## Definition of Cross-section

In general, **the cross-section** is an effective area that quantifies **the likelihood** of certain interaction between an incident object and a target object. The cross-section of a particle is the same as the cross section of a hard object, if the probabilities of hitting them with a ray are the same.

For a given event, the cross section **σ** is given by

** σ = μ/n**

where

**σ**is the cross-section of this event [m^{2}],**μ**is the attenuation coefficient due to the occurrence of this event [m^{-1}],**n**is the density of the target particles [m^{-3}].

In nuclear physics, the nuclear cross section of a nucleus is commonly used to characterize the **probability** that a nuclear reaction will occur. The cross-section is typically denoted **σ** and measured in units of area [m^{2}]. The standard unit for measuring a nuclear cross section is the **barn**, which is equal to **10 ^{−28} m² or 10^{−24} cm²**. It can be seen the concept of a nuclear cross section can be quantified physically in terms of

**“characteristic target area”**where a larger area means a larger probability of interaction.

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

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