Earlier in this history, we discussed chain reactions involving chemical
energy. A small bit of energy can ignite a chemical reaction that would
produce more than enough energy to ignite a neighboring section of the
system, which would in turn produce still more—and so on, and so on. In
this way the flame of a single match could start a fire in a leaf that
would burn down an entire forest, and the energy given off by the
burning forest would be enormously higher than the initial energy of the
match flame.
Might there not be such a thing as a “nuclear chain reaction”? Could one
initiate a nuclear reaction that would produce something that would
initiate more of the same that would produce something that would
initiate still more of the same and so on?
In that case, a nuclear reaction, once started, would continue of its
own accord, and in return for the trifling investment that would serve
to start it—a single neutron, perhaps—a vast amount of breakdowns would
result with the delivery of a vast amount of energy. Even if it were
necessary to expend quite a bit of energy to produce the 1 neutron that
would start the chain reaction, one would end with an enormous profit.
What’s more, since the nuclear reaction would spread from nucleus to
nucleus with millionths-of-a-second intervals, there would be, in a very
brief time, so many nuclei breaking down that there would be a vast
explosion. The explosion was sure to be millions of times as powerful as
ordinary chemical explosions involving the same quantity of exploding
material, since the latter used only the electromagnetic interaction,
while the former used the much stronger nuclear interaction.
The first to think seriously of such a nuclear chain reaction was the
Hungarian physicist Leo Szilard (1898-1964). He was working in Germany
in 1933 when Adolf Hitler came to power and, since he was Jewish, he
felt it would be wise to leave Germany. He went to Great Britain and
there, in 1934, he considered certain new types of nuclear reactions
that had been discovered.
In these, it sometimes happened that a fast neutron might strike a
nucleus with sufficient energy to cause it to emit 2 neutrons. In that
way the nucleus, absorbing 1 neutron and emitting 2, would become a
lighter isotope of the same element.
But what would happen if each of the 2 neutrons that emerged from the
original target nucleus struck new nuclei and forced the emission of a
pair of neutrons from each. There would now be a total of 4 neutrons
flying about and if each struck new nuclei there would next be 8
neutrons and so on. From the initial investment of a single neutron
there might soon be countless billions initiating nuclear reactions.
Szilard, fearing the inevitability of war and fearing further that the
brutal leaders of Germany might seek and use such a nuclear chain
reaction as a weapon in warfare, secretly applied for a patent on a
device intending to make use of such a nuclear chain reaction. He hoped
to turn it over to the British Government, which might then use its
possession as a way of restraining the Nazis and keeping the peace.
However, it wouldn’t have worked. It took the impact of a very energetic
neutron to bring about the emission of 2 neutrons. The neutrons that
then emerged from the nucleus simply didn’t have enough energy to keep
things going. (It was like trying to make wet wood catch fire.)
But what about uranium fission? Uranium fission was initiated by slow
neutrons. What if uranium fission also produced neutrons as well as
being initiated by a neutron? Would not the neutrons produced serve to
initiate new fissions that would produce new neutrons and so on
endlessly?
It seemed very likely that fission produced neutrons and indeed, Fermi,
at the conference where fission was first discussed, suggested it at
once. Massive nuclei possessed more neutrons per proton than less
massive ones did. If a massive nucleus was broken up into 2 considerably
less massive ones, there would be a surplus of neutrons. Suppose, for
instance, uranium-238 broke down into barium-138 and krypton-86.
Barium-138 contains 82 neutrons and krypton-86 50 neutrons for a total
of 132. The uranium-238 nucleus, however, contains 146 neutrons.
The uranium fission process was studied at once to see if neutrons were
actually given off and a number of different physicists, including
Szilard, found that they were.
Now Szilard was faced with a nuclear chain reaction he was certain would
work. Only slow neutrons were involved and the individual nuclear
breakdowns were far more energetic than anything else that had yet been
discovered. If a chain reaction could be started in a sizable piece of
uranium, unimaginable quantities of energy would be produced. Just 1
gram of uranium, undergoing complete fission, would deliver the energy
derived from the total burning of 3 tons of coal and would deliver that
energy in a tiny fraction of a second.
Szilard, who had come to the United States in 1937, clearly visualized
the tremendous explosive force of something that would have to be called
a “nuclear bomb”. Szilard dreaded the possibility that Hitler might
obtain the use of such a bomb through the agency of Germany’s nuclear
scientists.
Partly through Szilard’s efforts, physicists in the United States and in
other Western nations opposed to Hitler began a program of voluntary
secrecy in 1940, to avoid passing along any hints to Germany. What’s
more, Szilard enlisted the services of two other Hungarian refugees, the
physicists Eugene Paul Wigner (1902- ) and Edward Teller (1908- )
and all approached Einstein, who had also fled Germany and come to
America.
[Illustration: _Leo Szilard_]
[Illustration: _Eugene P. Wigner_]
Einstein was the most prestigious scientist then living and it was
thought a letter from him to the President of the United States would be
most persuasive. Einstein signed such a letter, which explained the
possibility of a nuclear bomb and urged that the United States not allow
a potential enemy to come into possession of it first.
Largely as a result of this letter, a huge research team was put
together in the United States, to which other Western nations also
contributed, with but one aim—to develop the nuclear bomb.