Activation (radioactivity)
In physics, activation is the conversion of stable into unstable, radioactive substances ( radionuclides ) through irradiation. Activation is always a consequence of nuclear reactions . In principle, any type of nuclear reaction can leave behind radioactive products.
Neutron radiation
Thermal neutrons
Neutron activation is of particular practical importance . This is generally understood to mean the capture of a thermal neutron, because most nuclides have large cross- sections for this purpose . Neutron activation in a research reactor or by means of a neutron source is an important, highly sensitive detection method for trace elements. It is also used to produce radionuclides for e.g. B. Medical purposes.
In many cases, for example in the structural materials of nuclear reactors , activation by neutron capture is an undesirable effect. It is also possible that activation of large particle accelerators generates radioactive substances in the soil which, under certain conditions, can then be transported with the groundwater .
Fast neutrons
The activation with fast neutrons is based on nuclear reactions of the types (n, p) or (n, alpha), with very high neutron energy (from about 10 MeV) also (n, 2n). It is occasionally used as an analytical method for elements for which thermal neutron capture does not result in a readily measurable radionuclide.
As an undesirable effect on the material of future fusion reactors , it will cause most of the radioactivity of these facilities.
Photon radiation
Gamma radiation and bremsstrahlung can have an activating effect through the nuclear photo effect. Depending on the element, this occurs from a gamma energy of about 2 MeV ; a neutron or a proton is ejected from the nucleus, which in turn triggers further nuclear reactions. In water-containing media this applies above all to the deuterium, which is always naturally contained as heavy water ; In addition, the existing water acts as a moderator and slows the neutrons down to 'thermal' energy, with which the subsequent reactions then occur.
In industrial irradiation systems for food, 60 Co is used as a radioactive radiation source . It emits gamma quanta of mean energy of 1.25 MeV. This energy is not enough to activate typical food components. There are a few nuclides with low threshold energy, but these are rarely found in food (e.g. indium) and their cross-section for activation is so small that this theoretical activation can hardly be measured in practice.
If high-energy particle radiation (e.g. electrons) triggers bremsstrahlung in heavy material, this can also cause the nuclear photo effect. For this reason, the maximum energy is required by law for irradiating food with electron beams (see Codex Alimentarius ).
Charged particle radiation
Charged particle beams such as alpha and beta radiation (from radioactive decay) as well as accelerated electrons in matter give off their energy piece by piece in successive collisions to the electrons in the atomic shell and only reach an atomic nucleus in a fraction of the cases. However, if the particle energy is sufficiently high, there is an indirect effect: the particles can generate bremsstrahlung when slowing down, which can cause U. can cause activation by nuclear photo effect.
The activation effect of particle radiation is generally insignificant compared to the direct harmful effects that result from the impact energy transferred to the material or tissue. A small part of the absorbed energy is converted directly into heat, but the greater part is consumed by ionization; hence the name 'ionizing radiation'. With the ions as particularly chemically reactive components (atoms or molecules), chemical reactions then take place that can have undesirable or desired effects.
Nuclear fission
The nuclear fission also leads to radioactive products, but is usually not referred to as activation.
Individual evidence
- ↑ N. Prolingheuer, M. Herbst, B. Heuel-Fabianek, R. Moormann, R. Nabbi, B. Schlögl, J. Vanderborght (2009): Estimating Dose Rates from Activated Groundwater at Accelerator Sites. Nuclear Technology , Vol. 168 / No. 3, pp. 924-930