We can understand why 235u is readily fissionable, and 238u is not, by usingthe following calculation.
(a) find the energy difference between 235u + nand 236u. we can regard this as the "excitation energy" of 236u.
(b) repeat (a) for 238u + nand 239u.(c) (2 pts) comparing your results for (a) and (b), explain why 235u will fission with very low energy neutrons, while 238u requires fast neutrons of 1 to 2mev of energy to fission.