Through Container THz Sensing: Applications for Explosives Screening

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1 VG- Through Container Sensing: Applications for Explosives Screening David J. Cook, Brian K. Decker, Gami Dadusc, and Mark G. Allen Physical Sciences Inc. New England Business Center Andover, MA January 5, Physical Sciences Inc. New England Business Center Andover, MA 11

2 Why is Interesting for Sensing Structure Penetrating Detect through clothing, within sealed packaging, container, shipping materials lack of Mie and Rayleigh scattering when compared to IR/VIS VG--1 Chemical Specificity Able to detect and classify illicit compounds need spectral signatures: rotational lines (gas); phonon bands (crystal); Non-Invasive/Non-Destructive/Non-Detectable Harmless undetectable to living beings, material under inspection is non-ionizing; need low power levels (<< 1 mw/cm ) for sensing Automated Threat Detection Straightforward chemometric-type spectral identification for automated alarm and classification use of spectral signatures leads to non-subjective algorithms Our approach is non-imaging!

3 Sensor Concept Portal Screening VG-- Application: Hidden explosives sensing Transmission mode for automated material receiving Backscatter (DIAL) mode for personnel screening Will use multi-laser, QCL's (from Tredicucci, SNS) for first prototype Our work to date is exploring basic spectroscopy of container materials, clothing, and explosives

4 CW Quantum Cascade Laser From Tredicucci, et al., Scuola Normale Superiore, Pisa VG--3 FTIR Spectrum Near Threshold (7 µw output power)

5 The PSI Time Domain Spectrometer VG-- 1 MHz 1 Watt 7 fs Ti:Sapphire laser Biased emitter source (Planken, et al., Rev. Sci. Inst., 73, 1715) ZnTe E-O Detection

6 Comparison of Predicted and Measured Water Vapor Absorption 1 atm, 3 K, 51 cm pathlength VG--5. Absorbance (cm -1 ) HITRAN Exp. Data Relative Humidity) Frequency ().5 3. F-53 Spectrometer data inversion correctly captures frequency and absorption scales

7 Container Penetration: Clothing VG-- Due to reduced scattering, radiation penetrates inhomogenous materials 1 1 Ripstop Nylon Dupont Nylon Vinyl Canvas Gortex Rayon Latex 1 1 Cotton Wool Leather F-9a F-91b Example spectra of clothing materials Transmission through many common clothing materials at relevant wavelengths

8 Container Penetration: Mailing Materials VG--7 USPS Tyvek UPS Padpak FEDEX Padded FEDEX Box Manila Envelope F-93b Example spectra mailing materials Good penetration of common mail materials at relevant wavelengths

9 Container Penetration: Packaging Materials VG-- Styrofoam Bubble Wrap Cardboard Paper Bag F-9a Example spectra packaging materials Good penetration of common mail materials at relevant wavelengths

10 Explosive Samples for Spectroscopy VG--9 Focus on pure high explosive substances, rather than on formulations (measure pure RDX rather than C-) Would ideally use free-standing pellets of pure polycrystalline material; concerned safety and durability--sandwich pellets between polyethylene windows previously unknown: Obtained 1 mm thick and 5 micron thick samples High Explosive Samples Studied RDX TNT HMX PETN NH NO 3

11 Target Spectral Signatures: PETN and TNT VG--1 PETN (5 µm thick) TNT (5 µm thick) 1 1 E x tin c tio n G F-79a

12 Target Spectral Signatures: HMX and RDX VG--11 RDX (5 µm thick) HMX (5 µm thick) F-7a F-77a

13 Target Spectral Signatures: NH NO 3 VG F-a NH NO 3 (5 µm thick) Restrahlen bands lead to strong absorption for > 1.5

14 New Transmission Strategies: VG--13 Goals: Control Concentration (avoid saturation) Increase pathlength (eliminate etalons) Decrease scattering Methods: Mixed powder pellets using PTFE & Polyethylene Viscous suspensions (Apiezon grease & Fomblin grease) Do not want to dissolve the material under study

15 Revised Sampling Methods VG PTFE (1 µm) Pellet PTFE (1 µm) Pellet Fomblin RT-15 Apiezon M Paraffin Wax Polyethylene Pellet Absorbance (cm -1 ) 1 PTFE (1 µm) Pellet PTFE (1 µm) Pellet Fomblin RT-15 Apiezon M Paraffin Wax Polyethylene Pellet G-a G-b

16 Polyethylene Pellet Results VG PETN HMX RDX 1 TATB mg of explosives in 13 mm dia. 9 mm thick pellets. G-315

17 Summary VG--1 TD- spectrometer used to obtain transmission measurements of package, clothing materials, as well as pure explosives Sample preparation and data collection strategies can reduce etalons. More work is necessary to reduce scattering in transmission measurements Target explosives exhibit strong, characteristic absorption peaks between.5 and. Based on these results, plan to develop multi-qcl DIAL system for hidden explosive detection Future work TD- reflection measurements QCL-based transmission/reflection measurements