Biomedical Engineering Reference
In-Depth Information
T
HE
I
NTERACTIONS OF
P
HOTONS WITH
B
ULK
M
ATTER
So far, we have mainly considered only single interactions with a
single atom. Now it is time to look at what happens in matter, which
is comprised of a whole lot of atoms. First, however, a few words
about the concept of dose.
The concept of dose
When radiation interacts with matter, energy is lost and much of it is
transferred to the atoms and molecules of the matter. The lost energy
may be transferred at, or very close to, the site of an interaction, or it
may be transferred some distance away from the site of the original
interaction by secondary photons and particles. Some of the energy is
carried away by photons and particles that exit the target material and
can have no further impact upon it.
Dose is a measure of the amount of energy deposited in a small
volume at a point of interest as a result of the radiation - be that
energy deposited locally, or brought to the point of interest by
secondary radiation generated at some distance from the primary
interactions. Dose is expressed in units of Gray (written
Gy
), where
one Gray is equal to 1 J
-1 3
kg .
In principle, one could measure the dose by measuring the energy
deposited in a volume which is small with respect to the spatial
variations in dose, and then dividing the measured energy by the
measured mass of the material in the small volume. There are, as
ever, all sorts of technical details connected with actually making
⋅
such a measurement. Suffice it to say that the most common
approach is to make measurements with a small ionization chamber
4
3
The Gray is an SI unit, primarily created for the purposes of radiotherapy.
In times gone by, the unit of dose was the rad. Its definition was such that
1 rad is equal to one hundredth of a Gray
which may be written 1 cGy.
4
An ionization chamber is a small cavity with generally a central electrode
or a pair of parallel flat electrodes. Radiation passing through the cavity
ionizes the gas, creating many ion
−
electron pairs, the number of which is
proportional to the dose deposited by the radiation. A voltage is applied
between the electrode and the cavity wall (which is made to be conducting)
and this voltage gradient causes the electrons and ions to drift apart
towards opposite electrodes. The charge carried by the electrons reaching
the positive electrode is measured with an electrometer and the dose is
equal to the charge collected multiplied by a calibration factor and by a
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