(You can encounter this situation in ordinary life in the case of billiard balls. A billiard ball, colliding with a cannon ball, will just bounce, moving just as rapidly afterward as before, though in a different direction. If a billiard ball strikes another billiard ball, it will set the target ball moving and bounce off itself with less speed.)
The energy of the molecules in the atmosphere depends on temperature. Neutrons that match that energy and have the ordinary quantity to be expected at room temperature are called “thermal” (from a Greek word meaning “heat”) neutrons. The comparatively light nuclei against which the neutrons bounce and slow down are “moderators” because they moderate the neutron’s energy.
Fermi and his co-workers were the first to moderate neutrons, produce thermal neutrons, and use them, in 1935, to bombard nuclei. He quickly noted how large nuclear cross sections became when thermal neutrons were the bombarding particles.
It might seem that hope could now rise in connection with the practical use of energy derived from nuclear reactions. Neutrons could bring about nuclear reactions, even when they themselves possessed very little energy, so output might conceivably be more than input for each neutron that struck. Furthermore because of the large cross sections involved, thermal neutrons missed far less frequently than high-energy charged particles did.
But there was a catch. Before neutrons could be used, however low-energy and however sure to hit, they had to be produced; and in order to produce neutrons they had to be knocked out of nuclei by bombardment with high-energy protons or some other such method. The energy formed by the neutrons was at first never more than the tiniest fraction of the energies that went into forming the neutrons in the first place.
It was as though you could indeed light a candle with a single match, but you still had to look through 300,000 useless pieces of wood before you found a match. The candle would still be impractical.
Even with the existence of neutron bombardment, involving low energy and high cross section, Rutherford could, with justice, feel right down to the time of his death that nuclear energy would never be made available for practical use.
And yet, among the experiments that Fermi was trying in 1934 was that of sending his neutrons crashing into uranium atoms. Rutherford had no way of telling (and neither had Fermi) that this, finally, was the route to the unimaginable.
FOOTNOTES
[1]The attempt to work out the structure of the nucleus resulted in a false, but useful, theory that persisted throughout the 1920s. The great advances in nuclear science in this decade were made in the light of this false theory and, for the sake of historical accuracy, they are so presented here. The theory now believed correct will be presented shortly, and you will see how matters can be changed from the earlier concept to the later one.