Biomedical Engineering Reference
In-Depth Information
contrast to 10 MeV, at 1 MeV it is relatively unlikely that a given proton will experi-
ence an energy loss 20% or more different from the mean (i.e., outside an interval
±5
keV about the mean of 25 keV). The relation between single-collision energy-
loss spectra, stopping power, and energy straggling was discussed in Section 5.4.
The physics behind Fig. 7.4 is of fundamental importance to radiation biology.
As pointed out in Section 7.2, radiation damages living tissue directly on a scale
of microns and below. At this level, energy straggling, as well as (related) delta-ray
effects, play important roles in the interaction of radiation with matter. The subject
of microdosimetry (Section 12.10) deals with the distribution and fluctuations in
energy loss and deposition in small volumes of tissue.
7.6
Range Straggling
Range straggling for heavy charged particles can be measured with the experimen-
tal arrangement shown in Fig. 7.5(a). A monoenergetic beam is directed on an
absorber whose thickness can be varied by using additional layers of the material.
A count-rate meter is used to measure the relative number of beam particles that
emerge from the absorber as a function of its thickness. A plot of relative count
Fig. 7.5
(a) Experimental arrangement for observing range
straggling, (b) Plot of relative count rate vs. absorber thickness,
showing the mean and extrapolated ranges.
