Biomedical Engineering Reference
In-Depth Information
In non-elastic collisions, the nucleus may disintegrate in a number of
ways, but generally a relatively light fragment is knocked out with
considerable speed leaving behind a heavy fragment which stays
close to where the interaction took place and is heavily ionizing, as
illustrated in Figure 10.4. The relative energy carried away by the
fragments that are produced is given in Table 10.1.
Table 10.1. Fractional energy loss taken up by various
particles when 150 MeV protons strike a
16
O nucleus.
Data taken from Selzer (1993).
particle
fraction of energy
(%)
protons
57
neutrons
20
alpha particles
2.9
deuterons
1.6
tritium
0.2
helium-3
0.2
other charged recoil fragments
1.6
−
T
HE
D
EPTH
D
OSE
D
ISTRIBUTION OF A
B
ROAD
P
ROTON
B
EAM
I now want to address the dose characteristics of a broad beam of
protons with a uniform lateral intensity distribution. Such a beam is
produced by passive scattering (see below), but it can equally well be
produced by scanning by keeping the weights of all pencil beams of a
given energy the same (see below).
The Bragg peak
The dose deposited by protons
rises sharply near the end of
their range, giving rise to the
so-called Bragg peak, named
after Sir William Henry Bragg
(who should not to be
confused with his son, Sir
William Lawrence Bragg, also
a physicist.) An example of a
100
100
80
80
60
60
40
40
20
20
0
0
0
0
5
5
10
10
15
15
Depth (cm)
Depth (cm)
Figure 10.5. Depth dose distribution
of a mono-energetic ~150 MeV proton
typical dose distribution of
beam in water, showing the charac-
a near-monoenergetic proton
teristic Bragg peak. Figure courtesy
beam is shown in Figure 10.5.
of B. Gottschalk, HCL, USA.
