Mod 1.5 Radiation Detection

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Radiation Protection Distance Learning Project

Module 1.5 – Methods of Radiation Detection

INTERNATIONAL ATOMIC ENERGY AGENCY

AUSTRALIAN NUCLEAR SCIENCE AND

TECHNOLOGY ORGANISATION

DISTANCE LEARNING MATERIALS

RADIATION PROTECTION

Module – 1.5

Prepared by Safety and Radiation Science

Australian Nuclear Science and Technology Organisation

With the assistance of consultants in Australia and the United

Kingdom, and IAEA experts from Korea, Mongolia, New Zealand,

the Philippines and Thailand

Production /var/www/apps/conversion/tmp/scratch_4/319323153.doc Printed:26/05/20

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Use of this material, acknowledging the IAEA and ANSTO as the source, is

permitted for non-profit training


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Acknowledgements

This material has been prepared by an extensive team of international

experts from the fields of radiation protection, medicine and education. It is

not possible to mention by name all those who have had an input to; the

initial planning; writing modules and workbooks; preparing and marking final

assignments; proof-reading and editing; using the material and providing

valuable feedback; organizing and hosting review meetings; and general

administrative support.

The support of ANSTO management in providing the resource and

encouragement is gratefully acknowledged.

The enthusiastic support of IAEA Technical Officers has been vital to the

success of this project


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Radiation Protection Distance Learning Project Page 1


PART 1

BASIC KNOWLEDGE

MODULE 1.5

METHODS OF RADIATION DETECTION

CONTENTS

OVERVIEW.......................................................................................................4

TIME ALLOCATION..........................................................................................4

MATERIALS......................................................................................................5

LEARNING OBJECTIVES................................................................................5

1. MECHANISMS USED FOR DETECTING RADIATION..........................6

1.1Ionization...............................................................................................6

1.2Scintillation............................................................................................6

1.3Thermoluminescence............................................................................7

1.4Chemical Mechanisms..........................................................................7

1.5Heating..................................................................................................7

1.6Biological Mechanisms..........................................................................8

1.7Summary of Detection Mechanisms.....................................................8

SELF-CHECK 1................................................................................................9

2. DETECTORS BASED ON IONIZATION..................................................9

2.1Gas-Filled Detectors..............................................................................9

2.1.1How they work..............................................................................10

2.1.1.1 The recombination region.....................................................11

2.1.1.2 Ion chamber region..............................................................12

2.1.1.3 Proportional region...............................................................12

2.1.1.4 Geiger-Müller region.............................................................13


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2.1.1.5 Continuous discharge region...............................................13

SELF-CHECK 2..............................................................................................13

2.1.2Resolving time, dead time and recovery time..............................15

2.1.3Types of gas-filled detectors........................................................16

2.1.3.1 Ionization chambers.............................................................16

2.1.3.2 Proportional Counters..........................................................18

2.1.3.3 Geiger-Müller (G-M) counters..............................................19

2.1.4Summary of gas-filled detectors..................................................21

PRACTICAL ACTIVITY..................................................................................22

SELF-CHECK 3..............................................................................................23

2.2Solid State Conductivity Detectors......................................................24

2.2.1How they work..............................................................................24

2.2.2Types of detectors........................................................................26

2.2.2.1 Diffused junction diodes.......................................................27

2.2.2.2 Surface barrier detectors......................................................28

2.2.2.3 Ion implantation detectors....................................................29

2.2.2.4 Lithium drifted detectors.......................................................29

2.2.2.5 High Purity Germanium Detectors.......................................31

2.2.3Summary of solid state conductivity detectors............................31

2.3Solid State Detectors Versus Gas-filled Detectors..............................32

SELF-CHECK 4..............................................................................................33

3. DETECTORS BASED ON SCINTILLATION.........................................34

3.1How they work.....................................................................................34

3.2Types of Scintillation Detector.............................................................34

3.2.1Zinc sulphide detectors................................................................35

3.2.2Sodium iodide detectors..............................................................36

3.2.3Plastic organic scintillators...........................................................37

3.2.4Liquid organic scintillators............................................................37


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3.3Summary of Scintillation Detectors.....................................................38

3.4Photomultiplier Tubes..........................................................................38

3.4.1How they work..............................................................................38

SELF-CHECK 5..............................................................................................39

4. NEUTRON DETECTORS......................................................................40

4.1How they work.....................................................................................40

4.2Types of Neutron Detectors.................................................................41

4.2.1Boron trifluoride proportional counters........................................41

4.2.2Helium proportional counters.......................................................42

4.2.3Gas recoil proportional counter....................................................43

4.2.4Bubble detectors..........................................................................43

4.3Summary of Neutron Detectors...........................................................44

SELF-CHECK 6..............................................................................................45

5. ELECTRONIC COMPONENTS.............................................................45

5.1Voltage Supply.....................................................................................465.2Direct Current Amplifier.......................................................................46

5.3Pre-amplifier........................................................................................47

5.4Pulse Amplifier....................................................................................47

5.5Discriminator........................................................................................47

5.6Counting Devices................................................................................48

SELF-CHECK 7..............................................................................................48

KEY POINTS..................................................................................................49

FINAL ASSIGNMENT....................................................................................52

GLOSSARY OF TERMS................................................................................53


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OVERVIEW

To control radiological hazards in the workplace and in the general

environment, it is necessary to be able to detect and measure the amount of

ionizing radiation present. As we cannot detect ionizing radiation with any of

our body senses, we must use the ability of this radiation to interact with

various materials as a way of detecting and measuring it. This module

discusses these various interactions and introduces the main physical

principles used to detect ionizing radiation. It also discusses some of the

detection systems commonly used in the workplace.

Before studying this module, it is important that you have read and

understood the information in the previous four modules. In particular, you

will need to have a good understanding of the information in Module 1.4

Interaction of Radiation with Matter.

An understanding of how the different types of detectors work will also assist

you in your choice of detector and its method of use. Hence, this module

provides the background information necessary to understand the later

module, Module 2.4 Use of Radiation Monitoring Instrumentation. It is

therefore important that you understand the information in this module before

continuing with the course.

There is one practical activity associated with this module. You will find the

relevant information in your workbook. Remember that this practical activity

must be completed before you can gain full credit for this course. You should

therefore make sure that you arrange a suitable time with your supervisor for

doing this practical work.

TIME ALLOCATION


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It is estimated that you will need to spendeight to tenhours to complete this

modulebefore commencing the assignment. This time estimate is for

guidanceonly. Do not worry if you need more time to understand the

module.

MATERIALS

You will need to have writing materials to take notes and answer questions

and assignments in the module.

LEARNING OBJECTIVES

When you have completed this module you will be able to do the following:

1)Describe the six basic methods of radiation detection.

2)State the basic operating principle of gas-filled detectors.

3)List the three main types of gas-filled detectors and describe their

operation.

4)Explain the terms resolving time, response time and dead time in relation

to gas-filled detectors.

5)Set up a Geiger-Müller detector and measure its operational

characteristics.

6)State the basic operating principle of solid state conductivity detectors.

7)State the basic operating principle of scintillation detectors.

8)Describe the function of a photomultiplier tube.

9)State the basic operating principles of neutron detectors.


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10)State the advantages and disadvantages of each type of radiation

detector.

11)Select the types of detectors most suitable for the detection of the

different types of radiation.

12)Match the components of a typical counting system with their functions.


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NOW YOU ARE READY TO START WORK

1. MECHANISMS USED FOR DETECTING RADIATION

Since ionizing radiation cannot be detected by our bodies’ senses we rely on

the detection of changes produced by the radiation when it interacts with

materials. Radiation detectors operate by detecting a change in the

absorbing medium that is caused by the transfer of energy from the ionizing

radiation to this medium. There are several effects caused by ionizing

radiation which allow us to detect and measure the radiation and these are

as follows:

• Ionization;

• Scintillation;

• Thermoluminescence;

• Chemical mechanisms;

• Heating; and

• Biological mechanisms.

1.1 Ionization

Ionization is caused directly by alpha and beta radiation and indirectly by

x-rays, gamma and neutron radiation. The ion pairs which are produced can

be collected, and the number of ion pairs collected can be related to the

amount of radiation causing the ionization. Many radiation monitoring

instruments use ionization as the detection mechanism.


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1.2 Scintillation

Scintillation is the production of light following the movement of electrons

from high energy levels orbits to lower energy levels within an absorbing

material. The electrons have moved into higher energy orbits by the process

of excitation. (Remember from module 1.4 Interaction of Radiation with

Matter that excitation occurs when the energy from ionizing radiation causes

electrons to move into higher energy levels temporarily). The light released

can be converted to an electrical signal. The size of the electrical signal

depends on the number of electrons moved into higher energy orbits and

can therefore be related to the amount of radiation causing the scintillation.

Scintillation is a very important detection mechanism for radiation monitoring

and detectors which use this mechanism are known asscintillation

detectors.

1.3 Thermoluminescence

When electrons in certain materials absorb energy they will move into higher

energy levels or ‘forbidden bands’. They remain trapped in these bands until

the material is heated to a certain temperature. The heat energy releases

the electrons and the material emits light as the electrons move back to their

original level. The light is converted to an electrical signal which can be

related to the amount of incident radiation. Thermoluminescent materials

are used for monitoring personal doses (i.e. dose to individual people) and

will be discussed further in Module 2.5 Personal Dosimetry.

1.4 Chemical Mechanisms

Ionizing radiation can cause chemical changes. This effect is observed in

the use of photographic film for personal dosimetry, medical x-rays and


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industrial radiography. In some cases, ionizing radiation increases the rate

at which chemical reactions occur and this mechanism can be used for

measuring high doses during the irradiation of medical equipment.

1.5 Heating

Ionizing radiation can increase the temperature of the absorbing medium

and careful measurement of this increase can give a radiation dose

measurement. This technique (known ascalorimetry) is not suitable for

routine measuring equipment in radiation protection as high doses are

needed to cause even small temperature rises. It is however used as a

primary standard for instrument calibration (see Module 2.4 Use of Radiation

Monitoring Instruments).

1.6 Biological Mechanisms

High doses of radiation can cause biological changes in living cells. This will

be discussed further in Module 1.6 Biological Effects of Ionizing Radiation.

Biological changes are only used for dose estimation in extreme

circumstances where personnel are suspected of having accidentally

received a high dose.

1.7 Summary of Detection Mechanisms

Table 1 summarizes the mechanisms which can be used to detect ionizing

radiation.

Table 1

Summary of the Mechanisms Used for Radiation Detection

Mechanism Main Use Type of Instrument Detector

Ionization Radiation

monitoring

1. Ion chamber 1.Gas-filled


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instruments 2. Proportional counter

3. Geiger-Müller counter

4. Solid state

2.Gas-filled

3.Gas-filled

4.Semiconductor

Scintillation Radiation

monitoring

instruments

Scintillation counter Crystal or liquid

Thermoluminescence Personal

dosimetry

Thermoluminescent

Dosimeter (TLD)

Crystal

Chemical Personal

dosimetry

Photographic film Photographic

emulsion

Heating Primary standard

and instrument

calibration

Calorimeter Solid or liquid

Biological Accident

situations

Biological tissue Biological tissue

SELF-CHECK 1

Now see how much you have understood by answering the following

questions in your workbook:

1.Why do we need to monitor ionizing radiation?

2.How do radiation detectors operate?

3. a) List the six mechanisms of radiation detection.

b) Which of these are important for radiation monitoring instruments?

c) Which of these are important for monitoring personal dose?

Now check your answers with the model answers in your workbook.


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2. DETECTORS BASED ON IONIZATION

As already mentioned, many radiation monitoring instruments use the

ionization as the detection mechanism, and the two types of detectors which

are most commonly used are gas-filled detectors and solid state

conductivity detectors.

2.1 Gas-Filled Detectors

Gas-filled detectors consist of a chamber filled with a gas (often air) and two

voltage plates known aselectrodes. The positive electrode is called the

anodeand is often in the centre of the chamber. It is electrically insulated

from the outer casing. The outer casing of the chamber is often the negative

electrode orcathode. Figure 1 shows a simple diagram of a gas-filled

detector.


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Figure 1

A Simple Gas-Filled Detector

2.1.1 How they work

Incoming radiation interacts with the walls of the chamber or the gas

particles and produces ion pairs. When a voltage is applied between the

electrodes, positive ions are attracted towards the negatively charged

cathode, and the electrons are attracted towards the positively charged

anode. A charge builds up on the anode, causing a voltage change in the

circuit. This change in voltage is referred to as apulse, and the presence of

this pulse causes a current to flow in the external circuit. By detecting either

this pulse or current, we can detect the presence of ionizing radiation.

The size of the pulse depends on the number of electrons collected by the

anode and this can depend on the amount of ionizing radiation entering the

chamber as well as its type and energy.

In addition, the size of the pulse is also a function of the voltage applied

between the cathode and the anode. Figure 2 shows how the pulse size (or

height) varies as the applied voltage is increased.


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Gas-filled


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As the voltage across the electrodes is increased, more of the ions reach the

electrodes and the pulse size increases (see Figure 2). However,

recombination of ions is still significant and so this region is known as the

recombination region. Gas-filled detectors are not normally operated in

this region as the recombination of ions makes it very difficult to measure the

quantity of incoming radiation.

2.1.1.2 Ion chamber region

When the voltage is large enough, almost all the ions generated will reach

the electrodes and the loss of ions through recombination is negligible. In

this region nearly all the ions are being collected and the pulse size no

longer increases with applied voltage. Instead, it levels out to a plateau

known as theion chamber region (see Figure 2).

The current flowing in the circuit also reaches a maximum value known as

the saturation current. This saturation current is proportional to the amount

of radiation entering the chamber and if the amount of radiation is increased,

then the saturation current is also increased.

2.1.1.3 Proportional region

As the voltage is increased past the ion chamber region, the pulse size starts

to increase again. We can explain why the pulse size increases if we

consider what is happening to the ions. As the applied voltage is increased,

the ions not only gain enough energy to reach the electrodes, but they also

gain enough energy to be accelerated. This acceleration causes more ion

pairs to be produced through secondary ionization of the particles in the gas.

This process is known asgas multiplicationand it results in the collection

of more ions and therefore a larger pulse.


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Figure 3 shows a single electron being accelerated towards the anode and

producing anavalanche of ions.

Figure 3

Gas Multiplication in the Proportional Region

The increase in the number of ions collected is a function of the applied

voltage. However, the total pulse size that results is also proportional to the

initial number of ions produced in the gas. For this reason, the region of a

gas-filled detector where this occurs is known as the proportional region.

2.1.1.4 Geiger-Müller region

If the voltage is increased still further, the gas multiplication is so great that a

single ionizing particle produces multiple avalanches along the length of the

anode, resulting in a very large pulse. The region where this occurs is

known as theGeiger-Müller (G-M) region and in this region, the size of the

pulse is the same, regardless of the quantity of energy originally deposited.

Instead, the pulse size is controlled more by the external circuit than by the

gas-filled chamber, and the pulse height shows a very small rise as the

voltage is increased (see Figure 2).


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2.1.1.5 Continuous discharge region

If the voltage is increased beyond the Geiger-Müller plateau, then the voltage

is high enough to ionize the gas molecules directly and a large signal is

generated even when the radiation field is removed. This is called the

continuous discharge regionand as the readings can be misleading,

radiation detectors should not be operated in this region.

Radiation detectors must not be operated in the continuous discharge

region.

SELF-CHECK 2



1. What is the basic detection mechanism on which all gas-filled detectors

depend?

2. Fill in the gaps with a suitable word or phrase:

Gas-filled detectors consist of a chamber filled with a ____ and two

voltage plates known as __________. The positive electrode is called

the _______ and the negative electrode is called the __________.

Incoming radiation interacts with the walls of the chamber or the gas

particles and produces ____ _______. When a voltage is applied

between the electrodes, the _________ _____ are attracted towards

the negatively charged electrode, and the _________ are attracted

towards the positively charged electrode. A charge builds up, causing a

voltage change in the circuit. This change in voltage is referred to as a

__________, and this causes a current to flow in the external circuit.


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By detecting either this voltage change or current, we can detect the

presence of ionizing radiation.

3. a) List the five regions of a gas-filled detector.

b) Complete the following table by matching the different regions with

their descriptions.

Region Description

1. a) The region where the applied voltage is large

enough to cause acceleration of ions.

2. b) The region where almost all ions are being

collected and the current reaches a constant

value known as the saturation current.

3. c) The region where many ions do not reach the

electrodes.

4. d) The region where the voltage is high enough to

ionize the gas molecules directly.

5. e) The region where the size of the pulse is the

same regardless of the amount of energy

deposited.

4. Which of these regions are used for detecting ionizing radiation?


2.1.2 Resolving time, dead time and recovery time

Before considering the operation of the different types of gas-filled detectors,

it is important to understand the concepts of dead time, recovery time and

resolving time. This is because if resolving time of a detector is too long, at


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high counting rates the pulses from the detector may be spaced so closely in

time that a lot of the information is lost. This means that the total counts may

then be grossly underestimated.

The resolving timeof a detector is defined as the minimum amount of time

which must separate two events in order that they are recorded as two

separate processes. The resolving time depends on the following factors:

• Thedead time of the detector (i.e. length of time for the signal or pulse to

build up sufficiently for it to be detected); and

• Therecovery time(i.e. length of time for the detector to recover from an

ionization event and return to its original condition).

Figure 4 shows how the dead time and recovery time for a Geiger-Müller

detector combine to give resolving time.

Figure 4

Resolving Time for a G-M Tube

The detector resolving time will depend on what interactions are taking place

in the detector. However, the overall resolving time of a whole instrument will


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Pulse Voltage

Dead Time Recovery

TimeResolving Time


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also depend on the dead times associated with electronic components of the

counting system.

In practice, the terms dead time and resolving time are used

interchangeably. Whichever term is used, the quoted value is taken to be a

measure of the ability of the instrument to distinguish two separate events

which occur very close together in time. Manufacturers’ specifications often

use the term dead time to describe the combined behaviour of the detector-

counting system.

2.1.3 Types of gas-filled detectors

In the following section we will look at the three types of gas-filled detectors

used in radiation monitoring instruments. The three types are as follows:

• Ionization chambers;

• Proportional counters; and

• Geiger-Müller counters.

2.1.3.1 Ionization chambers

Ionization chambers (more commonly known asion chambers) are

designed to operate at saturation current in the ion chamber region shown in

Figure 2. The average current output is measured and is proportional to the

amount of radiation to which the chamber has been exposed. Since the

output is not dependent on the voltage, there is no need for a highly stable

power supply. However, it is important that the voltage is steady enough to

ensure that saturation current is maintained.

To prevent the ion chamber from operating in the proportional region, the

applied voltage is limited to less than that required to cause secondary


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ionization of the gas molecules. If a 25V power supply is used, the energy

gained by an electron accelerating between the electrodes cannot be greater

than 25 eV. This energy is not enough to cause further ionization.

The currents produced in ion chambers are very small, typically in the order

of 10-12A, and so must be amplified for measurement purposes. Hence

instruments which incorporate ion chamber detectors require quite complex

solid state circuitry to amplify these extremely small direct currents.

The design of ion chambers and the choice of filling gas depend on the

particular application of the instrument. In portable radiation monitoring

instruments, the chamber is usually air-filled and constructed of low atomic

number materials. If the instrument is to be used to measure alpha or beta

radiation, the chamber must have thin walls or a thinend window. The only

way that this type of detector can be used to discriminate between the types

of radiation is by placing a shield over the thin end window to prevent the

alphas or betas from entering the chamber.

A typical beta/gamma portable radiation monitoring instrument containing a

ion chamber detector is shown in Figure 5. Note the sliding metal plate

which is incorporated into the design of the instrument. By sliding this plate

over the end window of the ion chamber, you can distinguish between beta

and gamma radiation.


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Front View Rear View

Figure 5

A Typical Beta Gamma Radiation Monitoring Instrument Incorporating

an Ion Chamber Detector

Note that ion chambers may also be incorporated in other instruments to

distinguish between the different energies of the incoming radiation. This

process is known asspectroscopy

2.1.3.2 Proportional Counters

Aproportional counter operates in the proportional region as shown in

Figure 2. The effect of the gas multiplication may increase the number of

ions produced by 104

. This means that for each electron produced by the

original ionization event, there may be ten thousand additional electrons

produced. Therefore each ionization event can be distinguished and

counted.

The output from a proportional counter is a series of pulses which are

counted by a counting circuit. In general, the resolving time is generally very

short for these counters (less than a microsecond) so high pulse rates can


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Thin endwindow

Sliding

metal plate


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be counted. However, the amplitude of each pulse is very small (of the order

of millivolts) and pre-amplification is required before the pulses can be

counted.

Have another look at Figure 2. You will notice that the slope of the graph in

the proportional region is quite steep. This means that a slight variation in

the applied voltage will have an effect on the pulse height. It is therefore

important that a stable high voltage supply is used as this will ensure that

any change in output is related to the change in incoming radiation rather

than a change in the applied voltage.

As noted earlier, the total pulse size is proportional to the energy deposited

by the radiation. Hence, proportional counters can be used with a pulse

height discrimination circuit to distinguish between the types of radiation on

the basis of their ionizing ability. For example, if the instrument is exposed to

both alpha and beta radiation of about the same energy, the alpha radiation

will produce a much greater number of ion pairs for the same path length so

the pulse height will be much larger. If different external circuits are used,

proportional counters can also be used to distinguish between the different

energies of the incoming radiation (i.e perform spectroscopy)

Gas flow proportional counters are often used for counting samples (see

Figure 6). The counting chamber has a very thin end window to allow alpha

and beta particles to enter the chamber. The term gas flow is used because

there must be a continuous flow of gas into the chamber to replace the gas

that has diffused out through the thin end window. The type of gas used is

usually a mixture of one of the inert gases with a hydrocarbon. For example,

the gas commonly known as P-10 is frequently used. This gas consists of

90% argon and 10% methane.


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Figure 6

A Gas Flow Proportional Counter

2.1.3.3 Geiger-Müller (G-M) counters

Geiger-Müller counters operate in the Geiger-Müller (G-M) region shown in

Figure 2 and use a filling gas such as the P-10 used in proportional counters.

The height of the output pulse is independent of the energy of the ionizing

particle. This means that it is not possible to distinguish electronically

between alpha and beta radiation. Nor is it possible to measure or

discriminate between the energies of the incoming radiation.

In the Geiger-Müller region, a discharge takes place all along the anode.

This discharge must bequenched to prevent it from continuing by itself and

to prevent multiple pulses forming. Use of an appropriate gas such as

organic gases (e.g. ethyl alcohol) or halogens (e.g. chlorine, bromine) in

addition to the filling gas will provide this quenching. Organic gases are used

up during the quenching process, therefore organically quenched tubes have

a limited useful lifetime of about 109 counts. Halogens have the advantage


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of not being used up in the quenching process so halogen-filled tubes have a

much longer lifetime and are more useful in high count rate situations.

Geiger-Müller counters can be made in a variety of shapes and sizes. For

the majority of applications, the counter is cylindrical and is known as a GM

tube(see Figure 7).

Figure 7

A Radiation Monitoring Instrument with A Geiger-Müller Tube

A small G-M tube can be sufficiently sensitive to measure low dose rates

whereas an ionization chamber with similar sensitivity would need to be

much larger. If the counter is to be used to detect alpha and beta radiation, it

must have a thin window to let the radiation enter the tube.

A G-M counter usually counts pulses in the same way as the proportional

counter. However, it can be modified to measure average current as done by

the ionization chamber. One advantage of G-M counters is that the output

pulse is in the order of a few volts, so the signal does not require pre-

amplification and the circuitry can be kept simple. This means that Geiger-

Müller counters are very rugged and are therefore commonly used in the

workplace for monitoring gamma radiation.


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G-M Tube

with thin end


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If G-M detectors are to be used as dose or dose rate meters, they must have

a response similar to human tissue over a wide energy range. As G-M tubes

over-respond at energies below about 200 keV, they are usually encased in

suitable filtration material to ensure that the energy response is as linear as

possible. This is calledenergy compensation.

One of the disadvantages of a G-M counter is their long resolving time. This

is usually of the order of 100 to 300 microseconds which means that this

counter is not suitable for high counting rates where pulses are forming very

quickly. A condition calledfoldback can occur in high radiation fields where

the pulses are being produced so quickly that they attach themselves to the

tail of the previous pulse before the anode has been cleared of charge.

Pulses following the initial pulse are therefore too small to be registered. If

an instrument is turned on in a high radiation field, it will initially show a rise

in reading, but this rapidly falls back to zero wrongly indicating that the field is

safe.

A G-M counter may read zero in an area of high dose rate because of

foldback.

Additional circuitry can be built into the detector to prevent this potentially

hazardous situation. Unless the instrument specifications provided by the

manufacturer clearly state that foldback will not occur, you should always

assume that it may be a problem.

2.1.4 Summary of gas-filled detectors

Many portable radiation monitoring instruments use gas-filled detectors.

Table 2 summarizes their properties and characteristics. Remember that it is

important to consider the thickness of the window through which the


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radiation enters the detector if you are hoping to detect alpha or beta

radiation.


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Table 2

Summary of Gas-filled Detectors

Detector Type of

Radiation

Efficiency Comments

Ionization

Chambers

Alpha High (with suitably thin

end window)

Used for counting and

spectroscopy.

Beta Moderate (with suitably

thin end window)

Used in portable radiation

monitoring instruments.

Gamma


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This is a practical assignment in which you will learn how to set up a G-M

tube so that it will operate correctly and give consistent results. The details

are found in your workbook. Contact your supervisor to arrange a

convenient time to complete this practical task.

SELF-CHECK 3


questions in your workbook.

1.a) Match the following terms with their descriptions:

Term Description

1. Resolving time a) The minimum amount of time which

must separate two events in order that

they are recorded as two separate

processes.

2. Dead time b) The length of time for the detector

to recover from an ionization event andreturn to its original condition.

3. Recovery time c) The length of time for the signal or

pulse to build up sufficiently for it to be

detected.

b) Why is it important to consider the resolving time of a gas-filled

detector?

2.Which two types of gas-filled detectors require current amplification?

3.a) Which type of gas-filled detector requires a stable high voltage power

supply?

b) Why is this so?


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4.Which type of gas-filled detector can be used with a pulse height

discriminator circuit to distinguish between the types of ionizing

radiation?

5.Which types of detectors require thin end windows to discriminate

between different radiation types?

6.a) Which type of detector is commonly used to detect gamma radiation in

the workplace?

b) Why is this so?

7.a) Explain the term foldback in relation to Geiger-Müller counters.

b) Why is it important to consider the possibility of foldback?


2.2 Solid State Conductivity Detectors

Conductivity refers to the ability of a material to conduct an electric current

and materials which have good conductivity (e.g. metals), are known as

conductors. Materials which have poor conductivity (e.g. wood) are known

asinsulators. Asemiconductor is a material with properties somewhere in

between these two extremes, and although there are a number of

substances with semiconducting properties, the ones most commonly used

for radiation detection are the crystalline solids of silicon and germanium.

Solid state conductivity detectors are so named because they consist of

semiconducting crystalline solids. When ionizing radiation interacts with

these solids, the overall conductivity of the material is increased. If this


16


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increase is then measured, it can be related to the amount of incident

radiation.

2.2.1 How they work

To understand how solid state conductivity detectors work we need to

consider the interaction of ionizing radiation with semiconducting materials

on a microscopic scale. As you may remember for Module 1.1 Structure of

Matter, electrons can only exist in definite energy levels. In solids, these

energy levels are referred to asenergy bands. These energy bands are

separated by areas known asforbidden bands, and the highest energy

band in which electrons usually exist is known as thevalence band.

Ionizing radiation can give enough energy to an electron in a semiconducting

crystalline solid to move it from its usual energy level (in the valence band)

through normally forbidden levels (in the forbidden band) and up into a

higher energy state (known as theconduction band). As it does so it

leaves a vacancy (or hole) in the valence band (see Figure 8).

Figure 8


16

Valence Band

Conduction Band

Forbidden Band

Incident radiation

KEY

= Electron

=


34/68



The Formation of an Electron-Hole Pair by Ionizing Radiation

The raising of the electron to the conduction band is known asionization

and the resulting electron-hole pair can be compared to the ion pair in a gas-

filled detector. In the same way as positive and negative ions move between

electrodes in a gas-filled detector, the ion pair will ‘move’ in a solid state

detector when a voltage is applied. This movement results in a pulse in the

external circuit which can be measured.

Note that in reality, the positively charged material cannot move in a

crystalline solid. What actually happens is that holes are filled by

neighbouring electrons which move across and leave another hole behind.

In this way, the holes appear to move.

Solid state conductivity detectors consist of semiconductors which have had

their conductivity enhanced. In general, the conductivity is improved by

introducing impurities to the semiconductor material. This process is known

asdoping and the introduced impurities provide either extra electrons or

extra holes. If the impurity provides extra electrons to the valence band (e.g.

in the case of arsenic or phosphorus doping), then these loosely bound

electrons can move to the conduction band with only a small amount of

energy being deposited in the material. In this way, the main conduction

mechanism is the movement of negative charges and the material is known

as ann type semiconductor. If the impurity provides extra holes to the

valence band (e.g. in the case of boron or gallium doping), then the main

conduction mechanism is the movement of positive holes and the material is

known as ap type semiconductor.

Solid state conductivity detectors actually consist of p and n type material

joined together. An electric voltage is applied across the junction so that the


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holes and electrons move away from the junction. The area around the

junction is free of holes and electrons and is known as thedepletion layer.

This depletion layer is the part of the material which will detect any incident

radiation (see Figure 9).

Figure 9

The Basis of a Solid State Conductivity Detector

When ionizing radiation passes through the depletion layer, it forms electron-

hole pairs. The electron-hole pairs move apart causing a pulse in the

external circuit. This pulse can then be measured. In this way, the depletion

layer forms thesensitive volume of the solid state detector and it is

equivalent to the chamber in a gas-filled detector.

2.2.2 Types of detectors

There are many different types of solid state conductivity detectors available

for detecting ionizing radiation. The types of solid state conductivity

detectors considered in this module are:

• Diffused junction diodes;


16

+

+

+

+

-

-

-

-

Electrons Holes

V

Depletion Layer

p typen type

+


36/68



• Surface barrier detectors;

• Ion implantation detectors;

• Lithium drifted detectors; and

• High purity germanium detectors.

2.2.2.1 Diffused junction diodes

Indiffused junction diodes the p type impurity has been allowed to diffuse

or spread into the n type material. This creates a depletion region just below

the crystal surface (typically about 1µm below the surface) as shown in

Figure 10. The surface layer represents a dead layer orwindow through

which the radiation must pass before entering the sensitive volume.

Figure 10

The Basis of a Diffused Junction Diode

This window can be a disadvantage in charged particle spectroscopy

because some of the lower energy particles may not be detected. To avoid

this disadvantage, diffused junction diodes have been replaced in many

charged particle spectroscopy applications by surface barrier detectors (see

Section 2.2.2.2). However, diffused junction diodes (made of either silicon or


16

n-type material

depletion layer

1µm p type material

Incident radiation

+

V

-

window


37/68



germanium) are still used for charged particle detection as they are more

rugged than surface barrier detectors.

Another practical use of silicon diffused junction diodes has recently been

discovered. These diodes (often referred to assilicon PIN photodiodes)

can be incorporated in electronic dosimeters (see Figure 11) to measure the

amount of gamma radiation being received by a person over time (i.e. to

measure personal gammadose).

Figure 11

An Electronic Dosimeter Incorporating a Silicon PIN Photodiode

Note that electronic dosimeters are also manufactured with G-M detectors.

The main advantage of using a solid state detector is that the instrument is

lighter in weight.

2.2.2.2 Surface barrier detectors

Surface barrier detectors have a very thin layer of p type material

deposited on n type material (see Figure 12). Again the incident radiation

needs pass through this layer to reach the sensitive volume but due to the

thinness of this layer in this type of detector, charged particles can easily be

detected.


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Figure 12

The Basis of a Surface Barrier Detector

As well as being extremely efficient at detecting charged particles, surface

barrier detectors are also very good at separating out the different energies

of the incoming radiation (i.e. they have very goodenergy resolution). For

example, surface barrier detectors can separate the three families of alpha

particles from Am-241 with energies of 5.486, 5.443 and 5.389 MeV.

One of the main problems with surface barrier detectors is that the crystal

surface must be kept very clean and free from oil or other foreign material.

They are also very sensitive to light because light photons can reach the

sensitive volume and create electron-hole pairs.

2.2.2.3 Ion implantation detectors

An alternative method of introducing impurities at the surface of a

semiconductor is to expose the surface to a beam of ions produced by an

accelerator. For example, a silicon crystal exposed to boron ions will have a

layer of p type material formed close to the surface. This method of doping

is calledion implantation and it gives a much more stable crystal which is

less likely to be affected by environmental conditions. In fact, these type of


16

n type layer

Depletion layer

Very thin p type layer

Incident radiation

-V

+


39/68



detectors are very rugged, and they can be manufactured with thin end

windows for alpha and beta detection.

Ion implantation detectors have a wide variety of applications including alpha

spectroscopy, low energy beta detection and heavy ion detection.

2.2.2.4 Lithium drifted detectors

Surface barrier and ion implantation detectors are very good for the

spectroscopy of charged particles but, due to inherent impurities in the

semiconductor crystals, they do not have a large enough sensitive volume

for photon (i.e. gamma ray and x-ray) spectroscopy. To counteract the

effects due to these impurities, semiconductors can have lithium added to

them to create a bigger sensitive volume. The area between the p type and

n type materials is then known as thelithium driftedorintrinsic region and

the size of this intrinsic region determines the sensitive volume of the

detector (see Figure 13).

Note that, although adding lithium to a semiconductor is considered to

provide a much bigger sensitive volume, in real terms, the size of the whole

detector is actually very small. This means that one advantage of this type

of solid state conductivity detector is that the detector dimensions can be

kept much smaller than the equivalent gas-filled detector.


16

p type

regionn

t

y

p

e

l

a

y

e

r

Intrinsic

region

+ V -

n type region


40/68



Figure 13

The Basis of a Lithium Drifted Detector

When lithium is added to a germanium crystal, the detector is called a

lithium drifted germanium (orGe(Li))detector. At room temperature the

lithium atoms will continue to move through the germanium crystal changing

the nature of the intrinsic region, so it is important that a Ge(Li) detector is

always kept at a very low temperature (using liquid nitrogen), even when not

being used. Lithium drifted germanium detectors are efficient detectors of

gamma radiation and have excellent energy resolution.

Lithium drifted silicon(or Si(Li)) detectors, as their name suggests,

consist of a lithium drifted silicon crystal. These Si(Li) detectors are very

similar to the Ge(Li) detectors but have the advantage that they can be

stored at room temperature without any damage to the crystal. They can be

operated at room temperature but their performance is greatly improved if

they are cooled by liquid nitrogen before use. Silicon has a much lower

atomic number that germanium so it is less likely to interact with gamma

radiation. Lithium drifted silicon detectors are therefore not as efficient at

detecting gamma radiation as Ge(Li) detectors. However, they make good

detectors for very low energy gamma rays (less than about 150 keV), x-rays,

and beta particles.

2.2.2.5 High Purity Germanium Detectors

Pure germanium has a high efficiency for the detection of gamma radiation.

Hence if the impurities in a germanium crystal are kept low, it is possible to

obtain depletion layers (sensitive volumes) that are comparable with those in


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a Ge(Li) detector. This type of detector is called ahigh purity germanium

(orHPGe) detector (see Figure 14).

Figure 14

A Typical High Purity Germanium Detector (HPGe) Arrangement

Like the Ge(Li) detector, the HPGe detector acts as an efficient gamma

detector with excellent energy resolution. Similarly, both detectors require

cooling with liquid nitrogen for efficient operation but one advantage of the

HPGe detector is that it may be stored at room temperature when not in use.

2.2.3 Summary of solid state conductivity detectors

Table 3 summarizes the properties of the various solid state conductivity

detectors.

Table 3

Summary of Solid State Conductivity Detectors

Detector Main Uses Advantages Disadvantages

DiffusedJunction Diode

Chargedparticle

• More rugged thansurface barrier

• Lower energyparticles not detected


16

Liquid Nitrogen

Dewar

HPGe

detector


42/68



detection

Silicon PIN

Photodiode

Photon

detection

• Lightweight

Surface Barrier Alpha and beta

spectroscopy

• Efficient at detecting

charged particles.

• Very good energy

resolution

• Surface must be kept

very clean

• Very sensitive to light

Ion

Implantation

Alpha

spectroscopy

Low energy

beta

monitoring

• Less likely to be

affected by

environmental

conditions

• Very rugged

Lithium Drifted

Germanium

Ge(Li)

Gamma

spectroscopy

• Efficient detectors of

gamma radiation

• Excellent energy

resolution

• Must be kept at liquid

nitrogen temperatures

at all times

Lithium Drifted

Silicon Si(Li)

Beta, gamma

and x-ray

spectroscopy

• Good detectors for

very low energy

gamma rays (


43/68



• They have much higher efficiency for gamma radiation.

• The sensitive volume of the detector can be chosen to suit the application.

The main disadvantages of solid state detectors are that:

• They may need to be cooled to liquid nitrogen temperatures for operation.

• They are sometimes less portable than gas-filled detectors.

SELF-CHECK 4



1. Fill in the gaps with a suitable word:

a)A material which has poor electrical conductivity is known as an

____________.

b)A material which has medium conductivity is known as a

____________.

c)A material which has good conductivity is known as a

____________.

2. Name the two semiconductor materials commonly used in solid state

conductivity detectors.

3. a) What is doping?

b) Why are semiconductor materials doped?

4. In what way are semiconductor detectors like gas-filled detectors?

5. Explain the term energy resolution.

6. Which type of solid state conductivity detector would be best for

detecting the following types of radiation?


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a)Alpha particles.

b)Beta particles.

c)Low energy gamma and x- rays.

d)Medium and high energy gamma rays.

7. What are the advantages of solid state conductivity detectors over gas-

filled detectors?


3. DETECTORS BASED ON SCINTILLATION

3.1 How they work

Scintillation detectors rely on the fact that some materials (known as

phosphors) will emit visible light when electrons change energy levels. If

you remember from Module 1.4, Interaction of Radiation with Matter, ionizing

radiation can give electrons sufficient energy to move into a higher energy

shell. In a phosphor, these electrons do not remain in the higher energy

level for very long. Instead, they fall back to their original level and, as they

do so, they emit photons of visible light (see Figure 15)


16

Lower energy shell

Higher energy shell

Incident radiation

Electron goes up to higher

energy level

Electron returns to lower energy level

with visible light released


45/68



Figure

The Scintillation Process

The number of photons of light emitted, and therefore the intensity of the

light, is proportional to the energy of the incoming radiation. Hence,

scintillation detectors can be used not only to detect radiation but also to

separate out the energies (i.e they can be used for spectroscopy purposes).

3.2 Types of Scintillation Detector

The phosphors which can be used in radiation detectors must have certain

properties, as follows:

• They must convert a large fraction of the absorbed energy into light

energy.

• The time between the excitation of the electron and the emission of the

light photon must be short.

• They must allow the light photons produced to pass through the material.

• The emitted light must be able to be converted easily and efficiently to an

electrical signal.

A variety of materials meet these criteria and these make up the basis of

scintillation detectors. The types of scintillation detectors discussed in this

module are as follows:

• Zinc sulphide detectors;

• Sodium iodide detectors;

• Plastic organic scintillators; and

• Liquid organic scintillators.


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Some of these phosphors may have small quantities of impurities (called

activators) added to control the way in which the electrons move back to the

lower energy levels. This ensures that the photons emitted will be visible

light photons.

3.2.1 Zinc sulphide detectors

Zinc sulphide (ZnS) detectors commonly have atoms of silver added as

activators. These type of detectors, known asZnS(Ag) detectors, are very

efficient for detecting ionizing radiation. However, as this type of material

does not allow the visible light photons to pass through it very easily, it can

only be used in thin layers (see Figure 16). Although this means that these

detectors are useful for detecting alpha particles and heavy ions, the main

disadvantage is that the thin layer can easily be pierced by sharp objects.

Figure 15

A Zinc Sulphide Detector


16

Thin film Zinc

Sulphide detector


47/68



3.2.2 Sodium iodide detectors

Sodium iodide detectors with thallium atoms addedNaI(Tl) are very efficient

for the detection of gamma radiation, even more so than solid state

conductivity detectors. However, the crystal will absorb moisture from the

atmosphere and deteriorate very rapidly. It is therefore ‘canned’ in an air-

tight container. The container is often made of aluminium (see Figure 17)

and may have a thin end window.

Figure 16

A Portable Sodium Iodide Detector Used for Gamma Ray Detection

The NaI(Tl) crystals can be made in various thicknesses. A thin crystal of

thickness 3 mm is very efficient for detecting gamma radiation up to about

150 keV. A thicker crystal is needed for maintaining a high efficiency for

higher energy gammas. A NaI(Tl) detector is easier to use in the work

environment than a solid state conductivity detector because it does not

need to be cooled. It also has a much better measuring efficiency,

particularly at higher energies. However, its energy resolution is poor when

compared with a solid state detector.

Figure 18 shows a typical sodium iodide detector with the related circuitry for

gamma spectroscopy.


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Sodium iodide

detector


48/68



Figure 17

A Typical Sodium Iodide Detector for Gamma Spectroscopy

3.2.3 Plastic organic scintillators

Plastic organic scintillators are cheap and can be manufactured in a variety

of different shapes and sizes. They are often used in conjunction with

ZnS(Ag) detectors for monitoring alpha and beta radiation.

3.2.4 Liquid organic scintillators

Liquid organic scintillators have a special use for monitoring alpha and beta

radiation, particularly low energy beta radiation such as carbon-14 and

tritium. Use of a liquid scintillator allows the contaminant of interest to be

mixed directly with the scintillant and can lead to very high detection

efficiency.

3.3 Summary of Scintillation Detectors

Table 4 summarizes the properties of the various scintillation detectors.

Table 4

Summary of Scintillation Detectors

Detector Main Uses Advantages Disadvantages


16

Sodium

Iodide

detector


49/68



Zinc Sulphide Detection of

alpha particles

and heavy ions

• Efficient for detecting

alpha particles and

heavy ions

• Thin layer can

easily be pierced

by sharp objects

Sodium Iodide Gamma

spectroscopy

Gamma detection

• More efficient for

detecting gammaradiation than solid

state conductivity

detectors

• Does not need to be

cooled

• Poorer energy

resolution thansolid state

conductivity

detectors

Plastic Organic Monitoring alpha

and beta radiation

• Cheap

• Can be manufactured

in different shapes

and sizes

Liquid Organic Monitoring alpha

and low energy

beta radiation

• High detection

efficiency when

contaminant is mixed

with the scintillant

3.4 Photomultiplier Tubes

Photomultiplier (or PM) tubes are necessary in scintillation circuits to

convert photons of light from the scintillator into electrical pulses. They are

also used to increase the size of the initial signal. Although, strictly speaking,

PM tubes are really part of the electronic system rather than the detection

mechanism, they are included in this section because they are of specific

use with scintillation detectors and are not part of general counting circuits.

3.4.1 How they work

Firstly, the incident radiation interacts with the phosphor to produce a light

photon. This light photon then hits a surface coated with light-sensitive

material known as aphotocathode. The energy from this light photon is

absorbed by an electron in the light sensitive material and this electron gains


16


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enough energy to leave the photocathode. The ejected electron forms the

basis of the electrical signal but, in practical terms, the signal must be

amplified by a series ofdynodes. Each dynode is basically an anode which

releases about four electrons for every electron it collects. The dynode

system requires a very stable high voltage power supply in order to operate

consistently. Figure 19 shows how the energy is transferred from the original

scintillation event to the external electrical circuit.

Figure 18

A Scintillation Detector Arrangement Incorporating a Photomultiplier

Tube

SELF-CHECK 5



1. Fill in the gaps with a suitable word or phrase:

Scintillation detectors rely on the fact that some materials (known as

_______) will emit visible light when electrons change _______ _____.

When ionizing radiation interacts with the electrons in this type of

material, they are given enough energy to move to a _______ energy


16


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shell. These electrons do not remain in here for very long. Instead,

they fall back to their original level and, as they do so, they emit

_______ ______.

2. Can scintillation detectors be used for spectroscopy purposes? Explain

your answer.

3. Which type of scintillator would you use for detecting alpha particles

and heavy ions?

4. Which type of scintillator would be best for detecting gamma radiation?

5. Which type of scintillator would be best for detecting tritium?

6. a) Give two advantages of using a sodium iodide detector compared

with a germanium detector.

b) What is the major disadvantage of sodium iodide detectors?

7. Why are photomultiplier tubes needed for scintillation detection

systems?


4. NEUTRON DETECTORS

4.1 How they work

Neutrons are uncharged particles and so do not cause ionization directly.

However, when they interact with material they produce secondary ionizing

particles and it is the detection of these particles which allows us to detect

neutrons.

The most common interactions used in neutron detectors are;


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• the reaction with boron-10, which produces alpha radiation;

• the reaction with helium-3, which produces a proton; and

• elastic scattering by hydrogen nuclei.

The first two interactions are most likely to occur for neutrons with energies

up to about 0.5 eV. These neutrons are at the bottom of the intermediate

neutron range and in the thermal neutron (0.025 eV) range. Elastic

scattering can be used to detect fast neutrons.

4.2 Types of Neutron Detectors

There are several factors to be taken into account when designing a suitable

neutron detector:

• Moderating material must be used to slow down fast neutrons (without

absorbing them) so that they will interact with the detector material.

• The detector material must have a high cross section (i.e. a high

possibility) for the particular reaction to occur so that detectors can be

built which are not too large.

• The heavy charged particles formed during the interaction with the

detecting material must all be stopped within the active volume of the

detector.

Four types of neutron detectors which fit this criteria are as follows:

• Boron trifluoride proportional counters

• Helium proportional counters

• Gas recoil proportional counters

• Bubble detectors


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4.2.1 Boron trifluoride proportional counters

Boron trifluoride proportional counters consist of a gas-filled proportional

counter filled with boron-10 enriched boron trifluoride (BF3). This gas

provides the filling gas for the detector as well as the target for incoming

thermal neutrons. The nuclear reaction which occurs in the detector is given

by Equation 1:

10B + n→ 7Li + 1

This reaction is often written as10B(n,)7Li.

The lithium nucleus and the alpha particle both have sufficient energy to

cause secondary ionization in the filling gas, and these secondary ionization

events can then be detected. Note that, some neutron interactions produce

a 0.48 MeV gamma ray. Hence, a suitable discrimination circuit is necessary

to distinguish between the incoming neutrons and resultant gamma rays.

Boron trifluoride proportional counters can also be used to detect

intermediate and fast neutrons (up to 10 MeV). However, in this case, the

detector must be surrounded by a moderating material such as polyethylene

to slow down the neutrons before capture (see Figure 20). Cadmium filters

are used to provide a uniform energy response.


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Figure 19

A Boron Trifluoride Counter

As well as being good detectors for thermal, intermediate and fast neutrons,

boron trifluoride proportional counters can be used for neutron spectroscopy

purposes.

4.2.2 Helium proportional counters

Helium proportional counters are similar in many respects to boron trifluoride

proportional counters. The main detection mechanism involves thermal

neutron capture, but if a moderator is used, helium proportional counters can

be used to detect intermediate and fast neutrons.

As the name suggests, helium proportional counters use helium as both the

target and filling gas. The important reaction is given in Equation 2:

3He + n→ 3H + p [2]

This is often written as3He(n,p)3H.

In this reaction, a tritium nucleus and proton are formed and it is these

charged particles which produce the secondary ionization.


16

Cadmium loaded

polyethylene sphere

Proportional counter

filled with Boron-10

enriched BF3 gas


55/68



Again, helium proportional counters can be used for neutron spectroscopy

purposes.

4.2.3 Gas recoil proportional counter

Gas recoil proportional counters use elastic scattering with hydrogen nuclei

as their mechanism of detection. In these counters, neutrons with energies

greater than about 500 keV are detected using a proportional counter filled

with a hydrogen-rich gas such as methane. A fast neutron will collide with a

hydrogen nucleus (which is just a single proton) giving it energy. This

nucleus will then go on to produce secondary ionization.

In some gas recoil proportional counters the hydrogen atoms may be

supplied by using material such as polyethylene in the walls of the counter.

The counter is enclosed in a thin sheet of cadmium which absorbs thermal

neutrons.

4.2.4 Bubble detectors

Bubble detectors contain microscopic liquid drops suspended in a gel-like

material. Incoming neutrons transfer enough energy to the liquid drops to

make them suddenly boil and change into a bubble. These bubbles are

visible to the eye and can be counted (see Figure 21).


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Figure 20

Bubble Detectors Before (left) and After (right) Neutron Exposure

The actual neutron dose is proportional to the density of bubbles, and these

remain fixed in the material until the dosimeter is reset.

Bubble dosimeters are mainly used for personal dosimetry. However, they

can also be used for environmental monitoring.

4.3 Summary of Neutron Detectors

Table 5 summarizes the properties of the various neutron detectors

Table 5

Summary of Neutron Detectors

Detector Main Uses Comments

Boron trifluoride

proportional counters

• Detection of thermal neutrons

• Can be used to detect neutrons

up to 10 MeV with suitable

moderator

• Neutron spectroscopy

• Gamma rays may be

produced so a

discriminator circuit is

necessary

Helium proportional

counters

• Detection of thermal neutrons

• Can be used to detect neutrons

up to 10 MeV with suitable

moderator

• Neutron spectroscopy

• Gamma rays may be

produced so a

discriminator circuit is

necessary


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Gas recoil

proportional counters

• Detection of fast (< 500 keV)

neutrons

Bubble detectors • Personal dosimetry

• Environmental monitoring

SELF-CHECK 6



1. Why can’t neutrons be detected using the detectors learnt about in

previous sections?

2. a) What modifications are needed to proportional counters to allow

them to detect neutrons?

b) Why is a material such as polyethylene often used to surround the

detector tube?

3. Which type of neutrons are most likely to interact with boron or helium?

4. Which type of neutrons are most likely to undergo elastic collisions with

hydrogen nuclei?

5. Which type of neutron detector can be used for personal dosimetry?


5. ELECTRONIC COMPONENTS

So far in this module we have been considering different types of detectors.

Once the energy of the ionizing radiation has been converted to an electrical

signal, various electronic components must be added to the detection system

to provide a meaningful reading.


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The electrical signals produced by detectors are of two types –direct

current (as in the ion chamber) andpulse. If the signal is measured as a

direct current, the average current over many interactions is recorded and

the circuit is said to be operating incurrent mode. If the signal is measured

as a change or drop in voltage, individual interactions are recorded and the

size of the pulse is dependent on the number of electrons collected. This is

known aspulse mode.

Direct current output is measured by a direct current amplifier whereas a

pulsed output is measured in a counting circuit. Figure 22 shows the

components of a general counting circuit.

Figure 21

A General Counting Circuit

In this section we will look briefly at the functions of a direct current amplifier

and the different components of a counting circuit. Remember that in a field

instrument all components are usually built into the instrument, which may


16

High Voltage

Supply

Detector

Scaler

Pre-ampAmplifier

Discriminator Rate meter

MCA


59/68



limit the capabilities of the instrument. A laboratory instrument can have

more functional components attached.

5.1 Voltage Supply

A stable direct voltage supply is necessary to supply the operating potential

for the detector. Field instruments are dependent on batteries to supply the

voltage. The level of the high voltage and the degree of stability required

depends on the type of detector. The voltage supply is known traditionally as

the HV (high voltage) supply.

5.2 Direct Current Amplifier

A direct current amplifier increases a very low current, in the order of 10-12 A,

to a current which can be measured by an ammeter, in the order of 10-3 A.

This requires a high gain. (The gain of an amplifier is the ratio of the size of

the output pulse to the size of the input pulse.) Since the necessary gain is

so high, any slight change in the input signal can have a large effect on the

output measurements. For this reason high stan

Documents

Mod 1.5 Radiation Detection