gamma ray-4

Light interaction

High-energy (from 80 to 500 GeV) gamma rays arriving from far-distant quasars are used to estimate the extragalactic background light in the universe: The highest-energy rays interact more readily with the background light photons and thus the density of the background light may be estimated by analyzing the incoming gamma ray spectrums.^[15]

Gamma ray production

Gamma rays can be produced by a wide range of phenomena, both nuclear and non-nuclear.

Radioactive decay (gamma decay)

Main article: Nuclear isomer

Gamma rays are produced during gamma decay, which normally occurs after other forms of decay occur, such as alpha or beta decay. An excited nucleus can decay by the emission of an α or β particle. The daughter nucleus that results is usually left in an excited state. It can then decay to a lower energy state by emitting a gamma ray photon, in a process called gamma decay.

The emission of a gamma ray from an excited nucleus typically, requires only 10⁻¹² seconds, and is thus nearly instantaneous. Gamma decay may also follow nuclear reactions such as neutron capture, nuclear fission, or nuclear fusion. Gamma decay is also a mode of relaxation of many excited states of atomic nuclei following other types of radioactive decay, such as beta decay, so long as these states possess the necessary component of nuclear spin. When high-energy gamma rays, electrons, or protons bombard materials, the excited atoms emit characteristic "secondary" gamma rays, which are products of the creation of excited nuclear states in the bombarded atoms. Such transitions, a form of nuclear gamma fluorescence, form a topic in nuclear physics called gamma spectroscopy). Formation of fluorescent gamma rays are a rapid subtype of radioactive gamma decay.

In certain cases, the excited nuclear state that follows the emission of a beta particle or other type of excitation, may be more stable than average, and is termed a metastable excited state, if its decay takes (at least) 100 to 1000 times longer than the average 10⁻¹² seconds. Such relatively long-lived excited nuclei are termed nuclear isomers, and their decays are termed isomeric transitions. Such nuclei have half-lifes that are more easily measurable, and rare nuclear isomers are able to stay in their excited state for minutes, hours, days, or occasionally far longer, before emitting a gamma ray. The process of isomeric transition is therefore similar to any gamma emission, but differs in that it involves the intermediate metastable excited state(s) of the nuclei. Metastable states are often characterized by high nuclear spin, requiring a change in spin of several units or more with gamma decay, instead of a single unit transition that occurs in only 10⁻¹² seconds. The rate of gamma decay is also slowed when the energy of excitation of the nucleus is small.^[16]

An emitted gamma ray from any type of excited state may transfer its energy directly to any electrons, but most probably to one of the K shell electrons of the atom, causing it to be ejected from that atom, in a process generally termed the photoelectric effect (external gamma rays and ultraviolet rays may also cause this effect). The photoelectric effect should not be confused with the internal conversion process, in which a gamma ray photon is not produced as an intermediate particle (rather, a "virtual gamma ray" may be thought to mediate the process).

Decay scheme of 60Co

Gamma rays, X-rays, visible light, and radio waves are all forms of electromagnetic radiation. The only difference is the frequency and hence the energy of those photons. Gamma rays are generally the most energetic of these, although a broad overlap with X-ray energies occurs. An example of gamma ray production follows:

First 60Co decays to excited 60Ni by beta decay emission of an electron of 0.31 MeV. Then the excited 60Ni decays to the ground state (see nuclear shell model) by emitting gamma rays in succession of 1.17 MeV followed by 1.33 MeV. This path is followed 99.88% of the time:

60 27Co	→	60 28Ni*	+	e−	+	ν e	+	γ	+	1.17 MeV
60 28Ni*	→	60 28Ni					+	γ	+	1.33 MeV

Another example is the alpha decay of 241Am to form 237Np; which is followed by gamma emission. In some cases, the gamma emission spectrum of the daughter nucleus is quite simple, (e.g. 60Co/60Ni) while in other cases, such as with (241Am/237Np and 192Ir/192Pt), the gamma emission spectrum is complex, revealing that a series of nuclear energy levels exist.

Because a beta decay is accompanied by the emission of a neutrino which also carries a varying amount of energy away, the beta emission spectrum does not have sharp lines, but instead is broad. Hence, it is not possible to describe the different energy levels found in the nucleus, using beta decay energies alone.

Gamma spectroscopy is the study of the energetic transitions in atomic nuclei, which are generally associated with the absorption or emission of gamma rays. As in optical spectroscopy (see Franck Condon effect) the absorption of gamma rays by a nucleus is especially likely (i.e., peaks in a "resonance") when the energy of the gamma ray is the same as that of an energy transition in the nucleus. In the case of gamma rays, such a resonance is seen in the technique of Mössbauer spectroscopy. In the Mössbauer effect the narrow resonance absorption for nuclear gamma absorption can be successfully attained by physically immobilizing atomic nuclei in a crystal. The immobilization of nuclei at both ends of a gamma resonance interaction is required so that no gamma energy is lost to the kinetic energy of recoiling nuclei at either the emitting or absorbing end of a gamma transition. Such loss of energy causes gamma ray resonance absorption to fail. However, when emitted gamma rays carry essentially all of the energy of the atomic nuclear de-excitation that produces them, this energy is also sufficient to excite the same energy state in a second immobilized nucleus of the same type.