Electromagnetic waves, from microwaves through ultraviolet radiation, are absorbed by matter. The absorbed energy increases an object’s thermal energy and its temperature, which is why objects sitting in the sun get warm.

In contrast to visible-light photon energies of a few eV, the energies of the alpha and beta particles and the gamma-ray photons of nuclear decay are typically in the range 0.1–10 MeV, a factor of roughly ${10}^6$ larger. These energies are much larger than the ionization energies of atoms and molecules. Rather than simply being absorbed and increasing an object’s thermal energy, nuclear radiation ionizes matter and breaks molecular bonds. Nuclear radiation (and also x rays, which behave much the same in matter) is called ionizing radiation.

An alpha or beta particle traveling through matter creates a trail of ionization, as shown in FIGURE 42.14. Because the ionization energy of an atom is $\approx 10\text{eV}$, a particle with 1 MeV of kinetic energy can ionize $\approx 100\text{,}000$ atoms or molecules before finally stopping. The low-mass electrons are kicked sideways, but the much more massive positive ions barely move and form the trail. This ionization is the basis for the Geiger counter, one of the most well-known detectors of nuclear radiation. FIGURE 42.15 shows how a Geiger counter works. The important thing to remember is that a Geiger counter detects only ionizing radiation.

FIGURE 42.14 Alpha and beta particles create a trail of ionization as they pass through matter.

FIGURE 42.15 A Geiger counter.

Ionizing radiation damages materials. Ions drive chemical reactions that wouldn’t otherwise occur. Broken molecular bonds alter the workings of molecular machinery, especially in large biological molecules. It is through these mechanisms—ionization and bond breaking—that nuclear radiation can cause mutations or tumors. We’ll look at the biological issues in Section 42.7.