A type of nuclear reaction in which an incoming subatomic particle bounces off the nucleus of an atom and transfers part of its energy to it while the composition of the nucleus remains unchanged. After such a collision, the nucleus of the atom becomes excited and then releases the excess energy by emitting a photon or by some other change. The particle is slowed down by losing some of its energy on impact. This principle is used in nuclear power plants to slow down fast neutrons. Slow neutrons then have a higher chance of splitting the uranium 235 nucleus.