Given a total interaction cross section σ (typically measured in barns), the mean free path of a prompt neutron is formula_1 where "n" is the nuclear number density. Most interactions are scattering events, so that a given neutron obeys a random walk until it either escapes from the medium or causes a fission reaction. So long as other loss mechanisms are not significant, then, the radius of a spherical critical mass is rather roughly given by the product of the mean free path formula_2 and the square root of one plus the number of scattering events per fission event (call this "s"), since the net distance travelled in a random walk is proportional to the square root of the number of steps: