Gamma radiation


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Gamma radiation (illustrative representation)

Gamma radiation - also written ɣ radiation - is in the narrower sense a particularly penetrating electromagnetic radiation , which arises from spontaneous transformations ("decay") of the atomic nuclei of many naturally occurring or artificially generated 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 a broader sense, gamma radiation is used to denote 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 of origin of the radiation is not known (for example in astronomy) or is indifferent 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).

The small Greek letter (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 that remains behind ( daughter nucleus ) is in an excited state after a radioactive alpha or beta decay ; this is true for many, but not 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 called a gamma transition or "gamma decay", although the nucleus in no way "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 higher- energy quantum.

spectrum

Measured gamma spectrum of 60 Co , 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 can be imagined as a pulsating rugby ball - builds up an oscillating electromagnetic quadrupole field . However, a gamma quantum can only absorb dipole oscillations; its emission is therefore relatively unlikely. According to the energy-time uncertainty relation , the lifetime of a transition is inversely proportional to its energy uncertainty or line width :

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The lifetimes of excited nuclear 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. It is, although “long” in the core physical sense, from a practical point of view mostly very short (fractions of a second). If you want to use gamma radiation for research, medical or technical purposes - for example the cascade of two photons of 1.17 and 1.33 MeV emitted by the 2.5 MeV state of the nuclide 60 Ni - you therefore need a preparation of the beta emitter 60 Co . This nuclide decays to the desired 60 Ni state with a half-life of 5.26 years .

60 Co decay scheme

For this practical reason, gamma rays (not only for 60 Ni, but in general, also in scientific and technical documents, tables, nuclide maps , etc.) are always assigned to the mother nuclide of the preceding alpha or beta decay, in the example 60 Co: Man speaks of Cobalt-60 radiation, cobalt cannon , etc., even if it is only about the gamma radiation that is emitted by the daughter core 60 Ni.

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

Pair annihilation

In 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. This gamma quantum carry along the energy of the mass corresponding to the destroyed particles less eventual binding energy if the two particles were already bound to each other or one another, "circling", and plus any pre-existing kinetic energy .

Gamma-ray bursts in astronomy

Gamma-ray bursts (English gamma ray bursts) - also known as gamma-ray explosions - are one of the most energetic phenomena in the universe is your formation mechanism is only partially understood.. The spectrum is continuous with photon energies from about 1 keV up 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 classical sense to identify the origin of the radiation (gamma radiation from nuclear processes, X-rays from high-energy processes with electrons). Other authors, on the other hand, 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 to shield .

In contrast to the Bragg curve for charged particle radiation, 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 compared 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. Secondary radiation such as released electrons and X-rays occur in the tissue . 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 occur on the irradiated organism itself ( somatic damage) or, through damage to the genetic make-up , on its offspring as genetic damage.

The functionality of the cells is initially mostly retained even with high radiation doses. As soon as the cell divides or produces proteins, changes to the genetic material and damage to cell organelles can lead to the death of the cell. The radiation disease affects so only after some time fatal if certain vital cell types that regularly die off even in healthy people and re-formed, no longer exist in sufficient numbers. Blood cells are particularly affected by this. Alternatively, the mutations caused by the radiation can lead to uncontrolled cell division, with the dividing cells mostly losing their original biological function. It caused tumors that beyond metastases can form ( cancer ).

Applications

Gamma emitters used in technology are mainly 60 Co , 75 Se , 169 Yb and 192 Ir . 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 .

medicine

Gamma radiation from radioactive sources is used in radiation therapy . The radiation energy in teletherapy must be as high as possible, values ​​of up to 23 MeV are possible; is used e.g. B. 60 Co , which emits gamma quanta with energies 1.17 MeV and 1.33 MeV. However, due to the need for high-energy photons and the safety problems associated with radioactive emitters, gamma radiation in teletherapy is usually obtained as electron bremsstrahlung on a tungsten plate and is also referred to as high-energy X-ray radiation . The electron beam is generated with a linear accelerator . In contrast to radioactive sources, this can be switched on or off as part of the treatment.

In brachytherapy ("irradiation from the inside"), gamma radiation is applied by means of small preparations introduced into the body, usually 192 Ir .

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

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. In the level measurement with gamma radiation one uses this circumstance, because the measured radiation intensity depends on whether there is in the considered vascular a medium or not.

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

Border protection and border searches

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

On behalf of the Ministry for State Security of the German Democratic Republic , so-called gamma cannons with the radioactive 137 Cs were installed at the border control points on the inner-German border . These x-rayed the vehicles driving from east to west in order to track down refugees from the GDR .

Sterilization, germ reduction, radiation-chemical crosslinking

For radiation sterilization and cross-linking of polymer -Kunststoffen be gamma irradiation facilities used. They work almost exclusively with 60 Co, which is produced from 59 Co in nuclear reactors through neutron capture . The radiation safety of 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, the onion irradiation, which was carried out in the GDR 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 German reunification, these approvals expired.

There are large-scale irradiation systems for. 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. In addition, if the half-life of the excited state is 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 radiation 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 .

See also

literature

  • Werner Stolz, radioactivity. Basics - Measurement - Applications , Teubner, 5th edition 2005, ISBN 3-519-53022-8
Nuclear physics
Research history
  • Milorad Mlađenović, The History of Early Nuclear Physics (1896–1931) , World Scientific 1992, ISBN 981-02-0807-3
Radiation protection
  • Hanno Krieger: Fundamentals of radiation physics and radiation protection . Vieweg + Teubner 2007, ISBN 978-3-8351-0199-9
  • Claus Grupen, 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 Radiation Science. 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

Commons : Gamma Radiation  - collection of images, videos and audio files
Wiktionary: Gamma radiation  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Information letter from BG RCI ( Memento of April 2, 2015 in the Internet Archive ) (PDF; 136 kB).
  2. There is no escape mirror from December 19, 1994
This version was added to the list of articles worth reading on June 15, 2007 .