Avery-high-energy gamma ray (VHEGR) isgamma radiation withphoton energies of 100 GeV (gigaelectronvolt) to 100 TeV (teraelectronvolt), i.e., 1011 to 1014electronvolts.[1] This is approximately equal towavelengths between 10−17 and 10−20 meters, or frequencies of 2 × 1025 to 2 × 1028 Hz. Such energy levels have been detected from emissions from astronomical sources such as somebinary star systems containing acompact object.[1] For example, radiation emitted fromCygnus X-3 has been measured at ranges from GeV toexaelectronvolt-levels.[1] Other astronomical sources includeBL Lacertae,[2]3C 66A[3]Markarian 421 andMarkarian 501.[4] Various other sources exist that are not associated with known bodies. For example, theH.E.S.S. catalog contained 64 sources in November 2011.[5]
Instruments to detect this radiation commonly measure theCherenkov radiation produced by secondary particles generated from an energetic photon entering the Earth's atmosphere.[3] This method is called imaging atmospheric Cherenkov technique orIACT. 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.[3] Instruments include the plannedCherenkov Telescope Array,GT-48 in Crimea,MAGIC onLa Palma,High Energy Stereoscopic System (HESS) in Namibia[6]VERITAS[7] andChicago Air Shower Array which closed in 2001. Cosmic rays also produce 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.[8]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.[8]
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 ofmuons.[7]
Very-high-energy gamma rays are too low energy to show theLandau–Pomeranchuk–Migdal effect. Only magnetic fields perpendicular to the path of the photon causes pair production, so that photons coming in parallel to the geomagnetic field lines can survive intact until they meet the atmosphere. These photons that come through the magnetic window can make a Landau–Pomeranchuk–Migdal shower.[9]
| Class | energy | energy | energy | frequency | wavelength | comparison | properties |
|---|---|---|---|---|---|---|---|
| eV | eV | Joules | Hertz | meters | |||
| 1 | 1 | 0.1602aJ | 241.8 THz | 1.2398 μm | near infrared photon | for comparison | |
| 100 GeV | 1 × 1011 | 0.01602 μJ | 2.42 × 1025 Hz | 1.2 × 10−17 m | Z boson | ||
| Very-high-energy gamma rays | |||||||
| 1 TeV | 1 × 1012 | 0.1602 μJ | 2.42 × 1026 Hz | 1.2 × 10−18 m | flying mosquito | produces Cherenkov light | |
| 10 TeV | 1 × 1013 | 1.602 μJ | 2.42 × 1027 Hz | 1.2 × 10−19 m | air shower reaches ground | ||
| 100 TeV | 1 × 1014 | 0.01602 mJ | 2.42 × 1028 Hz | 1.2 × 10−20 m | ping pong ball falling off a bat | causes nitrogen to fluoresce | |
| Ultra-high-energy gamma rays | |||||||
| 1 PeV | 1 × 1015 | 0.1602 mJ | 2.42 × 1029 Hz | 1.2 × 10−21 m | |||
| 10 PeV | 1 × 1016 | 1.602 mJ | 2.42 × 1030 Hz | 1.2 × 10−22 m | potential energy of golf ball on a tee | ||
| 100 PeV | 1 × 1017 | 0.01602 J | 2.42 × 1031 Hz | 1.2 × 10−23 m | penetrate geomagnetic field | ||
| 1 EeV | 1 × 1018 | 0.1602 J | 2.42 × 1032 Hz | 1.2 × 10−24 m | |||
| 10 EeV | 1 × 1019 | 1.602 J | 2.42 × 1033 Hz | 1.2 × 10−25 m | air rifle shot |
Very-high-energy gamma rays are of importance because they may reveal the source ofcosmic rays. They travel in a straight line (in space-time) 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 neutralpions which in turn decay toultra-high-energy photons.[8]
The ratio of primary cosmic rayhadrons 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 interactions with thecosmic microwave background by way of theGreisen–Zatsepin–Kuzmin limit cutoff above 50 EeV.[9]
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