gamma rays

Gamma rays (GRs) consists of electromagnetic radiation (EMR) with extremely short wavelengths, one criteria being any with a wavelength of 0-10 pm. In the electromagnetic spectrum, they include everything beyond X-rays in frequency and photon energy. They are generally the result of nuclear reactions, and are present in cosmic rays. They are often specified by their photon energy in electron volts (eV), as opposed to using their frequency or wavelength. Equivalent to the above-cited wavelength-range is "everything beyond 124 keV", but astronomers often cite the line between X-rays and gamma rays as 100 keV (i.e., a round number of eV): this discrepancy is largely moot because discussions of EMR in the 100-124 keV photon-energy range, the energy-of-interest is likely to be stated.

There is no consensus across all scientists/technologists on the boundary between EMR termed X-rays and gamma rays: the distinction originally arose from the manner in which they are produced: X-rays were from high-voltage vacuum tubes producing cathode rays, and gamma rays are one type of emission occurring during radioactive decay, of radioactive substances such as radium. However, both these types of emissions consist of EMR, and the wavelength ranges from these two sources overlap. Any fixed boundary is arbitrary: mechanisms that produce this short-wavelength EMR don't completely adhere to such absolute limits, and regarding astronomical observation, there will be particular instruments and sources that span any particular chosen boundary.

The targets of gamma-ray astronomy include gamma-ray transients (gamma-ray bursts, GRBs) and a gamma-ray background (cosmic gamma ray background as well as a galactic gamma ray background due to the interaction of cosmic rays with the interstellar medium) as well as gamma-ray sources, such as the Crab Nebula.

Gamma rays from space (like X-rays) are virtually blocked by Earth's atmosphere, which helps make Earth habitable, and gamma-ray telescopes are either space-based or are designed to sense the results of gamma-ray interactions with the atmosphere (Cherenkov detectors). Among the space telescopes/observatories:

• Swift (GRB observation).
• INTEGRAL.
• Fermi (FGST).
• AGILE (past).
• GRANAT (past).
• Compton Gamma Ray Observatory (CGRO) (past).
• Cos-B (past)

Ground detectors:

• High-altitude Water Cherenkov Observatory (HAWC).
• High Energy Stereoscopic System (HESS).
• VERITAS.
• Cherenkov Telescope Array (CTA).
• Fenton Hill Observatory (FHO) (GRB observations).
• Milagro (past).