Artificial transmutation

Through the ages the alchemists claimed that they could change one metal into another, particularly base metals into gold. One possibility was to change lead into gold but any gold found after the reaction probably came from the spectacle frames of the experimenters!

However in 1919 an experiment was performed that actually did change one material into another – although not into gold – when alpha particles colliding with nitrogen nuclei were found to give off protons.

Two reactions are possible:

By studying the tracks of the particles emitted in the reaction in a cloud chamber it was shown that it was the second reaction that had occurred: the alpha particles had actually been absorbed by the nucleus and had not simply 'chipped off' a proton.

A simplified diagram of the apparatus is shown above.

The alpha source produced a reaction in the gas and the resulting protons were detected using a zinc sulphide screen. With heavy elements alpha particle scattering occurs instead.

1 mg of radium emits about 37 000 000 alpha particles per second but even this enormous number does not cause many interactions because of the 'large' spaces between the nuclei. It required the development of particle accelerators before sufficiently high particle densities could be obtained. In 1932, using a 400 000 V accelerator, Cockcroft, Walton and Rutherford succeeded in transmuting a nucleus of lithium into two helium nuclei.