# Basic Physics of Nuclear Medicine/Scintillation Detectors

![](PET-detectorsystem.png)

The second type of radiation detector we will discuss is called the
scintillation detector.
Scintillations "wikilink") are minute
flashes of light which are produced by certain materials when they
absorb radiation. These materials are variously called fluorescent
materials, fluors, scintillators or
phosphors.

If we had a radioactive source and a scintillator in the lab we could
darken the room, move the scintillator close to the source and see the
scintillations. These small flashes of light might be green or blue or
some other colour depending on the scintillator. We could also count the
number of flashes produced to gain an estimate of the radioactivity of
the source, that is the more flashes of light seen the more radiation
present.

The scintillation detector was possibly the first radiation detector
discovered. You might have heard the story of the discovery of X-rays by
Wilhelm Roentgen in 1895. He was working
one evening in his laboratory in Wurzburg, Germany with a device which
fired a beam of electrons at a target inside an evacuated glass tube.
While working with this device he noticed that some platino-barium
cyanide crystals, which he just happened to have close by, began to glow
-- and that they stopped glowing when he switched the device off.
Roentgen had accidentally discovered a new form of radiation. He had
also accidentally discovered a scintillator detector.

Although scintillations can be seen we have a more sophisticated way of
counting and measuring them today by using some form of
photodetector.

We will learn about the construction and mode of operation of this type
of detector in this chapter. In addition, we will see how it can be used
not just for detecting the presence of ionizing radiation but also for
measuring the **energy** of that radiation.

Before we do however it is useful to note that scintillators are very
widely used in the medical radiations field. For example the X-ray
cassette used in radiography contains a
scintillator (called an **intensifying screen**) in close contact with a
photographic film. A second example is the **X-ray Image Intensifier**
used in fluoroscopy which contains
scintillators called phosphors. Scintillators are also used in some CT
Scanners and as we will see in the
next
chapter,
in the Gamma Camera and PET
Scanner. Their application
is not limited to the medical radiations field in that scintillators are
also used as screens in television sets and
computer monitors and for generating
light in fluorescent tubes -- to
mention just two common applications. What other applications can you
think of?

So scintillators are a lot more common than you might initially think
and you will therefore find the information presented here useful to you
not just for your studies of nuclear medicine.

## Fluorescent Materials

Some fluorescent materials are listed in the following table.
Thallium-activated sodium iodide, NaI(Tl)
is a crystalline material which is widely used for the detection of
gamma-rays in scintillation detectors. We will be looking at this in
more detail below.

Another crystalline material sodium-activated caesium
iodide, CsI(Na) is widely used for X-ray
detection in devices such as the X-ray image
intensifier. Another one called
calcium tungstate, CaWO~4~ has been widely used in X-ray cassettes
although this substance has been replaced by other scintillators such as
lanthanum oxybromide in many modern cassettes.

  Material                     Form
  ---------------------------- ---------
  NaI (Tl)                     crystal
  CsI (Na)                     crystal
  CaWO~4~                      crystal
  ZnS (Ag)                     powder
  p-terphenyl in toluene       liquid
  p-terphenyl in polystyrene   plastic

Notice that some scintillation materials are activated with certain
elements. What this means is that the base material has a small amount
of the activation element present. The term
doped "wikilink") is sometimes used instead
of **activated**. This activating element is used to influence the
wavelength (colour) of the light produced by the scintillator.

Silver-activated zinc sulphide is a
scintillator in powder form and p-terphenyl in toluene is a liquid
scintillator. The advantage
of such forms of scintillators is that the radioactive material can be
placed in close contact with the scintillating material. For example if
a radioactive sample happened to be in liquid form we could mix it with
a liquid scintillator so as to optimise the chances of detection of the
emitted radiation and hence have a very sensitive detector.

A final example is p-terphenyl in polystyrene which is a scintillator in
the form of a plastic. This form can be easily made into different
shapes like most plastics and is therefore useful when detectors of
particular shapes are required.

## Photomultiplier Tube

A scintillation crystal coupled to a photomultiplier
tube (PMT) is illustrated in the
following figure. The overall device is typically cylindrical in shape
and the figure shows a cross-section through this cylinder:

![](NM9_1.gif "NM9_1.gif")

The scintillation crystal, NaI(Tl) is very delicate and this is one of
the reasons it is housed in an aluminium casing. The inside wall of the
casing is designed so that any light which strikes it is reflected
downwards towards the PMT.

The PMT itself consists of a photocathode, a focussing grid, an array of
dynodes and an anode housed in an evacuated glass tube. The function of
the photocathode is to convert the light flashes produced by radiation
attenuation in the scintillation crystal into electrons. The grid
focuses these electrons onto the first dynode and the dynode array is
used for electron multiplication. We will consider this process in more
detail below. Finally the anode collects the electrons produced by the
array of dynodes.

The electrical circuitry which is typically attached to a PMT is shown
in the next figure:

![](NM9_2.gif "NM9_2.gif")

It consists of a high voltage supply, a resistor divider chain and a
load resistor, R~L~. The high voltage supply generates a dc voltage,
V~dc~ which can be up to 1,000 volts. It is applied to the resistor
divider chain which consists of an array of resistors, each of which has
the same resistance, R. The function of this chain of resistors is to
divide up V~dc~ into equal voltages which are supplied to the dynodes.
As a result voltages which increase in equal steps are applied to the
array of dynodes. The load resistor is used so that an output voltage,
V~out~ can be generated.

Finally the operation of the device is illustrated in the figure below:

![](NM9_3.gif "NM9_3.gif")

The ionizing radiation produces flashes of light in the scintillation
crystal. This light strikes the photocathode and is converted into
electrons. The electrons are directed by the grid onto the first dynode.

Dynodes are made from certain alloys which emit electrons when their
surface is struck by electrons with the advantage that more electrons
are emitted than are absorbed. A dynode used in a PMT typically emits
between two and five electrons for each electron which strikes it.

So when an electron from the photocathode strikes the first dynode
between two and five electrons are emitted and are directed towards the
second dynode in the array (three are illustrated in the figure). This
electron multiplication process is repeated at the second dynode so that
we end up with nine electrons for example heading towards the third
dynode. An electron avalanche therefore develops so that a sizeable
number of electrons eventually hits the anode at the bottom of the
dynode chain.

These electrons flow through the load resistor, R~L~ and constitute an
electric current which according to Ohm\'s Law generates a voltage,
V~out~ which is measured by electronic circuitry (which we will describe
later).

A number of photographs of devices based on scintillation detection are
shown below:

  ------------------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  !An X-ray cassette used in radiography.{width="150"}   !A NaI crystal coupled to a photomultiplier tube.{width="150"}                 !A number of identical photomultiplier tubes from a Gamma Camera.{width="150"}
  !A single channel analyser.{width="150"}                                !A single channel analyser with a crystal-PMT assembly.{width="150"}   !A well detector.{width="150"}
  ------------------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

The **important feature** of the scintillation detector is that this
output voltage, V~out~ is directly proportional to the energy deposited
by the radiation in the crystal. We will see what a useful feature this
is below. Before we do so we will briefly analyze the operation of this
device.

## Mathematical Model

A simple mathematical model will be presented below which will help us
get a better handle on the performance of a scintillation detector. We
will do this by quantifying the performance of the scintillator, the
photocathode and the dynodes.

Let\'s use the following symbols to characterize each stage of the
detection process:

-   *m*: number of light photons produced in crystal
-   *k*: optical efficiency of the crystal, that is the efficiency with
    which the crystal transmits light
-   *l*: quantum efficiency of the
    photocathode, that is the efficiency with which the photocathode
    converts light photons to electrons
-   *n*: number of dynodes
-   *R*: dynode multiplication factor, that is the number of secondary
    electrons emitted by a dynode per primary electron absorbed.

Therefore the charge collected at the anode is given by the following
equation:

```{=html}
<div class="center">
```
$Q = m k l R^n e\,\!$

```{=html}
</div>
```
where *e*: the electronic charge.

For example supposing a 100 keV gamma-ray is absorbed in the crystal.
The number of light photons produced, m, might be about 1,000 for a
typical scintillation crystal. A typical crystal might have an optical
efficiency, k, of 0.5 -- in other words 50% of the light produced
reaches the photocathode which might have a quantum efficiency of 0.15.
A typical PMT has ten dynodes and let us assume that the dynode
multiplication factor is 4.5.

Therefore

```{=html}
<div class="center">
```
$Q = 1000(0.5)(0.15)(4.5^{10})(1.6 \cdot 10^{-19})\ \text{C}$

```{=html}
</div>
```
```{=html}
<div class="center">
```
$\therefore Q = 41 \cdot 10^{-12}\ \text{C}$

```{=html}
</div>
```
```{=html}
<div class="center">
```
$\therefore Q \approx 40\ \text{pC}$

```{=html}
</div>
```
This amount of charge is very small. Even though we have used a
sophisticated photodetector like a PMT we still end up with quite a
small electrical signal.

A very sensitive amplifier is therefore needed to amplify this signal.
This type of amplifier is generally called a **pre-amplifier** and we
will refer to it again later.

## Output Voltage

We noted above that the voltage measured across the resistor, R~L~, is
proportional to the energy deposited in the scintillation crystal by the
radiation. Let us consider how the radiation might deposit its energy in
the crystal.

Let us consider a situation where gamma-rays are detected by the
crystal. We learnt in Chapter
5
that there were two interaction mechanisms involved in gamma-ray
attenuation -- the Photoelectric Effect and the Compton Effect. You will
remember that the Photoelectric Effect involves the total absorption of
the energy of a gamma-ray, while the Compton Effect involves just
partial absorption of this energy. Since the output voltage of a
scintillation detector is proportional to the energy deposited by the
gamma-rays it is reasonable to expect that Photoelectric Effects in the
crystal will generate distinct and relatively large output voltages and
that Compton Effects will result in lower output voltages.

The usual way of presenting this information is by plotting a graph of
the count rate versus the output voltage pulse height as shown in the
following figure:

![](NM9_8.gif "NM9_8.gif")

This plot illustrates what is obtained for a monoenergetic
gamma-emitting radioisotope, for example ^99m^Tc -- which, as we have
noted before emits a single gamma-ray with an energy of 140 keV.

Before we look at it in detail remember that we noted above that the
output voltage from this detector is proportional to the energy
deposited by the radiation in the crystal. The horizontal axis can
therefore be used to represent the output voltage or the gamma-ray
energy. Both of these quantities are shown in the figure to help with
this discussion. In addition note that this plot is often called a
**Gamma-Ray Energy Spectrum**.

The figure above contains two regions. One called the **Photopeak** and
the other called the **Compton Smear**. The Photopeak results because of
Photoelectric absorption of the gamma-rays from the radioactive source
-- remember that we are dealing with a monoenergetic emitter in this
example. It consists of a peak representing the gamma-ray energy (140
keV in our example). If our radioisotope emitted gamma-rays of two
energies we would have two photopeaks in our spectrum and so on.

Notice that the peak has a statistical spread. This has to do with how
good our detector is and we will not get into any detail about it here
other than to note that the extent of this spread is a measure of the
quality of our detector. A high quality (and more expensive!) detector
will have a narrower statistical spread in the photopeaks which it
measures.

The other component of our spectrum is the Compton Smear. It represents
a range of output voltages which are lower than that for the Photopeak.
It is therefore indicative of the partial absorption of the energy of
gamma-rays in the crystal. In some Compton Effects a substantial
scattering with a valence electron can occur which gives rise to
relatively large voltage pulses. In other Compton Effects the gamma-ray
just grazes off a valence electron with minimal energy transfer and
hence a relatively small voltage pulse is generated. In between these
two extremes are a range of scattering events involving a range of
energy transfers and hence a range of voltage pulse heights. A \'smear\'
therefore manifests itself on the gamma-ray energy spectrum.

It is important to note that the spectrum illustrated in the figure is
simplified for the sake of this introductory discussion and that actual
spectra are a little more complex -- see figure below for an example:

![](NM9_4a.gif "NM9_4a.gif"){width="320"}

You will find though that your understanding of actual spectra can
easily develop on the basis of the simple picture we have painted here.

It is also important to appreciate the additional information which this
type of radiation detector provides relative to a gas-filled detector.
In essence gas-filled detectors can be used to tell us if any radiation
is present as well as the amount of that radiation. Scintillation
detectors also give us this information but they tell us about the
**energy** of this radiation as well. This additional information can be
used for many diverse applications such as the identification of unknown
radioisotopes and the production of nuclear medicine images. Let us stay
a little bit longer though with the fundamental features of how
scintillation detectors work.

The photopeak of the Gamma-Ray Energy Spectrum is generally of interest
in nuclear medicine. This peak is the main signature of the radioisotope
being used and its isolation from the Compton Smear is normally achieved
using a technique called **Pulse Height Analysis**.

## Pulse Height Analysis

This is an electronic technique which allows a spectrum to be acquired
using two types of circuitry. One circuit is called a **Lower Level
Discriminator** which only allows voltages pulses through it which are
higher than its setting. The other is called an **Upper Level
Discriminator** which only allows voltage pulses though which are lower
than its setting.

The result of using both these circuits in combination is a
variable-width window which can be placed anywhere along a spectrum. For
example if we wished to obtain information from the photopeak only of
our simplified spectrum we would place the discrimination controls as
shown in the following figure:

![](NM9_9.gif "NM9_9.gif")

A final point to note here is that since the scintillation detector is
widely used to obtain information about the energies of the radiation
emitted from a radioactive source it is frequently referred to as a
**Scintillation Spectrometer**.

## Scintillation Spectrometer

Types of scintillation spectrometer fall into two basic categories --
the relatively straight-forward **Single Channel Analyser** and the more
sophisticated **Multi-Channel Analyser**.

The Single Channel Analyser is the type of instrument we have been
describing so far in this discussion. A block diagram of the instrument
is shown below:

![](NM9_10.gif "NM9_10.gif"){width="320"}

It consists of a scintillation crystal coupled to a photomultiplier tube
which is powered by a high voltage circuit (H.V.). The output voltages
are initially amplified by a sensitive pre-amplifier (**Pre-Amp**) as we
noted above before being amplified further and conditioned by the
amplifier (**Amp**).

The voltage pulses are then in a suitable form for the pulse height
analyser (**P.H.A.**) -- the output pulses from which can be fed to a
**Scaler** and a **Ratemeter** for display of the information about the
portion of the spectrum we have allowed to pass through the PHA. The
Ratemeter is a display device just like the speedometer in a car and
indicates the number of pulses generated per unit time. The Scaler on
the other hand usually consists of a digital display which shows the
number of voltage pulses produced in a specified period of time.

We can illustrate the operation of this circuitry by considering how it
might be used to generate a Gamma-Ray Energy Spectrum. What we would do
is set up the LLD and ULD so as to define a narrow window and place this
to pass the lowest voltage pulses produced by the detector through to
the Scaler and Ratemeter. In other words we would place a narrow window
at the extreme left of the spectrum and acquire information about the
lowest energy gamma-ray interactions in the crystal. We would then
adjust the LLD and ULD settings to acquire information about the
interactions of the next highest energy. We would proceed in this
fashion to scan the whole spectrum.

A more sophisticated detector circuit is illustrated in the following
figure:

![](NM9_11.gif "NM9_11.gif"){width="320"}

It is quite similar to that in the previous figure with the exception
that the PHA, Scaler and Ratemeter are replaced by a Multi-Channel
Analyser and a computer. The Multi-Channel Analyser (**MCA**) is a
circuit which is capable of setting up a large number of individual
windows to look at a complete spectrum in one go. The MCA might consist
of 1024 individual windows for example and the computer might consist of
a personal computer which can acquire information simultaneously from
each window and display it as an energy spectrum. The computer generally
contains software which allows us to manipulate the resultant
information in a variety of ways. Indeed the ^137^Cs spectrum shown
above was generated using this approach.

## External links

