where I o is the initial flux, path length Δx ≡ x − x o, the second equality defines an interaction mean free path λ, the third uses the number of targets per unit volume η to define an area cross-section σ, and the last uses the target mass density ρ to define a density mean free path τ. Hence one converts between these quantities via Q = 1/ λ = ησ = ρ/τ, as shown in the figure at left.