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Articles - Explosives Simulants Review
 


Introduction

The increased use of explosives and improvised explosive devices (IEDs) by terror groups over the past decade has led to the technological upgrading of existing detection systems, as well as to the development of new ones. The growing threats, as well as the security response, compel us to continuously modify and update our training programs and improve our calibration tools and methods.
This article deals with the use of cutting edge explosives simulants as training aids and as components of calibration bags, to significantly increase explosives and IED detection rates using both manual methods and advanced technological systems.
The Essential Role of Explosives Simulants
When training manpower to carry out manual searches for explosives or to operate technological detection systems, it is undoubtedly preferable to utilize the actual item they are being trained to detect, as a training aid. However, in the case of explosives, safety considerations and logistical issues prevent us from doing so. In order to maintain a high level of training effectiveness nevertheless, it is crucial to expose the trainees to training aids that are as similar as possible to "the real thing".
In order to significantly increase the rate of detection by security agents who are required to identify explosives and IEDs using their senses of sight (general appearance, color) and touch (texture, weight per volume) it is essential to provide them with the opportunity of experiencing explosives simulants that replicate actual explosives as closely as possible, as part of their training. Similarly, it is necessary to ensure that operators of Explosives Detection Systems (EDS) and Trace Detection Systems (TDS) become familiar with the specific X-ray transmission properties of the explosives they are called upon to identify, including Z effective (Zeff) and CT#.  Anything less, such as displaying a photograph of explosive materials, showing an X-ray image or using a putty-like substance for demonstration purposes, will result in security agents who are ill-equipped to identify the threats at the crucial moment, thus potentially endangering human lives.
High-quality explosives simulants that eliminate the safety problems associated with the use of real explosives were developed precisely in order to enable optimal training efficiency. These simulants replicate characteristics such as color, texture, molecular weight, dielectric constant and others, to ensure truly effective training of security manpower, as well as precise calibration that takes into consideration key operational parameters.


Explosives Simulants and their Properties

Visual and Tactile Properties

Different types of explosives differ in their color, light reflection properties and surface texture. Some look and feel like powders and come in various colors, while others are molded or are in liquid form – with varying degrees of transparency and in different colors.

                                                        

         Smokeless powder                       Semtex H plastic explosives
                   simulant                                                simulant

X-ray Absorption Properties

X-rays are a form of electromagnetic radiation: energy in the form of photons, which is transported through space as a combination of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of the energy propagation.
Attenuation is the reduction in the intensity of an X-ray beam as it traverses matter, as a result of either the absorption or the deflection of photons.

 

 

N    Number of transmitted photons
N0   Number of incident photons
 µ   Linear attenuation coefficient
 x   Absorber thickness


CT# Property

As in medical applications, the analysis of 3D computed tomography (CT) images requires an attenuation scale for each pixel. In explosives detection, the CT# is a normalized index of X-ray attenuation based on scale of -1000 (air) to +1000 (bone), with water being 0. The scale used in the analysis of images produced by CT-based explosives scanners is adapted to the characteristics of baggage and typical packed items

Zeffective Property

The Z effective number is a valuable parameter in the analysis of the type of compound being investigated, based on X-ray interactions. Similar to an atomic number used to distinguish single elements, the Z effective number is used to identify molecules and mixtures comprising two or more elements. The following formula is used for calculation:

 


a   Electron fraction of element from total number of electron
z   Atomic number of the elements
An example of a Z effective number for a TNT molecule is presented below:

                                                                                       
Chemical formula: C7H5N3O6                              TNT Explosive

 

Seven carbon atoms (Z=6), five hydrogen atoms (Z=1), three nitrogen atoms (Z=7), six oxygen atoms (Z=8). The total number of electrons is 7*6+5*1+3*7+6*8 = 116

The Evolution of Explosives Simulants

 Explosives Simulants for Bulk Detection

Explosives simulants were initially developed when the need to train security personnel to detect explosives first arose. The introduction of the first technological explosives detection systems motivated further development, which continued and expanded in parallel to the upgrading of these technologies.
First generation simulants replicated properties important for detection by humans, such as color, texture and typical external appearance. In the absence of technological solutions, these properties served as the basis for the identification of explosives.
Subsequently, with the production of the first generation X-ray based explosives detection scanners, second generation simulants were developed to meet the specific requirements of technological detection systems. These new simulants replicated X-ray absorption properties as interpreted by the screener's eyes.
The added possibility of developing calibration tools and databases of concealed IEDS, to be used for training, testing and quality assurance purposes, further broadened the potential uses of explosive simulants.
However, the complexity involved in the development of second generation simulants, replicating actual explosives in a broader range of characteristics, presented a dilemma. While there was logic behind the use of two separate lines of simulants – one intended for manual searches and the other for technological detection, innovative security concepts integrating the human element and advanced technologies supported the use of one set of simulants meeting both needs, despite the greater challenge presented by their production.
The choice of simulants was therefore dependent on the application: where detection was based solely on X-ray images, with no human contact – texture and color were meaningless. However, in integrated security operations, all properties (first- and second generation) were necessary to increase detection rates.
The later development of dual energy X-ray scanners capable of distinguishing between organic and inorganic materials led to the deployment of a new type of automatic detection systems, requiring third generation simulants. These upgraded versions offered enhanced detection based on Z effective properties, and not on density alone. This also presented the opportunity of new applications, such as pre-certification and training on the proper implementation of integrated security procedures when using automatic explosives detection scanners.
However, the deployment of these dual energy scanners – both automatic and operator decision – presented a major obstacle: false alarms, as well as certain detection limitations. The development of multi-view CT explosives scanners effectively overcame these challenges, as the CT scanners enabled advanced analysis without interference from other layers, such as exist in one view scanners. This led to the determination of a precise CT# scale enabling to accurately distinguish between inert materials and explosives.
Further scanner upgrading necessitated the development of fourth generation simulants, providing highly selective detection windows and typical computed tomography properties for accurate CT#, while preserving all other X-ray properties. The challenge was to unify the sophisticated algorithms based on real explosives' properties and cover all the detection parameters. The new detection thresholds required enhancing the simulants' performance level in relation to previous applications, as well as new ones, above all – calibration and verification. 

 
Explosives Simulants for Trace Detection

TDS systems identify the presence of explosives by implementing a different approach. Instead of trying to analyze the IED through its internal components, residue of explosives molecules remaining on the IED's surface during its assembly or through particles migration processes are collected using a vacuum device or swabs. This residue is then analyzed by sensitive analytical equipment that investigates its chemical and physical properties.
When evaluating the effectiveness of the trace detection approach, we must consider both stages: not only the analysis, but also the sampling. Understanding the importance of each stage to detection accuracy enables to optimally train operators and validate biosensors (in case of canines and other animals) and technological sensors.


                                                              



Triacetone Triperoxide

Analysis

The first trace detectors comprised color reagents, which produce a change in color when they come into contact with explosives. These kits are suitable mainly for trace amounts that can be seen with the naked eye. Two basic types of simulants were developed for these reagents. The first involves the use of a non-explosive compound that reacts with the reagent to provide a color indication. The second type, which is also easier to produce,  involves mixing a trace amount of real explosives with an inert chemical, producing a substance that will not detonate at trace detectors' detection levels and will therefore not be categorized as a hazardous material by the authorities.
The dilution method became widespread with the deployment of a second major trace technology: the mass spectrometer (MS), which detects trace molecules according to specific parameters of molecular weight and drifting time. MS technology enabled upgrading calibration and validation using diluted explosives.
The use of diluted explosives also facilitates the training of canines to detect explosives. Since the precise way dogs sense various chemicals is complex and not fully understood, the dilution method is preferable, as it ensures the canines are indeed being trained to detect the real explosive material, focusing on its unique active elements.


Sampling

Sampling efficiency is dependent on explosives' surface adhesion properties; it is not always a simple case of sampling a powder. For example, plastic bonded explosives like C4 contain a very "sticky" binder, which binds the RDX explosives powder inside. When this explosive comes into contact with a surface, a thick layer of the binder with the RDX powder remains attached to the sampled surface. Sampling in this case is different than sampling free powder, such as TNT, which has much lower adhesive forces to the surface.
Improved sampling tools are currently under development, and are expected to be available in the near future.


Summary

This article has demonstrated the importance of selecting the most effective simulants for specific applications. To summarize, when choosing a simulant, the main questions we should ask ourselves are:

(1) Does the simulant accurately represent the defined threat? 

(2) Which type of security procedure are we implementing?

  • Manual search
  • Technological search
  • Integration of both 

(3) If a technological search: is it bulk or trace detection?

If bulk detection:

  • Does the visual appearance of the simulant closely replicate that of the
    explosive material (color, texture, feel)?
  • Is its X-ray absorption the same as that of the explosive material? This question can be answered by comparing the simulant's scanned gray scale image parameters to the explosive's.
  • Will a dual energy X-ray scanner scanning this simulant be able to distinguish between an organic and inorganic material, as it would with explosive material?
  • Will a CT scanner scanning the simulant display the real CT# of the explosive material?


If trace detection:

  • Can we use simulants based on powder only, or based on other properties, like adhesion, as well?
  • Is the explosive contamination level low enough to be safe, yet high enough to ensure detection?


(4) If the simulant is intended for K9 training, is it free of other chemicals that may interfere with the dogs' senses?
Remember! Selecting the right simulant is critical to training effectiveness and calibration accuracy, and may save lives!

 Remember! Selecting the right simulant is critical to training effectiveness and calibration accuracy, and may save lives!


About The Author
Mr. Avi Icar is the founder and CEO of A.I. Explosives Inspection & Services. He is an honors graduate of Tel Aviv University's School of Chemistry, and has served as an officer in an Explosive Ordnance Disposal (EOD) Unit of the Israel Defense Forces, and subsequently as an EOD technician and detection technology integrator at the Israel Security Agency.

 
 
 
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