Difference Between Neutron Capture and Absorption

The key difference between neutron capture and absorption is that neutron capture refers to the combination of a neutron and a heavy nucleus via collision whereas neutron absorption refers to the formation of a compound nucleus when a nucleus completely absorbs a neutron.

Neutron capture and neutron absorption are two types of nuclear reactions. Both these process involve the combination of a nucleus and a neutron to form a compound nucleus; however, the method of combination is different from each other. In the neutron capture process, a collision occurs whereas, in the neutron absorption process, a fission occurs.

1. Overview and Key Difference
2. What is Neutron Capture
3. What is Neutron Absorption
4. Side by Side Comparison – Neutron Capture vs Absorption in Tabular Form
5. Summary

What is Neutron Capture?

Neutron capture is a technique used in nuclear reactors where an atomic nucleus undergoes collision with a high-speed neutron. Here, an atomic nucleus of a heavy element collides with one or more neutrons and merge to form a heavier atomic nucleus. Thus, this process is very important in cosmic nucleosynthesis.

Neutron has no electrical charge. This means neutrons are neutral (which led to naming them as neutrons). Therefore, they can easily enter a foreign atomic nucleus. If they were positively charged as protons, then the protons already present in the nuclei will repel the incoming neutrons.

In systems where we can observe a small neutron flux (ex: nuclear reactor), an atomic nucleus captures a single neutron (other than capturing two or more neutrons). For example, when naturally occurring gold isotopes are irradiated by neutrons, an unstable isotope of gold forms in an excited state, which then quickly undergoes radioactive decay to obtain its ground state. Here, the mass number increases by one because ¹⁹⁷Au converts into ¹⁹⁸Au. Gamma rays are emitted during the radioactive decay process. Moreover, if we use thermal neutrons in this neutron flux, then the process is called thermal capture rather than neutron capture.

Difference Between Neutron Capture and Absorption

Figure 01: Neutron Capture Process in Stars

In systems where we can observe a high neutron flux, such as in stars, the atomic nuclei have no time for radioactive decay between neutron-capture processes. Therefore, the mass number of atomic nuclei rises gradually, rather than decreasing like in nuclear reactors. However, the atomic number stays the same because protons are not involved in this process. Hence, we can observe the same chemical element (the type of chemical element is determined by the atomic number).

What is Neutron Absorption?

Neutron absorption is a technique used in nuclear reactors in which an atom completely absorbs a neutron to form a compound nucleus. It is the most important type of nuclear reaction we use in nuclear reactors. Here, the mode of decay of the newly formed atomic nucleus does not depend on the method that the neutron absorption occurred. Therefore, we can observe a variety of emissions followed by the absorption. E.g. radioactive capture results in gamma radiation.

Generally, the end product of neutron absorption reaction tends to divide into two parts while releasing some neutrons and a considerable amount of energy. This process primarily follows the kinetics of fission reactions.

What is the Difference Between Neutron Capture and Absorption?

Neutron capture and neutron absorption are two types of nuclear reactions. The key difference between neutron capture and absorption is that neutron capture refers to the combination of a neutron and a heavy nucleus via collision whereas neutron absorption refers to the formation of a compound nucleus when a nucleus completely absorbs a neutron.

Moreover, another significan difference between neutron capture and absorption is that in the neutron capture process, collision occurs whereas, in the neutron absorption process, fission occurs.

Summary – Neutron Capture vs Absorption

Reference:

1. International, British Electricity. “Nuclear Physics and Basic Technology.” Nuclear Power Generation, 1992, pp. 1–110., doi:10.1016/b978-0-08-040519-3.50008-1.

Image Courtesy:

1. “Rapid neutron capture” By Daveturnr – Own work (CC0) via Commons Wikimedia
2. “Alpha Decay” By Inductiveload – Own work (Public Domain) via Commons Wikimedia