Biomedical Engineering Reference
In-Depth Information
T
HE
P
HYSICAL
P
ROPERTIES OF
P
ROTONS
When protons of a given energy pass through matter, they are subject
to three main phenomena: Coulomb interactions with atomic
electrons, Coulomb interactions with atomic nuclei, and nuclear
interactions with atomic nuclei.
Coulomb interactions of protons with atomic electrons
Protons gradually lose energy, and hence deposit dose, as they
penetrate matter. This energy loss is mainly due to Coulomb
interactions of the protons with the orbiting electrons of atoms. The
opposite charges of the protons and electrons cause the protons to
attract the electrons and “suck” some of them out of the atoms. This
results in ionization of atoms and, even more importantly, setting
loose electrons that go
on to ionize further
atoms in the neighbor-
hood of the initial ioni-
zation, just as was
described in chapter
4. This process is shown
schematically in Figure
10.2, and should be
compared to Figure 4.6
of Chapter 4. On ave-
rage, the protons lose relatively little energy in individual ioniza-
tions and are very little deflected; they suffer some 100,000s of inter-
actions per centimeter of material before eventually losing all their
energy and coming to rest.
Figure 10.2. Coulomb interaction of a
proton (
blue
) with an atomic electron (
red
).
The particle sizes are schematic only; they
are totally out of proportion in the figure.
Indeed you could not see any of the
particles if the figure was drawn to scale.
A monoenergetic proton beam will penetrate matter of a given density
up to a well-defined depth, which is determined by the beam energy.
The fact that the depth of penetration is related to the proton energy in
a one-to-one manner is the key to the practical use of protons in
radiation therapy, for it allows the penetration of the beam within the
patient to be controlled, at the sub-millimeter level if necessary, by
simply controlling the energy of the protons incident upon the patient.
Coulomb interactions of protons with atomic nuclei
Because protons are so much heavier than electrons they are hardly
deflected at all by Coulomb interactions with atomic electrons - as
just described. However, they also experience a repulsive force when
they pass close to a positively charged
nucleus
of an atom (see Figure
