Biomedical Engineering Reference
In-Depth Information
Fig. 6.5
Three calculated tracks of 5-keV
electrons in liquid water. Each electron starts
from the origin and initially travels along the
horizontal axis toward the right. Each dot gives
the position of a chemically active species at
10
-11
s. [From J. E. Turner, J. L. Magee,
H. A. Wright, A. Chatterjee, R. N. Hamm, and
R. H. Ritchie, “Physical and Chemical
Development of Electron Tracks in Liquid
Water,”
Rad. Res.
96
, 437-449 (1983). Courtesy
Oak Ridge National Laboratory, operated by
Martin Marietta Energy Systems, Inc., for the
Department of Energy.]
where the primary electron is moving faster. This is generally true of charged-
particle tracks, since the stopping power is smaller at high energies than near the
end of the range. Note also the clustering of events, particularly in the first part of
the tracks. Such groupings, called “spurs,” are due to the production of a secondary
electron with just enough energy to produce several additional ionizations and ex-
citations. The range of the original secondary electron is usually not great enough
to take it appreciably away from the region through which the primary electron
