Shielding against ionizing radiation reduce the radiation intensity and are used to protect persons (see radiation protection), other living beings, objects or components against radiation damage as well as to reduce the background in radiation measurements.
Charged particles
Charged particles, for example alpha or beta radiation, each have a specific range in matter $ R $ , which depends on particle type, particle energy and material. Shielding thicker than $ R $ completely suppresses the incident radiation. The range in air of atmospheric pressure is z. B. for electrons of energy 10 MeV about 3 m, for alpha radiation of the same energy only 10 cm.
The definite range is explained by the fact that the particles do not release their energy to the absorber all at once, but in many small steps – by exciting outer shell electrons and, to a lesser extent, by knocking out inner electrons from the atom (see also Linear Energy Transfer, LET). Elastic scattering processes at the electron shell and Coulomb scattering at atomic nuclei also contribute something to absorption by lengthening the path of charged particles in the absorber.
For nonrelativistic particles of mass $ m $ and charge $ z $, the linear energy transfer $ D_ $ is inversely proportional to the energy $ E $ and depends on the density $ \rho $ and the ratio of the atomic number $ Z $ to the mass number $ A $ of the material:
The inverse dependence on the energy (decreasing continuously along the path) is reflected in the Bragg curve. The abrupt drop beyond the Bragg peak determines the range $ R $ . For alpha particles, this corresponds to the emission energy $ E_0 $ in air to Geiger’s empirical non-relativistic range law:
$ R = 3,1 \cdot E_0^ $ ( $ E_0 $ in MeV, $ R $ in mm)
At high energies and large absorber nuclear charge number, bremsstrahlung also provides a noticeable contribution to the energy dissipation $ D $ :
For electrons this range starts at about the energy 5 MeV, for protons only in the TeV range:
X-rays and gamma rays
High-energy electromagnetic radiation such as X-rays or gamma radiation is attenuated approximately exponentially in matter. Therefore, it has no definite range, but its reduction can be described by a half-thickness depending on quantum energy and material. This amounts to z. B. for quanta of 2 MeV in lead about 1.3 cm. (However, at high quantum energies, the radiation dose initially increases slightly with depth when entering matter, see buildup effect.).
The exponential attenuation – described by the mass attenuation coefficient – is essentially based on three different interaction processes, whose relative contributions depend on the quantum energy and on atomic number (nuclear charge number) Z and mass number A of the shielding material. Two other mechanisms, nuclear photoelectric effect and normal elastic scattering, are usually negligible for the shielding effect.
The most important single process is
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for energies up to ca. 1 MeV the photoelectric effect; attenuation
At low and high energies, heavy elements, i.e. elements with a high nuclear charge number, therefore absorb particularly well. That’s why most gamma and x-ray shielding is made of lead. Glass with high lead content is used for viewing windows. In the medium energy range, where Compton scattering is important, the properties of shielding materials differ only slightly. Neutron-poor light nuclides are somewhat more effective here at the same mass occupancy, but are not very common in practice because of the necessary greater layer thickness.
When radiation is attenuated, electrons are released as secondary particles and, at very high energies, protons and neutrons are also released due to the nuclear photoelectric effect. They are u. U. for your part new sources for gamma radiation and tertiary particles. To avoid spallation reactions with heavy elements, shielding against high-energy radiation is made of a combination of heavy and light materials.
Neutrons
Neutron radiation generally penetrates matter easily, because its interaction is only with the atomic nuclei, not with the electron shell. Shielding against neutrons usually works by a combination of scattering (elastic or inelastic) and absorption in nuclear reactions.
Thermal neutrons can be shielded very effectively with materials containing boron or cadmium, since these elements have large absorption cross sections for this purpose. A cadmium sheet of 1 mm thickness reduces the passing thermal neutron flux to about 10 -5 -fold, i.e. to 1 hundred thousandth. However, neutron radiation with a purely thermal energy spectrum hardly ever occurs, because free neutrons always have much higher energy when they are released.
For fast or mixed neutron radiation, a hydrogen-containing material such as water, kerosene or polyethylene, which acts as a moderator, is suitable when mixed with z. B. of a boron compound. For neutrons in the MeV range it is usually more material- and space-saving to slow them down first by inelastic scattering in e.g. iron and to arrange moderator and boron or cadmium only behind it.
Neutrinos
Because of the exclusively weak interaction of neutrinos, a shielding against these particles is not possible with practical means on earth. Since their interaction with matter, also tissue of living beings, is so extremely small, a shielding for radiation protection is not necessary.
Secondary radiations
Interactions of radiation of high particle or quantum energy with matter produce secondary radiation, which in many cases is more difficult to shield than primary radiation.
- The deceleration of charged particles in matter produces bremsstrahlung.
- Activation of the shielding material – especially important in the case of neutrons – leads to the emission of the corresponding radiation.
- Spallation (the shattering of atomic nuclei by collisions with energies of the order of GeV) produces particle showers of different compositions. The spallation of heavy elements creates new medium-heavy atomic nuclei, which cause further showers. On the other hand, the fragments of light elements are mainly protons.
Special shieldings
Low radiation shielding materials
With radiation measurements, z. B. in gamma spectroscopy, shielding is usually necessary to keep radiation other than that under investigation away from the measuring device. For measurements of weak sources, materials must be used that emit as little or no gamma radiation of their own as possible. [1] Possibly occurring electrons or alpha particles from the shielding material can be suppressed by an additional inner lining of the measuring chamber with plastic, for example. However, two very common materials, steel and lead, usually have some natural radioactivity that also produces gamma radiation. In the case of lead, this is caused by the lead isotope 210 Pb (half-life 22 years), which is contained in lead ore as a product of the uranium-radium decay series [2] , and in the case of steel by various radionuclides contained in the alloy. Therefore, for the aforementioned applications, low-radiation lead or steel is used, which has either been purified by elaborate technical processes, has been produced from special ores, or whose production has long been (d. h. many half-lives) back, such as hull material or lead ballast from sunken ships. [3]
Swimming pool reactors and decay ponds
Swimming pool reactors use a layer of water several meters thick as coolant and shielding. At nuclear power plants, spent fuel is stored in similar decay ponds. The water in the pools serves,