Absorption Cross Section & Irradiation: Explained

[Chuckmaster]

Can someone explain what absorption cross section is and how it's figured into an equation? And what is irradiation?

 

[BCRion]

The cross section is conceptually the area a nucleus presents to a target, having units of length squared. You can interpret this as roughly the relative probability of a nuclear intereaction occurring. Things with a high cross section have a much higher likelyhood than things with a low one.

Every kind of nuclear reaction has a cross section. An absorption cross section in most nuclear engineering roughly describes the probability of a neutron (or other particle) being absorbed by a nucleus.

An equation where this often shows up is in the 1D linear attenuation equation. The cross section (sigma) is usually referred to as a microscopic cross section as it is independent of material properties. The macroscopic cross section (capital sigma or sometimes mu) is the product of the number density A (units of m^-3) and the microscopic cross section (units of m^2) giving units of inverse length (m^-1). The macroscopic cross section is useful because it is a material property.

Say you have a beam of neutrons of intensity I0 incident on a thick slab of material. When the beam strikes the material, it will penetrate and some of the neutrons will be absorbed. The intensity of the beam at any point in the material is given as:

I(x) = I0*exp(-(macroscopic cross section)*x)

Where:

I(x) = beam intensity at position x
I0 = beam intensity at surface of target
x = distance in target.

Irradiation is the process of subjecting a material to some kind of radiation whether it be neutrons, beta particles, x-rays, microwaves, etc.

 

[raining Pete]

Sure, I can try to explain. Absorption cross section is a measure of how likely a particle or object is to absorb radiation or particles that pass through it. It is usually represented by the symbol σ and is measured in units of area. The higher the absorption cross section, the more likely it is that the particle or object will absorb the radiation or particles.

Absorption cross section is often used in equations that involve the interaction of radiation with matter. For example, in the Beer-Lambert law, which describes the relationship between the intensity of light and the concentration of a substance in a solution, the absorption cross section is used to calculate the amount of light that is absorbed by the substance.

Irradiation, on the other hand, refers to the process of exposing something to radiation. This can be through direct exposure to a source of radiation, such as a radioactive material, or through indirect exposure, such as being in an environment with high levels of background radiation. The amount of radiation absorbed by an object or material during irradiation can be calculated using the absorption cross section.

I hope this helps to clarify things for you. Let me know if you have any other questions.