9 - The Rutherford era: the radioactivists

Summary

The Rutherford era will always be remembered for the extraordinary achievements in radioactivity and the beginnings of the new field of nuclear physics. Jeffrey Hughes (1993) refers to the protagonists in this story as the radioactivists; they were to transform the field and open up new, and expensive, areas of basic research.

Rutherford and nuclear transformations

The α-particles remained the projectiles of choice used by Rutherford and his colleagues to study the nucleus, but the experiments were dependent upon securing sources of radioactive materials, which were generally in short supply and expensive. In 1908 it was discovered that zinc sulphide laced with 0.01% copper was extremely sensitive to α-particles and emitted most of its luminosity in the yellow-green region of the optical spectrum to which the eye is most sensitive – about a quarter of the incident energy was converted into light (Hendry, 1984). This became Rutherford's preferred method of detecting α-particles and was the technique which Geiger and Marsden used in their experimental demonstration of the validity of the Rutherford scattering law (Section 7.5.2) (Geiger and Marsden, 1913). The energies of the α-particles could be estimated from their ranges R in air at a standard temperature and pressure, which Rutherford and his colleagues took to be 15?C at normal pressure (Box 9.1). It can be seen from the abscissa of Figure 8.3 that the typical ranges of the particles were between 2 and 8 cm, corresponding to particle energies of roughly 4 to 8 MeV. The ranges of the α-particles could be measured in terms of the mass traversed by the particle per unit cross-section ζ until they were brought to rest by the process of ionisation losses. To measure ζ, the amount of absorbing material between the source of the particles and the screen was increased until there was a sudden drop in the number of scintillations counted. The range ζ could then be translated into a physical distance R in air at the standard reference conditions. The ranges of the α-particles were characteristic of each radioactive decay.