Gamma rays somewhere in the galaxy

Gamma radiation

Gamma radiation (illustrative representation)

Gamma radiation - also Ɣ radiation written - in narrower Sense is a particularly penetrating electromagnetic radiation that arises from spontaneous transformations ("decay") of the atomic nuclei of many naturally occurring or artificially produced radioactive nuclides.

The name comes from the division of ionizing radiation from radioactive decay into alpha radiation, beta radiation and gamma radiation with their increasing ability to penetrate matter. Alpha and beta radiation consist of charged particles and therefore interact much more strongly with matter than the uncharged photons or quanta of gamma radiation. Accordingly, the latter have a significantly higher penetration capacity.

in the further In the sense, gamma radiation denotes any electromagnetic radiation with quantum energies above about 200 keV, regardless of how it originates. This corresponds to wavelengths shorter than 0.005 nm (5 pm). In this general sense, the term is used in particular when the process by which the radiation is generated is not known (for example in astronomy) or is irrelevant to the specific task (for example in radiation protection), but it should be expressed that higher energies than with X-rays ( around 100 eV to 300 keV) are present.

The small Greek letter $ \ gamma $ (gamma) is generally used as a formula symbol for a photon of any energy and type of origin.

Emergence

Radioactivity: "gamma decay"

Gamma radiation in the original sense of the word arises when the nucleus (daughter nucleus) that remains after a radioactive alpha or beta decay is in an excited state; this is true for many, but not for all alpha and beta decays. The excited nucleus vibrates or rotates - to put it clearly - for a long time. During the transition to a less highly excited state or the ground state, it releases the released energy in the form of gamma radiation (see decay scheme). This change in state of the nucleus is known as a gamma transition or "gamma decay", although the nucleus by no means "disintegrates into its components" because the number of its neutrons and protons remains constant.

The excited state can also have arisen in other ways, such as neutron capture or other nuclear reactions or the previous absorption of a more energetic $ \ gamma $ -quant.

spectrum

Measured gamma spectrum of 60Co, lines at 1173 and 1332 keV

The wavelengths or energies of the gamma rays are discrete and are characteristic of the respective radionuclide, comparable to the optical line spectrum of chemical elements. The measurement of the gamma spectrum of an unknown substance (gamma spectroscopy) is therefore suitable for providing information about the types and proportions of the radionuclides it contains.

The sharp energies of the gamma spectral lines are explained by the fact that the lifetimes of gamma transitions are comparatively long in terms of nuclear physics. The excited nucleus - which you can imagine like a pulsating rugby ball - builds up an oscillating electromagnetic quadrupole field. A gamma quantum can only absorb dipole oscillations; its emission is therefore relatively unlikely. According to the energy-time uncertainty relation, the lifetime $ \ tau $ of a transition is inversely proportional to its energy uncertainty or line width $ \ Gamma $:

$ \ Gamma = \ frac {\ hbar} {\ tau} $.

The lifetimes of excited core states are always greater than about 10−15 Seconds and therefore lead to discrete photon energies with half-widths below 0.3 eV.

Designation according to the mother nuclide of the alpha or beta decay

The average delay or half-life between the alpha or beta decay and the gamma transition depends on the nuclide and the respective excited state. Even if it is “long” in the core physical sense, from a practical point of view it is usually very short (fractions of a second). If you want to use gamma radiation for research, medical or technical purposes - for example that of the 2.5 MeV state of the nuclide 60Ni sent out cascade two photons of 1.17 and 1.33 MeV - you therefore need a preparation of the beta emitter 60Co. This nuclide decays to the desired one with a half-life of 5.26 years 60Ni state.

For this practical reason, gamma rays (not only in the 60Ni, but in general, also in scientific and technical documents, tables, nuclide maps, etc.) always that mothernuclide of the preceding alpha or beta decay, in the example dem 60Co, assigned: One speaks of cobalt-60 radiation, cobalt cannon, etc., even if it is only about the gamma radiation from the daughter nucleus 60Ni is emitted.

The rare cases of excited atomic nuclei whose gamma transitions have half-lives of seconds, minutes or even longer are called metastable or referred to as core isomers. Only in these cases is the actual gamma-emitting nuclide used as a designation. One example is the technetium isotope 99mTc, which is used in medical diagnostics (see scintigraphy).

Pair annihilation

During pair annihilation, the reaction of a particle with the associated antiparticle, photons also arise (alone or in addition to other possible reaction products), which are also called gamma radiation. These gamma quanta together carry the energy that corresponds to the mass of the destroyed particles, minus the possible binding energy, if the two particles were already bound to each other or "orbited" each other, plus any kinetic energy that may have existed before.

Gamma-ray bursts in astronomy

Gamma ray bursts - also known as gamma ray explosions - represent one of the most energetic phenomena in space. The mechanism of their formation has only rudimentarily been clarified. The spectrum is continuous with photon energies from about 1 keV to the MeV range. Among other things, it contains X-rays. It is not about gamma radiation in the narrower, nuclear physical sense (see introduction).

Terminology: gamma rays and x-rays

The energy ranges of natural gamma and X-rays overlap, which results in a certain blurring of these terms. Some authors continue to use the terms in the classic sense to identify the origin of the radiation (gamma radiation from nuclear processes, X-rays from high-energy processes with electrons). Other authors, however, differentiate according to the quantum energy, the dividing line then being around 100 to 250 kiloelectron volts. However, there is no precise definition for this. To avoid misunderstandings, it is therefore always useful to explicitly state the quantum energy and its creation process. On the other hand, precisely this exact information in popular scientific literature regularly leads to difficulties in understanding, because many readers are overwhelmed with keV information or terms such as bremsstrahlung or synchrotron radiation, while the terms gamma and x-ray radiation are generally known. Authors therefore have to weigh up between comprehensibility and vagueness of their wording.

Interaction with matter

Gamma radiation is the most complex ionizing radiation that needs to be shielded.

In contrast to the Bragg curve for charged particle radiations, the intensity (and thus the energy input) of the gamma radiation decreases exponentially with the depth of penetration. This means that the number of gamma rays is halved after each half-value thickness. The half-value thickness depends on the wavelength of the gamma radiation and the atomic number of the shielding material: lead is therefore the most common material used for radiation protection against gamma radiation. Its half-value thickness for gamma radiation with an energy of 2 MeV is 14 mm. This clearly shows the much more penetrating effect in comparison to charged particle radiation.

The most important interaction processes when gamma radiation passes through matter are photoionization, Compton scattering and pair formation.

Biological effect

If gamma radiation is absorbed in human, animal or plant tissue, its energy becomes effective in ionization and other processes. This occurs in the tissue Secondary radiation like released electrons and X-rays. Overall, the breaking of chemical bonds results in effects that are mostly harmful to the organism. The extent of the overall effect is described by the dose equivalent. The consequences can be on the irradiated organism itself (somatic Damage) or, through damage to the genetic make-up, to its offspring as genetic Damage occur.

The functionality of the cells is initially mostly retained, even with high doses of radiation. As soon as the cell divides or produces proteins, changes in the genetic material and damage to cell organelles can lead to the death of the cell. Radiation sickness therefore only has a fatal effect after some time, when certain, vital cell types, which also regularly die off and are newly formed in healthy people, are no longer available in sufficient numbers. Blood cells are particularly affected by this. Alternatively, mutations caused by radiation can lead to uncontrolled cell division, with the dividing cells mostly losing their original biological function. Tumors develop, which can also form metastases (cancer).

Applications

medicine

Gamma rays from radioactive sources are used in radiation therapy. The radiation energy in teletherapy must be as high as possible; is used e.g. B. 60Co, which emits gamma quanta with energies 1.17 and 1.33 MeV. However, due to the need for the highest possible energy photons and the safety problems associated with radioactive emitters, bremsstrahlung from linear accelerators for electrons is now mostly used in teletherapy. Gamma rays are also used in brachytherapy using small preparations introduced into the body (usually 192Ir).

In scintigraphy and single-photon emission computed tomography, short-lived gamma emitters such as 99mTc, 123I, 131I, 133Xe or 111In used for diagnostic purposes.

Sensor technology and material testing

Gamma rays can penetrate matter without being reflected or refracted. Part of the radiation is absorbed as it passes through, depending on the density and thickness of the medium. This fact is used when measuring the level with gamma radiation, because the measured radiation intensity depends on whether there is a medium in the vessel under consideration or not.

Another application of gamma rays is found in radiographic testing, with the help of which one can detect deposits, corrosion damage or erosion damage on the inside of apparatus and pipelines.

Radionuclide Identifying Devices are used in border protection, which allow conclusions to be drawn about the transported radioactive substances via the gamma radiation.

Gamma emitters used in technology are mainly 60Co, 75Se, 169Yb and 192Ir.[1] A disadvantage of gamma rays is that the radiation sources cannot be switched off. When using gamma radiation in operation, extensive radiation protection measures have to be taken because of its dangerousness.

Sterilization, germ reduction, radiation-chemical crosslinking

For radiation sterilization and for the cross-linking of polymer plastics Gamma irradiation systems used. You work almost exclusively with 60Co that out 59Co is produced in nuclear reactors by neutron capture. Radiation safety in the systems is achieved by the fact that the radiation sources can be lowered into a deep water basin or a deep, shaft-shaped concrete bunker.

The gamma sterilization of medical products, e.g. B. welded emergency kits, has the advantage over other methods that it can be done in the sales packaging.

In the field of food irradiation, onion irradiation, which was carried out in the German Democratic Republic from 1986 to 1990, should be mentioned in particular. There was a specialized gamma irradiation system at the Queis agricultural production cooperative in Spickendorf. In the GDR, many other foods were also irradiated (poultry, spices, whole egg powder, etc.); labeling of the products was not intended. With the accession to the Federal Republic of Germany, all these approvals expired.

There are large irradiation facilities, for example. B. in the Netherlands and in South Africa.

Mössbauer spectroscopy

The recoil that the atomic nucleus normally receives when the gamma quantum is emitted can, under certain circumstances, be taken over by the entire crystal lattice in which it is embedded. As a result, the amount of energy that the photon loses through recoil becomes negligibly small. If the half-life of the excited state is also high, gamma rays with extremely sharp energy are created. The Mössbauer spectroscopy, which is important in chemical analysis, is based on this.

proof

Gamma radiation can be detected through its interaction with matter, e.g. B. with particle detectors such as the ionization chamber or the Geiger-Müller counter tube, scintillation counters, semiconductor detectors or Cherenkov counters.

Research history

In 1900, Paul Villard found a component in the radioactive radiation discovered four years earlier by Antoine Henri Becquerel, which could not be deflected by magnetic fields and showed a very high permeability of matter. Since it was the third ray component found, Ernest Rutherford coined the term Gamma radiation.

By diffraction of gamma rays on crystals, Rutherford and Edward Andrade succeeded in 1914 in showing that it is a form of electromagnetic radiation. The wavelengths found were very short and comparable to those of X-rays.

literature

  • Werner Stolz, Radioactivity. Basics - Measurement - Applications, Teubner, 5th edition 2005, ISBN 3-519-53022-8
Nuclear physics
  • Theo Mayer-Kuckuk, Nuclear physics, Teubner, 6th edition 1994, ISBN 3-519-03223-6
  • Klaus Bethge, Nuclear physics, Springer 1996, ISBN 3-540-61236-X
  • Jean-Louis Basdevant, James Rich, Michael Spiro, Fundamentals in Nuclear Physics: From Nuclear Structure to Cosmology, Springer 2005, ISBN 0-387-01672-4
Research history
  • Milorad Mlađenović, The History of Early Nuclear Physics (1896-1931), World Scientific 1992, ISBN 981-02-0807-3
Radiation protection
  • Hanno Krieger: Basics of radiation physics and radiation protection. Vieweg + Teubner 2007, ISBN 978-3-8351-0199-9
  • Claus groups, Basic course in radiation protection. Practical knowledge for handling radioactive substances, Springer 2003, ISBN 3-540-00827-6
  • James E Martin, Physics for Radiation Protection, Wiley 2006, ISBN 0-471-35373-6
medicine
  • Günter Goretzki, Medical radiology. Physical-technical basics, Urban & Fischer 2004, ISBN 3-437-47200-3
  • Thomas Herrmann, Michael Baumann, Wolfgang Dörr, Clinical radiation biology - in a nutshell, Urban & Fischer February 2006, ISBN 3-437-23960-0

Web links

Individual evidence

  1. ^ Information letter from BG RCI (PDF; 139 kB).