How do materials like lead absorb radiation
Shields against ionizing radiation serve to protect people (see radiation protection), other living beings, objects or components against radiation damage, as well as to reduce the background during radiation measurements. Neutron radiation is also to be counted as ionizing radiation here.
Ion radiation, e.g. alpha radiation, emits its energy in many individual impacts when it penetrates matter. For a given ion energy, it has a certain range in every material, at the end of which the particles are slowed down to thermal energy and are harmless. The range of alpha particles from radioactive sources in room air is a few centimeters.
Fast electrons, e.g. beta particles, also give off their energy in matter in many individual collisions. They have longer, but also well-defined ranges in matter. The range for 10 MeV electrons is 3 m in room air and 0.6 mm in iron. See also beta decay.
X-rays or gamma rays are attenuated in matter according to an exponential law (absorption law). So it cannot be completely shielded, but there is a half-value thickness for a given photon energy and a given material. For example, this is about 1.1 cm for the gamma radiation of cobalt-60 (1.17 and 1.33 MeV) in lead; with 11 cm lead, ten half-life thicknesses, one achieves a weakening by a factor of 210 = 1024.
Photons give their energy to the atomic electrons. Their attenuation increases sharply with an increasing number of electrons per atom, the atomic number. That is why materials with a high atomic number, such as lead, or concrete with iron ore or similar additives, are used. Viewing windows in screens are made of glass with a high lead content.
Shields containing boron or cadmium are used for thermal, i.e. slow neutrons, as these elements have large cross-sections for neutron capture. For fast or mixed neutron radiation, a hydrogen-containing material such as water, paraffin or polyethylene, which acts as a moderator, in a mixture with a boron compound is suitable. Depending on the energy spectrum of the neutrons, it can also be more material and space-saving to first slow down the fast neutrons by (inelastic) scattering in a layer of, for example, iron and only to arrange the moderator and boron or cadmium behind it.
Because of the only weak interaction of the neutrinos, shielding against these particles by practical means on earth is not possible, but also not necessary.
The radiation does not disappear "without a trace" in the shields, but gives off its energy, which can be released in the form of other radiation. Fast electrons generate braking X-rays, while neutrons generate gamma radiation when they are captured in atomic nuclei. Therefore, the above information is only an initial guide to designing shields.
- Krieger, H. (2004): Basics of radiation physics and radiation protection. 628 pp., ISBN 3-519-00487-9
- Shultis, K., Faw, R.E. (2000): Radiation shielding. American Nuclear Society, XVI, 537 pp. ISBN 0-89448-456-7
Category: nuclear physics
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