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:
14
7N
+ 2He
4
13
6C
+
1
1H
14
7N
+ 2He
4
17
8O
+
1
1H
+
4
2He
or
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.
alpha source
protons
alpha particles
zinc sulphide screen
nitrogen
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.
7
3Li
+
1
1H
4
2 2He