Transport Mean Free Path
From the equation for the probability of the first collision in dx (P(x)dx = Σtdx . e-Σt.x = Σt e-Σt.x dx) we can calculate the mean free path that is traveled by a neutron between two collisions. This quantity is usually designated by the symbol λ and it is equal to the average value of x, the distance traveled by a neutron without any interaction, over the interaction probability distribution.
whereby one can distinguish λs, λa, λf, etc. This quantity is also known as the relaxation length, because it is the distance in which the intensity of the neutrons that have not caused a reaction has decreased with a factor e.
The transport mean free path (λtr) is an average distance a neutron will move in its original direction after infinite number of scattering collisions.
is average value of the cosine of the angle in the lab system at which neutrons are scattered in the medium. It can be calculated for most of the neutron energies as (A is the mass number of target nucleus):