## Neutron Absorption

**The neutron absorption reaction** is the most important type of reactions that take place in a nuclear reactor. The absorption reactions are reactions, where the neutron is completely absorbed and compound nucleus is formed. This is the very important feature, because the **mode of decay** of such compound nucleus **does not depend on the way the compound nucleus was formed.** Therefore a variety of emissions or decays may follow. The most important absorption reactions are divided by the exit channel into two following reactions:

_{a}

**Radiative Capture.**Most absorption reactions result in the loss of a neutron coupled with the production of one or more gamma rays. This is referred to as a**capture reaction**, and it is denoted by**σ**._{γ}

**Neutron-induced Fission Reaction.**Some nuclei (fissionable nuclei) may undergo a fission event, leading to two or more fission fragments (nuclei of intermediate atomic weight) and a few neutrons. In a fissionable material, the neutron may simply be captured, or it may cause nuclear fission. For fissionable materials we thus divide the absorption cross section as**σ**._{a}= σ_{γ}+ σ_{f}

## Neutron Absorption Cross-section

The likelihood of a neutron absorption is represented by the **absorption cross section** as **σ _{a}**. The relative likelihoods of an absorption reaction or a neutron scattering are represented by dividing the total cross section into scattering and absorption cross sections:

**σ _{t} = σ_{s} + σ_{a}**

Given a collision, **σ _{a} / σ_{t} **is the probability that the neutron will be

absorbed and

**σ**/

_{s}**σ**is the probability that the neutron will be scattered.

_{t}Table of cross-sections.

Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library

Xenon – 135. Neutron absorption and scattering. Comparison of cross-sections.Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1.1 Nuclear Data Library