Was there nothing that could be done to make nuclear bombardment more
efficient and increase the chance of obtaining useful energy out of
nuclear reactions?
In 1928 the Russian-American physicist George Gamow (1904-1968)
suggested that protons might be used as bombarding agents in place of
alpha particles. Protons were only one-fourth as massive as alpha
particles and the collision might be correspondingly less effective; on
the other hand, protons had only half the positive charge of alpha
particles and would not be as strongly repelled by the nuclei. Then,
too, protons were much more easily available than alpha particles. To
get a supply of protons one only had to ionize the very common hydrogen
atoms, i.e., get rid of the single electron of the hydrogen atom, and a
single proton is left.
[Illustration: _Artificial radioactivity._]
Aluminum-27 (14N,13P) + Helium-4 (2N,2P) (Alpha particle)
→ (16N,15P)
→ N + Phosphorus-30 (Radioactive) (15N,15P)
→ Positron + Silicon-30
Neutron=N, Proton=P
Of course, protons obtained by the ionization of hydrogen atoms have
very little energy, but could energy be imparted to them? Protons carry
a positive charge and a force can therefore be exerted upon them by an
electric or magnetic field. In a device that makes use of such fields,
protons can be accelerated (made to go faster and faster), and thus gain
more and more energy. In the end, if enough energy is gained, the proton
could do more damage than the alpha particle, despite the former’s
smaller mass. Combine that with the smaller repulsion involved and the
greater ease of obtaining protons—and the weight of convenience and
usefulness would swing far in the direction of the proton.
Physicists began to try to design “particle accelerators” and the first
practical device of this sort was produced in 1929 by the two British
physicists John Douglas Cockcroft (1897-1967) and Ernest Thomas Sinton
Walton (1903- ). Their device, called an “electrostatic accelerator”,
produced protons that were sufficiently energetic to initiate nuclear
reactions. In 1931 they used their accelerated protons to disrupt the
nucleus of lithium-7. It was the first nuclear reaction to be brought
about by man-made bombarding particles.
Other types of particle accelerators were also being developed at this
time. The most famous was the one built in 1930 by the American
physicist Ernest Orlando Lawrence (1901-1958). In this device a magnet
was used to make the protons move in gradually expanding circles,
gaining energy with each lap until they finally moved out beyond the
influence of the magnet and then hurtled out of the instrument in a
straight line at maximum energy. This instrument was called a
“cyclotron”.
[Illustration: _Inventors of one of the first accelerators, Ernest T. S.
Walton, left, and John D. Cockcroft, right, with Lord Ernest Rutherford
at Cambridge University in the early 1930s._]
[Illustration: _The bombardment of lithium-7 with protons was the first
nuclear reaction caused by man-made particles._]
Lithium-7 (4N,3P) + Hydrogen-1 (Proton)
→ Helium-4 (2N,2P) (Alpha particle) + Helium-4 (2N,2P) (Alpha
particle)
Neutron=N, Proton=P
The cyclotron was rapidly improved, using larger magnets and
increasingly sophisticated design. There are now, at this time of
writing, “proton synchrotrons” (descendants of that first cyclotron)
that produce particles with over a million times the energy of those
produced by Lawrence’s first cyclotron. Of course, the first cyclotron
was only a quarter of a meter wide, while the largest today has a
diameter of some 2000 meters.
As particle accelerators grew larger, more efficient, and more powerful,
they became ever more useful in studying the structure of the nucleus
and the nature of the subatomic particles themselves. They did not
serve, however, to bring the dream of useful nuclear energy any closer.
Though they brought about the liberation of vastly more nuclear energy
than Rutherford’s initial bombardments could, they also consumed a great
deal more energy in the process.
It is not surprising that Rutherford, the pioneer in nuclear
bombardment, was pessimistic. To the end of his days (he died in 1937)
he maintained that it would be forever impossible to tap the energy of
the nucleus for use by man. Hopes that “nuclear power” might some day
run the world’s industries were, in his view, an idle dream.
[Illustration: _Ernest O. Lawrence holds a model of the first cyclotron
in 1930, a year after its conception._]