```
σa = (N * σ)
```
Where:
* σa is the absorption cross-section in barns (b)
* N is the number of atoms of the element per cubic centimeter
* σ is the microscopic absorption cross-section in barns per atom
The microscopic absorption cross-section can be found in the literature or calculated using theoretical methods.
For a mixture of elements, the total absorption cross-section is the sum of the absorption cross-sections of each element:
```
σatot = Σi(σai)
```
Where:
* σatot is the total absorption cross-section in barns (b)
* σai is the absorption cross-section of the i-th element in barns (b)
The absorption cross-section is an important parameter in neutron transport calculations. It is used to determine the rate at which neutrons are absorbed by a material.
