In the early days of the nuclear business, people struggled to understand how often a U-235 nucleus.

In the early days of the nuclear business, people struggled
to understand how often a U-235 nucleus would split apart and how often it
would not. Eventually, they found that the best way for the nucleus to split
apart was for it to absorb a thermal neutron. Using the values for the ssion
and capture cross sections that we presented in Chapter 5, estimate the
probability that a thermal neutron will cause a U-235 nucleus to split apart.
If the nucleus happens to absorb a fast neutron, is the probability of the
nucleus splitting apart exactly the same?