Mean-energy neutron moderation count ($n=\log (E_f/E_i)/\log f$)

If every step retains the same conditional mean fraction $f$ of the incident kinetic energy, the mean after $n$ steps is $f^n{E}_i$. Solving for a target mean energy gives the displayed count. It describes an ensemble mean, not a guaranteed collision number for each neutron; averaging the logarithmic loss is another statistical convention.