The boron neutron trap
In order to protect against neutron produced in reactors, engineers add boron to the water entering the concrete composition of the surrounding walls. The curve shows that the probability of neutron capture by a nucleus of boron-10 (a naturally occurring isotope of boron) exceeds more than 10 000 times that of a hydrogen nucleus, whatever the neutron energy. But if ratios barely vary, the probabilities themselves greatly increase when the neutron slows down, even so much that a boron-10 nucleus seems huge to a slow neutron. There is sharing of roles: the protons of hydrogen from water slow the neutrons by collisions until they are captured by boron. To make them visible on the diagram, the recoil protons have been magnified relatively to the boron nucleus.
IN2P3 (Source Janis data base)