The term radioisotope is shorthand for radioactive isotope. Isotopes are forms of an element whose atoms differ from each other in the number of neutrons contained in their nuclei and, hence, in their atomic masses. Hydrogen-1, hydrogen-2, and hydrogen-3 are all isotopes of each other.

Isotopes may be stable or radioactive. That is, they may exist essentially unchanged forever (stable), or they may spontaneously emit an alpha particle or beta particle and/or a gamma ray, changing in the process into a new substance. Hydrogen-1 and hydrogen-2 are stable isotopes, but hydrogen-3 is radioactive.

The first naturally occurring radioisotopes were discovered in the late 1890s. Scientists found that all isotopes of the heaviest elements—uranium, radium, radon, thorium, and protactinium, for example—are radioactive. This discovery raised the question as to whether stable isotopes of other elements could be converted to radioactive forms.

By the 1930s, the techniques...