Biomedical Engineering Reference
In-Depth Information
Absolute dosimetry
There are three main classes of absolute dosimeter, namely:
Calorimeters
These devices measure the amount of
heat generated per unit mass of absorbing material - which is
usually either graphite or pure water. This is very close to a direct
measurement of dose; the main correction is for the so-called “heat
defect” which estimates the small fraction of the energy which
goes, not into heat, but into induction of chemical changes in the
absorbing medium.
Faraday cups
The Faraday cup measures the total
charge deposited in a block of conducting material when protons
stop entirely within the material. Since the proton charge is very
accurately known, one can directly ascertain the number of protons
that stopped in the material. Armed with this information, and
knowledge of the stopping power of the protons coming in to the
Faraday cup, one can immediately compute the dose that would be
delivered to material placed just in front of the device. The main
correction is for the change in collected charge due to charged
particles such as electrons which escape the stopping material by
being scattered backwards and, to a lesser extent, for the change in
charge due to absorbed secondary particles emanating from
material in front of the Faraday cup.
Ionization chambers
Ionization chambers consist of a pair of
electrodes between which is sandwiched a known amount of gas
such as air. The gas is ionized by the radiation traversing it, and
an electric potential applied across the electrodes separates the
positive ions and the electrons which drift towards opposite
electrodes where they are collected and the total charge measured.
Ionization chambers may intercept the whole beam, or may be
used to measure the dose in a small volume. In the former case,
they use large diameter parallel plates which integrate the dose
across the full beam. For measurements of small volumes,
ionization chambers can be made quite small, containing perhaps a
fraction of a milliliter of gas. They are often cylindrical in shape
with a wire forming the central electrode, They may also feature a
parallel plate geometry which is useful for measurements of dose
close to the entrance surface. Converting the current coming from
an ionization chamber into dose requires knowledge of the mass of
gas contained in the cavity, and the so-called “w value” which is
the amount of energy required to ionize an atom of the gas.
