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.
[Illustration: _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.
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.
(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.