rford used naturally occurring alpha particles from radium as his
projectiles because they were the most effective he could then find. But
these natural alpha particles have several drawbacks: they are
positively charged, like the nucleus itself, and are therefore more or
less repulsed depending on the proton number of the element being
bombarded; they do not move fast enough to penetrate the nuclei of
heavier elements (those with many protons); and, for various other
reasons (some of them unexplained), are inefficient in breaking up the
nucleus. It is estimated that only 1 out of 300,000 of these alpha
particles will react with nitrogen.

Physicists immediately began the search for artificial means to
accelerate a wider variety of nuclear particles to high energies.

Protons, because they have a +1 charge rather than the +2 charge of the
alpha particles, are repulsed less strongly by the positive charge on
the nucleus, and are therefore more useful as bombarding projectiles. In
1929, E. T. S. Walton and J. D. Cockcroft passed an electric discharge
through hydrogen gas, thereby removing electrons from the hydrogen atom;
this left a beam of protons (i. e., hydrogen ions), which was then
accelerated by high voltages. This Cockcroft-Walton voltage multiplier
accelerated the protons to fairly high energies (about 800,000 electron
volts), but the protons still had a plus charge and their energies were
still not high enough to overcome the repulsive forces (Coulombic
repulsion) of the heavier nuclei.

A later development, the Van de Graaff electrostatic generator, produced
a beam of hydrogen ions and other positively charged ions, and electrons
at even higher energies. An early model of the linear accelerator also
gave a beam of heavy positive ions at high energies. These were the next
two instruments devised in the search for efficient bombarding
projectiles. However, the impasse continued: neither instrument allowed
scientists to crack the nuclei of the heavier elements.

Ernest O. Lawrence's cyclotron, built in 1931, was the first device
capable of accelerating positive ions to the very high energies needed.
Its basic principle of operation is not difficult to understand. A
charged particle accelerated in a cyclotron is analogous to a ball being
whirled on a string fastened to the top of a pole. A negative electric
field attracts the positively charged particle (ball) towards it and
then switches off until the particle