Rutherford 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 swings halfway around; the field then becomes negative in front of the particle again, and again attracts it. As the particle moves faster and faster it spirals outward in an ever increasing circle, something like a tether ball unwinding from a pole. The energies achieved would have seemed fantastic to earlier scientists. The Bevatron, a modern offspring of the first cyclotron, accelerates protons to 99.13% the speed of light, thereby giving them 6.2 billion electron volts (BeV).
Another instrument, the heavy-ion linear accelerator (Hilac), accelerates ions as heavy as neon to about 15% the speed of light. It is called a linear accelerator because it accelerates particles in a straight line. Stanford University is currently (1963) in the process of building a linear accelerator approximately two miles long which will accelerate charged particles to 99.9% the speed of light.
But highly accelerated charged particles did not solve all of science's questions about the inner workings of the nucleus.
In 1932, during the early search for more efficient ways to bombard nuclei, James Chadwick discovered the neutron. This particle, which is neutral in charge and is approximately the same mass as a proton, has the remarkable quality of efficiently producing nuclear reactions even at very low energies. No one exactly knowns why. At low energies, protons, alpha particles, or other charged particles do not interact with nuclei because they cannot penetrate the electrostatic energy barriers. For example, slow positive particles pick up electrons, become neutral, and lose their ability to cause nuclear transformations. Slow neutrons, on the other hand, can enter nearly all atomic nuclei and induce fission of certain of the heavier ones. It is, in fact, these properties of the neutron which have made possible the utilization of atomic energy.