Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3) / Mass and Energy; The Neutron; The Structure of the Nucleus

by Isaac Asimov

There was one way of getting around this and this was explained in 1935 by the American physicist J. Robert Oppenheimer (1904-1967) and by his student Melba Phillips.

Use is made here of the nucleus of the hydrogen-2 (deuterium) nucleus. That nucleus, often called a “deuteron”, is made up of 1 proton plus 1 neutron and has a mass number of 2 and an atomic number of 1. Since it has a unit positive charge, it can be accelerated just as an isolated proton can be.

Suppose, then, that a deuteron is accelerated to a high energy and is aimed right at a positively charged nucleus. That nucleus repels the deuteron, and it particularly repels the proton part. The nuclear interaction that holds together a single proton and a single neutron is comparatively weak as nuclear interactions go, and the repulsion of the nucleus that the deuteron is approaching may force the proton out of the deuteron altogether. The proton veers off, but the neutron, unaffected, keeps right on going and, with all the energy it had gained as part of the deuteron acceleration, smashes into the nucleus.

Within a few months of their discovery, energetic neutrons were being used to bring about nuclear reactions.

Actually, though, physicists didn’t have to worry about making neutrons energetic. This was a hangover from their work with positively charged particles such as protons and alpha particles. These charged particles had to be energetic to overcome the repulsion of the nucleus and to smash into it with enough force to break it up.

Neutrons, however, didn’t have to overcome any repulsion. No matter how little energy they had, if they were correctly aimed (and some always were, through sheer chance) they would approach and strike the nucleus.

In fact, the more slowly they travelled, the longer they would stay in the vicinity of a nucleus and the more likely they were to be captured by some nearby nucleus through the attraction of the nuclear interaction. The influence of the nucleus in capturing the neutron was greater the slower the neutron, so that it was almost as though the nucleus were larger and easier to hit for a slow neutron than a fast one. Eventually, physicists began to speak of “nuclear cross sections” and to say that particular nuclei had a cross section of such and such a size for this bombarding particle or that.

The effectiveness of slow neutrons was discovered in 1934 by the Italian-American physicist Enrico Fermi (1901-1954).

Of course, there was the difficulty that neutrons couldn’t be slowed down once they were formed, and as formed they generally had too much energy (according to the new way of looking at things). At least they couldn’t be slowed down by electromagnetic methods—but there were other ways.

A neutron didn’t always enter a nucleus that it encountered. Sometimes, if it struck the nucleus a hard, glancing blow, it bounced off. If the nucleus struck by the neutron is many times as massive as the neutron, the neutron bounced off with all its speed practically intact. On the other hand, if the neutron hits a nucleus not very much more massive than itself, the nucleus rebounds and absorbs some of the energy, so that the neutron bounces away with less energy than it had. If the neutron rebounds from a number of comparatively light nuclei, it eventually loses virtually all its energy and finally moves about quite slowly, possessing no more energy than the atoms that surround it.