Definition of cross sections 1 - Moodle

Definition of cross sections 3The microscopic cross section must have the dimension of a surface to make Eq. (3)dimensionally consistent. They are generally expressed in barn (b), with 1 b = 10 −24 cm 2 . Itis also common to define the macroscopic cross section Σ x to group all the characteristics ofthe target in a single value. It is defined as Σ x = N σ x .If the material of the target is an homogeneous mixture of different types i of nuclides, theresulting macroscopic cross section is(4)Σ x = ∑ iN i σ x,i .Moreover, we define the total macroscopic cross section as the sum of cross sections fromall nuclear reactions. We write(5)Σ = ∑ xΣ x .ENE6101: Week 4 Cross sections – 5/37