Basic Physics of Nuclear Medicine/Print version

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Ionisation Chamber

The ionisation chamber consists of a gas-filled detector energised by a relatively low dc voltage. We will first of all make an estimate of the voltage pulse height generated by this type of detector. We will then consider some applications of ionisation chambers.

When a beta-particle interacts with the gas the energy required to produce one ion pair is about 30 eV. Therefore when a beta-particle of energy 1 MeV is completely absorbed in the gas the number of ion pairs produced is:

$n = \frac{1\ \text{MeV}}{30\ \text{eV}} = \frac{1 \cdot 10^6}{30} \approx 3 \cdot 10^4\ \text{ion pairs}$

The electric charge produced in the gas is therefore

$Q = n \cdot e$

$\therefore (3 \cdot 10^4\ \text{ion pairs}) \cdot (1.6 \cdot 10^{-19}\ \text{C})$

$\therefore Q = 5 \cdot 10^{-15}\ \text{C}$

If the capacitance of the ionisation chamber (remember that we compared a gas-filled detector to a capacitor above) is 100 pF then the amplitude of the voltage pulse generated is:

$V = \frac{Q}{C} = \frac{5 \cdot 10^{-15}\ \text{C}}{100 \cdot 10^{-12}\ \text{F}} = 5 \cdot 10^{-5}\ \text{V}$

$\therefore V = 50\ \mu\text{V}$

Because such a small voltage is generated it is necessary to use a very sensitive amplifier in the electronic circuitry connected to the chamber.

We will now learn about two applications of ionisation chambers. The first one is for the measurement of radiation exposures. You will remember from Chapter 4 that the unit of radiation exposure (be it the SI or the traditional unit) is defined in terms of the amount of electric charge produced in a unit mass of a air. An ionization chamber filled with air is the natural instrument to use for such measurements.

The second application is the measurement of radioactivity. The ionisation chamber used here is configured in what is called a re-entrant arrangement (see figure below) so that the sample of radioactive material can be placed within the detector using a holder and hence most of the emitted radiation can be detected. The instrument is widely referred to as an Isotope Calibrator and the trickle of electric current generated by such a detector is calibrated so that a reading in units of radioactivity (for example MBq or mCi) can be obtained. Most well-run Nuclear Medicine Departments will have at least one of these devices so that doses of radioactivity can be checked prior to administration to patients.

Here are some photographs of ionisation chambers designed for various applications:

An exposure-area product detector used in radiography.	A range of ionisation chambers of different volumes using for measuring radiation exposure.	An exposure meter used in radiography.
An isotope calibrator used in nuclear medicine - the blue cylinder on the left contains the re-entrant chamber.	An exposure meter used in radiography.