# Basic Physics of Nuclear Medicine/Gas-Filled Radiation Detectors

![](Geiger_counter.jpg "Geiger_counter.jpg")

We have learned in the last two chapters about how radiation interacts
with matter and we are now in a position to apply our understanding to
the detection of radiation.

One of the major outcomes of the interaction of radiation with matter is
the creation of ions as we saw in Chapter
5.
This outcome is exploited in gas-filled detectors as you will see in
this chapter. The detector in this case is essentially a gas, in that it
is the atoms of a gas which are ionised by the radiation. We will see in
the next
chapter
that solids can also be used as radiation detectors but for now we will
deal with gases and be introduced to detectors such as the Ionization
Chamber and the Geiger
Counter.

Before considering these specific types of gas-filled detectors we will
first of all consider the situation from a very general perspective.

## Gas-Filled Detectors

As we noted above the radiation interacts with gas atoms in this form of
detector and causes ions to be produced. On the basis of what we covered
in Chapter
5
it is easy to appreciate that it is the Photoelectric and Compton
Effects that cause the ionisations when the radiation consists of
gamma-rays with energies useful for diagnostic purposes.

There are actually two particles generated when an ion is produced - the
positive ion itself and an electron. These two particles are
collectively called an **ion pair**. The detection of the production of
ion pairs in the gas is the basis upon which gas detectors operate. The
manner in which this is done is by using an electric field to sweep the
electrons away to a positively charged electrode and the ions to a
negatively charged electrode.

Let us consider a very simple arrangement as shown in the following
figure:

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![](NM8_1.gif "NM8_1.gif"){width="320"}

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Here we have two electrodes with the gas between them. Something like a
capacitor with a gas dielectric.

The gas which is used is typically an inert gas, for example
argon or xenon. The reason
for using an inert gas is so that chemical reactions will not occur
within the gas following the ionisations which could change the
characteristics of our detector.

A dc voltage is placed between the two electrodes. As a result when the
radiation interacts with a gas atom the electron will move towards the
positive electrode and the ion will move towards the negative electrode.
But will these charges reach their respective electrodes? The answer is
obviously dependent on the magnitude of the dc voltage. For example if
at one extreme we had a dc voltage of a microvolt (that is, one
millionth of a volt) the resultant electric field may be insufficient to
move the ion pair very far and the two particles may recombine to reform
the gas atom. At the other extreme suppose we applied a million volts
between the two electrodes. In this case we are likely to get sparks
flying between the two electrodes - a lightning bolt if you like - and
our detector might act something like a neon
sign. Somewhere in between these two extremes
though we should be able to provide a sufficient attractive force for
the ion and electron to move to their respective electrodes without
recombination or sparking occurring.

We will look at this subject in more detail below. Before we do let us
see how the concept of the simple detector illustrated above is applied
in practice. The gas-filled chamber is generally cylindrical in shape in
real detectors. This shape has been found to be more efficient than the
parallel electrode arrangement shown above.

A cross-sectional view through this cylinder is shown in the following
figure:

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![](NM8_2.gif "NM8_2.gif"){width="320"}

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The positive electrode consists of a thin wire running through the
centre of the cylinder and the negative electrode consists of the wall
of the cylinder. In principle we could make such a detector by getting a
section of a metal pipe, mounting a wire through its centre, filling it
with an inert gas and sealing the ends of the pipe. Actual detectors are
a little bit more complex however but let us not get side-tracked at
this stage.

We apply a dc voltage via a battery or via a dc voltage supply and
connect it as shown in the figure using a resistor, R. Now, assume that
a gamma-ray enters the detector. Ion pairs will be produced in the gas -
the ions heading towards the outer wall and the electrons heading
towards the centre wire. Let us think about the electrons for a moment.
When they hit the centre wire we can simply think of them as entering
the wire and flowing through the resistor to get to the positive
terminal of the dc voltage supply. These electrons flowing through the
resistor constitute an electric current and as a result of Ohm\'s
Law a voltage is generated across the
resistor. This voltage is amplified by an amplifier and some type of
device is used to register the amplified voltage. A loud-speaker is a
fairly simple device to use for this purpose and the generation of a
voltage pulse is manifest by a click from the loud-speaker. Other
display devices include a **ratemeter** which displays the number of
voltage pulses generated per unit time - something like a speedometer in
a car - and a pulse counter (or **scaler**) which counts the number of
voltage pulses generated in a set period of time. A voltage pulse is
frequently referred to in practice as a **count** and the number of
voltage pulses generated per unit time is frequently called the **count
rate**.

## DC Voltage Dependence

If we were to build a detector and electronic circuit as shown in the
figure above we could conduct an experiment that would allow us to
explore the effect of the dc voltage on the magnitude of the voltage
pulses produced across the resistor, R. Note that the term **pulse
height** is frequently used in this field to refer to the magnitude of
voltage pulses.

Ideally, we could generate a result similar to that illustrated in the
following figure:

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![](NM8_3.gif "NM8_3.gif"){width="320"}

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The graph illustrates the dependence of the pulse height on the dc
voltage. Note that the vertical axis representing the pulse height is on
a logarithmic scale for the sake of compressing a large linear scale
onto a reasonably-sized graph.

The experimental results can be divided into five regions as shown. We
will now consider each region in turn.

-   **Region A**     Here V~dc~ is relatively low so that recombination
    of positive ions and electrons occurs. As a result not all ion pairs
    are collected and the voltage pulse height is relatively low. It
    does increase as the dc voltage increases however as the amount of
    recombination reduces.

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-   **Region B**     V~dc~ is sufficiently high in this region so that
    only a negligible amount of recombination occurs. This is the region
    where a type of detector called the **Ionization Chamber** operates.

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-   **Region C**     V~dc~ is sufficiently high in this region so that
    electrons approaching the centre wire attain sufficient energy
    between collisions with the electrons of gas atoms to produce new
    ion pairs. Thus the number of electrons is increased so that the
    electric charge passing through the resistor, R, may be up to a
    thousand times greater than the charge produced initially by the
    radiation interaction. This is the region where a type of detector
    called the Proportional Counter
    operates.

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-   **Region D**     V~dc~ is so high that even a minimally-ionizing
    particle will produce a very large voltage pulse. The initial
    ionization produced by the radiation triggers a complete gas
    breakdown as an avalanche of electrons heads towards and spreads
    along the centre wire. This region is called the **Geiger-Müller
    Region**, and is exploited in the Geiger Counter.

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-   **Region E**     Here V~dc~ is high enough for the gas to completely
    breakdown and it cannot be used to detect radiation.

We will now consider features of the Ionisation Chamber and the Geiger
Counter in more detail.

## 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:

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

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The electric charge produced in the gas is therefore

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$Q = n \cdot e$

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

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

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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:

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

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

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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.

![](NM8_10.gif "NM8_10.gif"){width="320"}

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

  --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------
  !An exposure-area product detector used in radiography.{width="150"}                                                                                                                 !A range of ionisation chambers of different volumes using for measuring radiation exposure.{width="150"}   !An exposure meter used in radiography.{width="150"}
  !An isotope calibrator used in nuclear medicine - the blue cylinder on the left contains the re-entrant chamber.{width="150"}   !An exposure meter used in radiography.{width="150"}                                                                                                                                      !A contemporary Geiger counter.{width="150"}
  --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------------------------------

## Geiger Counter

We saw earlier that the Geiger Counter operates at relatively high dc
voltages (for example 400-900 volts) and that an avalanche of electrons
is generated following the absorption of radiation in the gas. The
voltage pulses produced by this detector are relatively large since the
gas effectively acts as an amplifier of the electric charge produced.

There are four features of this detector which we will discuss. The
first is that a sensitive amplifier (as was the case with the Ionization
Chamber) is not required for this detector because of the gas
amplification noted above.

The second feature results from the fact that the generation of the
electron avalanche must be stopped in order to reform the detector. In
other words when a radiation particle/photon is absorbed by the gas a
complete gas breakdown occurs which implies that the gas is incapable of
detecting the next particle/photon which enters the detector. So in the
extreme case one minute we have a radiation detector and the following
moment we do not.

A means of stopping the electron avalanche is therefore required - a
process called **Quenching**. One means of doing this is by
electronically lowering the dc voltage following an avalanche. A more
widely used method of quenching is to add a small amount of a
**quenching gas** to the inert gas. For example the gas could be argon
with ethyl alcohol added. The ethyl
alcohol is in vapour form and since it consists of relatively large
molecules energy which would in their absence give rise to sustaining
the electron avalanche is absorbed by these molecules. The large
molecules act like a brake in effect.

Irrespective of the type of quenching used the detector is insensitive
for a small period of time following absorption of a radiation
particle/photon. This period of time is called the Dead
Time and this is the third feature of this
detector which we will consider. Dead times are relatively short but
nevertheless significant - being typically of the order of 200-400 µs.
As a result the reading obtained with this detector is less than it
should be. The true count rate, T, can be obtained using the following
equation:

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$T = \frac{A}{1 - \tau A}$

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where A is the (actual) reading and τ is the dead time. Some instruments
perform this calculation automatically.

The fourth feature to note about this detector is the dependence of its
performance on the dc voltage. The Geiger-Müller Region of our figure
above is shown in more detail below:

![](NM8_12.gif "NM8_12.gif"){width="320"}

Notice that it contains a plateau where the count rate obtained is
independent of the dc voltage. The centre of this plateau is where most
detectors are operated. It is clear that the count rate from the
detector is not affected if the dc voltage fluctuates about the
operating voltage. This implies that a relatively straight-forward dc
voltage supply can be used. This feature coupled with the fact that a
sensitive amplifier is not needed translates in practice to a relatively
inexpensive radiation detector.

## External Links

-   Inside a smoke
    detector - about the
    ion chamber used in smoke detectors - from the How Stuff Works
    website.
-   Ionisation
    Chambers - a
    brief description from the Triumf Safety Group.
-   Radiation and
    Radioactivity -
    a self-paced lesson developed by the University of Michigan\'s
    Student Chapter of the Health Physics Society with a section on gas
    filled detectors.
-   The Geiger
    Counter - a
    brief overview from the NASA Goddard Space Flight Center, USA.

