James C. Weatherall

Spectral Signatures for Identifying Explosives With Wideband Millimeter-Wave Illumination

Millimeter-wave imaging systems used in airports, government buildings, and other facilities for personnel screening use advanced imaging technology (AIT) to detect explosives and weapons concealed under clothing. Additional information in the imaging data can be applied to identify the composition of the detected objects. The method described here demonstrates that material data in the form of the dielectric constant can be derived from the variation of reflectivity in millimeter waves over a range of frequencies from 18 to 40 GHz. By fitting the reflectivity to an optical model, the thickness and dielectric constant, including attenuation, can be computed. The method is applied to samples of inert substances and a military sheet explosive to show that detected anomalies can be distinguished as distinct materials through their dielectric constants. For absorptive materials, a frequency band of lower frequencies, 2-18 GHz, can be applied to detect the dielectric, as is demonstrated in the case of a commercial explosive. Used with AIT, the technique can facilitate the evaluation of threats at personnel checkpoints.

Identifying explosives by dielectric properties obtained through wide-band millimeter-wave illumination

A method for extracting dielectric constant from free-space 18 - 40 GHz millimeter-wave reflection data is demonstrated. The reflection coefficient is a function of frequency because of propagation effects, and numerically fitting data to a theoretical model based on geometric optics gives a solution for the complex dielectric constant and target thickness. The discriminative value is illustrated with inert substances and military sheet explosive. In principle, the measurement of reflectivity across multiple frequencies can be incorporated into Advanced Imaging Technology (AIT) systems to automatically identify the composition of anomalies detected on persons at screening checkpoints.

Development of a contrast phantom for active millimeter-wave imaging systems

As the development of active millimeter wave imaging systems continues, it is necessary to validate materials that simulate the expected response of explosives. While physics-based models have been used to develop simulants, it is desirable to image both the explosive and simulant together in a controlled fashion in order to demonstrate success. To this end, a millimeter wave contrast phantom has been created to calibrate image grayscale while controlling the configuration of the explosive and simulant such that direct comparison of their respective returns can be performed. The physics of the phantom are described, with millimeter wave images presented to show successful development of the phantom and simulant validation at GHz frequencies.

Measurements of the dielectric properties of explosives and inert materials at millimeter wave frequencies (V-band and above) using free space reflection methods

We present a free space material measurement system operating in the E band (60-90 GHz) frequency range that uses calibration standards placed at the sample location to define the measurement reference plane directly at the sample surface. Measurement signal to noise is improved by using an aperture in radar absorbing material (RAM) to simplify the RF measurement environment. Measurements are provided that extend earlier work done in the 18-40 GHz frequency range. Data is extracted using numerical fitting of reflection-only data to a theoretical model based on geometric optics. System calibration, and results are presented.

Identifying explosives using broadband millimeter-wave imaging

Millimeter wave imaging is employed in Advanced Technology Imaging (AIT) systems to screen personnel for concealed explosives and weapons. AIT systems deployed in airports auto-detect potential threats by highlighting their location on a generic outline of a person using imaging data collected over a range of frequency. We show how the spectral information from the imaging data can be used to identify the composition of an anomalous object, in particular if it is an explosive material. The discriminative value of the technique was illustrated on military sheet explosive using millimeter-wave reflection data at frequencies 18 – 40 GHz, and commercial explosives using 2 – 18 GHz, but the free-space measurement was limited to a single horn with a large-area sample. This work extends the method to imaging data collected at high resolution with a 18 – 40 GHz imaging system. The identification of explosives is accomplished by extracting the dielectric constant from the free-space, multifrequency data. The reflection coefficient is a function of frequency because of propagation effects associated with the material’s complex dielectric constant, which include interference from multiple reflections and energy loss in the sample. The dielectric constant is obtained by numerically fitting the reflection coefficient as a function of frequency to an optical model. In principal, the implementation of this technique in standoff imaging systems would allow threat assessment to be accomplished within the scope of millimeter-wave screening.

Developing an ANSI standard for image quality tools for the testing of active millimeter wave imaging systems

In 2016, the millimeter wave (MMW) imaging community initiated the formation of a standard for millimeter wave image quality metrics. This new standard, American National Standards Institute (ANSI) N42.59, will apply to active MMW systems for security screening of humans. The Electromagnetic Signatures of Explosives Laboratory at the Transportation Security Laboratory is supporting the ANSI standards process via the creation of initial prototypes for round-robin testing with MMW imaging system manufacturers and experts. Results obtained for these prototypes will be used to inform the community and lead to consensus objective standards amongst stakeholders. Images collected with laboratory systems are presented along with results of preliminary image analysis. Future directions for object design, data collection and image processing are discussed.

Toward the development of an image quality tool for active millimeter wave imaging systems

Preliminary design considerations for an image quality tool to complement millimeter wave imaging systems are presented. The tool is planned for use in confirming operating parameters; confirmation of continuity for imaging component design changes, and analysis of new components and detection algorithms. Potential embodiments of an image quality tool may contain materials that mimic human skin in order to provide a realistic signal return for testing, which may also help reduce or eliminate the need for mock passengers for developmental testing. Two candidate materials, a dielectric liquid and an iron-loaded epoxy, have been identified and reflection measurements have been performed using laboratory systems in the range 18 - 40 GHz. Results show good agreement with both laboratory and literature data on human skin, particularly in the range of operation of two commercially available millimeter wave imaging systems. Issues related to the practical use of liquids and magnetic materials for image quality tools are discussed.

Identifying Raman and Infrared Vibrational Motions of Erythritol Tetranitrate

The vibrational bands of erythritol tetranitrate (ETN) were measured experimentally with both Raman spectroscopy and attenuated total reflectance Fourier transform infrared (ATR FT-IR) spectroscopy. Seventy-two (3N-6) vibrational modes were predicted for ETN using density functional theory calculations performed using the B3LYP/ 6-31G* density functional basis set and geometry optimization. Raman spectroscopy and ATR FT-IR were used to measure observable Raman and IR signatures between 140 and 3100 wavenumbers (cm−1). Within this spectral range, 32 Raman bands and 21 IR bands were measured and identified by their predicted vibrational motion. The spectroscopic and theoretical analysis of ETN performed will advance the detection and identification capabilities of field measuring instruments for this explosive.