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Figure 2.7 Penetrating radiation and nonpenetrating radiation.

The term penetrating radiation may be used to describe X‐ray and gamma radiation, as they have the potential to penetrate considerable thickness of a material. Although we have just described some of the many ways photons interact with matter, the likelihood of any of these interactions occurring over a short distance is small. An individual photon may travel several centimeters or farther into tissue before it interacts. In contrast, charged particles (alpha, beta) undergo many closely spaced interactions. This sharply limits their penetration (Figure 2.7).

Interaction of charged particles with matter

      Because of the strong electrical force between a charged particle and the atoms of an absorber, charged particles can be stopped by matter with relative ease. Compared to photons, they transfer a greater amount of energy in a shorter distance and come to rest more rapidly. For this reason, they are referred to as nonpenetrating radiation (see depiction of alpha and beta particles in Figure 2.7). In contrast to a photon of 100 keV which has a HVL of 4 cm in soft tissue, an electron of this energy would penetrate less than 0.00014 cm in soft tissue [1].

      Excitation

Charged particles (alphas, betas, and positrons) interact with the electrons surrounding the atom’s nucleus by transferring some of their kinetic energy to the electrons. The energy transferred from a low‐energy particle is often only sufficient to bump an electron from an inner to an outer shell of the atom. This process is called excitation. Following excitation, the displaced electron promptly returns to the lower‐energy shell, releasing its recently acquired energy as an X‐ray in a process called de‐excitation (Figure 2.8). Because the acquired energy is equal to the difference in binding energies of the electron shells and the binding energies of the electron shells are determined by the atomic structure of the element, the X‐ray is referred to as a characteristic X‐ray.

      Ionization

Charged particles of sufficient energy may also transfer enough energy to an electron (generally one in an outer shell) to eject the electron from the atom. This process is called ionization (Figure 2.9). This hole in the outer shell is rapidly filled with an unbound electron. If an inner shell electron is ionized (a much less frequent occurrence) an outer shell electron will “drop” into the inner shell hole and a characteristic X‐ray will be emitted. Ionization is not limited to the interaction of charged particles and matter, the photoelectric effect and Compton interactions are examples of photon interactions with matter that produce ionization.

       Specific ionization

When radiation causes the ejection of an electron from an atom of the absorber, the resulting positively charged atom and free negatively charged electron are called an ion pair (Figure 2.9). The amount of energy transferred per ion pair created, W, is characteristic of the materials in the absorber. For example, approximately 33 eV (range 25 to 40 eV) is transferred to the absorber for each ion pair created in air or water. It is often convenient to refer to the number of ion pairs created per unit distance the radiation travels as its specific ionization (SI).

Figure 2.8 Excitation and de‐excitation.

Figure 2.9 Ionization. The ejected electron and the positively charged atom are called an “ion pair” (demarcated by dashed line).

Particles with more charge (alpha particles) have a higher specific ionization than lighter particles (electrons).

       Linear energy transfer

      Linear energy transfer (LET) is the amount of energy transferred in a given distance by a particle moving through an absorber. Linear energy transfer is related to specific ionization:

      Alpha particles are classified as high LET radiation, beta particles and photons as low LET radiation.

       Range

Range is the distance radiation travels through the absorber. Particles that are lighter, have less charge (such as beta particles), and/or have greater energy travel farther than particles that are heavier, have a greater charge (such as alpha particles), and/or have less energy (Figure 2.10).

      In traversing an absorber, an electron loses energy at each interaction with the atoms of the absorber. The energy loss per interaction is variable. Therefore, the total distance traveled by electrons of the same energy can vary by as much as 3% to 4%. This variation in range is called the straggling of the ranges. The heavier alpha particles are not affected to a significant degree and demonstrate very little straggling of range.

      Annihilation

This interaction in matter most often involves a positron (positive electron) and an electron (negatron). After a positron has transferred most of its kinetic energy by ionization and excitation, it combines with a free or loosely bound negative electron. Recall that electrons and positrons have equal mass but opposite electric charge. This interaction is explosive, as the combined mass

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