Can ionizing radiation destroy semiconductors

Ionizing radiation is radiation that can ionize atoms or molecules.

Particle radiation and electromagnetic radiation are counted as ionizing radiation if the energy of the particles or the quantum energy is sufficient to release electrons from an atom or molecule - also via intermediate reactions. Radiation is ionizing above the substance-specific ionization energy of around 5 electron volts (eV). In the electromagnetic spectrum, this corresponds to a wavelength of less than approximately 200 nm; therefore only gamma and X-rays as well as shorter-wave ultraviolet radiation have enough quantum energy to release electrons from the atomic shells and thus also to separate atomic bonds. Electromagnetic waves in the infrared, radar and radio waves are in principle not able to change or even break down molecules. Molecules that would be broken down by such low-energy photons cannot exist at room temperature.

Interaction with matter

The eponymous mechanism of ionizing radiation is the fact that it can knock electrons out of atomic shells, whereby the affected atom or molecule becomes an electropositive ion (cation). If the energy is sufficiently high, multiple ionizations occur which, for. B. generate the fog tracks in a cloud chamber. High-energy electrons generate bremsstrahlung in matter, which itself also has an ionizing effect.

Ionizing radiation not only ionizes matter, it can also destroy chemical compounds, that is, break up larger molecules, creating chemical radicals. This is where their biologically harmful effect lies. Ions generated by radiation are also unstable and strive to get the missing electrons out of their surroundings, whereby either the original molecules / atoms are restored (recombination) or other molecules are created by splitting off atoms. Fragments of broken molecules, on the other hand, rarely come together again. They react / combine with other molecules, whereby these usually also lose their biological function.

charged: direct ionizing

Photon: directly ionizing

Neutron: indirectly ionizing

Image: Interaction of ionizing radiation with matter.
In the case of the incident neutron, some typical intermediate processes in hydrogen-containing material are shown.

In the picture, gamma quanta are represented by wavy lines, charged particles and neutrons by straight lines or straight lines. The small circles represent ionization processes.

Photons (gamma quanta) do not ionize continuously on their way like alpha or beta particles (see particle radiation). The interaction of a gamma quantum with matter occurs through one of the following three processes:

  1. Compton effect (see picture: two Compton scatterings taking place one behind the other). With every Compton scattering, the quantum gives off energy to a struck electron and flies on in a different direction with reduced energy.
  2. Photo effect: With the photo effect, the photon gives up all of its energy to an electron. This effect occurs on metal surfaces and in semiconductors even with quantum energies much lower than 5 eV.
  3. Pair formation: In pair formation, the photon disappears; its energy leads to the formation of a particle-antiparticle pair. For example, the creation of an electron-positron pair requires a quantum energy of more than 1 MeV.

Sizes and units of measure

Gray Gy
(SI unit of absorbed dose). The Gray replaces the old designation "Rad" ("radiation-absorbed dose"). It indicates how much energy is absorbed by one kilogram of the irradiated matter. 1 Gray = 1 J / kg = 100 rad; 1 wheel = 0.01 gray
radiation absorbed dose; old unit of absorbed dose, replaced by Gray (Gy)
roentgen-equivalent man; old unit of personal dose, replaced by Sievert (Sv)
old unit of ion dose
Sievert Sv
Unit of dose equivalent; solves the old name Rem (roentgen-equivalent-man) from.

Sources of Ionizing Radiation

Different natural or artificial sources emit ionizing radiation:

  • Cosmic radiation: mainly fast charged particles, secondary radiation through interaction with the atmosphere reaches the earth's surface; responsible for radiation exposure during air traffic
  • Radiation from the sun: Ultraviolet (UV-B is almost completely absorbed and leads, among other things, to sunburn, UV-C is completely absorbed in the atmosphere and, by separating the molecular oxygen, leads to the ozone layer), particle radiation (leads to auroras)
  • Radioactive substances: Alpha radiation, beta radiation, gamma radiation, neutrons, X-rays resulting from electron capture
  • Nuclear reactions in nuclear reactors and accelerator systems: photons and neutrons are important - charged particles usually do not penetrate the housing or walls.
  • X-rays (photons): from X-ray tubes, bremsstrahlung from electron accelerators, cosmic sources; X-rays are also produced unintentionally in high-voltage tubes and picture tubes.

Health effects

Radicals generated by ionizing radiation usually cause greater damage through subsequent chemical reactions than the destruction of the first molecule by radiation alone. This may be desirable, for example in the fight against cancer, as it favors the death of affected cells, in this case ideally tumor cells.

Above a certain radiation dose, so many molecules with a biological function are destroyed at once that the affected cells are no longer viable or too many substances that are too toxic are created, which kill the cell. Changes in the genetic material are also common, which with a certain probability can lead to cancer, but above all to mutations that can lead to malformations in offspring or developing embryos / fetuses and total sterility (infertility) (see also radiation risk). In principle, all ionizing radiation is harmful to the health of living beings. In Germany, radiation protection is regulated by law.

The Short term sequence Too high a dose of radiation is called radiation sickness. It manifests itself in a weakened immune system and burns. At the molecular level, among other things, the damaging effect of radicals produced by radiolysis is involved. Radiation sickness occurs from a short-term exposure of 0.25 Sv. 4 Sv as short-term exposure is usually fatal.

Ionizing radiation occurs in low doses as natural background radiation. This consists, among other things, of cosmic radiation and radiation from radioactive substances that occur naturally in the earth's crust and atmosphere, such as the radioactive component in carbon. The currently measurable dose rate of the background radiation is globally higher than the previously natural background radiation, since above-ground nuclear weapons tests and the Chernobyl catastrophe caused radionuclides to be distributed worldwide in the atmosphere, on land and in water.

The radioactive noble gas radon is continuously created from spontaneous nuclear fission reactions in the earth's crust. It emerges from the earth's crust, can diffuse through concrete, rock layers and masonry, and is therefore often found in higher concentrations, e.g. in cellars over uranium deposits (see radon in houses) and in underground uranium mining due to its high density. If there is insufficient ventilation, it leads to harmful radiation doses with an increased risk of leukemia, since it spontaneously (also in the lungs) decomposes under alpha radiation to form radioactive substances. The radon concentration in basement rooms can be sufficiently reduced by ventilation.

Ultraviolet radiation is generated artificially by the sun and mostly by mercury vapor lamps. It leads to radiation damage on the skin, corneal damage and conjunctivitis. While UV-A (longer-wave ultraviolet near the visible spectral range) is also used therapeutically and cosmetically (against acne, for tanning), the shorter-wave components are, due to the thinning of the ozone layer in the stratosphere, a greater proportion also from the sun to the earth's surface responsible for an increased risk of skin cancer. The shorter-wave ultraviolet radiation from the sun is almost completely absorbed by the earth's atmosphere; however, sun exposure of the skin and eyes, especially in the mountains, requires sun protection (absorbent skin creams, sunglasses).
Natural and artificial ultraviolet radiation also leads to the disintegration of plastics (paints, plastics) and organic dyes (bleaching).

Biological and chemical applications of ionizing radiation

In biology mainly the mutation promoting and sterilizing effect is used. In plant breeding, for example, “radiation-induced mutations” are used to create mutants that can produce modified species. One field of application is “sterile insect technology”, or SIT for short. Male insect pests are sterilized and then released in the target area. The absence of offspring leads to a decrease in the population. The advantage here is that no harmful chemicals are used and other insects remain unaffected.

A dark chapter in German history is the forced sterilization of people using X-rays.

Furthermore, ionizing radiation is also suitable for the sterilization of devices, implants, food and drinking water (sterilization and degradation of organic substances by ultraviolet). This kills microorganisms. However, strict requirements apply to the radiation sterilization of food. The growth of a seedling can be improved by weak radiation, whereas excessive radiation has a growth-inhibiting effect.

Lower organisms such as bacteria can endure much higher doses of radiation than humans. Record holder is Deinococcus radioduransthat can even live in the cooling water of nuclear reactors.

In the production of polymers, irradiation enables crosslinking without generating heat. Large components can also be networked with radiation that penetrates far. Beta radiation (radiation-crosslinked insulating materials) and ultraviolet radiation (curing of synthetic resin lacquer layers) are used, among other things. When activators are added, some polymer reactions can also be initiated by irradiation with visible light.

The change in color of gemstones, glasses and pigmented plastics due to radioactivity is also interesting.

Photolithography (e.g. in microelectronics and circuit board production) uses crosslinking reactions (positive lacquer) or decomposition reactions (negative lacquer), which are caused by ultraviolet, X-ray, ion or beta radiation.

Ultraviolet radiation can also be used for chlorine-free bleaching of cellulose; In the past, the sun's light was used to bleach white textiles. In both cases, the coloring (dirt) components of the fabrics are broken down chemically and converted into volatile or washable substances.

Categories: Radioactivity | Radiation biology