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8/12/2019 Strahlenfolter Stalking - TI - People Screening Using Terahertz Technology - Aparatura.ro

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People screening using terahertz technology

Colin Baker, William R. Tribe, Thomas Lo, Bryan E. Cole, Simon Chandler, Michael C. Kemp*

TeraView Ltd, Platinum Building, St Johns Innovation Park,

Cambridge, CB4 0WS, United Kingdom

ABSTRACT

There is a need for ever more effective security screening to detect an increasing variety of threats. Many techniquesemploying different parts of the electromagnetic spectrum from radio up to X- and gamma-ray are in use. Terahertzradiation, which lies between microwave and infrared, is the last part to be exploited for want, until the last few years, of

suitable sources and detectors. Terahertz imaging and spectroscopy has been shown to have the potential to use very lowlevels of this non-ionising radiation to detect and identify objects hidden under clothing. This paper describes recentwork on the development of prototype systems using terahertz to provide new capabilities in people screening - both atsecurity checkpoints and stand-off detection for remote detection of explosives and both metallic and non-metallicweapons.

Keywords:Terahertz, security screening, THz stand-off detection, THz wand, explosives detection

1. INTRODUCTION

The terahertz (THz) region of the electromagnetic spectrum is typically considered to occupy 300 GHz to 10 THz,bridging the gap between millimeter waves and the infrared. Recent developments have started to address the previouslack of sources and sensitive detectors in this range.

1,2Consequently, the technological advances have been accompanied

by much interest in possible applications in the security,3,4pharmaceutical,5non-destructive testing (NDT),6and medical

industries.7,8

There are unique properties of THz radiation that make it a potentially powerful technique in security screening. Firstly,THz radiation penetrates many everyday physical barriers such as typical clothing and packing materials with modest

attenuation.3

Secondly, many chemical substances and explosive materials exhibit characteristic spectral responses atTHz frequencies that can be used for threat object identification.4Additionally, being at sub-millimeter wavelengths,

THz radiation is of low energy and non-ionising. THz techniques, therefore, combine safe-to-use high-resolutionimaging, and the identification through spectroscopy of threat materials even when hidden in packages or under

clothes.

THz imaging systems can be passive, simply detecting the THz part of the thermal black-body radiation given off byobjects. Alternatively, active schemes can be implemented whereby the object being imaged is illuminated by a THz

source. Whilst passive systems may be effective for 2-D low frequency (e.g. 100 GHz) millimeter wave imaging,chemical/structural analysis of suspect objects is restricted to higher frequency techniques, since there are virtually no

spectroscopic features in solids below 500 GHz. Active techniques, such as terahertz pulsed imaging (TPI), and terahertzcontinuous wave (TCW) imaging can be several times more sensitive. Pulsed techniques enable 3-D imaging, much likeradar by using time of flight analysis providing increased contrast and discrimination.

Thus by combining imaging and spectroscopy THz has the potential to be an important tool for the detection of hiddenmetallic and non-metallicobjects, such as ceramics and explosives, for people screening.

In this paper, we discuss some of the important factors involved in addressing practical systems in the security industry,and describe two ongoing development programmes for use in people screening applications. The first is a stand-offdetection system, aimed at detecting threats such as explosives using spectroscopyat a distance.The second project is ahand-wand which is intended to operate much like a hand-held metal detector wand, but which will also detect non-metallic weapons and explosives.

*[email protected]; phone: +44 1223 435380; fax: +44 1223 435382; www.teraview.com

Invited Paper

Terahertz for Military and Security Applications III, edited by R. Jennifer Hwu, Dwight L. Woolard,

Mark J. Rosker, Proceedings of SPIE Vol. 5790 (SPIE, Bellingham, WA, 2005)0277-786X/05/$15 doi: 10.1117/12.602976

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2. TERAHERTZ SYSTEMS

Established experimental THz systems are largely based on photoconductive switches, which rely on the production offew-cycle THz pulses using an ultrafast (femtosecond) laser to excite a biased photoconductive antenna. This technique

is inherently broadband, with the emitted power distributed over several THz (typically 0.1 4 THz). Pulsed THzemission in photoconductive antennas is produced when the current density, j, of a biased semiconductor is modulated

on subpicosecond timescales ETHzdj/dt.9 The change in current density, and hence photocurrent, arises from twoprocesses: the rapid change of the carrier density via femtosecond laser illumination, and the acceleration of

photogenerated carriers under an external electric field.10

Coherent detection of the incident THz radiation can beperformed in a similar photoconductive antenna circuit.11-14By gating the photoconductive gap with a femtosecond pulsesynchronised to the THz emission, a dc signal that is proportional to the THz electric field may be measured. Further, byvarying the optical path length to the receiver, the entire THz time domain can be sampled. Hence both the amplitude

and phase of the incident THz wave can be obtained, and a dynamic range of ~60 dB demonstrated using time-gateddetection.15

Femtosecond

Pulsed Laser

Beamsplitter

Long Delay

Stage

Detector

Emitter

Target

15Hz FastScan Delay

Stage

Lock-in

Amplifier

PC

Signal

Reference

Parabolic

Mirror

Parabolic

Mirror

THz Beam

Femtosecond

Pulsed Laser

Beamsplitter

Long Delay

Stage

Detector

Emitter

Target

15Hz FastScan Delay

Stage

Lock-in

Amplifier

PC

Signal

Reference

Parabolic

Mirror

Parabolic

Mirror

THz Beam

Fig. 1. Schematic of a THz photoconductive system. In this example, specifically designed for stand-off explosives detection, the THzbeam is manipulated for reflection spectroscopy of a target material.

In a typical pulsed THz system (see figure 1), the beam from the ultrafast femtosecond laser is split by a beamsplitterinto two components, a pump beam and probe beam, used to illuminate the emitter and receiver respectively. A

motorised delay stage is then incorporated into the probe beam to vary the difference in optical delay around zerobetween the incoming THz pulse and the probe laser pulse at the detector. A rapid scanning delay line is often utilisedsince it allows both the delay position and the lock-in output to be digitized and re-interpolated to obtain the THz field asa function of optical delay in real time. The frequency Fourier transform can then also be displayed in real-time on a

computer display. Typically, the output from the THz emitter is coupled from the rear surface of the device using a high-resistivity silicon lens, which, in combination with off-axis parabolic mirrors/lenses, allows the THz beam to be

manipulated as required. Additionally, to couple the THz radiation from free-space into the receiver, a second siliconlens in contact with the rear of the receiver chip is used.

The THz emitter and detector are manufactured from SI and LT GaAs materials respectively, using, for the detectors,recently developed materials processing techniques.18A bias of 50 V was applied across the emitter electrodes, and thegated output signal from the receiver was fed to a lock-in amplifier.

2 Proc. of SPIE Vol. 5790


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3. SPECTROSCOPY OF EXPLOSIVE MATERIALS

To validate that THz technology is capable of detecting explosives, we first measured the spectrum of various commonexplosives in transmission. This was performed using our commercially available transmission spectrometer, TPI spectra

1000. In transmission the THz field is modified by dispersion and the absorption of the media under examination. The

ratio of the electric field strength beforeEr()and after transmissionEs(), is given by:

+=

c

dndt

E

En

r

s

)1)((

2

)(exp

)(

)()(

( )2)(

1)(

)(4

+=

n

ntn

where dis the thickness of the sample, the frequency of the radiation, c the speed of light, and tn()is the reflection loss

at the sample surface as defined above. Hence, in transmission spectroscopy both the refractive index n() and the

absorption coefficient()can be determined from the measured amplitude and phase information by:

++

=2)1)((

)(4log

)(

)(log

2)(

n

n

E

E

d r

s

+=

)(

)(log1)(

r

s

E

E

d

cn

In figure 2 we show the absorption spectra (solid line) and refractive index (dashed line) of Semtex-H. The features

appearing in the absorption spectra of Semtex-H can be understood from the constituent explosives. Semtex-H consistsof approximately equal amounts of RDX and PETN, with the remainder comprising of poly(butadiene-styrene) and oil.

It can be seen that the absorption of Semtex-H is the sum of its active constituents, shown as the dotted line.

10 20 30 40 50 60 70 80 90 100 110

0

10

20

30

40

50

6070

800.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3

Frequency/THz

Wavenumber/cm-1

Absorption/cm

-1

1.5

1.6

1.7

1.8 Refractiveindex

Fig. 2. The THz absorption spectra (solid line) and refractive index (dashed line) of Semtex-H. The dotted line shows the combined

absorption spectra of RDX and PETN.

Despite the high sensitivity and dynamic range of THz pulsed spectroscopy (TPS) techniques any practicalimplementation of a people screening system will need to work in reflection rather than transmission geometry. From therefractive index, as determined from transmission spectroscopy, the reflectance can be determined. The reflectance,

Proc. of SPIE Vol. 5790 3


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defined as the frequency dependent ratio of the reflected intensity to the incident intensity, is related approximately to therefractive index by:

2

1)(

1)()(

+

n

nR

Thus using transmission spectroscopy it is possible to determine the expected result from a reflection spectroscopy

measurement. Figure 3 shows the expected reflectance of Semtex-H as determined from the above equation.

0.50 0.75 1.00 1.25 1.50 1.750.040

0.045

0.050

0.055

0.060

0.065

0.070

Re

flectance

Frequency/THz

Fig. 3. The calculated reflection spectrum of Semtex-H, as determined by the above equation using transmission spectroscopy.

4. TERAHERTZ HAND-HELD WAND

As reported in reference [17] we have recently developed a prototype THz hand-held detection and identification wandthat, as well as metals, can also detect and identify hidden non-metallic objects and explosives.

Such a system would have a number of benefits over current checkpoint screening approaches. The key benefit would be

improved detection of explosives and non-metallic weapons without the need for hand searches, which are both timeconsuming and invasive to passengers. Other benefits include the more reliable identification of threat materials, reducedfalse alarms, and reduced inconvenience to passengers. These would contribute both to reducing staff costs andincreasing throughput.



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Fig. 4. Photograph of the THz wand prototype. The system is based around the TPI-core system.

The wand prototype, as shown in figure 4, is driven by a TeraView Ltd TPI core system adapted to support multipledevices and to meet the specific needs of the application. In our prototype, the output of an ultrafast femtosecond laser(as described in section 3) is split into two beams, a pump and probe beam by an 80% reflective high-energybeamsplitter. A rapid scanning delay line (oscillating at 30 Hz), and a conventional long delay stage is incorporated intothe probe beam. The pump beam is coupled into a single optical fibre to illuminate the THz photoconductive emitter.

However, the probe beam is further split into six beams of equal power by beamsplitters, and coupled into multiple fibresto illuminate six photoconductive detectors. The photoconductive THz emitter is coupled to a silicon lens located at thecentre of a head unit, and generates a quasi-collimated beam of THz radiation. The six photoconductive THz detectorsare incorporated in the head unit, and surround the emitter, each with a silicon lens to capture the reflected and scattered

THz signal in multiple directions from the target. The signals from each detector can be analysed independently to formboth spatial and spectroscopic information of the target.

Much of the development work has been creating a compact design/layout of the optical components and the probe head,whilst in parallel developing more powerful sources and highly sensitive detectors.

Using the THz wand we have successfully identified targets consisting of pieces of metal, ceramic, plastic, and SX2plastic sheet explosive, hidden beneath up to three layers of cotton, polyester, or leather. A typical measured B-scan isshown in figure 5 for the case of an SX2 target concealed beneath two layers of leather. The horizontal axis represents

the optical time delay, and relates directly to the wand target distance. The vertical axis represents time of acquisition. Asthe wand is moved over the target region at a constant speed, the time of acquisition relates directly to the wand positionwith respect to the target. The scale shows the THz field amplitude at each point.

In figure 5, THz reflections from both the front and back sides of the target are evident (since SX2 is largely transparentat low frequencies). The higher refractive index of the SX2 compared to the surrounding air means that the back-side

reflection is somewhat delayed in time, with the result that the back-side of the target appears beyond the level of thetarget skin. The front-side of the target generates a reflection earlier in time and the combination of these two yields avery clear gap in the otherwise uninterrupted skin reflection. The clothing layers are also clearly visible in the B-scandata.



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Fig. 5. THz B-scan plots showing the THz reflection obtained through 2 layers of leather both with (shown right), and without (shownleft) a piece of sheet explosive SX2 against the skin.

To demonstrate the spectroscopic capabilities of the THz wand, figure 6 shows measured reflection spectra (acquired in atime of 1/30 seconds) from the RDX-based sheet explosive SX2. The absolute reflectance was determined by division of

the measured power spectrum, by a power spectrum from a mirror reference, measured in an otherwise identical fashion.A clear feature is present in the reflection spectra centered at 0.8 THz, which agrees well with the predicted reflectionspectra of SX2, calculated from experimental transmission data measured, as described in Section 3.

Currently algorithms are being developed for automatic identification of hidden objects based on edge detection of hardobjects beneath layered materials, and matching spectra with known features of common explosives.

Fig. 6. The measured reflection spectrum of SX2 acquired with the THz wand.



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5. STAND-OFF EXPLOSIVES DETECTION

For many THz applications, including security screening, it may be necessary to perform measurements in reflectionover a distance through the atmosphere. In this paper we demonstrate for the first time the spectroscopic detection of

concealed explosives at a stand-off distance of 1 m, both real time, in reflection, and under normal atmosphericconditions.

A schematic of the pulsed THz photoconductive system used is shown in figure 1. In our setup, a Coherent Vitesse laserwas used to generate pulses of average power 800 mW, at 800 nm centre wavelength, 80 MHz repetition rate, and with abandwidth-limited pulse duration of 80 fs. As described in Section 2, the output of the ultrafast laser was split into two

components, a pump beam and probe beam, used to illuminate the emitter and receiver respectively. This was achievedby using a high-energy beamsplitter, with 80% of the beam being reflected into the probe beam, and 20% transmitted as

the pump beam. A rapid scanning delay line oscillating at 15 Hz, and a 1000 mm delay stage were incorporated into theprobe beam. The optical laser powers at the emitter and receiver were controlled by using neutral density filter wheels inboth the pump and probe beams. In this arrangement, the pump and probe laser powers incident on the photoconductivedevices were measured to be 5 mW, and 20 mW respectively.

The optical scheme for the THz radiation was as follows: the generated THz radiation from the photoconductive emitter

was coupled from the rear surface of the device using a high-resistivity silicon hyperhemispherical lens, which, in

combination with an f /1 off-axis gold parabolic mirror, collimated the THz beam. This was then focused and directedonto a target object at a stand-off distance of 1 m by second off-axis parabolic reflector. The reflected THz beam fromthe target object then followed an identical reverse scheme, using two parabolic mirrors and a hyperhemispherical siliconlens, allowing the THz beam to be focused onto the photoconductive gap of the receiver. This allows us to performsingle point reflection spectroscopy of targets at a stand-off distance of 1 m.

As a demonstration of the performance (measured in rapid scan and collected in a time of 1/15 seconds), figure 7 showsthe measured power spectrum with a mirror placed at the focal plane. From Figure 7, we show that, at a stand-offdistance of 1 m, a system dynamic range of >60 dB, and a spectral bandwidth extending from 0.1 THz to about 3.0 THzis demonstrated. Sharp absorption features can be seen in the THz spectrum shown in figure 7, which correspond toatmospheric water vapor line these can be removed algorithmically from the spectrum. These absorption bands in the

THz spectrum are occurring after traversing a total path length of 2.4 m through the atmosphere. It can be seen that thereare numerous water attenuation windows through which the signal is relatively unattenuated.

0 1 2 3 410

-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

Power

Frequency/THz

Fig. 7. The power spectrum from the stand-off system with a mirror as the target material. The sharp absorption lines in the spectrum

are due to atmospheric water vapour, with the THz path length equal to 2.4 m.



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In figure 8, we show a reflection measurement from two explosive samples, (a) Semtex-H, and (b) SX2, taken using thestand-off instrument at a distance of 1m, measured in rapid scan and collected in a time of 1/15 seconds. As before, theabsolute reflectance was determined by division of the measured power spectrum, by a power spectrum from a mirror

reference, measured in an otherwise identical fashion (see figure 7). Also shown in figures 8 (a) and (b) are the predictedreflection spectra of Semtex-H and SX2. The agreement between calculation and experiment is excellent, with featuresbeing observed in both spectra at 0.8 THz, 1.05 THz and 1.4 THz. For the measured spectra, data processing techniques

were applied real time to the dataset to remove the water vapor absorption lines i.e. both the data acquisition and dataprocessing was performed in 1/15 seconds.

0.50 0.75 1.00 1.25 1.50 1.75 2.000.02

0.04

0.06

0.08

Reflectance

Frequency/THz

0.50 0.75 1.00 1.25 1.50 1.75 2.000.02

0.04

0.06

0.08

0.10

Reflectance

Frequency/THz

Fig. 8.The measured (solid line) and calculated (dashed line) reflectance spectra of (a) Semtex-H, and (b) SX2 at a stand-off distanceof 1 m, under normal atmospheric conditions, taken real time. The calculated spectra were derived from transmission spectroscopy

data.

Also, there is agreement between the frequencies of the spectral features of Semtex-H and SX2. This is to be expectedbecause both explosives contain substantial components of RDX, which dominate the spectral features over thefrequency range of the figure. Since SX2 is predominately RDX (over 80% by volume), whereas Semtex-H contains

only around 40%, the spectral features in SX2 are stronger than those from Semtex-H.

0.50 0.75 1.00 1.25 1.500.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Reflectance

Frequency/THz

Increasing layers of cotton

0.50 0.75 1.00 1.25 1.50-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.200 layers Cotton

1 layers Cotton

2 layers Cotton

3 layers Cotton

4 layers Cotton

DerivativeReflectance

Frequency/THz

Fig. 9. (a) The reflectance of SX2 behind cotton clothing, and (b) the first derivative of the reflectance of SX2 behind cotton clothing.The RDX spectral feature is clearly visible in (b) through 4 layers of cotton.

Clearly clothing is crucial to the prospect of practical stand-off detection. We have performed a series of measurementsof explosives hidden behind increasing numbers of clothing layers. Shown in figure 9(a) is the effect of increasingnumbers of clothing layers on the 1 m spectrum of SX2 sheet explosive. The clothing layers were from a typical

cotton/polyester shirt loosely placed in front of the explosives, in a manner consistent with normal wear. As before, thesemeasurements were taken at a distance of 1 m, in a time 1/15 second, and with automated water vapor removal. The

(a) (b)

(a) (b)



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rapidly diminishing reflectance, as the frequency rises, is a straightforward consequence of the exponentially increasingattenuation of clothing layers with frequency the clothing layers acting as a low-pass filter to the spectrum. The effectsof this can be visually removed by taking the derivative the reflectance spectrum, as shown in figure 9(b). The

reflectance feature of SX2 centered at 0.8 THz manifests itself as an oscillation in the derivative spectrum. Althoughdifficult to recognize from the linear scaling of figure 9(a), the derivative spectra (figure 9(b)) clearly shows that we canconvincingly identify spectral features through several layers of clothing.

As we have discussed in this paper, there are characteristic spectral features in many common types of explosivesmaterials at THz frequencies. Relatively few other materials have strong spectral features in the range of 0.5 to 1.0 THz.

A substance that does exhibit interesting features in this range is D-tartaric acid, as shown in figure 10. Thismeasurement was performed in reflection at a stand-off distance of 1 m, and was acquired under normal atmosphericconditions, hence data processing algorithms to remove water vapor noise features. Both acquisition and data processingwere performed in a time of 1/15 second. For comparison the calculated reflectance spectra of D-tartaric acid is shown asthe dashed line in figure 10, as determined from transmission spectroscopy. Again we can see an excellent agreementbetween calculation and experiment. Although the form of the reflection spectrum of D-tartaric acid is similar to that of

RDX, the center frequency and magnitude is different, demonstrating that we can spectrally distinguish betweenexplosives and other materials in reflection.

0.50 0.75 1.00 1.25 1.50 1.750.02

0.04

0.06

0.08

0.10

0.12

Reflectance

Frequency/THz

Fig. 10. The measured (solid line) and calculated (dashed line) reflection spectrum of tartaric acid at a stand-off distance of 1 m. Thisdemonstrates the ability of the technique to distinguish between materials.

6. CONCLUSION

In this paper, we have discussed some of the important factors involved in addressing practical systems in the security

industry, and presented results from two prototype THz systems specifically designed for security applications. Firstly, aTHz hand held wand for detection of concealed objects, to be used much like a metal detector. Using the THz wand wehave successfully identified explosive, metallic and non-metallic targets hidden beneath several layers of clothing. Wehave also successfully measured spectroscopic features from SX2 sheet explosive in reflection using the wand at a 30 Hzscan rate. Secondly, we have demonstrated a prototype terahertz stand-off detection system. Using this apparatus wehave verified spectroscopic detection of explosives at a stand-off distance of 1 m, both real time, in reflection, undernormal atmospheric conditions, and behind several layers of clothing.

It is envisaged these systems will become more sophisticated and sensitive as the software and technology evolves.However, indications are that such systems will represent a significant new capability for people screening.



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ACKNOWLEDGEMENTS

Parts of this work were carried out under contract for the UK Government whose support is gratefully acknowledged.

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