV	2011 (estimated)	[1]
Extreme Universe Space Observatory	International Space Station	1,000,000 km³	2°	1019–1021 eV	2015 (estimated)	[1]
Baksan Neutrino Observatory	Baksan, Russia	3,000 m³	?	?	1977–Present	—

Observation challenges

Neutrinos interact only very rarely with matter. The enormous flux of solar neutrinos racing through the Earth is sufficient to produce only 1 interaction for 10³⁶ target atoms, and each interaction produces only a few photons or one transmuted atom. The observation of neutrino interactions requires a large detector mass, 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 elementary particles produced by cosmic rays striking the atmosphere, and particles produced by radioactive decay. To reduce the amount of 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, about 1°. To improve the angular resolution, a large array of neutrino detectors may be used.

Scientific Problems of Neutrino Astronomy

• Solar neutrino problem - In detecting solar neutrinos, it became clear that the number detected was half or a third than that predicted by models of the solar interior. The problem was solved by revising the properties of neutrinos and understanding the limits of the detection mechanisms - only one third of the forms of neutrinos coming in was being detected and all neutrinos oscillate between the three forms.
• Cosmic neutrino background - (CNB) is the background particle radiation composed of neutrinos as a relic of the big bang which decoupled from matter when the universe was 2 seconds old.
• Hot dark matter - since many neutrinos come from stellar cores and supernovae, they are released at great temperature/energy. As neutrinos do not interact with matter electromagnetically, they are by definition dark matter.
• Warm dark matter - possibly so called sterile neutrinos.

See also

• List of neutrino experiments

Notes

1. ^ ^{a b c d e f g} Gelmini, Graciela B. (May 2010). "Through Neutrino Eyes". Scientific American: pp. 38–45. http://www.scientificamerican.com/article.cfm?id=through-neutrino-eyes.