Biomedical Engineering Reference
In-Depth Information
7
Phenomena Associated with Charged-Particle Tracks
7.1
Delta Rays
A heavy charged particle or an electron traversing matter sometimes produces a
secondary electron with enough energy to leave the immediate vicinity of the pri-
mary particle's path and produce a noticeable track of its own. Such a secondary
electron is called a delta ray. Figure 7.1 shows a number of examples of delta rays
along calculated tracks of protons and alpha particles at several energies with the
same speeds. The 20-MeV alpha particle produces a very-high-energy delta ray,
which itself produces another delta ray. There is no sharp distinction in how one
designates one secondary electron along a track as a delta ray and another not, ex-
cept that its track be noticeable or distinct from that of the primary charged particle.
Delta rays can also be seen along the electron tracks in Fig. 6.7. (Note the difference
in scale compared with Fig. 7.1.)
7.2
Restricted Stopping Power
As will be discussed in Chapter 12, radiation dose is defined as the energy absorbed
per unit mass in an irradiated material. Absorbed energy thus plays a preeminent
role in dosimetry and in radiation protection.
Stopping power gives the energy
lost
by a charged particle in a medium. This is
not always equal to the energy
absorbed
in a target, especially if the target is small
compared with the ranges of secondary electrons produced. On the biological scale,
many living cells have diameters of the order of microns (10
-4
cm). Subcellular
structures can be many times smaller; the DNA double helix, for example, has a
diameter of about 20 Å. Delta rays and other secondary electrons can effectively
transport energy out of the original site in which it is lost by a primary particle.
The concept of restricted stopping power has been introduced to associate en-
ergy loss in a target more closely with the energy that is actually absorbed there.
The restricted stopping power, written
(-d
E
/d
x
)
, is defined as the linear rate of
energy loss due only to collisions in which the energy transfer does not exceed a
