Gamma rays

Electromagnetic radiation emitted from excited atomic nuclei as an integral part of the process whereby the nucleus rearranges itself into a state of lower excitation (that is, energy content). See Nuclear structure, Radioactivity

The gamma ray is an electromagnetic radiation pulse—a photon—of very short wavelength. The electric (E) and magnetic (H) fields associated with the individual radiations oscillate in planes mutually perpendicular to each other and also the direction of propagation with a frequency &ngr; which characterizes the energy of the radiation. The E and H fields exhibit various specified phase-and-amplitude relations, which define the character of the radiation as either electric (EL) or magnetic (ML). The second term in the designation indicates the order of the radiation as 2^L-pole, where the orders are monopole (2⁰), dipole (2¹), quadrupole (2²), and so on. The most common radiations are dipole and quadrupole. Gamma rays range in energy from a few kiloelectronvolts to 100 MeV, although most radiations are in the range 50–6000 keV. As such, they lie at the very upper high-frequency end of the family of electromagnetic radiations, which include also radio waves, light rays, and x-rays. See Electromagnetic radiation, Multipole radiation, Photon

The dual nature of gamma rays is well understood in terms of the wavelike and particlelike behavior of the radiations. For a gamma ray of intrinsic frequency &ngr;, the wavelength is λ = c/&ngr;, where c is the velocity of light; energy is E = h&ngr;, where h is Planck's constant. The photon has no rest mass or electric charge but, following the concept of mass-energy equivalence set forth by Einstein, has associated with it a momentum given by p = h&ngr;/c = E/c. See Light, Quantum mechanics, Relativity

Various nuclear species exhibit distinctly different nuclear configurations; the excited states, and thus the γ-rays which they produce, are also different. Precise measurements of the γ-ray energies resulting from nuclear decays may therefore be used to identify the γ-emitting nucleus. This has ramifications for nuclear research and also for a wide variety of more practical applications. One of the most useful studies of the nucleus involves the bombardment of target nuclei by energetic nuclear projectiles to form final nuclei in various excited states. Measurements of the decay γ-rays are routinely used to identify the various final nuclei according to their characteristic γ-rays.

In practical applications, the presence of γ-rays is used to detect the location or presence of radioactive atoms which have been deliberately introduced into the sample. In irradiation studies, for example, the sample is activated by placing it in the neutron flux from a reactor. The resultant γ-rays are identified according to isotope, and thus the composition of the original sample can be inferred. Such studies have been used to identify trace elements found as impurities in industrial production, or in ecological studies of the environment, such as minute quantities of tin or arsenic in plant and animal tissue.

In tracer studies, a small quantity of radioactive atoms is introduced into fluid systems (such as the human blood stream), and the flow rate and diffusion can be mapped out by following the radioactivity. Local concentrations, as in tumors, can also be determined.

For the three types of interaction with matter which together are responsible for the observable absorption of γ-rays, namely, Compton scattering, the photoelectric effect, and pair production, See Compton effect, Electron-positron pair production, Photoemission

gamma rays

Gamma rays, like X-rays, are usually described in terms of photon energy, h ν, where ν is the frequency of the radiation and h is the Planck constant. The γ-ray region of the electromagnetic spectrum spans many decades of photon energy – from about 10⁵ electronvolts (eV) to more than 10¹⁵ eV. The range is customarily subdivided into a number of energy bands that are related to changes in telescope technology: low energy γ-rays, 10⁵–10⁷ eV;

medium energy γ-rays, 10⁷–10⁹ eV;

high energy γ-rays, 10⁹–10¹¹ eV;

very high energy γ-rays, 10¹¹–10¹⁴ eV;

ultra high energy γ-rays, >10¹⁴ eV.

Low energy γ-ray photons (h ν ˜ 10⁶ eV) are the most penetrating photons available to the astronomer. Traditionally γ-ray source spectra have been measured in units of photons per unit area per second per unit energy interval, i.e. treating the photons as individual events. At present the general move is to unify the measurement system by use of the jansky.