Seminar

Detection of plastic explosives

Author: Ga²per Renko

Mentor: prof. dr. Samo Korpar

Ljubljana, 2009

ABSTRACT

This seminar explores the role of radiation methods in adressing the problemof detection plastic explosives. X-ray backscatter, gamma ray Compton eectand neutron in, gamma out techniques for explosive detection are described,with also detector examples for each technique. Other methods of detection arealso mentioned.

Contents

1 Introduction 1

2 Plastic explosives 2

2.1 History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2 Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.2.1 RDX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22.2.2 PENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.3 Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3.1 Land mines . . . . . . . . . . . . . . . . . . . . . . . . . . 42.3.2 Terrorism . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Detection 5

3.1 X-ray backscatter technique . . . . . . . . . . . . . . . . . . . . . 53.1.1 Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63.1.2 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.2 Gamma ray Compton eect technique . . . . . . . . . . . . . . . 83.2.1 Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83.2.2 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.3 "Neutron in, gamma out technique . . . . . . . . . . . . . . . . 103.3.1 Physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.3.2 Detector . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.4 Other detection techniques . . . . . . . . . . . . . . . . . . . . . 113.4.1 Manual detection with a metal detector . . . . . . . . . . 113.4.2 Dogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.4.3 Rats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.4.4 Honey bees . . . . . . . . . . . . . . . . . . . . . . . . . . 113.4.5 Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.4.6 Acoustic detection . . . . . . . . . . . . . . . . . . . . . . 12

4 Conclusion 12

1 Introduction

Plastic explosive is a specialized form of explosive material. It is a soft and handmoldable solid material. Plastic explosives are properly known as putty explo-sives or putties. Common plastic explosives include Semtex and C-4. Plasticexplosives are especially suited for explosive demolition as they can be easilyformed into the best shapes for cutting structural members and have a highenough velocity of detonation (velocity at which the shock wave front travelsthrough a detonated explosive) and density for metal cutting work. They aregenerally not used for ordinary blasting as they tend to be signicantly moreexpensive than other materials that perform just as well in that eld. Also,when an explosive is combined with a plasticizer, its power is generally lowerthan when it is pure.

Plastic explosive cannot explode without a detonator, so they are safe tohandle and store.

2 Plastic explosives

2.1 History

The rst plastic explosive was gelignite (known also as blasting gelatin and jelly),invented by Alfred Nobel (who had earlier invented Nobel's Blasting Powderalso known as dynamite) in 1875. Its composition makes it easily moldable andsafe to handle without protection, as long as it is not near anything capableof detonating it. One of the cheapest explosives, it burns slowly and cannotexplode without a detonator. Due to its widespread civilian use in quarries andmining, it has historically been often used by revolutionaries, insurgents, andguerrillas such as Irish Republican Army.

2.2 Composition

Plastic explosives are a thick, exible, moldable solid material that can beshaped and will retain that shape after forming, much like clay. Putties nor-mally contain mostly RDX explosive, but may include some PETN (Semtex, forexample).

2.2.1 RDX

RDX, an initialism for Research Department Explosive, is an explosive ni-troamine widely used in military and industrial applications. RDX is alsoknown as cyclonite, hexogen and T4. Its chemical name is cyclotrimethylen-etrinitramine and chemical formula C3H6N6O6, IUPAC nomenclature is 1,3,5-Trinitroperhydro-1,3,5-triazine (gure 1 and 2).

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Figure 1: Sheme of RDX. [1]Figure 2: Sheme of RDX, atomsin red are oxygen, black car-bon, blue nitrogenium and whitehydrogenium.[1].

In its pure, synthesized state RDX is a white, crystalline solid. As an ex-plosive, it is usually used in mixtures with other explosives and plasticizers,phlegmatizers or desensitizers. It is stable in storage and is considered oneof the most powerful and brisant of the military high explosives. RDX formsthe base for a number of common military explosives, such as Composition A,Composition B, Composition C (C-4 is in this group), Composition CH6, DBX,Semtex. The velocity of detonation of pure RDX at a density of 1.76 g/cm³ is8750 m/s. It has explosion energy 6432 kJ/kg (equal to 1.5 kg TNT).

2.2.2 PENT

Also known as PENTA, TEN, corpent, nitropenta, with chemical name pen-taerythritol tetranitrate, IUPAC nomenclature [3-Nitrooxy-2,2-bis(nitrooxymethyl)propyl]nitrate (gure 3 and 4).

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Figure 3: Sheme of PENT.[1]. Figure 4: Sheme of PENT, atomsin red are oxygen, black car-bon, blue nitrogenium and whitehydrogenium.[1].

As a mixture with RDX and other minor additives, it forms another plasticexplosive called Semtex as well. Detonation velocity is 8350 m/s at denisty of1.73 g/cm3. It has explosion energy 5810 kJ/kg (1.24 kg TNT).

2.3 Use

Plastic explosive is commonly used for the demolition of obstacles and forti-cations. It is also commercially used for shock hardening (process used tostrengthen metals, wherein a shock wave produces atomic-scale defects in thematerial's crystalline structure, making materials stier, but more brittle). Twoways of usage which demend precise detection techniques are use in land mines[2] and use in terrorism [3].

2.3.1 Land mines

A land mine is usually a weight-triggered explosive device which is intendedto damage a target, either human or vehicle, by means of a blast or fragmentimpact. They can remain dangerous many years after a conict has ended,harming the economy and citizens of many developing nations. Land minescontinue to kill nearly 20,000 people every year, even decades after the warended and injure a lot more people and animals.

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Figure 5: Land mine examples [5]. Figure 6: Anti tank land mine com-position [4].

2.3.2 Terrorism

One of the preferred weapons of the modern (and historical) terrorist is thebomb. It allowes the terrorist to strike at considerable distance with great de-structive eect. There are few incidents, where plastic explosives (mostly Sem-tex) mixed with larger quantity of poorer grade explosives were used. Manch-ester bombing of 15th June 1996 used by IRA injured more than 200 peoplewhen it detonated and was the largest bomb to go o in mainland Britainsince World War 2. On 23rd October 1983 a truck laden with compressed gascylinders and explosives was driven into the headquarters of the First Battalion8th US marines, it was to be one of the largest conventional bomb explosionsin history. The resulting detonation killed 241 marines and other US militarysta and collapsed the 7-storey building. Bombs do not have to be large to beeective, the bomb that destroyed the Pan Am ight over Lockerbie on 21st De-cember 1988 was probably made up of a block of Semtex the size of a 200 gramblock of butter. The resulting explosive decompression destroyed the airlinerand killed 270 people [3].

3 Detection

3.1 X-ray backscatter technique

This technique is used for land mine detection and has been developed basedon the physics of X-ray backscatetering. The main requirement for a land minedetection system is that it utilizes the dierent backscattering X-ray character-istics of materials with dierent densities.

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3.1.1 Physics

When X-rays pass through matter they will be attenuated, i.e. absorbed orscattered [6]. For the purpose of this work, X-ray scattering is the eect ofinterest. Although scattering can be the strongest of all attenuation processesat typical energies of interest, signals from a single detector channel are weak.This is due to the limited opening of a detector channel and to absorptionprocesses in the test object. The greatest losses occur on the way back from thescatter volume to the detector because due to the Compton eect the scatteredphotons have a lower energy than the incoming photons. X-ray backscattertechnologies oer the following advantages :

. the scatter signal is directly proportional to the density of the material inthe irradiated volume

. it requires only single-sided access and

. high image contrasts are achievable.In contrast to other X-ray technologies such as radiography an XBT image

normally is obtained pixel by pixel. A scanning mechanism for a pencil X-raybeam is used to generate images which consist of a series of parallel layers. Veryoften, a single layer image may be sucient to identify the details of interest.

3.1.2 Detector

To demonstrate the capabilities of X-ray backscatter technologies, mobile pro-totype scanner has been developed (gure 7). For this purpose, a scanner headwas mounted on a trailer. A high ux 450 kV X-ray tube and a multi-channelX-ray detector system are the major components. The prototype scanner Com-Scan450 has been in use on military test sites several times. Tests were madeat varying soil conditions such as humus, sand, gravel etc. Tests were also madewith varying conditions of vegetation and of wetness. In the following, a limitednumber of examples clearly show the potential of the new technology.

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Figure 7: Sheme of prototype scanner ComScan450 [9].

Some examples of photos and images made by ComScan450 are on gures8, 9, 10 and 11.

Figure 8: Photography of an anti-personal mine [9].

Figure 9: X-ray backscatter imageof an anti personal mine[9].

Figure 10: Photography of an antitank mine [9].

Figure 11: X-ray backscatter imageof an anti tank mine [9].

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3.2 Gamma ray Compton eect technique

3.2.1 Physics

In relativistic quantum mechanics, the scattering of x-rays by a free electron isgiven by the Klein-Nishina formula (formula ([eq:1])). If we assume unpolarizedx-rays and unaligned electrons, this formula can be approximated as follows

dσKN

dΩ∼=

r2e(1 + cos2θ)

2 (1 + k (1− cosθ))2(1)

where k is angular wavenumber of photon and re is the classical radius ofthe electron. The total cross section is approximately

σKN∼= 8πr2e

(1 + 2k + 1.2k2

)3 (1 + 2k)

2 (2)

Note that for very low energies, we recover the Thomson cross section. Thereal dierence comes when we deal with atoms. In that case, if the scatter-ing leaves the atom in the ground state, we deal with coherent scattering (seeabove), whereas if the electron is ejected from the atoms, the scattering is (inco-herent) Compton scattering. At high energies, the total Compton cross sectionapproaches ZσKN , where Z is atomic number of atom.

The method uses directional information from a point-like positron sourceplaced in front of an object to be examined as schematically shown in gure[7]. A positron detector behind the source measures 511keV rays from positronannihilation to determine the direction of the correlated 511 keV gamma rayafter Compton scattering o the object in time coincidence to the positiondetermining gamma ray enables to map the matter distribution in the object.Gamma rays emitted by the source illuminate the object of inspection throughthis center hole. If a gamma ray is detected at a certain position of the positiondetector its 180counterpart may be scattered o an object in front of the set-up. For a given range of scattering angles this scattered gamma ray may bedetected in the backscatter detector and generates a coincidence trigger. Animage of the matter distribution in the eld of view - dened by the activearea of the position detector and the source to matter distance - is obtained byincrementing an image data array in the data acquisition computer.

3.2.2 Detector

The prototype device incorporates a 22Na source of 10 MBq activity in a mas-sive tungsten alloy shielding. The position detector has nine position sensitivesolid state photomultipliers. They cover a solid angle of about 5% of 4π. Thebackscatter detector contains eight semiconductor NaI(Tl) detector elements.

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Figure 12: The principle of gamma backscatter imaging employing positronannihilation radiation [7].

Figure 13: Image of a dummy landmine buried in forest soil with a metal key[7].

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3.3 "Neutron in, gamma out technique

3.3.1 Physics

All gamma rays that originate from inelastic scattering of fast neutrons onnuclei of elements, which comprise the suspected explosive material, arrive intothe detector within a well dened time interval, which is determined by thetime of ight of promp gamma ray from the reaction point to the detector. Allgamma rays that do not t into this time interval should be rejected, since theyoriginate from dierent sources.

Thus, for each gamma ray that arrives into the detector two parametersshould me measured: its energy and time of ight relative to the moment ofemission of the associated particle.

3.3.2 Detector

Neutron source Timed isotopic 252Cf neutron source and a portable neu-tron generator with build in detector of associated particles were developed.

252Cf decays with α decay and by spontaneous ssion in the manner shownon gure 14. The main diculty for detection of small objects is connected withits lack of position sensitivity.

Figure 14: Decay of 252Cf.

Second method is portable neutron generator with built in detector of asso-ciated particles. Neutrons are emitted in the D-T (deuterium-tritium) reaction

D + T→ α+ n0.

Neutron is emitted at about 180with respect to α particle. This neutronsproduce characteristic gamma rays in the surrounding materials. Only those

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gamma rays are accepted by the data acquisition system, that coincide withina narrow (few nanoseconds) time window with α particle. If the correspondingαparticle misses detector, or if a γparticle arrives in the detector too late or tooearly, such γ rays are rejected.

Detector Detector for γ rays is NaI(Tl) scintillation detector, which usescrystals that emit light when γ rays interact with the atoms in the crystals.Detector is connected to photomultiplier that convert and amplify the signal.

3.4 Other detection techniques

3.4.1 Manual detection with a metal detector

Explosive devices can be detected with metal detector, if device contains enoughmetal [10]. Some explosive device contains as little as 1g metal, so false alarmsare often. This technique is mainly used to detect land mines.

3.4.2 Dogs

Dog require extensive training, together with his handler [11]. This is usuallydone at a combined dog breeding and training center. Training is a game, wheredog gets a ball, if explosive is discovered. It takes from one and a half to twoyears to train a dog and to establish connection with a handler. The dogs willneed supplementary (maintenance) training all the time, particularly if they areto work with devices containing dierent kinds of explosives. If the handler isnot well, the dog cannot work. After some time, usually between 1 and 2 hours,the dog will be bored and will need some time to recover his interest in thegame. And also dogs can be distracted with other smells.

3.4.3 Rats

Like dogs, Giant pouched rats are being trained to sni out chemicals like TNT.[12]. These rats are currently working in mineelds in Mozambique and aretrained in Tanzania by APOPO. The rats are called HeroRATS. These animalsalso have the advantage of being far lower mass than the typical human. Theyare less likely to set o small mines intended to injure or kill people, if thebomb-sning animal crosses directly over the top of a buried mine.

3.4.4 Honey bees

Insects in the order Hymenoptera can be trained to perform a variety of tasks[13]. The olfactory senses of bees and wasps in particular have been shown torival the abilities of snier dogs. Snier bees and snier wasps have been trainedto detect substances such as explosive materials or illegal drugs, as well as somehuman and plant diseases.

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3.4.5 Bacteria

A bacterium, known as a bioreporter, has been genetically engineered to uo-resce under ultraviolet light in the presence of TNT [14].

3.4.6 Acoustic detection

It is possible to detect land mines by directing sound waves at the area to bedemined, which causes the land mines to vibrate, and then using a laser to searchfor vibrations on the surface by means of the Doppler shift - this technique istermed Scanning Laser Doppler Vibrometry [15].

4 Conclusion

There is a urge for development of fast and reliable system for detection ofplastic explosives. This is because of terrorism and for faster demining of landmines. A fusion of methods is needed as a solution to this problem.

References

[1] Web site: www.wikipedia.org, 2009.

[2] Explosive content of land mines, web page:http://www.nolandmines.com/explosivesinmines.htm, 2009.

[3] Bombs use in terrorism, web page:http://www.historyofwar.org/articles/weapons_terrorbomb.html,2009.

[4] Land mine composition, web page:http://science.howstuworks.com/landmine4.htm, 2009.

[5] Land mines, web page: http://www.generasia.com, 2009.

[6] S. Yuk, K. H. Kim, Y. Yi, Detection of buried landmine with X-ray backscattering technique, Nuclear Instruments and Methodsin Physics Research A 568 (2006) 388-392.

[7] J. Gerl, Gamma-ray imaging exploiting the Compton eect, Nu-clear Physics A 752 (2005) 688c-695c.

[8] Compton scattering, web page:http://xdb.lbl.gov/Section3/Sec_3-1.html, 2009.

[9] CamScan450, web site: http://www.ndt.net/article/ecndt02/96/96.htm,2009.

[10] Metal detector, web page: http://school.mech.uwa.edu.au/~jamest/demining/info/what-is.html, 2009.

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[11] Detection explosives with dogs, Web site:http://school.mech.uwa.edu.au/~jamest/demining/k9/dogs-in-use.html, 2009.

[12] Detection explosives with rats, Web site :http://www.apopo.org/home.php, 2009.

[13] Dtection explosives with bees, Web site:http://www.independent.co.uk/news/uk/crime/snier-bees-new-ying-squad-in-war-against-terror-477173.html, 2009.

[14] Detection of explosives with bacteria, Web site:http://school.mech.uwa.edu.au/~jamest/demining/others/ornl/rsb.html,2009.

[15] "Laser Doppler Vibrometer-Based Acoustic Landmine Detec-tion Using the Fast M-Sequence Transform, N Xiang and J MSabatier, IEEE Geosci. and Remote Sens. Letts. Vol. 1 (4) 2004

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