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Nuclear Chemistry (Theme 4: "-Spectrometry) NWU-NC-TH-04 (rev 00) of 35

Nuclear Chemistry

Document Reference: NWU-NC-TH-04

Theme 4

Measurement of Radioactivity

Part 2: "-Spectrometry

Credits: 2

Original document compiled by: Mr Barnard SmitAdapted for use NWU by: Prof Dr Arnaud Faanhof, March 2006


About the Nuclear Chemistry Module

Motivation

There are several organizations in the Republic of South Africa that require skilled personnel tounderstand and perform radiochemical as well radiometric analyses. However, most of the companies,if not all, have to provide in-house training as no formal training is available at tertiary institutions. Thishas two serious consequences regarding personnel:

(a) Due to this highly specialised field, very few tertiary organisations offer training courses in nuclearchemistry and its applications in the nuclear analytical disciplines. The subject is absent from theanalytical chemistry curricula of our Technicons and Universities. There are, therefore, very fewscientists and technicians available in the market that have been trained in Nuclear Chemistry andits applications.

(b) And even those scientists and analysts with some theoretical training, find it insufficient for thedemands and problems encountered in the real world, i.e. the nuclear and related industries. Theextensive knowledge of the basic techniques that is required in order to operate effectively a worldclass laboratory (ISO accredited), can not be obtained by formal training in South Africa and infact the whole African continent.

And, yet, there is a strong demand for the services that are to be provided to and by the nuclearindustries, which include the development new of technologies as well as interpretation of findings andrecommendations based on these. They include amongst others:

(i) radionuclide specific analysis due to the presence of large quantities of naturally radioactivematerial in nature and the fact that the radioactive components can become concentrated in theenvironment during mining and mineral processing associated with a higher standard of living,

(ii) the application of man-made radioisotopes in medicine due to the increased applications innuclear medicine,

(iii) the application of radioisotopes, x-rays and neutrons in industry in the form of non-destructivetesting,

(iv) the use of a variety of nuclear analytical techniques to determine element concentrations in air,water and foodstuffs, which may impact on human health and the environment: and

(v) the determination of the radiological risk to humans and the environment due to exposure toradioactive substances, which is detrimental to the health of people and the ecosystem.

In view of these opportunities, it is mandatory that formal training overcomes the problems due to thelack of skilled personnel in the South African Nuclear and related Industries. One way of doing this,is to provide adequate in-house training to enable the available staff to perform their complex activitieseffectively. An other way is to use existing in-house training programmes and/or develop new materialsfor the formal educational sector through which the training process can be managed. Accordingly, acourse curriculum has been developed for specific use at North West University at the BSc-Hons level.

The objective of the BSc-Hons programme is to provide students in a systematic and structuredmanner with the necessary theoretical knowledge and practical experience for them to be able tounderstand and perform most of the nuclear chemical work done to support industries as well aslegislators in their personnel demand.


Structure of the Nuclear Chemistry Module

The BSc(Hon)-course at NWU has been developed to cover 128 credits, of which 32 credits will bededicated to the development of practical experience in the basic principles of Applied RadiationScience and Technology.

The Nuclear Chemistry Module provides a number of themes, which in total will provide 24 credits. Percredit an average time-span of 10-hours is allocated, which basically comes down to a full 6-weeks ofself study and lecturer contacts. This course is sub-divided in 12 themes of 2 credits each, whichmeans that per theme 2,5 working days are allocated.

Each theme provides the students with the theoretical background as well as worked-out examples,seminars as well a specific tasks of which the answers are to be provided to the lecturer not later than1-week after the end of the individual themes.

The following structure is envisaged:

Week 1 Monday: Theme 1 2 hours lecture, 6(+) hours self-study

Tuesday: Theme 1 8(+) hours self-study

Wednesday: Theme 1 2(+) hours self-study, 2 hours seminar

Wednesday: Theme 2 2 hours lecture, 2(+) hours self-study

Thursday: Theme 2 8(+) hours self-study

Friday: Theme 2 6(+) hours self-study, 2 hours seminar

Week 2 - 6 Repetition of the week-1 schedule for themes 3 to12.

Accordingly, the training programme is modular, and consists of a number of separate butinterdependent themes. Some of these can be done in isolation, while completion of some areconsidered as a precondition for starting on another. The courses are grouped together as follows:

(a) Theory: Providing a theoretical foundation on which the other themes can develop, includingstatistical aids. The learner will notice that some of the topics dealt with may partly overlap withsome of the topics mastered in the Nuclear Physics Module, presented earlier in the BSc(Hon)course. The study guides are:

Theme 1 Theory required for the Nuclear Chemistry module 2 credits

Theme 2 Sensitivity requirements of radioactivity measurements 2 credits

Theme 6 Nuclear Reactions & the Nuclear Reactor 2 credits

(b) Radiometric techniques: Introducing the student to the different radioanalytical fields. The studyguides are:

Theme 3 Gross "$-Counting 2 credits

Theme 4 "-Spectrometry 2 credits

Theme 5 (-Spectrometry 2 credits

(c) Non-destructive testing: Introducing the student to various nuclear techniques applied in non-destructive testing. The study guides are:

Theme 12 Instrumental Neutron Activation Analysis (INAA) 2 credits

Theme 7 Neutron Radiography 2 credits

Theme 8 Neutron and X-ray Diffraction 2 credits


(d) Chemical techniques: Introducing the student to the basics of radiochemistry. The study guidesare:

Theme 9 Radiochemicals & radiopharmaceuticals 2 credits

Theme 10 Radioactive waste & NORMS 2 credits

Theme 11 Labeling techniques 2 credits

Each of study guides on the individual themes will be developed to meet the criteria of the BSc(Hon)-level. It is compiled in such a way that:

(I) the theoretical background is sufficient to comprehend the inherent limitations and potentialapplications of that theme,

(ii) the student will be able to read and apply the theory mastered and understand the examplesprovided in order to answer the instructions given in an appropriate and correct way, and

(iii) the student will be able to perform specific assignments within one week after the specific themehas been dealt with. Marks will be given, which form part of the overall evaluation of the coursemark.

It is envisaged that students will work systematically through this module under the guidance of thelecturers concerned. All the themes will, as far as practicable (holidays and staff availability), bepresented one after the other in order to allow students to master the subject as fast as possible.

Assignments will be given on completion of the entire module to test the student’s understanding ofthe interrelation between various themes. These can be either written or verbal presentationsdepending on the themes covered.

Mastering the contents of this module ( with a minimum mark of 60%) is a prerequisite for studentsthat want to continue their studies in Nuclear Chemistry at the MSc-level in Applied Radiation Scienceand Technology.

Development of Practical Skills

The Applied Radiation Science and Technology Course at the BSc(Hon)-level makes provision for a32 credit module in which practical skills are to be mastered. The Nuclear Chemistry module will cover8 credits, which boils down to a 2-week practical.

This module is still to be developed awaiting the establishment of a laboratory infrastructure at NWU(Mafikeng campus). In the meantime practicals are offered at Necsa in the following themes within theNuclear Chemistry module.

• Radioanalytical techniques and applications 2 credits

• Radiochemistry 2 credits

• Non-destructive testing 2 credits

• Human exposure 2 credits

It is envisaged that students will work systematically through the themes and complete theassignments provided for each theme within one week after the end of the practicals. Theseassignments will form part of the overall evaluation of the practical’s final mark.


Objective of this Syllabus (Theme 4)

This theme covers the radiometric technique of alpha-spectrometry in its entirety at the BSc(Hon) level.It is compiled in such a way that:

(i) the theoretical background is sufficient to comprehend the inherent limitations and potentialapplications of that technique,

(ii) the student will be able to read and apply the operating instructions of the instruments andmethods employed by a radioanalytical laboratory, and

(iii) the material covers the data processing that is required to convert laboratory readings to usefulanalytical results.

On completion of this module, the student will have a detailed knowledge of all the principles involvedin "-spectrometry, the different detectors that can be utilised, and the basics of pulse handling anddata processing. The student will also be able to apply this basic knowledge to select and maintaindetectors and the supporting hardware, to set criteria for the samples to be measured, to design andimplement procedures for regular calibration and performance tests, and to calculate the activity valuesfrom the raw data.

Completion of the following course is a prerequisite for this theme:

(a) NWU-NC-TH-01: Theme 1: Theory required for the Nuclear Chemistry Module

(a) NWU-NC-TH-03: Theme 3: Gross "$-Counting

Examples to support the theory are provided in green text, with worked-out solutions and/ordiscussions shown in blue print (e.g. Section 3). Specific questions are given in red print (e.g. Section4), which are to be answered and handed in within two weeks after the specific theme has beendealt with. Marks will be given, which form part of the overall evaluation of the course mark. A test willbe taken on completion of the theme at the end of the Nuclear Chemistry module. This will be eitherindividual questions or multiple choice ones, depending on the subject covered.


Contents of Theme 4

1 RADIATION DETECTORS FOR "-SPECTROMETRY . . . . . . . . . . . . . . . . . . . . . . . . 7

1.1 Interaction of "-Particles with Matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

1.2 Gas Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3 Liquid Scintillation Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.4 Solid State Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2 SOLID STATE DETECTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.1 Description of Solid State Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.2 Operational Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3 Problems due to Recoil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.4 Pulse Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.5 Analysis of the Pulse Height Spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3 PRACTICAL CONSIDERATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.1 Samples to be Counted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

3.2 Background Corrections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.3 Calibration Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.4 Analytical Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.5 Quality Assurance Programme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

3.6 Care and Maintenance of the Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

4 POINTS TO PONDER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Annexure 1 Example of the Chart of the Radionuclides . . . . . . . . . . . . . . . . . . . . . . . . 34

More information can be obtained in internal training material available at Necsa, which will form partof the Nuclear Chemistry Module at MSc-level:

RA-PFT-107 (01):A study of the cleaning of PIPS detectors that are used for "-spectrometry.

RA-PFT-107 (02):A study of the prevention of contamination of PIPS detectors used in "-spectrometry.

RA-VAL-107 (01):Validation of the method used for determining the counting efficiency of solid statedetectors.


1 RADIATION DETECTORS FOR "-SPECTROMETRY

This chapter describes in general terms the different techniques that are available to utilise theinteraction of "-particles with matter, in order to detect and measure them for analyticalpurposes.

1.1 Interaction of "-Particles with Matter

1.1.1 Energy transfer by ionisation

The heavy charged particles lose energy mainly by interaction with the electrons in the materialthrough electrostatic Coulomb forces. Some of the kinetic energy of the "-particle is transferredto the bound electron; which usually is sufficient to overcome the binding energy of the electron,and which results in its ejection from the atom or molecule. This leads to the formation of a freeelectron and an ion by the process of ionisation.

The way the available kinetic energy of the "-particle is shared with the electron, is subject tothe requirement that the total momentum and kinetic energy must be conserved. For the specialcase of a "head-on" collision for a 5 MeV "-particle, it can be shown that the energy that canbe transferred to the electron is given approximately by:

. . . . 1MeV = 1.10 eV6

where: m = relative mass of the electron; andM = relative mass of the "-particle (or 2 protons and 2 neutrons).

= (2 × mass of proton) + (2 × mass of neutron)

It is clear that the energy lost by an "-particle for a single ionisation event, is quite small. Thefollowing conclusions, which are important for understanding the techniques and problems of"-particle detection, can be drawn from this fact:

• The particle is not deflected significantly from its initial direction of travel; one can thereforeassume that it travels along a straight line through the material it is moving in.

• It would require a large number of ionisation events inside the material before the particle haslost all of its kinetic energy, and comes to a stop.

• Secondary ionisation, in this case, is caused mainly by low-energy electrons. These have avery high energy transfer.

1.1.2 Stopping power and range

A number of parameters can be covered in discussing the interaction of "-particles with matter.One can obtain valuable insight into the interaction process by looking at the effect of the energyof the particle and the type of absorber. These quantities are described below, and someexperimental values are presented in Table 1.1.

(a) Stopping power: This unit, also known as the "linear energy transfer" value, is the averageenergy that is lost by an "-particle when passing through a thin layer of a substance, dividedby the thickness of that layer of material.

(b) Range: This unit refers to the average distance from the source that the "-particles of acertain energy can travel in a particular material before they are stopped completely.

(c) Ionisation density: This value can only be measured in a gas. It indicates the number ofelectrons (and positive ions) that are formed per unit thickness of the layer of the relevantgas.


Table 1.1: Absorption of "-particles of different energy values in air and silicon

Material Property considered 1 MeV 5 MeV 10 MeV

Silicon Stopping power MeV/(g.cm )-2

keV.:m-11200

52600

26400

17

Range g.cm-2

:m1,1 @ 10 -3

4,75,5 @ 10 -3

2414 @ 10 -3

60

Air Ion pairs produced ions/(mg.cm )-2

ions.mm-15,4 @ 104

6,4 @ 1032,2 @ 104

2,6 @ 1031,3 @ 104

1,5 @ 103

Range g.cm-2

mm (20 C, 746 mm Hg)00,44 @ 10 -3

44,1 @ 10 -3

3512,6 @ 10 -3

110

Density: Silicon: 2,330 g.cm-3

Air: 1,183 mg.cm-3

All these parameters refers to a "layer of absorbing material". The thickness of this layer canbe measured either in its linear dimension (mm or cm), or by the amount of material it containsper unit of area (g.cm ).-2

The second unit (i.e. g.cm ) is unfamiliar, and "feels" difficult to work with. It has the advantage,-2

however, of eliminating some of the differences due to density effects, and making comparisonsbetween different materials easier. If one compares for example, the range of an "-particle insilicon and in air, the "linear values" differ a lot, while the "g.cm values" are about the same.-2

The one unit can be converted to the other by using the density of the medium.

The data in Table 1.1 illustrate how the stopping power,which is also related to the ionisation density, increases asthe energy of the "-particle decreases. The highestdensity is therefore near the end of the range, as shownin the figure. This has serious implications for "-spectrometry: Any barrier inside the source, or betweenthe source and the detector, will lead to an energy loss forthe particles. One can, however, not subtract a constantamount of energy from each "-particle because those withthe higher energy will loose less energy in the barrier.

Interaction of the "-particles with the electrons in atoms and molecules is the most importantissue to consider, and other mechanisms (eg. interaction with the nucleus) can be ignored. Thistype of interaction leads to the formation of free electrons and positive ions. The next Sectionsdescribe three different techniques to utilise this ionisation phenomenon for "-spectrometry.

1.2 Gas Detectors

1.2.1 Principles of operation

A gas-filled detector that can be used for "-spectrometry, typically consists of a metal box inwhich a very thin wire is strung. The wire is isolated from the box, and kept at a relatively highpositive bias with respect to the body of the detector (typically 500 volt or more). One face ofthe box is covered with a thin window to give access to the "-particles. The box is filled with gas(usually a 10 % mixture of methane in argon), and usually connected to the gas supply toreplenish the gas that leaks through the thin entrance window.

The primary electrons produced in the ionisation by the "-particle, have sufficient energy to


ionise more of the gas and produce secondary electrons. All these electrons are attracted bythe positive wire, collect sufficient kinetic energy to ionise other atoms, and produce moreelectrons by gas multiplication.

1.2.2 Fields of application

The gas-flow proportional counter described in the previous paragraph, has a number of seriouslimitations for "-spectrometry.

• The entrance window must be an electrical conductor, and mechanically strong enough towithstand normal handling and the gas pressure inside the counter. The minimum thicknessthat can be realised, still presents a significant barrier to the "-particles, and creates problemsfor spectrometry.

• The thickness of the gas layer must be larger than the range of the "-particles to be studied,to ensure that all its energy is dissipated inside the sensitive volume. This means a minimumof about 100 mm for the "-emitters found in nature. This is difficult to achieve; especially forthose particles that are emitted at an angle.

These problems can be eliminated by placing the sampleinside the detector. Since about 50 % of the particlesthat are emitted by the sample should be detected, thisarrangement is described as a 2B-counter. This type ofdetector is still considered to be the most accurateinstrument for determining the "-activity of a samplebecause there is no need to calibrate it: the countingefficiency is close to , = 0,500 (with a small correction forthose particles that are emitted in the horizontal direction.There are, however, a number of practical issues:

(a) The detector has to be opened for each new sample, and flushed to remove all the tracesof air before the measurements can start. This makes the method too time consuming forroutine applications.

(b) There is no guarantee that the performance of the detector will not be affected when it isopened. Plateau runs to verify the best bias voltage setting are therefore required regularly.

(c) The counting volume must be large to ensure complete absorption of all the "-particles(typically 10 cm high and 20 cm diameter), which makes it difficult to collect all theelectrons. This problem can be reduced by pressurising the detector. If it is operated at apressure of 200 kPa, the range of "-particles is halved, and a smaller counting chamber willbe sufficient.

The 2B-counter is mainly used for the absolute counting of calibration samples.

1.3 Liquid Scintillation Detectors

1.3.1 Principles of operation

In this type of instrument the kinetic energy of the incident "-particle is converted into a shortscintillation flash inside the body of the detector. The light from this flash is collected on a photo-multiplier tube (PMT), which converts it into electrons, which cause an electrical pulse at theoutput of the tube. Only the two scintillator types that are suitable for "-spectrometry, ZnS andliquid, are discussed in the next paragraph.

1.3.2 Different scintillators

(a) ZnS scintillator:


This detector consists of a very thin layer of ZnS with a suitablebinding material, that is spread evenly over an inert support.

The fact that ZnS is an opaque powder, presents problems inmaking the scintillating layer: On the one hand it should be as thinas possible to prevent the light pulse from losing some of itsintensity in the layer itself, but it should be thick enough on theother hand to ensure that the "-particles can loose all theirenergy in the scintillator.

The main advantage of this type, and the way the detector is usually constructed, is theabsence of a window on the entrance, and the low cost of the detector itself.

(b) Liquid scintillator:

The detector for liquid scintillation counting (LSC) is in the form of an organic liquid, and itis possible to dissolve the sample in this liquid. The composition of the scintillating liquid,also known as a LSC cocktail, can be modified to suit a particular requirement of theanalyst: basic or acidic aqueous solutions, organic solvents, soluble filters and many more.

The elimination of the entry window is a major advantage of this detector. The cost of usinga new "detector" for every sample, can be a problem.


The following issues are relevant for evaluating the possible application of scintillation counting,especially of LSC, for "-spectrometry:

(a) The efficiency for converting incident radiation energy into a scintillation pulse, is about tentimes higher for $-particles than for "-particles. This means that the scintillation pulse froma typical 5 MeV "-particle will have about the same intensity as one from a 500 keV $-particle. Overlap of pulses from "-particles and $-particles is therefore inevitable.

(b) There are a number of factors which can not be controlled by the analyst, but which willaffect the size of the electrical pulses that are produced:

• The light production in the detector is a statistical process, and there will be a naturalvariation in the intensity of the scintillation for the same excitation.

• Collection of light from an event close to the entrance window of the PMT will be moreefficient than for an event in the far corner of the detector volume.

• The efficiency for converting light into electrons varies over the area covered by thephotocathode. The same amount of light coming from different sections of the detector,will lead to pulses with different heights.

Even if the incident particles have the same energy, the combined effect of these variationscauses a large spread in the size of the output pulse. The end result is a poor performancein the factor that is critical for spectrometry: the ability to distinguish between "-particleswith a small energy difference.

(c) The light output (size of scintillations) is easily suppressed, or quenched, by impurities inthe sample, and by its chemical composition. This means that each sample must becalibrated before it is used. But the only way to do this, is to add a radioactive standardsolution to the sample, which makes the measuring procedure tedious.

Some of these problems (but by no means all of them), have been removed in an approach thatis marketed under the trade name of "Perals". The sample (or detector) container for thisinstrument is very small, and is surrounded by mirrors that removes some of the variation in thelight collection efficiency.

The high ionisation density for "-particles is the cause for the poor energy conversion in thedetector, and the reason why "- and $-pulses overlap. It is also the cause of another effect: The


duration of an "-induced scintillation, is significantly longer than one caused by a $-particle. Itis therefore in principle possible to separate "- and $-scintillations based on the duration of eachpulse.

1.4 Solid State Detectors

This type of detector is extremely important for "-spectrometry, and will be discussed in detailin this Section.

1.4.1 Information on semi-conductors

The standard atomic model assumes a heavy nucleus surrounded by a number of electrons inorbitals around it. The energy of each orbital is fixed, and the atom can go from one energystate to another only in discrete energy steps. Every electron in the atom is bound to a particular

3 atom (e.g. in Ar), or is shared by a few atoms in a stable molecule (eg. in NH ).

This model is useful for single atoms (such as gases), but can not be applied for condensedmaterial. In the case of solids, the following model has been developed:

• Electrons are "shared" by all the atoms in the crystal; and are not tied to one atom only.

• Electron orbitals are no longer discrete energy levels associated with individual atoms.

• Orbitals have collapsed (degenerated) into two relatively sharp energy regions, that areseparated by an energy gap that depends on the type or composition of the material:

- valency band occupied by low energy electrons strongly bound to the crystal, and

- conduction band where electrons are rather loosely bound to the crystal.

This model can be applied to understand the electrical properties of three different types ofmaterial found in nature:

• Non-conductors: In these materials the valency band is only partially filled with electrons,while the conduction band is not occupied (i.e. empty). There are no mobile electrons to carrythe electrical current. Examples are ceramics and plastics.

• Conductors: In these the valency band is completely filled with electrons, and the conductionband is also partially occupied. There is a large number of mobile electrons available in theconduction band to carry electricity when an electrical voltage is applied. The metals arecommon examples of conducting materials.

• Semi-conductors: In these the valency band is completely filled, but the conduction bandis empty. There are usually not sufficient mobile electrons available, and the materialbehaves like a typical non-conductor at low voltages.

Small amounts of impurities in semi-conductor material have a dramatic effect on the populationof the conduction band, and hence the conductivity of the material. True "intrinsic" material canonly be obtained at impurity levels of less than 1 in 10 . It is clear that very sophisticated10

techniques are required to produce this material.

The electrical properties of a semi-conductor material depends on the external voltage applied


to it, and on the manner in which it is constructed.

• By increasing the voltage that is applied over a piece of semi-conducting material, the energyof electrons in the valency band is increased; but no effect is apparent as long as they stayin that band. As soon as the voltage is sufficient to lift (or "excite") some electrons to theconduction band, the material suddenly behaves like a conductor. The higher the appliedvoltage, the more electrons are moved to the conduction band, and more current is carried.

• A composite of semi-conductors with empty and partially filled conduction bands, will (at lowvoltages) conduct electricity in one direction only. This is used in transistors.

Effect of the applied voltage on the current through different materials

1.4.2 Semi-conductors as "-detectors

Energy (thermal, electrical, etc.) absorbed in a piece of perfect semi-conducting material willexcite some electrons from the valency band, leading to the formation of mobile electrons in theconduction band. This is also true if the energy is supplied in the form of ionising radiation. Andby applying a suitable voltage across the material, these electrons can be collected to form anexternal electrical pulse that can be detected electronically. The process is similar to gas-detectors, but a semi-conductor has a number of important advantages over similar gasdetectors:

• About 3,6 eV is required to form an electron ion pair in silicon at room temperature, ascompared to 32 eV in air. An "-particle with the same energy, will therefore produce 10 timesmore secondary electrons in such a detector.

• The density of silicon (2,3 g.cm ) is much larger than that of air (0,001 g.cm ). This means-3 -2

that the range is much less (see Table 1.1), and that a thinner layer of silicon will be requiredto ensure total absorption of "-particles.


Detectors for "-spectrometry are all based on high purity silicon as detector material.

This type of detector has many advantages over the others that have been discussed, and isthe one that is usually preferred for "-spectrometry:

• It can distinguish between "-particles with a very small energy difference; i.e. solid statedetectors have a high energy resolution.

• It is easy to make layers of material that are thick enough to ensure complete absorption ofthe "-particles.

• They are rugged, and do not require a continuous supply of counting gas.


2 SOLID STATE DETECTORS

2.1 Description of Solid State Detectors

There is a variety of solid state detectors for different applications; some of which are only usedfor sophisticated nuclear physics research. From an analytical point of view, only the two thatare important for "-spectrometry will be discussed:

• the Silicon Surface Barrier (or SSB) detector; and

• the Passivated Implanted Planar Silicon (or PIPS) detector.

2.1.1 Construction of detectors

Previous paragraphs mentioned the formation of electron/ion pairs in semi-conducting material,and the collection of this charge to produce a pulse. The charge collection requires electrodeson the opposite faces of the layer of material that acts as detector. Currently two techniques areused to apply these electrodes; resulting in the two types mentioned.

The principles of the process is well known, but the technical details on how it is done by thedifferent manufacturers, are confidential because it gives them a market advantage. The majordisadvantage of the SSB detector is the thin aluminium or gold window: It is extremely sensitiveto oil and grease, and any attempt to remove contamination usually leads to deterioration of theperformance due to a damaged window. This problem is largely overcome in the PIPS detectorbecause the conducting layer is actually implanted in the surface, and therefore not easilydamaged. Another advantage is the possibility of having a thinner layer of material for theentrance window; which will give less attenuation of "-particles.

The wafers are chemically cleaned and canned. Electrical contact to the electrodes are throughstandard co-axial connecters; typically the BNC or Microdot types.

At the junction of the p- and n-type material, the repulsion between the negative carriers (holesin p-type and electrons in n-type) creates a thin depletion layer that is free of carriers. Theapplication of a reverse bias widens this depletion region. This is the essential sensitive volumeof the detector: Any free electrons caused by ionising radiation in this layer is immediately sweptaway (collected) by the reverse bias, causing an electrical pulse in the external circuit. Themagnitude of the reverse bias (and hence the thickness of the depletion layer) is limited:Electrical breakdown results if this value is exceeded.

2.1.2 Parameters affecting the performance


(a) Depletion layer:

The thickness of this layer that is free of carriers (i.e. the sensitive volume) is determined bythe applied bias and by the resistivity of the semi-conducting material; which is a function ofthe impurity concentration. The higher the purity, the thicker the maximum depletion layerthat can be attained. SSB and PIPS detectors with depletion depths in excess of 500 :m canbe obtained with relative ease.

The search for a thick depletion layer is not primarily motivated by the range of the "-particles; which is only 60 :m at 10 MeV (see Table 1.1). The reason is that the twoelectrodes form an electrical condenser over the layer; and the thinner this layer, the largerthe capacitance value, and the more electronic noise is produced. This will adversely affectthe ability of the detector to distinguish between "-particles with a small energy difference(i.e. its resolution).

(b) Leakage current:

A silicon detector will have a small leakage current, even in the absence of ionising radiation.The magnitude of this current is about 1 to 10 nA.cm for PIPS detectors, and about 20 to-2

100 nA.cm for SSB detectors with a depletion depth of 100 :m at room temperature. This2

current is another source of electronic noise, and also an indicator of the performance of aparticular detector. It can be reduced by cooling the detector, as is done for Ge(Li) andHPGe detectors (germanium-lithium and high-purity germanium respectively). This is notoften done for analytical applications, however, because there are more serious problems(e.g. sample properties) affecting resolution.

(c) Surface area:

The active surface area of the depletion layer that is exposed to ionising radiation, affectsthe characteristics of that detector in two respects:

• The larger the surface area, the higher the efficiency for counting "-particles. This iscrucial in environmental analysis because of the low activity levels to be detected.

• A larger area means a bigger capacitance value, which reduces the resolution.

Detectors can be obtained with surface areas ranging from about 30 mm (diameter of2

6 mm) to 3000 mm (diameter of 60 mm).2

(d) Energy resolution:

This is a measure of the detector's ability to distinguish (or "resolve") between two "-particles with a very small energy difference. It is determined mainly by the active area,capacitance and leakage current of the detector, by the statistical nature of the conversionof radiation energy into electrical charge and of the charge collection process itself, and bythe noise in the electronic components. In most analytical applications the loss of resolutiondue to attenuation of "-particles in the source, is often more serious than the resolution ofthe detector.

Resolution values, expressed as the minimum energy difference that can be detected, rangefrom about 10 to 15 keV for detectors with a small area, to about 60 to 70 keV for the verylarge detectors.

(e) Breakdown voltage:

This defines an upper limit to the reverse bias that can be applied to a solid state detector.One would prefer to operate at a high value because this will: (i) provide a large depletiondepth and capacitance, and therefore good resolution, and (ii) ensure complete collectionof all the charge which will also improve the resolution. There is, however, a risk of anelectrical breakdown which can destroy the detector. One can consider operating the unitbelow the recommended bias voltage if resolution is not of prime importance.


2.2 Operational Requirements

The detectors discussed in Section 2.1, requires support facilities before they can be appliedas analytical instruments. This is the focus of this Section.

2.2.1 Vacuum system

The high ionisation density of "-particles leads to a large energy loss; even when movingthrough air. Using the data in Table 1.1 and considering that about 32 eV is required to produceone ion/electron pair in air, one can calculate that an "-particle of 5 MeV loses

32 × 2,6@10 = 83 keV3

or 1,7 % of its energy when moving through 1 mm of air. An energy loss of this magnitude cannot be tolerated when doing "-spectrometry. The only alternative is to remove all the air, i.e. tocarry out all the measurements with both sample and detector under vacuum.

Different types vacuum chambers are available, each servicing a single or up to four detectors.The design typically provides a hinged door that gives access to the chamber, a valve thatconnects the chamber either to a vacuum system (to pump out the air) or a vent port (to fill thechamber). This valve can be controlled manually or electronically via a control unit. The vent portis usually connected to air, but provision is sometimes made for filling the chamber with anothergas.

The pressure inside the chamber is measured and displayed, so that the user can see when thepressure is low enough for counting. The bias supply and count controls are often inter-lockedwith the pressure sensor. The user can load, start and leave the system; but the reverse biasand "count start" will be enabled automatically only when the pressure inside the chamber hasreached a preset limit.

The vacuum system is usually built on rotary vane pumps because of the cost considerations.These pumps use spring-loaded vanes running against the inside of a drum to pull air out, andoil to lubricate and seal the vanes. A number of issues must be considered when using this typeof pump:

• The rapid mechanical movements inside the pump causes a mist of fine oil droplets. Oncethe chamber is evacuated, there is no significant flow of air in the vacuum system any more,and these small droplets can diffuse back into the counting chamber. This effect, referred toas back streaming, can become significant during long counting periods. It results in a thinoil layer on the inside of the chamber and on the detector. Some systems use oil traps toreduce back streaming, eg. a membrane or cloth filter to catch the oil, or a large ball withbaffles as a trap. These components also reduce the pump speed of the system, whichincreases the time required for pump down.

• The pump will stop running during a power failure. If the chamber is not vented rapidly in suchan event, oil may be pushed back into the vacuum tubes and into the counting chamber. Thiscan destroy the detector(s).

• The pump reeds regular service and oil replacement.

2.2.2 Sample geometry control

It is essential that all the laboratory and calibration samples shall be counted in positions thatare identical regarding:

• the distance between the sample surface and detector face; and

• the surface area of the sample being counted.

There are different ways to satisfy these requirements; some of which are difficult to apply forroutine analytical work. The following two strategies have found general acceptance inlaboratories doing "-spectrometry.


(a) Samples mounted on racks:

The detector is mounted facing down inside a small cupboardwith horizontal slots on the sides. A tray carrying the sample atthe centre, is placed in these slots. The analyst can adjust thecounting efficiency by moving the tray to higher or lower slots. Itis less effective for low level measurements where one wants thecount samples as close as possible to the detector face, and it isdifficult to align the sample accurately with the detector.

(b) Special geometry rigs:

A ring that is divided into two "sections" as shown in the figure, is cut from a convenientplastic material.

• The top section fits snugly around the detector, andis designed to position every sample accurately.

• The aperture in the bottom section is cut to the dia-meter of the sample that has to be exposed forcounting. The thickness of this section is selected tosuit the application.

The unit is assembled by placing the sample in the centre of the carrier tray (typically ametal or plastic planchette), placing the geometry rig on top op the sample, and then slidingit upwards over the end cap of the detector. The unit is kept in this position by a spring-loaded support or a platform that can be locked in position.

An important issue to consider in the design of these rings, is the effect of the air in the gapbetween the sample and the detector face.

• During the evacuation phase, this air must be removed. The time required to pump it, canbe long and will delay the analysis, unless an escape route is provided.

• During the venting phase, air might enter below the sample before the space above it isfilled. This can push the sample against the detector face, and contaminate it with radio-active material. It might even cling to the surface; which will probably destroy a SSB.

2.2.3 Electronic modules

A useful "-spectrometer requires, in addition to the detector (see § 2.2.1) and operational units(see § 2.2.2), at least the following electronic support:

• Reverse bias supply: Adjustable, typically in the 70 to 100 volt range.

• Pulse generator: Unit delivers pulses of the same height at the input of the system: heightadjustable; can be switched in (testing) and out (analysis) as required. These pulses areprocessed together with those produced in the detector, and test the performance of theelectronic system.

• Pulse amplifier: Unit increases (i.e. "amplifies") the height of the small pulses delivered by thedetector to a size as required by the pulse analysis electrons. A crucial requirement is that theheight of every pulse should be increased by the same factor.

• Pulse processing: Units and operation to be discussed in Section 2.4.


2.3. Problems due to Recoil

2.3.1 Principles

The energy that becomes available in the "-decay process, is shared between the particle andthe product atom due to recoil, and these two particles move in opposite directions.From the requirement that both momentum and energy must be conserved, one can say that:

for momentum:

for energy:

where: M (or m) is the mass of the product atom (or "-particle);

V (or v) is the velocity of the product atom (or "-particle);

P "E (or E ) is the kinetic energy of the product atom (or "-particle); and

Q is the total amount of energy that is available.

From which: . . . . . . . . . . . (2.1)

If Equation (2.1) is applied to a nuclide with a mass number of 230 (mass of product = 226) anda decay energy of 5.000 MeV, it follows that:

P "E = 87 keV and E = 4,913 MeV

The recoil kinetic energy of the product atom is sufficient to break its chemical bonds in thesample material (typically 10 eV or less). If the "-particle was emitted in the direction of thesample support (bottom), the recoil atom will move in the direction of the detector. Since thespace between sample and detector has been evacuated, there is nothing to stop this atom fromreaching the detector window. The kinetic energy of these particles may be sufficient to implantthem in the surface layer of the detector.

The efficiency of a solid state detector (i.e. fraction of the "-particles reaching the entrancewindow) is typically 0,2. This means that about 20 % of the recoil atoms that break loose fromthe sample, can reach the detector. The situation is even worse for the radon isotopes: thesegas atoms are already in the space between sample detector face, and it is not even necessaryfor them to break free.

2.3.2 The effects of recoil

These recoil atoms in the detector face can do no harm by themselves. A problem arises, how-ever, because Radioanalysis is often involved in the study of uranium and thorium and theirdecay products, and the recoil atoms are usually radioactive too. The "-particles emitted bythese recoil atoms inside the detector face, have a 50 % probability of being detected, and willcause an increase in background of the detector.

The severity of this problem is determined by the activity of the recoil material, and not directlyby the amount that is present. Since the ratio of activity/amount is inversely proportional to thehalf-life of the nuclide, this factor must also be considered. The following general situations canbe identified:

(a) The product has a long half-life: The specific activity of this material (activity per unit mass)is low because of the long half-life. The increase in background will be small.

(b) The product has a half-life of less than a week: The effect will be severe in this casebecause of the high specific activity of the material. One can, however, leave the detectorfor a few weeks, and use it only later when all the surface contamination has decreased toinsignificant levels.


(c) The product has an intermediate half-life: The effect will be less severe than for (b), but thehalf-life does not allow one the option to wait the contamination out. A few examples of thistype, and which are often encountered by Radioanalysis, are:

½• Po (T = 138 d) out of Ra.210 226

½ ½• Th (T = 19 d) is produced from the $-emitter Ac (T = 22 a) out of Pa.227 227 231

½• Th (T = 1,9 a) out of Th.228 232

2.3.3 Recoil suppression

The following two techniques can be applied (often simultaneously) to prevent the recoil atomsfrom reaching the detector.

(a) Run the counting chamber at a pressure where all the recoil atoms will be stopped in the airbefore they reach the detector face.

The range of charged particles and ions in a gas is determined by the amount of materialin the layer; which is proportional to the pressure of the gas. The followings values areobtained if one accepts that the range of a 100 keV recoil atom is about 80 :m in air (20 C0

and 750 mm Hg pressure).

Pressure (mm Hg) < 0,10 6 60 750Range (mm) > 600 10 1 0,080

At the pressure where the vacuum chamber is operated (typically less than 0,1 mm Hg), therange of the recoil atoms is about a metre or more, and a large proportion will recoil into thedetector face. One would prefer an air layer less than 10 mm thick between sample anddetector, because the counting efficiency decreases rapidly for larger distances.

(b) Apply an electrical potential between the detector face and the sample.

The recoil atom will be stripped of some of its outer electrons, and will therefore be positivelycharged. If one keeps the entrance window at a positive potential relative to the body of thechamber, or the sample at a negative potential, these positive ions will be repelled from thedetector face back to the sample.

The second option is sometimes used because it can be retro-fitted to old equipment, andbecause of the fact that there is already a bias voltage over the detector.

2.4 Pulse Processing

2.4.1 Electronic modules

The electronic circuitry involving the detector and charge collection is shown schematicallybelow. The pulses delivered at the output are amplified before being processed.


The size (or height) of each pulse is proportional to the amount of ionisation in the detectorbody; which is proportional to the energy of the "-particle. This relationship between "-energyand pulse height must be maintained at all costs.Only the height of the pulses are important for "-spectrometry; and not the time when theevents occurred. The objective is therefore to convert the train of pulses produced by thedetector, into information that can be used by the analyst to determine the energy and intensityof the "-particles emitted by the count sample.

2.4.2 Analog to digital conversion (ADC)

The pulses delivered at the output of the detector are "sorted" or classified according to theirheights as shown in the illustration below. The number of pulses that fall into each category arethen counted; with each counter representing one pulse height interval.

There are two important concepts that are relevant to this process:

• The index number or "code" assigned to each of the counters. (These codes normally runfrom "1" to higher values as required. They are referred to as channels. One would typicallyuse 1000 channels for "-spectrometry.)

• The total number of pulses that were collected in each of these counters in the measuringperiod, i.e. the contents of a counter. (Typically up to 10 .) 6

If the contents of each channel is plotted(on the vertical axis) against the indexnumber of that channel (on horizontalaxis), one obtains a raw pulse heightspectrum as shown in this figure. Thisillustration was derived from the first one.It is, however, impossible to re-constructthe original train of pulses from this figure,because both their actual sizes and thetime when they occurred, have been lostin the processing of the data.

2.4.3 Spectrum storage

Modern spectrometer systems are based on personal computers (PC's); both for control of thepulse collecting hardware and for the storage of the spectrum data. While the sample is beingcounted and data are still flowing in, all this information is kept in the random access memory(RAM). The analyst can look at a live video display of the pulse height spectrum, and see howit is updated as each pulse from the detector is processed and stored.

At completion of the counting period, all these data are transferred to a mass storage device,


typically the PC hard disk, before the next sample can be counted using the same channels.The analyst can at any time retrieve and process these data. It is good practice to make back-ups of all the pulse height spectra, in case the hard disk develops a problem.

2.4.4 Multiplexing

The count rates encountered in the analytical application of "-spectrometry are typically lessthan 1 count per second (compared to gamma spectrometry where rates in excess of 10 c/s4

are common). The ADC and storage modules are therefore idle for most of the time. Mostmodern equipment allows a single ADC to service up 16 detectors simultaneously, using atechnique known as multiplexing. It can be described as follows for eight detectors:

• Each detector has its own bias supply and preamplifier.

• The required memory of 8K is divided into 8 sectors of 1K channels each.

• Signals from the detectors enter the mixer-router at eight separate inputs (to identify thedetector involved in each event), but are fed to the same ADC.

• If an address value of (say) 485 is generated for a 4,85 MeV "-particle in detector #3, anoutput address of 485 + 3K = 485 + 3036 =3521 is produced by the ADC.

• The contents of channel 3521, or channel 485 of sector 3, in incremented.

This approach saves significantly on the cost of hardware, without reducing the performanceof the system at low count rates. The user is often not aware of multiplexing.

2.5 Analysis of the Pulse Height Spectrum

Section 2.4 described how the events in the detector that resulted from the presence of a radio-active sample, were processed to produce a pulse height spectrum. This Section will focus onhow these data on hard disk can be applied to obtain two valuable bits of information about thesample: (i) which radionuclides are present, and (ii) what is the activity of each?

2.5.1 Peak form

The form of the peak in the pulse height spectrum that can be expected froma sample that emits mono-energetic "-particles without any attenuation, isillustrated. The peaks are usually symmetrical, and can be described as aGauss or normal distribution.

The width of a peak is important because it determines the resolution of thedetector, i.e. its ability to distinguish between two peaks that are separatedby a small energy difference. In spectrometry the peak width is alwaysdetermined as the full width at half maximum height or FWHM value.

2.5.2 Peak search

"-Spectrometry is usually applied to solid samples where theenergy loss (or "attenuation") of the particles inside thematerial can be serious; even for relatively thin samples.Those particles that are emitted at or on the surface of thesource, will give a peak as one expects from the resolution ofthe detector (see § 2.5.1). Particles from inside the sample willloose some energy, and cause pulses to the left of the "main"peak. And the deeper the particles are emitted, the bigger theenergy shift of the pulses as shown by the tail in the figure.

This effect is usually ignored in high energy gamma spectrometry. The attenuation of photonswith energy values higher than about 100 keV in a low to medium density matrix, is very small


for a sample thickness of less than 20 mm.

Three issues require attention when discussing this low energy tailing:

• The magnitude of this effect is dependent on the sample thickness and density; but these cannot be controlled effectively in the laboratory. The analyst is therefore confronted with tailingand peak forms which will differ from sample to sample.

• The technique of "-spectrometry is usually applied by Radioanalysis for environmentalstudies. The activity of a sample can be so low that, even for long counting periods, only alimited number of pulses are received. And this information is often not sufficient to determinethe form of the low-energy tail.

• The activity of the sample, and hence the number of pulses in the spectrum, can be increasedby using more material. This will also increase the attenuation and tailing, and therefore thedegree of overlap between adjacent peaks. One may loose more due to overlap than whatis gained by having more pulses.

These practical realities usually exclude the use of commercial software to determine the peakpositions, or to resolve overlapping peaks automatically. The analyst often has no option but todefine regions of interest (ROI's) manually for the different peaks. The following procedures areuseful for selecting these regions:

(a) When there are sufficient counts in the spectrum to define the peaks regions reasonablywell:

• The peak position (which relates to the energy of the "-particle) is accepted as eitherat the channel which indicates the maximum of the peak (well-defined peaks), or onechannel below the upper "edge" of the peak (peak poorly defined).

• The upper limit of the ROI is set one channel above the upper edge of this peak.

• The lower limit of the ROI is set just above the upper edge of the peak on the left (lowerenergy) of the peak being evaluated.

(b) When the number of counts are so low that there are no peaks "visible" at all in the pulseheight spectrum, the analyst is obliged to calculate the limits for the ROI's of the expectednuclides.

• Set the upper limit of the ROI at a channel number (rounded off upwards) correspondingto 10 keV more than the energy of the particle being investigated.

• Set the lower limit of this ROI at a channel number that corresponds to an energy value20 to 50 keV above the energy the preceding peak.

There are a two important issues to consider when applying any one of these methods to setROI's in the pulse height spectrum:

• The energy interval (or channel numbers) covered, should be the same for all the ROI's. Ifthe intervals are not the same, a different fraction of the tail will be dropped for each peak,which will lead to inaccurate peak area values.

• If the peak forms are poorly defined, it becomes difficult to determine the top of each peak.One can not use such a pulse height spectrum for the accurate measurement of peak positionand "-energy, and the reliable identification of the nuclide involved.

2.5.3 Peak position and energy

For solid state detectors one can assume a linear relationship between the energy of the "-

Aparticle being absorbed in the detector (E ), and the channel number (K) at the maximum ofthe corresponding peak in the pulse height spectrum.

AE = m@K + C (general equation)

Aor E = m@K (when the electronic modules in Section 2.4 are properly adjusted).


There is no attenuation (and energy loss) of the particles in the special case where both thesample and the entrance window at the detector are very thin, and the counting chamber is

"operated under vacuum. Under these circumstances, the energy of the particle (E ) as emittedby the sample is given by:

" AE = E = m@K . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2.2)

When recoil suppression is used (see § 2.3.3), the gap between the sample and the detectorface is filled with air at a reduced pressure to absorb the recoil ions, and to prevent them frombeing implanted in the detector face. This air layer will also attenuate the "-particles (i.e. reduce

" Atheir energies) so that E � E .

In the general case:

" XE = E + m@K + C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2.3)

"where: E energy of the "-particle as emitted by the decaying nuclide (keV);

XE energy loss of this particle in the source (= 0 for thin sample), in the air layerused for recoil suppression (= 0 for vacuum operation) and the entrance windowof the detector (= 0 for PIPS);

K channel number at the maximum of the peak in the pulse height spectrum thatis caused by this particle;

m the linear energy scale of the analog to digital conversion unit, typically set closeto 10 keV/channel for "-spectrometry; and

C the zero off-set of the analog to digital conversion (set close to zero).

It has been mentioned earlier (see § 1.1.2) that the stopping power of any material is dependenton the energy of the "-particle. This means that, given a fixed sample and counting geometry,

X Xthe value of E will be larger for the low-energy particles. This variation in E should also be

X "covered by Equation 2.3; but the empirical measurement of E as a function of E is not arealistic option. It has been found that the energy relationship, including external attenuation,can be expressed satisfactorily by:

" 1 2E = m @K + m @K + C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 (2.4)

1 2where m and m are empirical constants for a particular counting geometry.

2.5.4 Peak form and the resolving of overlapping peaks

Paragraph 2.5.2 describes the situation as it isusually experienced by Radioanalysis: too fewcounts in the pulse height spectrum to allowproper definition of the "-peaks. There are,however, also situations where one can clearlydefine the peaks, as is demonstrated in thediagram. This example, based on isotopes ofnatural uranium, illustrates how the tail of ahigh-energy peak overlaps with peaks at lowerenergy values.

In the pulse height spectrum as observed by the analyst, the contributions from the differentpeaks are no longer separated as shown. All the peaks have been added together, and theproblem is to unscramble the individual peaks from the combined data.

There are different software packages available for resolving these overlapping peaks. They arebased on the assumption that all the peaks in a pulse height spectrum have the same "form"after they have been normalised to the same peak height. The procedure typically involves anumber of steps:

• The general "form" of the peaks is determined from those parts that are relatively free


from overlap (eg. the top part of the U, and the lower tail of the U peak).234 238

• This curve is used to calculate and correct for the contribution of U to the U peak.234 235

• The tail of the U peak (now free from any U contribution) is calculated, and then sub-235 234

tracted from the U peak.238

• The data on the unscrambled peaks are used to obtain a better estimate of the general peakform. The whole process is repeated a few times, using a form that becomes more reliablefor every repetition.

It is relatively easy to describe and understand the process, but very difficult to translate it intoa computer program. Most of the software requires input from the operator in order to make thebest decisions.

2.5.5 Counting efficiency

Every "-particle that enters a solid state detector, will add one count to the corresponding peakin the pulse height spectrum. Some of these counts may fall in the tailing part of the peak, butthey still form part of that peak. This means that the intrinsic efficiency of this type of detectoris unity.

All the particles that are emitted by the decaying nuclides in the sample, do not reach theentrance window of the detector, because the window covers only a part of all the angles intowhich the particles are being emitted. It is important to note that this loss of particles is due togeometric restrictions only (i.e. size of sample and detector, and their distance apart), and notto their absorption in the sample or in material between sample and detector.

Note: If the attenuation becomes so serious that particles are being "lost", the tailing is so largethat one can no longer consider the measurement as "spectrometry". It should rather beclassified as gross "-counting as discussed in NWU-NC-TH-01).

All the "-particles are affected to the same extent by the counting geometry, regardless of theirenergy value. This means that the counting efficiency for a solid state detector is notdependent of the energy of the "-particles.

The standard efficiency relation is also relevant:

N = A @ Y @ , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (2.5)

where: N count rate in the peak, i.e. peak area ÷ count period (counts/sec);

A activity of particular nuclide in the sample (Bq);

Y yield, or fraction of the decays that produce particles of this energy; and

, counting efficiency, of fraction of the particles that reach the entrance window.

Note: The yield value for "-decay is usually 1,000. There are only a few exceptions like Bi212

where branching is significant.

The assumption that the counting efficiency (,) is independent of "-energy, is valid only if thearea under the tailing part is included in the "peak area" value for Equation 2.5. This is the casewhen the overlapping peaks are properly resolved as described in § 2.5.4. If, on the other hand,the approach of § 2.5.2 is applied, the bottom part of the tail may be excluded from the selectedROI. It is also possible to include part of the tailing from a higher energy particle in this ROI.


3 PRACTICAL CONSIDERATIONS

Some theoretical issues regarding solid state detectors for "-spectrometry were discussed inChapter 2. This chapter will focus on the practical application of this technique to the analysisof real samples.

3.1 Samples to be Counted

3.1.1 Requirements regarding thickness

The high stopping power for "-particles in the sample (see § 1.1.2) leads to tailing in the pulseheight spectrum (see § 2.5.2). This effect forces the analyst into a choice between two needsthat can not be satisfied simultaneously:

• One needs a large amount of material to obtain the maximum activity; which will ensuresufficient counts in the pulse height spectrum for proper definition and resolving of the peaksthat are present (see § 2.5.4).

• One needs a small amount of material to minimize attenuation in the sample, which willensure little overlap of peaks and reliable peak position and area values.

Experience has shown that tailing and peak overlap can still be tolerated for "typical" samples,if the thickness of the material is kept below 100 :g.cm . Above this limit, the overlap is so bad-2

that one can no longer produce reliable results. If the energy difference is very small for theparticles that are emitted by the nuclides in the sample, one might be forced to accept a limitthat is even lower than 100 :g.cm . There are a number of options one can consider to reduce-2

the impact of the thickness limitation:

(a) Remove most of the other components without losing the active material that has to bedetermined. One can, for example, dissolve the active ingredient, remove the insolublefraction by filtration, and prepare a sample for counting from the liquid. The major obstacleis the possible loss of some of the analyte.

(b) Separate the element(s) to be measured (eg. radium, thorium or uranium) chemically fromthe field sample, and convert the separated fraction into a form that is suitable for counting.The easier option to separate groups of similar elements together, is not recommended:Each element will have at least one active isotope, and all the contributions may lead to avery crowded pulse height spectrum with a lot of overlap that is difficult to resolve. There isagain the possibility to loose some of the element that is determined.

(c) Use less material, but count for longer periods. Counting periods of several days arecommon in "-spectrometry.

Very few samples are measured by "-spectrometry in the form in which they were submittedby the client; there is usually some form of processing required to make them suitable for thecounting. The terms field samples and count samples are used when necessary to make adistinction between the material as received in the laboratory and the final preparation that isloaded in the "-spectrometer.

3.1.2 Requirements regarding size

The count samples should meet a number of dimensional requirements to ensure reliableresults. Most of these are not inherent in the technique itself, but are imposed by the problemswith efficiency calibration procedures (to be discussed in Section 3.3). The following are of majorconcern:

• The sample itself must be flat with an even surface.

• The activity must be spread evenly over the full surface of the sample, or an area that isclearly defined as the active area.


• The active surface must have a regular shape (preferably circular), that is preferably smallerthan the entrance window of the detector.

• The thickness and size of the support on which the sample rests, is also limited by the insidedimensions of the counting chamber.

The analyst is often confronted with samples that are either larger than the detector window, orthat do not cover the full area of the support. A typical example would be the standard 47 mmfilter discs used for sampling dust particles in air or suspended material in water, and in whicha 3 to 4 mm ring around the edge was covered by an O-ring during sampling. The followingapproach is suggested for handling this problem:

• Determine the diameter of the area that was exposed (= D): Use the area that is slightlydarker due to dust, measure the dimensions of sampling device, or use whatever informationis available to estimate this value.

• Use a geometry rig as described in 2.2.2 (b) with known diameter (= d) and smaller than thedetector window, to "mask" the sample.

• Count the sample at least three times by placing the mask over different positions of thesample. Use these data to determine the average activity (= A*) and the standard deviation(as a measure of the homogeneity of the activity over the surface).

• Correct for the smaller area covered so that the true activity value (= A) is given by:

A = D / d . A*2 2

3.1.3 Requirements regarding composition

Considerations based on spectrum analysis or counting efficiency do not impose any require-ments on sample composition. One should, however, consider possible physical damage to thedetector itself:

(a) The samples must be dry, and free from corrosive substances; eg. HF and HCl. Thesefumes can also damage the valves, vacuum pumps and pressure measuring components.

(b) The evacuation and re-pressurising of the counting chamber, and especially the spaceabove and below the sample, can lead to pressure on the sample. This can cause thesample or its support to move upwards, and to touch the detector face. This will damage thedetector.

3.1.4 Operational measures in the laboratory

(a) Storage:

It is sometimes necessary to leave samples for a few days before they are counted (eg.Ra that has been separated from a water sample, to allow the decay products to reach226

radioactive equilibrium). Samples are usually kept for at least a month after being counted,to allow the analyst to investigate them should questions be asked during data processingor by the client. This requires special storage facilities and operational systems that willensure the identity and integrity of each sample.

(b) Identification:

Most of the count samples used for "-spectrometry, do not permit the analyst to write onthem; which can lead to serious problems in maintaining traceability. Special sample carriersthat are clearly labelled with the laboratory codes, can reduce this problem. The problem isdifficult to eliminate because of the possibility of human error when the carriers are loaded,and when the samples are transferred from the carrier to the counting chamber and back.A large number of these are required considering the need for storing the samples.

(c) Records:


It is clear that a lot of information is generated while the material is processed to deliver asuitable count sample, when it is measured, and when the data are processed. A lot can getlost and go wrong if this information is not properly recorded. The laboratory shall have adocumentation system for "-spectrometry that: (i) is transparent, accessible and understoodby all personnel; (ii) will ensure traceability; and (iii) is easy to use.

3.2 Background Corrections

3.2.1 Principles of background corrections

The very low range of "-particles makes it easy to shield the detector against external radiation.This leaves only the inside of the counting chamber, the body of the detector, and the samplesupport itself as possible sources of "-particles. It is possible to reduce the total alphabackground to less than 5 counts per day.

The main reasons for an increase in the background of a detector is recoil (see § 2.3), and themechanical transfer of radioactivity from a count sample to the detector or the geometry rig.

Since it is not possible to measure the background while the sample is being counted, theanalyst can only assume that the background remains constant over the measuring period, andthat a reading before (or after) the samples will be an acceptable indicator. This assumption isreasonable in view of the fact that the background is not affected by external radiation, and thatany contamination inside the counting chamber will be detected when the next backgroundreading is taken.

3.2.2 Value for defined regions of interest

The concept of "the background of the detector" has no practical significance; only the valueover a defined region of interest (ROI). These ROI-values are selected (see § 2.5.1 and 2.5.3)according to the position and overlap of peaks in the pulse height spectrum, and the same ROI'smust be used when determining the background reading.

The statistical error in the background measurement contributes to the gross error of analysis.It is usually kept within reasonable limits by using a count period for background that is at leastas long as for samples.


(a) Frequency of measurement:

"-Spectrometry is characterised by long count periods, and a large amount of time is lostin counting background. The question then arises: how long can one wait between thesemeasurements? The answer to this question is to be found in an unpleasant fact: If asignificant background increase is detected, all the results since the previous backgroundreading are suspect, and all the counting must be repeated. The analyst must decide whichwill probably "waste" the most time: frequent background readings, or re-counting all thosesamples?

The following guideline is suitable for low-activity measurements: If the detector has not becontaminated the past year, monthly background readings would be sufficient. If a jump inthe reading has been observed less than a year ago, weekly readings are recommended.

(b) Data processing and storage:

The same background radiation spectrum is relevant to all analytical methods. Each of thesewill use different ROI's, but one can usually not specify in advance all the ROI's that will beused. It is therefore not practical to record the background values over these ROI's. It ismore effective to store the full pulse height spectrum, and obtain background values overROI's from it when they are required.


(c) Management of deviations:

Sudden variations in the background are not expected. When they do occur, it might be anindication of serious problems in the counting system. It is therefore recommended that suchdeviations be investigated thoroughly in order to identify and eliminate the root cause.

3.3 Calibration Procedures

The reliability of the analytical results that are obtained from "-spectrometry, can not be betterthan the quality of calibration procedures. This Section covers the two issues that are crucial inthis regard: energy and efficiency calibration.

3.3.1 Energy calibration

Paragraph 2.5.3 describes the theoretical relation between the energy of the "-particle, and thechannel number for the maximum of the corresponding peak in the pulse height spectrum. Thevalues for the parameters in Equations (2.2) and (2.4) can not be calculated from basicprinciples. They are typically determined by applying the following procedure, known as anenergy calibration, with a set-up sample containing one or more known radionuclides:

• Count the calibration sample, and determine the position (i.e. the channel number, K) at thetop of each peak in the pulse height spectrum.

• Assign each of these peaks to a particular radionuclide in the calibration sample.

• Obtain the corresponding energy values (E) for the "-particles of these nuclides fromstandard tables or the Chart of the Nuclides.

• Replace these K- and E-values in the equations, and solve values for the parameters.

For most applications a linear relation with zero intercept (Eqn. 2.2) is sufficient. If recoilsuppression with a relatively thick air layer is used, a quadratic relation or a non-zero interceptmight be required (Eqn. 2.4).

The "-spectrometry software used to display the pulse height spectrum on screen, oftenconverts a selected channel number into the corresponding energy value by using the para-meters measured above. This allows the analyst to place the cursor on the top of a peak, andread the keV-value directly from the screen.

Many spectrometer systems are provided with a pulse generator that feeds electronic pulsesinto the pre-amplifier close to the input from the detector. This allows the user to check whetherthe energy calibration has drifted from the last measurement.

3.3.2 Efficiency calibration

Paragraph 2.5.5 describes the theoretical relation between the area of a peak in the pulseheight spectrum, and the activity of the corresponding radionuclide. The value for the yield (i.e.Y in Equation 2.5) can be obtained from standard tables. The value for the counting efficiency,however, can not calculated, and is not specified for a particular detector; the analyst has todetermine its value empirically.

The principles of this procedure, known as an efficiency calibration, is very simple to describe:A sample containing a known activity (= A) of any "-emitter is counted, and the efficiency value(= ,) is calculated using the peak area expressed as a count rate (= N).

Since this efficiency is determined by geometric considerations only (and not by the energy ofthe "-particles), the measured value is valid for all the other "-emitting nuclides. This value canbe used to determine the "-activity in any other sample, provided that it has the samedimensions and is at the same distance from the detector as the calibration sample.

Correct efficiency calibration is difficult to realise in practice because of the requirement that the


calibration and field samples must be dimensionally alike. It is extremely difficult to obtainreliable calibration samples:

• There is a requirement in the most quality standards that calibration "must be traceable tonational or international standards". Each method used in the laboratory produces anothertype of sample to be counted. But the organisations producing standards (eg. NIST orAmersham) can not satisfy these diverse needs from laboratories at a reasonable cost.

• Analyst are therefore compelled to prepare their own calibration samples as required by thedifferent analytical methods, using known amounts of radioactive solutions that are traceableto and certified by reputable suppliers. It is sometimes a daunting task to prepare calibrationsamples that represent the laboratory samples in every respect.

• A serious issue with in-house calibration samples is to provide objective evidence that noneof the activity that was used, has been lost in the production process. Or, alternatively, tomeasure the fraction that is retained in the final product.

Experience has shown that a good starting point for efficiency calibration is a certified standard:(i) that is larger than any of the laboratory samples to be measured, (ii) has a known area overwhich the activity is spread, and (iii) with a homogeneous distribution of the activity over the fullarea. The efficiency calibration procedure can be summarised as follows:

• Measure the effective aperture of the geometry rig that will be used when counting samples.Calculate the fraction of the active area of the standard, and hence the activity, that will beexposed by this rig.

• Count the standard for a suitable period for the required probable error, and obtain the peakarea for the selected ROI. Convert this peak area value to counting rate; with backgroundcorrections as required.

• Apply Equation 2.5 to calculate the counting efficiency value.

This procedure must be applied for every geometry rig that is used with this detector, becausethe counting efficiency varies with the size (diameter) of the sample and its distance from thedetector. The expression "counting efficiency of a detector" can be somewhat misleading, sinceit implies that there is only a single value. It would be better to refer to the "counting efficiencywith geometry X".


Long count periods are characteristic of this technique. This compels the laboratory to run alarge number of instruments in order to attain a reasonable throughput of work. This inevitablyleads to a large volume of calibration data that are produced and applied by different analysts;which can create chaos if not managed properly.

(a) Records:

The laboratory requires a known strategy and transparent documentation system to ensurethat essential data are recorded for: the introduction of new geometry rigs; replacement ofdefective detectors; regular energy and efficiency calibration; and changes to the system.The records must be readily available to all users that require data; yet secure to prevent anyunauthorised changes to it. Obsolete data must be archived so that changes can be traced,and old results processed again with other parameters.

(b) Application software:

The raw count results are combined with other laboratory data, and processed to obtainactivity concentration values in the field samples that were analysed. These operations areusually not very complex (but extremely tedious to do) and can be done more efficiently byspecial computer procedures (eg. in spreadsheets).

A different package is typically required for every method, but they all share the samecalibration data. It is more efficient to have all these data available in a central computer file


where it can be updated and read as needed, instead of typing it in every time an instrumentis used.

(c) Audits:

The proposed strategies are more efficient, but can easily flounder if the laboratory ignoresan important obstacle: human nature. It is therefore recommended that an internal auditsystem be designed and applied to ensure that the data are recorded and enter as and howit was agreed.

3.4 Analytical Procedures

The aim of this Section is to give an outline of the steps that are involved in obtaining activityconcentration values for the samples that have been submitted for analysis by a client.

3.4.1 Sample preparation and counting

It should be clear by now that very few samples that are processed by "-spectrometry, arecounted "as received". Most field samples require some physical preparation or chemicalseparation as mentioned in Section 3.1. There is a vast number of methods from which theanalyst can select one that will suit the needs of the client; and no attempt is made in thismodule to cover them. The important thing is that the method must produce a count sample thatis suitable for this technique.

The count process is relatively simple in principle. It only requires the analyst to make a fewdecisions before counting can commence:

• Which detector to use? The following are relevant issues in this selection process: Is thecounting to be done with or without recoil suppression? Is there a contamination hazard fromthe active samples? Is there a special need for low background values?

• Which geometry rig to use? This choice is usually prescribed by the analytical method. If not,the only issues to concern the analyst are that: (i) the aperture of the rig should be the sameas the active area of the sample, and (ii) there must be a valid efficiency calibration for thisgeometry of the selected detector.

• What count period to use? This choice depends on the needs of the client regarding thesensitivity of measurement: The longer the sample is counted, the lower the minimum activitythat can still be measured. The criteria for this selection should have been defined during thevalidation of the method.

3.4.2 Converting count rate data to activity values

(a) Once the counting is finished, the analyst sets ROI's over the relevant peaks in the pulseheight spectrum. This selection can be done according to: (i) the criteria in 2.5.4 for well-defined peaks; (ii) criteria in 2.5.2 when the peaks are poorly defined; or (iii) the instructionsgiven for the analytical method being used. It is usually possible to apply the same ROI's forall the samples in a particular batch.

(b) The value for the integral over all the channels in a ROI (i.e. sum of the counts) is calculated,and converted to the corresponding count rate (counts/s).

(c) The same process is repeated for the most recent background pulse height spectrum for thisdetector. The background rates are subtracted from the gross values in (b) to obtain the nettcount rate in each ROI. Note: Since the count rates are used, there is no requirement thatsamples and background are to be counted for the same period.

(d) This count rate and the counting efficiency for this geometry rig are combined to calculatethe activity of the count sample.


3.4.3 Corrections for chemical yield

The count sample processed in § 3.4.2, represents only that amount of activity that wasrecovered in the final preparation. If one considers the complexity of a typical separationprocedure, it is obvious that the potential loss of a significant amount of initial activity can notbe ignored. It is necessary to determine the fraction of the initial activity present in the fieldsample, that was recovered in the count sample, and make the necessary corrections for it.There are two strategies for measuring this yield of separation.

(a) By means of a carrier:

It is often necessary in trace element analysis to add an inactive carrier to the field sampleprior to separation, to prevent "losing" some of the material when working with these micro-scopic quantities. The fraction of the carrier that is recovered in the count sample, is therequired yield value. This approach is usually not suitable for "-spectrometry:

• The attenuation of "-particles in the count sample, restricts its thickness to less thanabout 100 :g.cm (see § 3.1.1). This small amount of inactive carrier is often difficult to-2

determine accurately by gravimetric means.

• "-Spectrometry is applied mainly to uranium and thorium and their decay products. Theseelements, with the exception of bismuth and lead, do not have stable isotopes that can beutilised as inactive carriers.

(b) By means of a radioactive tracer:

One can add a known quantity of a radioactive isotope of the same (or similar) element tothe field sample before the separation process is started, and then measure how much ofthis activity is recovered in the count sample. An isotope that decays by $-emission ispreferred as a tracer because:

• its radiation does not add peaks to the complex "-spectrum; and

• it can be measured accurately by gamma spectrometry or gross $-counting.

Examples: Ba for Ra, and Th for Th.131 226 234 232

3.4.4 Minimum detectable activity

The analyst who measures uranium and thorium and their decay products in environmentalsamples, is often confronted with extreme demands for sensitivity. In spite of all the complexseparation procedures and long counting periods that are applied, only a very small number ofcounts are often observed. The question then arises: How significant are these results? Thisissue has been discussed in § 3.5.4 of NWU-NC-TH-01.

It is strongly recommended that the detection limit (LLD) of the instrument is calculated fromthe observed background count rate and the count periods for sample and background; and theminimum detectable activity (MDA) value for each sample be obtained from this. Both themeasured activity concentration and the MDA values on each sample should be reported to theclient.

3.5 Quality Assurance Programme

3.5.1 Scope of the programme

A QA programme should cover all the parameters that can affect the reliability of the analyticalresults in order to be effective, but should require the minimum amount of time (i.e. costs ofoperation) in order to be efficient. These requirements are somewhat contradictory, and theanalyst is obliged to evaluate the probability that something can go wrong and the impact thedefect would have on the results. The following issues are relevant for "-spectrometry:


• Energy calibration (see § 2.5.3): the relation between the energy of the "-particle (keV) andthe position of the corresponding peak (channel number) in the pulse height spectrum. Somedrift in this relation is to be expected due to temperature effects and slow changes in theelectronic modules.

• Counting efficiency (see § 2.5.5): the relation between the activity of the nuclide in the countsample and the area of the corresponding peak in the pulse height spectrum. This value isdetermined by the geometry rig being used, but it can only change with time if there is aserious defect in the detector.

• Energy resolution (see § 2.5.1): the ability to resolve peaks that are close to each other inthe pulse height spectrum, expressed as the FWHM value of a count sample free fromattenuation. This value will deteriorate when the detector is damaged; but it is rather difficultto measure it because one can not separate the contributions from the detector and from thesample being counted.

• Background count rate (see § 3.2): This value will increase if the detector or inside of thecounting chamber has been contaminated; which will have a significant effect on the bestMDA value that can be attained.

3.5.2 Outline of a reasonable programme

The following QA programme will address the important issues (directly or indirectly), withoutusing so much instrument time that the productivity is affected severely:

(a) Measure and record the leakage current (see § 2.1.2 a) and the position of the pulser peak(see § 2.2.3) at weekly intervals.

• Peak position is outside specification: Repeat the energy calibration (see § 3.3.1).

• Leakage current is outside specification: Check background count rate, and FWHM fora low attenuation sample.

(b) Count a single nuclide test sample of 10 to 100 Bq at regular intervals (at least every month)using the same geometry rig every time. Record the position of the peak and calculate theactivity of the count sample.

• Peak position is outside specification: Repeat the energy calibration (see § 3.3.1).

• Activity is outside specification: The detector is probably defective and must be withdrawnuntil an investigation has identified and eliminated the root cause of the problem.

Note: The same test sample can be shared by different detectors if necessary.

It is necessary to verify one geometry rig only. If the performance of the detector isin (or out) for this one, it will be in (or out) for all the others too.

(c) Count a clean sample support with the same geometry rig as used in (b). Use a count periodthat is equal to or longer than the maximum value used for samples on this detector. See§ 3.2.3 on the suggested frequency of measurement and storage of spectra. Measure andrecord the integral count rate value over a defined ROI that starts just above the maximumof the noise region (but not more than 4 MeV) and extends to the last channel that is stored.If this "gross background" count rate is outside specification:

• clean the detector as described in Section 3.6; or

• reserve this detector for counting the more active samples.

3.5.3 Implementation of the programme

A number of practical issues should be considered when setting up and running the QAprogramme suggested in § 3.5.3:


(a) Performance specification:

It is usually difficult to decide on minimum or maximum limits on the measured values, thatwill be considered as being "acceptable". The specification of the manufacturer is of little usebecause it usually does not cover all the issues listed in § 3.5.1. The analyst is thereforeoften obliged to accept the known performance of the "new" detector (= X) as the properspecification for a "good" detector. The upper and lower performance limits are usually setat the (X + 3F) and (X - 3F) values.

(b) Corrective action:

The aim of the QA programme is to identify instruments that are out of control, and to with-draw them from use until the problem has been removed. This objective is often defeatedby poor laboratory discipline where the performance data are collected as agreed, but onlyprocessed later after all the samples have been counted to. But this is too late: Deviationsmust be identified and corrected before the samples are counted.

3.6 Care and Maintenance of the Detectors

3.6.1 Replacing defective detectors

It is relatively easy to replace a defective detector. The following guidelines apply.

• Use gloves when handling any detector. Traces of fat and sweat from the hands will increasethe leakage current and background, and can affect the resolution.

• The serial number and date of installation should be recorded.

• The new detector requires new energy and efficiency calibration; which must also be fullydocumented (see Section 3.3).

3.6.2 Preventing contamination of detectors

(a) Use recoil suppression (see § 2.3.3) where possible.

(b) Make sure that the count sample does not contain loose material that might come off, andadhere to the detector and inside of the counting chamber.

(c) Clean all geometry rigs at regular intervals.

(d) Use old and contaminated detectors for counting the more active samples. The effect ofmore contamination will be less obvious.

3.6.3 Preventing damage to detectors

If a detector has been properly installed (see § 3.6.1), and if the measures for the preventionof contamination are applied rigorously (see § 3.6.2), very little can happen to these detectors.The only remaining issues to consider, are the following:

(a) Never run a detector above the recommended bias voltage.

(b) Never switch on the bias voltage while the counting chamber is being pumped down, i.e.before the recommended operating pressure value is attained.

3.6.4 Cleaning a contaminated detector

The best method to clean a detector, is to prevent its contamination in the first place.

Surface barrier detectors (see § 2.1.1) can not be cleaned; such an attempt will probablydamage the thin metallic layer on the surface and destroy the detector. The "face" of a PIPSdetector, on the other hand, is a layer of silicon semi-conductor (see § 2.1.1) implanted withions, which is relatively stable. These detectors can be cleaned.


The cleaning should be done in steps; starting with the more gentle treatments. Two tests canbe carried out before the cleaning process, and also after every step to see whether it has beensuccessful: (i) FWHM with a source that is essentially attenuation free, to monitor the detectorresolution; and (ii) background, to see whether the specified value has been reached. Theprocedure is terminated as soon as the desired result has been obtained. The followingsequence of cleaning steps are recommended:

(a) Wipe the entrance window and body of the detector with a cotton swab soaked in ethanol.This will remove any loose surface contamination, as well as the film of oil that is caused byback streaming (see § 2.2.1).

(b) Leave the window surface (but not the rest of the body) in contact with water that containsa complexing agent (eg. EDTA). Leave for about a day to enhance the dissolution of thecontamination. Rinse with water and ethanol, and leave to dry.

(c) Leave the window surface (but not the rest of the body) for about 10 minutes in contact with1 M HCl solution. Rinse with water and ethanol, and leave to dry.

(d) Repeat step (c) for another 30 minutes.

4 POINTS TO PONDER

4.1 The calibration sample has an activity of (471,2 ± 6,8) Bq of Am, which is evenly spread over241

a circle with a diameter of 55 mm. The geometry rig to be used with laboratory samples has acircular aperture that is 10 mm in diameter. The calibration sample and a blank are counted for20 minutes each with this rig, and reading of 4484 and 36 counts respectively were observedin the prescribed ROI. Calculate:

(a) the counting efficiency; and

(b) the probable error on this value.

4.2 A sample must be analysed for Po. A volume of 500 mR is spiked with (471 ± 12) mBq of Po210 209

tracer, and the polonium is separated by electroplating on a silver disc. This disc is counted for16 hours on an "-spectrometer with negligible background, using a rig with 21,7 % countingefficiency. Analysis of the pulse height spectrum gave the following results:

ROI Integral

450 - 490 4841

492 - 532 1483

Use this information, and other essential data from the Table of Nuclides, to calculate:

(a) the chemical yield for polonium in the separation;

(b) the activity concentration of Po in the sample; and210

(c) the probable error on both these values.


Annexure 1 Example of the Chart of the Radionuclides


Annexure 1 Example of the Chart of the Radionuclides (continued)

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