It takes energy to move this electron away from the proton. It takes 13.6 electron-volts of energy to move this electron completely away from the proton. Electrons are very light objects, so we don't expect an electron-volt to represent very much energy. The energy associated with the radioactive decay ranges from thousands to millions of electron-volts per nucleus, which is why the decay of a single nucleus typically leads to a large number of ionizations. The radioactivity of a substance is measured in the number of nuclei that decay per unit time.
Specific activity measures the radioactivity of a unit weight of substance.
The units are curies per gram or becquerels per gram. This allows us to compare whether a substance is more or less radioactive than another. The specific activity of a radionuclide is inversely proportional to its atomic weight and its half-life. Environmental and biological measurements of radioactivity are generally expressed as concentrations of radioactivity in soil, water, air, or tissue. Examples of units include picocuries per liter, becquerels per cubic meter, picocuries per gram, and disintegrations per minute per 100 square centimeters.