Sensors and Instrumentation and Nondestructive Evaluation

Homeland Security Applications

Electromagnetic waves at millimeter-wave and terahertz frequencies (THz) offer promising sensing applications for homeland security. Frequencies from 0.1 to 10 THz are considered THz radiation. For gas-phase molecules, the rotational signatures peak at THz frequencies, with three to four orders of magnitude improvement in sensitivity than possible at microwave frequencies. At low pressures (~1 mtorr), the selectivity of detection is nearly 100% (zero false positives). With such high sensitivity and selectivity, THz systems can be used to monitor public facilities and high-occupancy buildings for toxic industrial chemicals, chemical agents, and trace explosives in a continuous and autonomous manner. These systems can also be used for wide-area monitoring of toxic chemicals in open air, although they lose some selectivity in this case due to broadening of spectral lines with pressure. Because of better penetration in materials than optics, they can also be used for detection and imaging of weapons concealed under clothing. Furthermore, the wavelengths in the THz range appear to resonate with biological macromolecular structures and DNA strands in a unique manner. These systems, because of their inherently fast response, can be used as “detect to protect” sensors for early detection and warning of bioaerosols such as spores, bacteria, viruses, and pathogens. Described below is the work we have done and are doing on THz (millimeter-wave) devices for diverse applications.

Chemical Sensing

Funded by DOE’s National Nuclear Security Administration (DOE-NNSA), the purpose of this work is to investigate the use of fingerprint-type molecular rotational signatures in the mm-wave spectrum to sense airborne chemicals. The mm-wave sensor, operating in the frequency range of 225-315 GHz, can work under all weather conditions and in smoky and dusty environments. The chemical plume to be detected is situated between the transmitter/detector and the reflector. Millimeter-wave absorption spectra of chemicals in the plume are determined by measuring the swept-frequency radar return signals with and without the plume in the beam path. The problem of pressure broadening, which hampered open-path spectroscopy in the past, has been mitigated in this work by designing a fast sweeping source over a broad frequency range. The heart of the system is a backward-wave oscillator (BWO) tube that can be tuned over 220]350 GHz. Using the BWO tube, we built a mm-wave radar system and field-tested it at the DOE Nevada Test Site at a standoff distance of 60 m. The mm-wave system detected chemical plumes very well; detection sensitivity for polar molecules such as methyl chloride was down to 12 ppm for a 4-m two-way pathlength.

Gas Leak Detection

Funded by DOE’s Office of Fossil Energy, National Energy Technology Laboratory, we have developed a microwave radar for remote and fast imaging of gas leaks for protection of natural gas pipeline infrastructure. The underlying principle is the change in radar reflection, refraction, and scattering properties of leak plumes with respect to the surrounding air. The problem of radar detection of gas leaks from a point source is akin to meteorological targets in weather prediction ¾ both deal with gas-phase media and the volume scattering from dielectric property changes. The source of radar returns in meteorological radars is the change of reflectivity, refraction, and scattering caused by storms, rain, cloud, atmospheric turbulence, and wind movement. Even in the absence of contrasting dielectric materials such as water and ice droplets in the atmosphere, the radars can detect clear-air turbulence and wind shear due to wave scattering caused by index-of-refraction inhomogeneities.

In collaboration with AOZT Finn-Trade Co. (St. Petersburg, Russia), we performed a theoretical and experimental investigation to show the feasibility of the radar technique. The gas dynamics of the leak jet were modeled first to determine the plume geometry and the variation of gas concentration in air with distance from the leak source. From the turbulence-induced static and dynamic changes of the index of refraction, the radar backscatter cross section of the plume was determined next. To verify the model predictions and to determine the detection sensitivity of gas leaks, a commercial X-band radar system was interfaced with a computer to enable data collection and specialized signal and image processing. Two gas-plume tests were conducted, one with a cold nitrogen plume at 60 m from a liquid nitrogen dewar for initial testing, and the other with a propane leak from a 50-L cylinder at 720 m from the radar. Because of condensation, the cold nitrogen plume was easily detected, even for small leaks. The propane leak test was more realistic in that it simulated the spread and dynamics of a methane leak as well as its radar cross section closely. The radar signals obtained between two distance channels corresponding to the plume and background atmosphere showed a clear change in signal level for the propane plume. The radar cross section of the propane plume, calculated from the radar return, agreed well with the model results. The results thus indicate the technical feasibility of the radar technique for remote and fast inspection of gas pipelines for leaks.

Radiation Detection

Funded by the DOE’s Initiatives for Proliferation Prevention Program, we have worked with the AOZT Finn-Trade Co., which pioneered the use of microwave (MW) pulsed radars for detecting radioactive plumes. The main source of radar return is the index-of-refraction inhomogeneities of the air space caused by the ionization of air molecules and the additional effects that the ions and radioactive particles produce in air to enhance the radar reflectivity and scattering. For example, aerosol particles can be attracted to the ions by electromagnetic induction. In addition, these charged particles serve as nuclei for condensation of water vapor, which offers a large radar cross section. The technique was demonstrated by using a 3.2-cm MW radar mounted on the top of a van at a distance of 9 km from a nuclear power plant. The radar imaged the weak radioactive cloud emanating from the ventilation stacks of the nuclear power plant during normal operation. The typical release level was 25-50 Ci in a 24-h period. The radar images of the nuclear power plant showed a clear difference between the times when the plant was operating and when it was idling.

Trace Explosives Detection

Funded by DOD’s Technical Support Working Group, a team consisting of Sarnoff Corp., Argonne National Laboratory, Sandia National Laboratories, and Dartmouth College is developing a portal design and detection scheme for fast screening of people for trace explosives. The objective is to demonstrate the feasibility of THz spectrometry for monitoring of trace explosives. In particular, we will identify the rotational spectral signatures of common explosive molecules and their absorption strengths. We will also determine the selectivity of these spectral lines with respect to confounding molecules in the background air. The outcome of this task will provide the basis for design and fabrication of the Sarnoff's Smith-Purcell THz absorption spectrometer.

Because the vapor pressures of explosive molecules are very low, we have built a heat cell for studying their THz signatures. Using a BWO-based spectrometer, we will test the signatures of explosive molecules in the frequency range of 100 to 300 GHz, where their rotational spectral lines are expected to peak from ab initio calculations.

Concealed Weapons Detection

Passive millimeter-wave image of a hammer. Click on image to view larger image.

Funded by DOE-NNSA, we investigated the potential of passive mm-wave imaging for detection of concealed devices. Radiometry in the millimeter wavelengths has been used for remote sensing of the earth and atmosphere. Earth resources such as vegetation, soil moisture, and snow cover, as well as weather patterns and military targets, have been imaged with this technology. Unlike visible or infrared (IR) techniques, mm-wave radiometer has the advantage of all-weather capability. Because the thermal contrasts of objects are different in the mm and IR bands, mm-wave radiometer can complement IR imaging systems.

In this work, we developed and tested a focal-plane imaging radiometer at 160 GHz. The figure below shows a scanned image of a hammer placed on the floor of the laboratory at a distance of 36 inches from the lens plane. The metallic head and wooden handle of the hammer are clearly visible. The image contrast is mainly due to differences in emissivity of the materials. Because mm-waves can penetrate clothing and plastic materials, a focal-plane imaging array camera holds the potential for detection of concealed devices under clothing, buried roadside bombs, and land mines.

Biological Detection

We have investigated the potential of real-time dielectric sensing for detection and classification of biological species. Though many methods of measuring the dielectric properties of materials exist in the literature, they lack specificity in identifying materials. Therefore, few chemical or biological sensors based on such dielectric measurements exist at this time. We have developed a new method that provides a degree of selectivity based on the extent to which the complex dielectric constant of a material affects the signal pattern of a resonator. Using a TE10 microwave cavity, we have demonstrated proof of principle by testing four powder samples: acetaminophen (powdered Tylenol caplet), isobutylphenyl propionic acid (powdered Ibuprofen caplet), bovine albumin (protein), and herring sperm DNA. The cavity showed distinct and repeatable response patterns for the four biomolecules, thus showing promise for the use of the dielectric method to identify biological macromolecules.

The method is applicable at any excitation frequency from radio-frequency to terahertz range; however, if the excitation frequency is selected to correspond to one of the resonance (relaxation-type or spectroscopic) frequencies of the material under investigation, the degree of selectivity and the sensitivity of detection can be improved significantly. The resonance-enhanced dielectric method holds the potential for a fast first screening of chemical or biological agents in the form of gas, powder, or aerosol.

Synchrotron Radiation-Based Terahertz Source

In collaboration with Argonne’s Advanced Photon Source and Energy Systems Division, we are developing a coherent high-power pulsed THz source based on ballistic electron bunch compression. The building blocks of a synchrotron-based THz source include an electron source that can produce short pulses of relativistic electrons, a dipole or an undulator magnet that is coupled to the electron source to produce coherent long-wavelength radiation, and photon transport optics and detectors for collecting and characterizing the radiation. In FY 2003, all the building blocks were commissioned and were tested individually. The electron source has been fully commissioned in terms of input RF power conditioning at 2856 MHz. The first electron-beam-based measurements demonstrated that the gun is behaving as the theory predicted. The complete system will be tested for THz radiation, followed by chemical and biological materials testing.

Superconductor-Based Terahertz Source

Collaborating with the Materials Science Division, we are developing a high-temperature superconductor-based miniature THz source. The purpose is to demonstrate a compact, tunable, solid-state terahertz source of sub-millimeter electromagnetic waves using strongly-driven Josephson vortices in layered, high-temperature superconductors, like Bi₂Sr₂CaCu₂O₈ (BSCCO). Although theoretical efforts have predicted large power output, two primary technical challenges must be overcome: (1) the large mismatch of impedance (light velocity or index of refraction) between BSCCO and free space, and (2) heating.

Last Modified: Thu, April 21, 2016 7:20 AM