42.5 Radiation and Radioactivity

Becquerel’s 1896 discovery of “rays” from crystals of uranium prompted a burst of activity. In England, J. J. Thomson and, especially, his student and protégé Ernest Rutherford worked to identify the unknown rays. Using combinations of electric and magnetic fields, much as Thomson had done in his investigations of cathode rays, they found three distinct types of radiation. FIGURE 42.13 shows the basic experimental procedure, and TABLE 42.3 summarizes the results.

FIGURE 42.13 Identifying radiation by its deflection in a magnetic field.

The figure illustrates Thompson and Rutherford's experimental setup for identifying radiation by its deflection in a magnetic field.
FIGURE 42.13 Full Alternative Text

TABLE 42.3 Three types of radiation

Radiation Identification Charge Stopped by
Alpha, α 4He nucleus +2e Sheet of paper
Beta, β Electron −e Few mm of aluminum
Gamma, γ High-energy photon 0 Many cm of lead

Within a few years, as Rutherford and others deduced the basic structure of the atom, it became clear that these emissions of radiation were coming from the atomic nucleus. We now define radioactivity or radioactive decay to be the spontaneous emission of particles or high-energy photons from unstable nuclei as they decay from higher-energy to lower-energy states. Radioactivity has nothing to do with the orbiting valence electrons.

NOTE   The term “radiation” merely means something that is radiated outward, similar to the word “radial.” Electromagnetic waves are often called “electromagnetic radiation.” Infrared waves from a hot object are referred to as “thermal radiation.” Thus it was no surprise that these new “rays” were also called radiation. Unfortunately, the general public has come to associate the word “radiation” with nuclear radiation, something to be feared. It is important, when you use the term, to be sure you’re not conveying a wrong impression to a listener or a reader.