Ultra-high-energy gamma ray

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Ultra-high-energy gamma rays are gamma rays with energies higher than 100 TeV (0.1 PeV). They have a frequency higher than 2.41x1028 Hz and a wavelength shorter than 1.2x10−20 m. As of 2014 they are theoretical only and have not been detected. The highest energy astronomical sourced gamma rays detected are very-high-energy gamma rays.


Ultra-high-energy gamma rays are of importance because they may reveal the source of cosmic rays. Discounting the relatively weak effect of gravity, 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.[1]

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.[2]

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.[2]

energy energy energy frequency wavelength comparison properties
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


  1. ^ Aharonian, Felix (24 August 2010). "The Fascinating TeV Sky" (PDF). WSPC - Proceedings. Retrieved 27 November 2011. 
  2. ^ a b Vankov, H. P.; N. Inoue2 and K. Shinozaki (2 February 2008). "Ultra-High Energy Gamma Rays in Geomagnetic Field and Atmosphere" (PDF). Retrieved 3 December 2011. 

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