Ultra-high-energy gamma ray
Ultrahigh energy gamma-ray (UHEGR) denotes gamma radiation with the shortest wavelengths (between 10−20 and 10−23 meter), with photon energies in the range from 1014 to 1017 electronvolts; a unit of energy used in particle physics. Individual photons will have energies from μJ to Joules. As a frequency this is as high as 3x1031Hz. Such energy levels have been detected from emissions from astronomical sources such as some binary star systems containing a compact object. For example, radiation emitted from Cygnus X-3 has been measured at exaelectronvolt-levels. Other astronomical sources include BL Lacertae, 3C 66A Markarian 421 and Markarian 501. Markarian 501 has so far produced the highest measured energy for a gamma ray of 16 TeV. Various other sources exist that are not associated with known bodies. For example, the H.E.S.S catalog contains 64 sources in November 2011.
Instruments to detect this radiation commonly measure the Cherenkov radiation produced by secondary particles generated from an energetic photon entering the Earth's atmosphere. This method is called imaging atmospheric Cherenkov technique or IACT. A high energy photon produces a cone of light confined to 1° of the original photon direction. About 10,000 m2 of the earth's surface is lit by each cone of light. A flux of 10−7 photons per square meter per second can be detected with current technology, provided the energy is above 0.1 TeV. Instruments include the planned Cherenkov Telescope Array, GT-48 in Crimea, MAGIC on La Palma, High Energy Stereoscopic System (HESS) in Namibia VERITAS and Chicago Air Shower Array which closed in 2001. Cosmic rays also produces similar flashes of light, but can be distinguished based on the shape of the light flash. Also having more than one telescope simultaneously observing the same spot can help exclude cosmic rays. Extensive air showers of particles can be detected for gamma rays above 100 TeV. Water scintillation detectors or dense arrays of particle detectors can be used to detect these particle showers.
Air showers of elementary particles made by gamma rays can also be distinguished from those produced by cosmic rays by the much greater depth of shower maximum, and the much lower quantity of muons.
Ultra high energy gamma rays interact with magnetic fields to produce positron electron pairs. In the earth's magnetic field a 1021eV photon is expected to interact about 5000 km above the earth's surface. The high energy particles then go on to produce more lower energy photons that can suffer the same fate. This effect creates a beam of several 1017eV gamma ray photons heading in the same direction as the original UHE photon. This beam is less than 0.1 m wide when it strikes the atmosphere. These gamma rays are too low energy to show the Landau–Pomeranchuk–Migdal effect. Only magnetic field perpendicular to the path of the photon causes pair production, so that photons coming in parallel to the geomagnetic field lines can survive intact till they meet the atmosphere. These photons that come through the magnetic window can make a Landau–Pomeranchuk–Migdal shower.
|eV||eV||Joules||Hertz||meters||in real world|
|1||1||0.1602x10 aJ||242 THz||1.2μm||near infrared photon||for comparison|
|100 GeV||1x1011||0.1602 μJ||2.41x1025Hz||1.2x10−17 m||Z boson|
|1 TeV||1x1012||1.602 μJ||2.41x1026Hz||1.2x10−18 m||flying mosquito||produces Cherenkov light|
|10 TeV||1x1013||16.02 μJ||2.41x1027Hz||1.2x10−19 m||air shower reaches ground
16 TeV highest energy detected
|100 TeV||1x1014||0.1602 mJ||2.41x1028Hz||1.2x10−20 m||ping pong ball falling off a bat||causes nitrogen to fluoresce|
|1 PeV||1x1015||1.602 mJ||2.41x1029Hz||1.2x10−21 m|
|10 PeV||1x1016||16.02 mJ||2.41x1030Hz||1.2x10−22 m||potential energy of golf ball on a tee|
|100 PeV||1x1017||0.1602 J||2.41x1031Hz||1.2x10−23 m||penetrate geomagnetic field|
|1 EeV||1x1018||1.602 J||2.41x1032Hz||1.2x10−24 m|
|10 EeV||1x1019||16.02 J||2.41x1033Hz||1.2x10−25 m||air rifle shot|
Ultrahigh energy gamma rays are of importance because they may reveal the source of cosmic rays. They travel in a straight line from their source to an observer. This is unlike cosmic rays which have their direction of travel scrambled by magnetic fields. Sources that produce cosmic rays will almost certainly produce gamma rays as well, as the cosmic ray particles interact with nuclei or electrons to produce photons or neutral pions which in turn decay to ultra high energy photons.
The ratio of primary cosmic ray hadrons to gamma rays also gives a clue as to the origin of cosmic rays. Although gamma rays could be produced near the source of cosmic rays, they could also be produced by interaction with cosmic microwave background by way of the Greisen–Zatsepin–Kuzmin limit cutoff above 50 EeV. The fraction of gamma rays compared to cosmic rays above 10EeV was under 2% as measured at the Pierre Auger Observatory.
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