The measurement of radioactivity must be accomplished indirectly, so use
is made of the physical, chemical, and electrical effects of radiation
on materials. One commonly used effect is that of ionization. Alpha and
beta particles ionize gases through which they pass, thereby making the
gases electrically conductive. A family of counters uses this principle:
the ionization chamber, the proportional counter, and the Geiger-Müller
counter.
Certain crystals, sodium iodide being an excellent example, emit flashes
of visible light when struck by ionizing radiation. These crystals are
used in scintillation counters.