The speed of a neutron plays a crucial role in causing fission in certain isotopes. The difference between slow neutrons and fast neutrons lies in their ability to induce fission and sustain a nuclear chain reaction.
Slow neutrons, also known as thermal neutrons, have relatively low kinetic energy. They have been moderated or slowed down to thermal energies through collisions with other particles in a substance like a moderator (e.g., water, graphite). When a slow neutron interacts with a fissile isotope, such as uranium-235 (^235U) or plutonium-239 (^239Pu), it has a higher probability of being captured by the nucleus. This capture leads to an unstable compound nucleus, which quickly undergoes fission, splitting into two or more smaller nuclei and releasing additional neutrons and a significant amount of energy. These additional neutrons can go on to induce fission in other fissile nuclei, resulting in a self-sustaining chain reaction.
On the other hand, fast neutrons have higher kinetic energy and have not been moderated. When a fast neutron interacts with a fissile isotope, the probability of capture is lower. Instead of being captured, fast neutrons are more likely to scatter or bounce off the nucleus without inducing fission. To increase the chances of fission, fast neutrons can be slowed down or moderated by passing them through a moderator material. Slowing down fast neutrons to thermal energies allows them to be captured by fissile isotopes more effectively, thus increasing the fission probability and sustaining a chain reaction.
In summary, slow or thermal neutrons are more effective in causing fission in certain isotopes because they have a higher capture cross-section and a greater probability of inducing fission. Fast neutrons, while less likely to cause fission on their own, can be moderated to thermal energies to enhance their fission-inducing capabilities and sustain a nuclear chain reaction.