Neutron capture

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Neutron capture (term in nuclear physics and nuclear technology ; English neutron capture ) or neutron attachment (term in astrophysics ) in the narrower sense is a nuclear reaction in which an atomic nucleus absorbs a neutron without releasing particles with a mass . The core emits the binding energy gained rather than gamma radiation . According to its formula notation - see examples below - this type of reaction is also called the n-gamma reaction .

However, neutron reactions with emission of mass particles are occasionally referred to as neutron capture, especially if their excitation function is similar to that of the n-gamma reactions. This applies, for example, to the n-alpha reaction at boron-10, as the name boron neutron capture therapy shows.

Nuclide map with cross-section for neutron capture

Since the neutron in contrast to the proton no electrical charge carries and is therefore not rejected by the atomic nucleus, it can him with low kinetic energy easy approach. The effective cross-section for the capture is even in general particularly large with thermal , i.e. very small, neutron energy.

In stars , neutron deposition takes place as an s or r process . It plays an important role in cosmic nucleosynthesis , because it explains the formation of elements with mass numbers above about 60, i.e. atoms that are heavier than iron or nickel atoms . These can be caused by thermonuclear reactions , i.e. H. by nuclear fusion , cannot be formed in stars .

Neutrons released in normal surroundings on earth are in the vast majority of cases, after being braked to thermal energy, captured by nuclei in this way. Technical applications of neutron capture are for example:

The picture on the right shows a nuclide map with color coding of the cross-section for neutron capture ( neutron capture cross-section ). The magical proton and neutron numbers are highlighted by double lines ; one recognizes that this cross-section of such magic atomic nuclei is usually small, but far from magic numbers it is large.

Neutron capture with small neutron flux

If the neutron flux is not too high , for example in the case of neutron radiation in a nuclear reactor , one neutron is captured by one atomic nucleus. The mass number (number of nucleons in the nucleus) increases by 1. For example, when natural gold , 197 Au, is irradiated , the gold isotope 198 Au is created in a highly excited state, which changes very quickly to the ground state of 198 Au by emitting a γ quantum . In formula notation:

or short:

The gold isotope 198 Au is a β - radiator , so its nucleus decays to the mercury isotope 198 Hg through the emission of an electron and an electron antineutrino .

The s-process inside stars mentioned above works essentially the same way.

Importance in nuclear engineering

On ordinary hydrogen there is a trapping reaction with a noticeable cross-section :


This absorption of hydrogen means that a light water reactor with natural uranium cannot become critical .

The neutron absorbers used for reactor control and in shielding against neutrons are mostly based on neutron capture.

In the nuclear fuel uranium , transuranic elements , especially plutonium, are formed through neutron capture (see also breeder reactor ).

Neutron capture with a large neutron flux

In the r-process in the star's interior, the neutron flux density is so high that the atomic nucleus has “no time” for beta decay between neutron capture, ie the mean time interval between neutron capture is short compared to the half-life of beta decay. The mass number increases sharply without the ordinal number increasing. Only then do the resulting highly unstable nuclides decay through several successive β - decays to form stable or more stable (longer-lived) nuclides with correspondingly higher atomic numbers.

See also


Individual evidence

  1. See BL Cohen, Concepts of Nuclear Physics , McGraw-Hill 1971, p. 338.