16.1
Introduction to dose measurement
16.2
One of the simplest and most accurate ways to measure radiation dose is with an ionization chamber. This is represented schematically in Figure 16.1.1, which shows a volume of air (gray) located inside some medium (red), such as water. The air in the chamber is typically vented in some way to the room and is thus at the ambient temperature, T, and pressure, P. In the chamber shown in the figure, there are two metal plates (electrodes), one at the top and one at the bottom of the chamber, that are held at some voltage relative to each other. A photon interacts with an electron in the medium and undergoes Compton scattering, producing an electron (blue). This electron then enters the air-filled ionization chamber. At this point, it experiences a force due to the electric field in the chamber. Recall that the voltage difference between the plates results in an electric field, which produces a force on the electron. The electron is attracted to the electrode at positive voltage (in the figure, this is the electrode on the bottom). The accumulation of many electrons on this electrode results in a charge, Q, which can be measured. Recall that charge has the units of Coulombs (C). The charges typically encountered in a therapy beam are in the range of nano-Coulombs (nC).