Biomedical Engineering Reference
In-Depth Information
5
Interaction of Heavy Charged Particles with Matter
This chapter and the next four deal with the mechanisms by which different types
of ionizing radiation interact with matter. Knowledge of the basic physics of radi-
ation interaction and energy transfer is fundamental to radiation detection, mea-
surement, and control, as well as to understanding the biological effects of radia-
tion on living tissue. We consider “heavy” charged particles first, that is charged
particles other than the electron and positron.
5.1
Energy-Loss Mechanisms
A heavy charged particle traversing matter loses energy primarily through the ion-
ization and excitation of atoms. (Except at low velocities, a heavy charged particle
loses a negligible amount of energy in nuclear collisions.) The moving charged par-
ticle exerts electromagnetic forces on atomic electrons and imparts energy to them.
The energy transferred may be sufficient to knock an electron out of an atom and
thus ionize it, or it may leave the atom in an excited, nonionized state. As we show
in the next section, a heavy charged particle can transfer only a small fraction of its
energy in a single electronic collision. Its deflection in the collision is negligible.
Thus, a heavy charged particle travels an almost straight path through matter, los-
ing energy almost continuously in small amounts through collisions with atomic
electrons, leaving ionized and excited atoms in its wake. Occasionally, however, as
observed in Rutherford's experiments with alpha-particle scattering from a gold
foil, a heavy charged particle will undergo a substantial deflection due to elastic
scattering from an atomic nucleus.
Electrons and positrons also lose energy almost continuously as they slow down
in matter. However, they can lose a large fraction of their energy in a single col-
lision with an atomic electron (having equal mass), thereby suffering relatively
large deflections. Because of their small mass, electrons are frequently scattered
through large angles by nuclei. In contrast to heavy charged particles, electrons
and positrons do not generally travel through matter in straight lines. An electron
can also be sharply deflected by an atomic nucleus, causing it to emit photons in
the process called bremsstrahlung (braking radiation). Figure 5.1 shows the con-
