Question

show neutrons are more effective than fast ones in inducing fission.why?

Answer 1

It is because slow neutrons have a highest fission probability (called cross-section) to be captured by a fissile nucleus then to trigger fission. The large fission cross-section compensates for the low proportion in the fuel.

One drawback of fast neutrons in reactors is that the probabilities of their capture by nuclei are comparatively small. Travelling in matter, neutrons see nuclei as targets. The apparent cross-section of these targets is much more smaller for fast neutrons than it is for slower neutrons. As a result, an intense neutron flux and a fuel rich in fissile elements are both needed to compensate for this lower probability.