Biomedical Engineering Reference
In-Depth Information
passes. Clustering occurs as a result of the broad shape of the single-collision spec-
tra in Fig. 5.3 with most of the area covering energy losses
70 eV.
Figure 6.6(a) gives a stereoscopic representation of another 5-keV electron track,
traveling out of the page toward the reader, calculated in water. The same track is
shown from the side in (b), except that the primary electron was forced to move
straight ahead in the calculation.
As another example, Fig. 6.7 displays ten tracks randomly calculated for 740-keV
electrons in liquid water. They are normally incident from the left at the origin
of the
X
-
Y
axes. The dots show the coordinates of every one-hundredth inelastic
event in the track projected onto the
X
-
Y
plane. Of the ten electrons comprising
the figure, nine slow down and stop in the phantom, and one is backscattered into
the space
x
<0
. The diverse, tortuous paths of the electrons are in stark contrast to
those of heavy charged particles. Under identical initial conditions, the latter travel
in almost straight lines to about the same depth.
Whereas the application of range-energy tables and graphs for shielding and
dosimetry with heavy charged particles is relatively straightforward, their use with
beta particles warrants a closed look. As related at the beginning of Section 6.5,
the range of an electron of given energy is the average pathlength that it travels in
coming to rest. Figure 6.7 illustrates that there is a considerable difference between
electron range and the depth of penetration in matter. The calculated distributions
(a) (b)
Fig. 6.6
(a) Stereoscopic view of a 5-keV electron track in water.
(b) Lateral view of same track in which the primary electron is
forced to always move straight ahead. (Courtesy R. N. Hamm,
Oak Ridge National Laboratory, operated by Martin Marietta
Energy Systems, Inc., for the Department of Energy.)
