Again I didn´t manage to make the calendar blogpost yesterday - and again I´m very sorry :/ Again I give you two for the price of one - to make it up ;)
Again there are cross-section graphs in the calendar - cross-sections are really important <3 (Later, maybe I should do a blogpost about these nuclear cross-sections; what they really mean...)
Today there are two different cross-sections, for two different uranium isotopes (different "versions" of uranium). Both pictures show the cross-section/probability that the nucleus will fission (in blue) or capture (in red), when hit by a neutron.
Both graphs show the cross-section on the y-axis, and the energy of the neutron hitting the nucleus on the x-axis.
Uranium-238 is not a fissile nucleus; the probability that it will a capture a neutron is much bigger than the probability (cross-section) that it will fission...:(
Luckily the situation is different for the uranium-235 nucleus; the probability that this nucleus will fission when hit by a neutron is bigger than the probability that it will capture (and "eat") the neutron <3 *smiling*