The Story of Nuclear Energy: Neutron Bombardment

Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3), by Isaac Asimov is part of HackerNoon’s Book Blog Post series. You can jump to any chapter in this book here. Volume II, THE NEUTRON: Neutron Bombardment

Neutron Bombardment

As soon as neutrons were discovered, it seemed to physicists that they had another possible bombarding particle of extraordinary properties. Since the neutron lacked any electric charge, it could not be repelled by either electrons on the outside of the atoms or by the nuclei at the center. The neutron was completely indifferent to the electromagnetic attraction and it just moved along in a straight line. If it happened to be headed toward a nucleus it would strike it no matter how heavy a charge that nucleus might have and very often it would, as a result, induce a nuclear reaction where a proton would not have been able to.

J. Robert Oppenheimer

To be sure, it seemed just at first that there was a disadvantage to the neutron’s lack of charge. It could not be accelerated directly by any device since that always depended on electromagnetic interaction to which the neutron was impervious.

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.

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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 109larger 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.

(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 110neutrons 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 111atoms. Rutherford had no way of telling (and neither had Fermi) that this, finally, was the route to the unimaginable.

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Isaac Asimov. 2015. Worlds Within Worlds: The Story of Nuclear Energy, Volume 2 (of 3). Urbana, Illinois: Project Gutenberg. Retrieved May 2022 from https://www.gutenberg.org/files/49820/49820-h/49820-h.htm#c28

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