Suppose that a fission neutron, emitted with an energy of 2 mev, slows down to an
energy of 1 ev as the result of successive collisions in a moderator. if, on the average,
the neutron gains in lethargy the amount \xi in each collision, how many collisions are
required if the moderator is

(a) hydrogen,
(b) graphite?

hint: you need to calculate the change in lethargy from 2 mev to 1 ev. then, calculate (or look up in a table) the average change in lethargy per collision to obtain the total number of collisions