Since neutrons are electrically neutral, the charges on atomic nuclei and electrons have no influence on their movement. Neutron radiation therefore penetrates matter relatively easily. The ionizing effect arises indirectly, mostly through the impact of light atomic nuclei or their components (e.g. protons ), which in turn have an ionizing effect. Such collisions make the neutrons lower in energy (slower).
The main effect of slow, especially thermal neutrons is based on their ability to attach to atomic nuclei ( neutron capture ). An isotope of the capturing atom is formed with a mass number increased by 1 . Many of the isotopes that are created in this way are radioactive , so that ionizing radiation can still occur a long time after neutron irradiation (depending on the half-life of the isotope) due to the decay.
The free state of the neutron always ends after a very short time with a neutron capture or another nuclear reaction . Only in a high vacuum does a free neutron have a “chance” to “experience” its radioactive decay .
The cosmic radiation is in the atmosphere or on the ground by interaction with molecules natural neutron radiation free. The natural decay of atomic nuclei rarely produces neutron radiation; they are produced artificially with the help of neutron sources . In the nuclear reactor , during nuclear fission, neutrons are released, as is during nuclear fusion .
Another powerful source are neutron bombs . With the help of neutron radiation, it can kill people in the target area, but leave buildings and infrastructure relatively undamaged.
In materials research, neutron beams are used to determine the atomic or molecular structure of solids ( neutron scattering ). To monitor the sub- criticality of a nuclear reactor, the neutron radiation can e.g. B. a radium-beryllium neutron source can be used. In radiation therapy , attempts were made to kill cancer cells with neutron beams; Because of the side effects in healthy tissue, this is rarely used.
See also: Research with Neutrons
Harmful effect on living things
The most important harmful effect of fast neutrons in living tissue is the elastic scattering of hydrogen . It generates recoil protons , which in turn strongly ionizing and thus be harmful in the tissue. An indirect damage by thermal neutron radiation comes through the gamma radiation concluded that when capture is formed of a neutron of hydrogen: 1 H + n → 2 H + 2.2 MeV .
The harmfulness of neutron radiation is taken into account by the high radiation weighting factors of the German Radiation Protection Ordinance with values from 5 to 20.
Harmful effect on structural materials
Neutron radiation generally has a negative impact on structural materials such as steel (see radiation damage ). By scattering on atomic nuclei, neutrons create defects in the crystal lattice, which usually lead to the embrittlement of the material. The activation and the associated conversion of alloy components can also have (mostly negative) effects on the material properties. These processes occur particularly in places with very high neutron fluence , such as reactor pressure vessels , their internals and the fuel rod cladding . Similar large neutron fluences occur in fusion reactors , whereby the energy of the neutrons is also particularly high here. Therefore, developing materials for future fusion power plants is a major challenge.
A shielding against neutron radiation, usually uses a combination of physical effects, and is composed of several materials in layers. A moderator , for example water, paraffin , graphite or plastic, slows down fast free neutrons. Thermal neutrons are absorbed by cadmium or boron , for example . The accompanying gamma radiation is reduced in particular by correspondingly thick layers of concrete , steel and lead .
- The "Radiation Protection Glossary" from Forschungszentrum Jülich explains many terms relating to ionizing radiation (alpha, beta, gamma radiation, regulations, radiation protection, etc.)
- name = ITER the ITER International Team: Materials Challenges for ITER. (PDF) Retrieved August 18, 2016 .