The Number of Elastic Collisions in a Finite Lethargy Interval

An integral equation is derived for the function giving the average number of collisions required to thermalize neutrons by elastic collisions with nuclei of relative mass M. An asymptotic solution is obtained which corrects the customary result by between and 1 collision (depending on M). Numerical results for small lethargy increments are presented for the cases M = 2 and M = 235.