Neutron capture

Neutron capture or thermal capture . It is a type of nuclear reaction in which a neutron collides with an atomic nucleus , so they combine to form a heavier nucleus.

The main condition for neutrons to be captured is that they and their target nuclei must move at similar speeds , that is, they must have similar temperatures .

A free neutron at a relatively low speed is an unstable particle , with a half-life of 15 minutes, so the neutron capture process is conditioned by this circumstance. When the neutron is captured by the nucleus it usually immediately releases excess energy through a Gamma decay event ; In addition, the new core may undergo beta decay for greater stability.


[ hide ]

  • 1 Types of neutron capture processes
  • 2 Process-r
  • 3 Process-s
    • 1 Neutron absorption
    • 2 Neutron activation
    • 3 Neutron Transmutation
    • 4 Uses
  • 4 Sources

Types of neutron capture processes

There are two types of neutron capture processes: a “fast” capture process r-process and a “slow” capture process s-process . These processes can generate, for the same white nucleus , different isotopes ; Furthermore, some isotopes can only result from one or the other process, but not both.


The r-process (for “fast”) is a neutron capture process for radioactive elements that occurs under conditions of high temperature and high neutron density . It is related to the -s and -p processes. In the r-process the nuclei are bombarded by a high neutron flux to create very unstable nuclei with large numbers of neutrons which, in turn, decay very rapidly to form stable nuclei but always very rich in neutrons .

It is believed that the process-r acts in the core iron of supernovas collapse (Ib types, Ic and II), where physical conditions exist. However, the low observed abundance of elements resulting from the r-process requires that either only a small fraction of the elements created in this way are released to the exterior of the supernova, or that in each supernova only small mechanisms are formed by this mechanism. quantities of items.

Due to the very high neutron flux in this process (of the order of 1022 neutrons per cm2 per second), the rate of isotopic formation is much higher than that of the subsequent beta decay , so the elements created by this path rapidly ascend the line stability N / Z ( number of neutrons / number of protons or atomic number ), even through areas of instability, where energy separation neutron (neutron English drip line) is zero. The neutrons accumulate, creating new isotopes until they reach the region where the atomic mass it is 270 (rutherfordium – darmstadtium zone), where they undergo spontaneous fissions due to the instability of the nucleus formed.

The observed element abundance peaks show evidence of rapid neutron capture followed by subsequent beta decay, since the r-process abundance peaks are 10 amu below those formed by the s-process, indicating that the ascent by the N / Z line gives rise to closed neutron layers with enough proton deficiency to make the peaks solvable.

The r-process involves multiple neutron capture, which produces an unstable nucleus that rapidly decays through a series of beta decays until it reaches a stable isotope. This process is relevant in stellar nucleosynthesis due to the large number of free neutrons present.


The s-process (slow, “slow”) involves the capture of a single neutron that produces a stable nucleus, or decays by beta decay in a stable nucleus before another neutron capture can occur. It is a type of nucleosynthesis that requires conditions of lower neutron density and lower temperature in stars than the r-process. Under these conditions, the neutron capture rate by the nuclei is slow when compared to the rate of beta decay.

Stable isotopes are obtained by moving along the stability valley within the isotope table . The s-process produces about half of the elements heavier than iron and therefore plays an important role in galactic chemical evolution . The s-process differs from the faster r-process in terms of reaction pathways and reaction conditions.

The s-process is believed to occur in stars more massive than the Sun , primarily those located on the asymptotic giant branch. Unlike the r-process, which can run for seconds in explosive environments, the s-process can take thousands of years.

The degree to which the s-process increases the atomic number of elements along the isotope table depends essentially on the star’s ability to produce neutrons and the initial amount of iron present. Iron is the necessary starting material for this type of neutron capture + beta decay, from which new elements are synthesized.

The main sources of neutrons are:

  • 13 C + α → 16 O + n
  • 22 Ne + α → 25 Mg + n

It is easy to see which is the main source of neutrons and which is the secondary one. The main source produces heavy elements beyond strontium (Sr) and yttrium (Y), to lead (Pb) in metal- poor stars . The main component’s production site is the least massive stars in the giant asymptotic branch. The secondary component of the s-process includes elements of the iron group to Sr and Y, and starts the end of the cycle of combustion of helium and carbon in the most massive stars.

The s-process is often treated mathematically using the so-called ‘local approximation’, which provides a theoretical model of the abundances of the different elements based on the assumption of a constant neutron flux within the stars, so that the ratio of abundances be inversely proportional to the neutron capture ratio per cross section for each isotope.

This approach is, as its name suggests, only valid locally for isotopes of similar masses. Due to the relatively low neutron fluxes expected for the s-process to occur (on the order of 105 to 1011 neutrons per cm2 per second), elements beyond the radioactive isotopes of thorium or uranium cannot be obtained . The cycle that ends the s-process is:

  • 209 Bi + n ° → 210 Bi + γ
  • 210 Bi → 210 Po + β-
  • 210 Po → 206 Pb + α

It is then when 206 Pb captures three neutrons giving 209 Pb, which in turn disintegrates emitting an electron resulting in 209 Bi, resuming the process.

Neutron absorption

The nucleus of atoms is made up of neutrons and protons . If a nucleus is bombarded with neutrons, it has a certain probability of incorporating it into its composition. That probability is given by an amount called the absorption cross section. When an isotope with n neutrons and z protons thus incorporates a new neutron, it becomes an isotope with n + 1 neutrons and z protons.

Neutron activation

When the resulting isotope is radioactive, the phenomenon is called Neutron Activation. This effect causes a series of radioactive isotopes to appear in places where neutrons are produced, such as nuclear power plants, since in many cases the isotopes that have been activated turn out to be unstable. A typical example of this neutron activation is Cobalt -60, produced by the iron that exists in the components of a nuclear reactor , and which is used routinely in Cobaltotherapies or Curieterapias for the treatment of cancer .

Neutron Transmutation

The Neutron Transmutation phenomenon is known for the process in which an activated radioactive [[isotope [[ decays and decays) giving rise to an isotope child of a different element .


Neutron capture can be used as a method of non-destructive analysis of materials. Different elements emit different characteristic radiation patterns when subjected to a neutron capture process. This makes it a very useful technological process in fields such as mining or security.



Leave a Comment