333

10

Unmasking the Illicit
Trafficking of Nuclear
and Other Radioactive
Materials

Stuart Thomson, Mark Reinhard, Mike Colella,
and Claudio Tuniz

CONTENTS

10.1 Introduction 334
10.1.1 The Nuclear and Radiological Terrorist Threat 334
10.1.2 Radioactive and Nuclear Materials 334
10.1.3 Categorization of Nuclear and Radiological Materials 335
10.1.4 Radiological Scenarios 336
10.1.5 The Illicit Trafficking of Radioactive Materials 338
10.1.6 The Role of Scientific Practitioners 339
10.2 Radiation Detection Strategies 339
10.2.1 Introduction 339
10.2.2 Radionuclides of Interest to Border Monitoring 340
10.2.3 Radiation Detection at Border Control Points 341
10.2.3.1 Gamma Ray Detectors 342
10.2.3.2 Stage One: Fixed Portal Monitors 344
10.2.3.3 Stage Two: Locating and Isolating the Source
of Radioactivity 345
10.2.3.4 Stage Three: Isotopic Analysis 345


N

UCLEAR
AND

R

ADIOLOGICAL

T

ERRORIST

T

HREAT

In the last 15 years we have seen vast changes in the worldwide political land-
scape. The end of the Cold War and the subsequent dissolution of the Soviet
Union saw a reshuffling of international alliances and the disintegration of former
political ties. With the end of the Cold War, many envisaged a new world order
and hoped security would be rooted within the United Nations. Clearly, this has
not occurred.
The reawakening of ethnic and religious tensions and the exacerbation of
global socioeconomic issues are causing conflicts in a number of critical regions
of the world. One phenomenon of particular concern is the upsurge in global

made). Naturally occurring radioactive materials (NORMs) include isotopes pro-
duced via the uranium series, the actinium series, and the thorium series, and the
low-abundance isotope of potassium,

40

K. Besides the NORMs of primordial
origin, there is a very weak (but measurable) concentration of natural radio-
nuclides, such as

3

H,

14

C, and

10

Be, produced by nuclear reactions of highly
energetic cosmic rays.
Anthropogenic radioactive materials are produced via appropriate nuclear
reactions. Examples include the production of

60

Co via neutron capture in a

DK594X_book.fm Page 334 Tuesday, June 6, 2006 9:53 AM
AND

R

ADIOLOGICAL

M

ATERIALS

The International Atomic Energy Agency (IAEA) classifies radioactive materials
into five separate categories: unirradiated direct use nuclear materials, irradiated
direct use nuclear materials, alternative nuclear materials, indirect use nuclear
materials, and radioactive sources [3].
Unirradiated direct use nuclear material does not contain substantial quantities
of fission products and can be readily used to construct a nuclear weapon or
improvised nuclear device (IND) [3]. This is primarily because these materials
require little or no further processing. Examples of such materials are highly
enriched uranium (HEU), containing the isotope

235

U at a concentration greater
than 20%, or plutonium containing less than 7%

240

Pu [3]. Irradiated direct use

to health [4]. Table 10.1 details the nomenclature typically used to categorize
nuclear and radioactive materials. There is no separate international classification
system to categorize materials according to the potential for malevolent use,
however, parameters to consider include those based on the radiological hazards,
in addition to issues related to portability, dispersability, and the potential for
theft. Table 10.2 details the results of a Monterey Institute of International Studies
report commissioned to determine the radioactive materials that pose the greatest
risk to public health and safety, focusing on the potential consequences of their
malevolent use [5]. More detailed guidance on the categorization of nuclear
material is available from the IAEA [3,4].

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336

Radionuclide Concentrations in Food and the Environment

10.1.4 R

ADIOLOGICAL

S

CENARIOS

In recent publications, four mechanisms by which terrorists can exploit current
nuclear and radioactive stockpiles and obtain suitable weapons have been dis-
cussed [1,6]. The mechanisms are
• The theft and detonation of an existing nuclear weapon.


233

U
Irradiated direct use nuclear
material
Irradiated nuclear fuel material In irradiated nuclear
fuel
Indirect use nuclear material Depleted uranium (DU) <0.7%

235

U
Natural uranium (NU) 0.7%

235

U
Low enriched uranium (LEU) >0.7%

235

U and <20%

235

U
Plutonium (

238

Co and

137

Cs
Radioactive sources
Category 2
Industrial

γ

radiography

192

Ir
High/medium dose rate brachytherapy

103

Pd,

60

Co,

137

Cs, and


Cf
Radioactive sources
Category 4
Thickness/fill level gauges

241

Am
Portable gauges (e.g., moisture, density)

137

Cs and

60

Co
Radioactive sources
Category 5 (least dangerous)
Medical diagnostic sources

131

I
Fire detectors

241

Am,


the public to radiation and cause contamination of the surrounding area. However,
all states require strict security for such facilities, particularly larger establish-
ments such as nuclear power plants. The large structural mass surrounding a
reactor core would facilitate the need for a large catastrophic event to cause a
reactor core breach. With this in mind, it would be extremely difficult for terrorists
to achieve such a feat. Nonetheless, some of the latest reactor construction

TABLE 10.2
Radioactive Sources of Greatest Concern (Adapted from Ferguson et al. [5])

Isotope Common Use Form Half-Life Emissions

137

Cs Teletherapy, blood irradiations, and
sterilization facilities
Solid, chloride
powder
30.1 yr

β

and

γ

radiation

60


α

and

γ

radiation

90

Sr Thermoelectric generators Solid, oxide
powder
28.8 yr

β

radiation

241

Am Well logging, thickness, moisture
and conveyor gauges
Solid, oxide
powder
433 yr

α

radiation


long periods. While the use of either an RDD or RED is considered the most
plausible terrorist act, the consensus is that such acts would generally result in a
small number of immediate deaths [9]. The benefit to terrorists using such a
device is the disruption it is likely to cause. A device used in a city is likely to
result in hysteria from the public, based on the fear that they may have been
exposed to radiation [1]. In the case of an RDD, the consequent contamination
from such a device would take considerable time to clean up, resulting in long-
term evacuation of the area, which is likely to have significant economic and
social impacts [1,9].
Protecting and accounting for both nuclear and other radioactive materials is
a major concern of the international community and there have been significant
efforts to modernize physical protection and accounting systems throughout the
world [9]. Individual states and international organizations have also been pro-
viding both technical and financial support to less wealthy nations. One example
is the recent commitment of the G8 group of nations to provide US$20 billion
over 10 years to help former Soviet Union states manage and secure their radio-
active materials [6]. However, the problem of securing radioactive materials is a
worldwide dilemma. In the past, many security measures applied to nonfissionable
radioactive sources aimed to prevent accidental access or petty theft of the sources
[10]. Any thought of terrorists using radionuclides as weapons were not persuasive
enough to enforce a move to more regulated systems. While many states are now
acting to address this issue, there still exist many thousands of unaccounted for
sources worldwide. These sources are termed “orphaned sources,” an expression
used by the IAEA to denote radioactive sources that are outside official regulatory
control, which may have been lost, discarded, or stolen [4]. Therefore orphaned
sources represent potential weapons for terrorists.

10.1.5 T

HE

Illicit Trafficking of Nuclear and Other Radioactive Materials

339

trafficking of radioactive materials may be occurring undetected [12]. These
figures illustrate the need for comprehensive programs worldwide to both secure
existing sources and to recover orphaned sources.

10.1.6 T

HE

R

OLE
OF

S

CIENTIFIC

P

RACTITIONERS

The need to develop strategic programs aimed at preventing, recovering, and
responding to terrorist acts involving nuclear and other radiological materials

Illicitly Trafficked Material
Confirmed Incidents
(1993 to 2003)

Nuclear material 182
Other radioactive material 300
Nuclear and other radioactive material 23
Radioactively contaminated material 30
Other 5

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340

Radionuclide Concentrations in Food and the Environment

and scientists to become more conscious of the challenges that each of these
agencies face. Another requirement is the need for distributing information about
new detector technologies to user groups in a form that is easily interpreted and
not overwhelmingly technical.
The global need for solutions in this area also necessitates the sharing of any
new capabilities or techniques between states. To this end, the IAEA has supported
international technical initiatives to explore suitable technologies and novel solu-
tions for user groups. (The relevant programs include the coordinated research
project (CRP) “Improvement of Technical Measures to Detect and Respond to
Illicit Trafficking of Nuclear and Other Radioactive Materials” and the “Illicit
Trafficking Radiation Detection Assessment Program” (ITRAP), and the Inter-
national Technical Working Group’s (ITWG) Nuclear Forensic Laboratories
(INFL) program. These programs have enabled cooperative research to be under-
TO

B

ORDER

M

ONITORING

Of the thousands of radionuclides associated with NORMs or anthropogenic
production, only a few are liable to be encountered by border monitoring staff.
These materials include isotopes produced in significant quantities for designated
applications in medicine or industry, and NORMs that are prevalent in many
substances commonly traded (e.g., ceramics, stoneware, and fertilizers).
The radionuclides of greatest interest, as determined by various agencies
associated with the IAEA [13], are listed below:
• Medical radionuclides:

18

F,

32

P,

51


131

I,

133

Xe,

153

Sm,

198

Au, and

201

Tl.
• Industrial or scientific radionuclides:

22

Na,

57

Co,


Am.
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Illicit Trafficking of Nuclear and Other Radioactive Materials 341
• NORMs:
40
K,
226
Ra,
232
Th, and
238
U.
• Nuclear materials:
233
U,
235
U,
237
Np, and
239
Pu.
Neutron sources based on mixed radionuclides may also be encountered due to
the widespread use of neutron emitters in mining and other underground gauging
applications. These include mixed radionuclide neutron sources like
241
AmBe and
238
PuBe, in which the
241

ments, including
• The inspection system should not unnecessarily impede or disrupt the
flow of general traffic.
• The analysis is performed in real time in order to enable customs and
law enforcement officers to act rapidly.
• There should be the ability to deal with the wide variety of traffic that
may pass through the corridor.
In establishing a system that satisfies this somewhat competing set of criteria,
a “staged” or “layered” approach is used. The first stage will typically employ a
system for the gross detection of radioactive material. This would consist of an
autonomously operated radiation detector system fixed in position along the
transport corridor. All traffic passes by the fixed monitor, thereby allowing non-
invasive testing for radioactive material. The detector would monitor changes in
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342 Radionuclide Concentrations in Food and the Environment
the ambient radiation level from that attributed to the local background. The
system would alarm when levels exceeded a set threshold. An alarm would then
initiate a personnel response that would confirm the alarm state. If confirmed,
stage two would commence.
The second stage of the detection strategy is to assess the radiological hazards
and potential health risks to which an operator may be exposed and then locate
the radioactive material. The assessment of radiological hazards is discussed
extensively in a number of publications such as those prepared by the International
Commission on Radiological Protection (ICRP) [16], the International Commis-
sion on Radiological Units and Measurements (ICRU) [17], and the U.S based
National Commission on Radiation Protection (NCRP) [18]. Locating the radio-
active materials is the domain of handheld instrumentation. By scanning the
instrument over the vehicle, cargo, or person, the location of the radiation can be
determined. Once this has been performed, stage three would commence.

Illicit Trafficking of Nuclear and Other Radioactive Materials 343
Detection efficiency relates to the relative sensitivity of the instrument to
respond to a particular intensity of radiation. This is an inherent property of the
detector that depends on the γ-ray absorbing properties of the material from which
the detector is manufactured, as well as the size or volume of the active region
of the detector.
The ability of the detector to absorb γ rays per unit volume increases with
the atomic number of the detector material’s constituent atoms and the density
of the resultant detector material. Solid-state materials such as sodium iodide (NaI)
are associated with high detection efficiencies due to the relatively high average
atomic numbers of sodium and iodine, that is, 32 (Z
Na
= 11, Z
I
= 53), and the fact
that the material is a dense solid. A lower detection efficiency can be expected in
other solid-state detectors, such as those based on silicon, due to the lower atomic
number of silicon by comparison (Z
Si
= 14). All gaseous-type detectors suffer from
low detection efficiency due to the low density of atoms in the gaseous state.
By increasing the volume of the detector, the overall ability to absorb radiation
and thereby contribute to a measurable signal will increase. This can be achieved
through the use of large-volume detectors or through the use of a bank of multiple
detectors operated in parallel. In addition to the inherent properties of a particular
type of detector, the overall sensitivity of the system will also be subject to the
operational conditions imposed by the monitoring facility. This will include the
distance of separation between the radioactive material and the detector, the length
of time of the measurement, and, in the case of a moving item, the speed of the
item relative to the detector.

on the compound semiconductors cadmium telluride (CdTe) and cadmium zinc
telluride (CdZnTe). Despite the relatively high average atomic number of the
constituent atoms and the existence of the material in a solid state, such detectors
suffer from relatively low detection efficiency due to technological limitations in
the production of detector crystals with volumes greater than 1 cm
3
.
The mainstay of handheld room temperature operable γ-ray spectroscopy
instruments is the thallium doped sodium iodide scintillation detector, NaI(Tl).
Its low energy resolution makes this detector unsuitable for many of the demands
of border monitoring. However, subject to the limitations of the other detector
types discussed above, it is often the detector of choice for γ-ray spectroscopy
in a border monitoring context.
Research efforts directed toward the realization of new detector materials
with energy resolutions superior to that of NaI(Tl), but with similar operating
characteristics, are currently under way. Some successes have been achieved in
lanthanum halide-based scintillator materials such as LaCl
3
and LaBr
3
[20,21].
10.2.3.2 Stage One: Fixed Portal Monitors
Fixed portal monitors are designed to screen vehicles, cargo, or people for the
presence of radioactive material as part of the first stage of any detection strategy.
This type of system will typically consist of an array of detectors located in close
proximity to the passing traffic, usually within vertical pillars or just below the
transport surface. To maximize the ability of the system to detect the presence
of radioactive materials, the detectors employed must have high detection effi-
ciency. Energy resolution is not important because this stage of detection is
concerned simply with detecting the presence of radioactive material.

radiation is required in order to identify the presence of a large radiological
hazard. The response readout of handheld survey instruments is usually in the
form of either an analog or digital display. Modern dose rate meters are also
equipped with alarms, which can be set at a predetermined dose rate. This alarm
is typically set to advise the user of unsafe radiation levels.
10.2.3.4 Stage Three: Isotopic Analysis
The existence of a unique signature for each radionuclide that emits γ-rays allows
the measurement of this signature to be used as a means of assaying the isotopic
composition of any detected radioactive material. The working basis of all γ-ray
spectrometers is collection of the energy deposited by a single γ-ray photon in
the active volume of the detector and the conversion of this energy into a voltage
pulse, the amplitude of which is proportional to the initial energy of the γ ray.
By sorting the voltage pulses according to the amplitude, a spectrum of different
γ-ray energy intensities can be displayed. Comparison of the spectrum against
reference libraries enables identification of the radionuclides.
A variety of different types of γ-ray detectors are available. This includes
those based on scintillator materials, such as thallium-activated sodium iodide,
as discussed previously, or those based on semiconductor materials such as HPGe
and CdZnTe. Packaging of these detectors into handheld analyzers, complete with
the ability to display the isotopic composition of an interrogated item in real time,
is highly desired by border control officers. Most handheld analyzers are based
on either NaI or CdZnTe.
Instruments with high resolution, typical of HPGe, are most ideally suited to
this application. The Stirling engine-cooled HPGe detectors, which have recently
become available, offer the desired high energy resolution at the expense of being
heavy and somewhat bulky in comparison to the other isotopic analyzers.
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346 Radionuclide Concentrations in Food and the Environment
10.2.4 MASKING OF ILLICIT MATERIALS

a reliable indicator of the presence of nuclear materials. Neutrons are detected
through the measurement of ionizing nuclear reaction products, such as protons
and α particles, produced when the neutron interacts with agents such as boron
trifluoride (BF
3
) gas or
3
He gas. Neutron coincidence counting is the technique
generally used in the nondestructive assay of bulk quantities of nuclear materials,
and in particular of plutonium.
10.2.5.2 Radiation Pagers
Pager and pocket γ and γ/neutron monitors are small, lightweight radiation detec-
tors that can be worn on the person of customs officers or border monitoring
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Illicit Trafficking of Nuclear and Other Radioactive Materials 347
staff. The units constantly measure the local background radiation level surround-
ing the instrument and can provide an alarm state when the radiation level exceeds
a set threshold. Some units may also include nonvolatile memory, which allows
storage of a history of the unit’s operation. Access to these data are usually
obtained by downloading to a computer.
The detection efficiency of these instruments is generally much less than that
of large-volume portal monitors. For this reason, such instruments are not consid-
ered as a replacement for the fixed portal monitors described above in “Stage One”
of the detection strategy. However, the ability to equip roaming officers with such
instruments may provide some additional benefit to border control situations
where narrow corridors with fixed portal monitors cannot be established.
In summary, the place of pagers or pocket monitors within the overall frame-
work of detection of illicitly trafficked nuclear or other radioactive materials is
dependent on individual user agency operational strategies. In addition to the

O), the geographical origin of a sample can sometimes be estab-
lished. This method has been demonstrated in principle for uranium oxides [23].
While the information obtained by the use of these techniques is indeed
valuable, it alone does not yield information on the origin of the material. The
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348 Radionuclide Concentrations in Food and the Environment
process of attribution requires not only experimental data, but also information
specific to individual manufacturing facilities. Therefore, the processing methods
used will contribute to the creation of products with unique chemical and physical
signatures that may assist the nuclear forensic scientist [24]. The rigorous char-
acterization that is required to certify radioactive materials during processing
means that extensive datasets exist within each facility. Recording this informa-
tion within specific forensic databases is a strategy being pursued by organizations
such as the IAEA and the Institute of Transuranium Elements (ITU). Other means
of identifying radioactive material is via the use of intelligence or regulatory
databases. Examples of these include the IAEA’s Illicit Trafficking Database
(ITDB) [11] and various registers maintained by individual national regulatory
authorities. Information from these sources, particularly that relating to missing
or stolen radioactive materials, may provide useful information for the purposes
of attribution.
10.3.2 AT THE SCENE
10.3.2.1 The Investigation Team
The presence of radiological material at a scene dictates the desire for a number
of specialists to be available during the investigation. The skill set required
includes health physicists to examine the scene for health and safety hazards,
bomb disposal experts to check and disable trigger devices, law enforcement
forensic experts to control sample collection and advise on sample handling,
storage, and transport, and nuclear forensic specialists to advise on radiological
evidence.

similar to those used for traditional forensics [2]. The use of a grid system,
photographic documentation, and precise drawings or mapping with referenced
coordinates (e.g., the global positioning system [GPS]) showing the location of
any evidence (radioactive or traditional) is essential. It is advisable that a radio-
logical survey, referenced to a grid system or map is used to determine the extent
of the contamination and the establishment of cordon and control areas [14]. The
use of radiation detection equipment, as described earlier, is vital for locating
and identifying the radioactive isotopes.
The objective of the crime scene expert is to collect evidence for analysis
that may provide clues to the origin of the material. Hence the radioactive material
and related samples, such as source containers with unique identifiers, are both
potential indicators as to the origin of radioactive material. If the radioactive
sample is contained within a vessel or lead-shielded container, the job of sample
collection is simplified. For this scenario, the material should be collected and
secured with care so as not to destroy any potential traditional forensic evidence
(e.g., fingerprints, fibers, biological evidence, etc.). In situations where the radio-
active material is widely dispersed, the task of collection is more difficult. If
possible, it is recommended that radioactive samples be extricated from nonra-
dioactive material (i.e., contaminants). However, this may not always be easily
achieved [28]. The handling of traditional evidence from this scenario is also
problematic due to the presence of radioactive material. In all cases, the collection
of either radioactive or traditional samples should be conducted, if possible, in a
manner that will not destroy or modify the other.
International organizations, such as the IAEA and the ITWG’s INFL program,
are working to provide advice and support to the international community on
issues pertaining to the collection of radiological samples. Details can be found
in a number of publications [29,30]. Moreover, both the IAEA and INFL are
currently fine tuning procedures for the sampling of seized materials, the shipping
of samples, and the analysis and evaluation of experimental results.
10.3.2.3 Sample Storage and Transportation

The role of the nuclear forensic scientist is to apply a number of appropriate
techniques to elucidate information that may provide clues about the material.
As with traditional forensics, there exist no steadfast procedures. The techniques
used depend on the physical and chemical properties of the samples. Figure 10.1
details, in flow chart form, the main stages of identification of materials of
unknown origin, which is based on procedures outlined in [34].
A number of publications have discussed at length the expertise and facilities
a laboratory must possess in order to conduct nuclear forensics [2,19,28]. Of
particular importance are mass spectrometry measurements such as inductively
coupled plasma mass spectrometry (ICP-MS), secondary ion mass spectrometry
(SIMS), accelerator mass spectrometry (AMS), and thermal ionization mass
spectrometry (TIMS), because these techniques can provide isotopic information
[7,35–37]. Radiometric techniques such as high-resolution γ spectroscopy
(HRGS) also play an important role and are invaluable in identifying radioactive
materials and the corresponding isotopes.
Ultimately, nondestructive analyses are preferred to those that modify or
destroy potential evidence. Moreover, analysis techniques may be precluded from
use if the quantity of sample required for the analysis would result in the destruc-
tion of a high proportion of a sample. Therefore, law enforcement officers must
decide whether the benefits of a technique are worth the loss or partial loss of a
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Illicit Trafficking of Nuclear and Other Radioactive Materials 351
sample. National laws that govern the use and preservation of evidence will need
to be considered prior to any analysis being undertaken.
A number of useful characterization techniques are listed in Table 10.4 and
classified with respect to the capability of providing imaging, bulk, and microan-
alytical data. A comprehensive list of techniques and their capabilities is outlined
in Table 10.5. Radiation detection will not be revisited in this section, as most
of the relevant issues have already been discussed. Guidance in the selection and

about a sample such as homogeneity or the presence of microscopic impurities.
It follows that documenting visual characteristics is one method of sample iden-
tification and this can be done by the use of photographic equipment.
TABLE 10.4
A List of Useful Instrumentation Likely to Be Used in a Nuclear
Forensic Laboratory
Analysis Type Measurements Suitable Techniques
Imaging Photography Conventional or digital photography
Radiography X-ray imaging
Microscopy Optical and electron microscopy
Bulk analysis Macroscopic characterization* See note
Elemental analysis TIMS, XRF, ICP-MS, PIXE
Trace elemental analysis
(inorganic)
ICP-MS, AMS, NAA, PIXE
Organic analysis GC-MS, CHNS analyzer, vibrational
spectroscopy, NMR
Isotopic analysis HRGS, ICP-MS, SIMS, AMS
Crystal structure XRD and ED
Bulk environmental sampling AMS, TIMS, ICP-MS, α, β, γ spectrometry
Particle environmental sampling SIMS, fission track TIMS, SEM, EDX/WDX
Microanalysis
(or particle
analysis)
Electron microscopy SEM, TEM
Elemental analysis WDX, EDX, SIMS
Particle analysis SIMS, TIMS, fission track, SEM, EDX/WDX
* Macroscopic characterization includes weight, density, radiography, viscosity, surface area, particle
size, and surface roughness measurements.
Note: AMS, accelerator mass spectrometry; CHNS, carbon-hydrogen-nitrogen-sulfur; ED, electron

powder samples. This information can be indicative of the
manufacturing process.
Chemical GC-MS
Infrared (IR) spectroscopy
NMR
MDL ~ parts per million
~5–15 µm spatial resolution
MDL ~ parts per million
Identification of trace organic constituents, structure and association
of chemical and molecular species (e.g., uranium oxide can be
found in many different forms — UO
2
, U
3
O
8
, or UO
3
).
Elemental Mass spectrometry:
SIMS
TIMS
ICP
Glow discharge (GD-MS)
EDX
XRF
MDL 0.1 ppb–10 ppm: 0.2–1 µm spatial
resolution
MDL ~ picogram to nanogram
MDL ~ picogram to nanogram

MDL ~ picogram to nanogram
MDL ~ picogram to nanogram
Fission or neutron-capture products: indisputable evidence that the
material has been in a nuclear reactor (e.g.,
236
U or
129
I are
indicative of nuclear processes).
Decay (daughter) products act as fingerprints for the type and
operating conditions of a given reactor and are used to determine
the age after enrichment/purification from the “parent” isotopes
in the material.
Note: EDX, energy dispersive x-ray; GC-MS, gas chromatography mass spectrometry; ICP
, inductively coupled plasma; NMR, nuclear magnetic resonance; SEM,
scanning electron microscopy; SIMS, secondary ion mass spectrometry; TEM, transmission electron microscopy;
TIMS, thermal ionization mass spectrometry; XRD,
x-ray diffraction; XRF, x-ray fluorescence.
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Illicit Trafficking of Nuclear and Other Radioactive Materials 355
Another useful imaging technique is electron microscopy. Scanning electron
microscopy (SEM) can provide visual information at a spatial resolution on the
order of 1 nm [38]. The technique is capable of identifying crystal morphologies
and therefore the presence of multiple phases [39]. It is also useful for studying
the distribution of particle size above the spatial resolution limit and can yield
qualitative information on sample porosity [40]. Topographical information is
obtained by the use of secondary electrons, while backscattered electrons yield
information related to the average atomic number of the area being imaged. The
latter is observed as contrast variations within the image.

converted to an aerosol and transported into a plasma, which results in a unique
vaporization, atomization, excitation, and ionization source for atomic emission
and mass spectrometry [44]. In ICP-AES, the radiation emitted by the analyte is
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356 Radionuclide Concentrations in Food and the Environment
measured at characteristic wavelengths and this signal is used to identify and
quantify the elements present. In ICP-MS, the tail of the plasma is extracted into
a low-pressure interface and the ions focused and transmitted to a mass analyzer
[45]. For ICP-AFS, a primary excitation source, such as a laser or cathode lamp,
is used to excite atomic fluorescence from atomic and ionic analyte species [44].
The success of ICP analysis techniques is primarily due to the low detection
limits (ppm to ppb depending on the element), the large number of elements that
can be analyzed, the precision of the techniques (relative standard deviation of
0.2% to 3%), and the wide dynamic concentration ranges (4 to 11 orders of
magnitude) for many elements [44]. However, ICP-MS is generally considered
the most powerful method of the three due to its superior detection limits and
the ability of this technique to provide isotopic information.
Inductively coupled plasma methods have benefited from the development of
a number of methods used to introduce a sample into the plasma. These methods
include separation techniques such as gas chromatography (GC) [46], liquid
chromatography (LC), capillary electrophoresis (CE) [47], ion exchange chro-
matography (IEC) [48], and ablation techniques for the direct analysis of solids,
such as laser [49], arc, and spark ablation [48]. The technique has also benefited
by the ongoing development of mass spectrometers. The development of multi-
collector ICP-MS (MC-ICP-MS) enables isotopes of interest, within the limits
set by the mass analyzer, to be analyzed simultaneously rather than sequentially.
The use of a time-of-flight analyzer (ICP-TOF-MS) enables all isotopes to be
analyzed simultaneously. Both MC-ICP-MS and ICP-TOF-MS are proving to
be valuable techniques for situations where a limited amount of sample is avail-

erally classified as either energy dispersive x-ray fluorescence (ED-XRF) or
wavelength dispersive x-ray fluorescence (WD-XRF). An energy dispersive
instrument utilizes an energy analyzing detector upon which all the resultant
x-rays are focused. A wavelength dispersive instrument uses a diffraction crystal
to focus x-rays of specific wavelength upon a detector. By rotating the crystal,
the wavelength range is scanned. While ED-XRF systems are faster and less
expensive, WD-XRF is more sensitive and has higher resolution [60].
Commercially available XRF systems typically employ x-ray tubes as the
excitation source; however, fixed radioactive sources can also be used, particularly
in portable devices. Characteristic x-rays can be measured using a variety of
detectors such as NaI, lithium doped silicon detectors, and Peltier-cooled PIN
detectors [61]. A PIN detector works in the same way as a p-n junction, but it
has an intrinsic layer between the p-n junctions, hence the term PIN. For portable
instruments solid-state room temperature detectors such as CdZnTe and mercuric
iodide are typically used [60,61]. Recent events such as the development of micro-
XRF, which enables small particles or fibers to be examined, have enabled the
technique to be expanded to analyzing trace quantities of samples.
X-ray fluorescence is used widely in traditional forensic analysis to analyze
samples such as glass from automobile headlights [62], fibers [63], environmental
forensics [64], coins [65], and printer toner [66]. It follows that similar applica-
tions will apply to the analysis of radioactive samples, however, few reports exist
in the literature at the present time.
The PIXE technique is analogous to XRF except high-energy particles are
used as a primary source. Sample irradiation is usually achieved by the use of
protons with energies between 2 and 4 MeV. An accelerator is typically used to
produce such particles. The use of protons means that the technique probes only
the top 10 to 60 µm of a sample, depending on the energy of the incident beam
and the energy of characteristic x-rays. Hence it is important that the analyzed
region is representative of the whole sample [67]. The detection of the resultant
x-rays is usually performed using energy dispersive semiconductor detectors such