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This paper investigates the accuracy of deriving the composite optical constants of binary mixtures from only the complex indices of refraction of the neat materials. These optical constants enable the reflectance spectra of the binary mixtures to be modeled for multiple scenarios (

Direct comparisons are made between an active laser-based swept-wavelength ECQCL and passive LWIR hyperspectral imager for dynamic chemical plume detection at a standoff distance of 1.5 km in complex terrain.

To support stand-off detection, spectral libraries are being developed that include the optical constants, n(ν) and k(ν). Variable angle spectroscopic ellipsometry and single-angle reflectance are used to derive these data, enabling the reference spectra to be modeled for different parameters.

This paper updates recent work to populate spectral databases for (near-) infrared sensing of solids in different modalities including passive and active sensing of minerals, ores and other mining chemicals and minerals.

Ultrafast laser ablation coupled with optical emission spectroscopy is currently under development for standoff detection of elements and their isotopes for a wide range of application areas. In this work, we explore the role of ambient air pressure during the interaction of an ultrafast laser beam with two metallic targets (brass and uranium) with significantly different oxygen gas-phase reactivity. Plasma plumes were generated by focusing ∼800 nm, ∼35 fs pulses from an ultrafast laser system using a long focal length lens (f = 1 m). Two-dimensional spectral imaging was performed over the pressure range of 30 mTorr to 700 Torr air to evaluate emission dynamics, plasma chemistry, signal-to-background ratio, and characteristic parameters (i.e. excitation temperature, electron density). An increase in ambient air pressure during ultrafast laser ablation leads to plume confinement and subsequent changes in the emission dynamics due to plasma chemistry as well as changes in plasma generation conditions (i.e. focused fs laser ablation versus filament ablation). Ablation crater morphologies were also investigated via scanning electron microscopy. Results indicate atomic emission intensity and signal-to-background ratios peak at moderate pressure levels (∼50–100 Torr air) for both targets studied, although plasma chemistry influences uranium emission signatures. The emission features of the uranium plasma at pressures ≥10 Torr showed the presence of oxide molecules. We also find filament ablation leads to wider, more shallow craters compared to focused laser ablation. Our study provides unique insight into the interplay between plume dynamics, confinement, and plasma chemistry of fs laser-produced plasmas and how these phenomena evolve with changing ambient air pressure.

Binary mixtures of liquids may be encountered in industrial or remote sensing scenarios and present challenges to positive identification compared to neat single-component liquids. Our investigation examines whether one can predict the optical properties of the mixture, i.e. its complex index of refraction, by assuming a linear superposition of the real and imaginary components of the index of refraction in proportion to the ratio of each constituent. To investigate this hypothesis various liquid mixtures were created using mass ratios. The mixtures were then characterized as to their complex index of refraction and used in numerical modeling calculations of thin liquid mixture films on surfaces and compared with composed mixtures using linear n and k synthetic mixtures where the n and k components of the complex index of refraction were combined in similar ratios. The comparison of modeling and experimental results is presented with recommendations for further investigation.

Knowledge of the bulk optical constants n and k of solids or liquids allows researchers to accurately predict the absorption, reflection, and scattering properties of materials for different physical forms. Indeed, chemically complex materials such as minerals can have an almost limitless variety of morphologies, particle sizes, shapes, and compositions, and the optical properties of such species can be predicted if the optical constants are known. For species such as minerals, there can be additional challenges due to e.g. hydration or dehydration during the course of the optical constants measurement. Here, we describe the protocols to obtain the bulk optical constants n and k of uranium-bearing minerals and ores such as uraninite or autunite. If quality n and k data are at hand, the (infrared) reflectance spectra can be predicted for different particle sizes and morphologies and the modeling results for various scenarios can be derived.

This paper presents the methods for developing synthetic spectral libraries using the real n(λ) and imaginary k(λ) parts of the complex refractive index for sensors, including hyperspectral sensors, and presents results for solid materials.

Gas-phase oxidation of uranium (U), plume chemistry, and the corresponding impact on optical emission features of the U plasma are investigated. Plasmas were produced via nanosecond laser ablation of a natural U target in a chamber where U oxidation was controlled by varying the oxygen partial pressure in an argon cover gas. Monochromatic imaging of U atoms and monoxide (UO) molecules was performed using narrow bandpass optical filters. Results reveal the spatio-temporal evolution of atomic and molecular species in the plasma. U oxides are found to be formed further from the target (in comparison to U atoms), where lower temperatures favor molecular recombination. Segregation between the distribution of U atoms and UO species is observed at later times of plasma evolution, and is more apparent at lower oxygen partial pressures. At higher oxygen partial pressures, significant variation in plume morphology is noticed for UO species, which can be attributed to the higher oxide (UxOy) formation further from the target. The monochromatic images of U atoms and UO molecules and corresponding spectral features at various oxygen partial pressures presented here provide unique insight into gas-phase, high-temperature U oxidation and chemistry, with implications for a wide range of nuclear applications, from stand-off detection of radioisotopes to forensics and safeguards.

Rapid and sensitive standoff measurement techniques are needed for detection of trace chemicals in outdoor plume releases, for example from industrial emissions, unintended chemical leaks or spills, burning of biomass materials, or chemical warfare attacks. Here, we present results from 235 m standoff detection of transient plumes for 5 gas-phase chemicals: Freon 152a (1,1-difluoroethane), Freon 134a (1,1,1,2-tetrafluoroethane), methanol (CH3OH), nitrous oxide (N2O), and ammonia (NH3). A swept-wavelength external cavity quantum cascade laser (ECQCL) measures infrared absorption spectra over the range 955-1195 cm−1 (8.37- 10.47 µm), from which chemical concentrations are determined via spectral fits. The fast 400 Hz scan rate of the swept-ECQCL enables measurement above the turbulence time-scales, reducing noise and allowing plume fluctuations to be measured. For high-speed plume detection, noise-equivalent column densities of 1-2 ppm*m are demonstrated with 2.5 ms time resolution, improving to 100-400 ppb*m with 100 ms averaging.

Laser ablation in conjunction with optical emission spectroscopy is a potential non-contact, stand-off detection method for all elements in the periodic table and certain isotopes such as radionuclides. Currently, significant development efforts are on-going to use ultrafast laser filaments for remote detection of materials. The application of filaments is of particular interest in extending the range of stand-off capability associated with elemental and isotopic detection via laser-induced breakdown spectroscopy. In this study, we characterize the expansion dynamics and chemical evolution of filament-produced uranium (U) plasmas. Laser filaments are generated in the laboratory by loosely focusing 35 femtosecond (fs), 6 milli Joule (mJ) pulses in air. Time-resolved, two-dimensional plume and spectral imaging was performed to study hydrodynamics and evolution of U atomic and UO molecular emission in filament-produced U plasmas. Our results highlight that filament ablation of U plasmas gives a cylindrical plume morphology with an appearance of plume splitting into slow and fast moving components at later times of its evolution. Emission from the slow-moving component shows no distinct spectral features (i.e. broadband-like) and is contributed in part by nanoparticles generated during ultrafast laser ablation. Additionally, we find U atoms and U oxide molecules (i.e. UO, UxOy) co-exist in the filament produced plasma, which can be attributed to the generation of low-temperature plasma conditions during filament ablation.

A swept-wavelength external cavity quanhim cascade laser measures broadband infrared absolution spectra from 2050-2300 cm-1 at a 100 Hz rate to determine time-resolved temperabrre and column densities of CO, CO2, H2O, and N2O in high-explosive detonations.

We report on recent work on spectral databases that can be used for (near-) infrared sensing of solids and liquids relevant to both passive/active sensing of materials related to mining, oil and gas exploration.

We characterize performance of a standoff detection system based on a swept-wavelength external cavity quantum cascade laser (ECQCL) tuned from 955-1195 cm-1 (8.37-10.47 pm) at 400 Hz for detection of transient chemical plumes.

We investigate the oxidation of uranium (U) species, the physical conditions leading to uranium monoxide (UO) formation and the interplay between plume hydrodynamics and plasma chemistry in a laser-produced U plasma. Plasmas are produced by ablation of metallic U using nanosecond laser pulses. An ambient gas environment with varying oxygen partial pressures in 100 Torr inert Ar gas is used for controlling the plasma oxidation chemistry. Optical emission spectroscopic analysis of U atomic and monoxide species shows a reduction in the emission intensity and persistence with increasing oxygen partial pressure. Spectral modelling is used for identifying the physical conditions in the plasma that favor UO formation. The optimal temperature for UO formation is found to be in the temperature range of ∼1500-5000 K. The spectrally integrated and spectrally filtered (monochromatic) imaging of U atomic and molecular species reveals the evolutionary paths of various species in the plasma. Our results also highlight that oxidation in U plasmas predominantly occurs at the cooler periphery and is delayed with respect to plasma formation, and the dissipation of molecular species strongly depends on oxygen partial pressure.

A swept-wavelength external cavity quantum cascade laser (ECQCL) is used to perform standoff detection of combustion gases in a plume generated from an outdoor high-explosive (HE) open detonation. The swept-ECQCL system was located at a standoff distance of 830 m from a 41 kg charge of LX-14 (polymer-bonded high explosive) and was used to measure the infrared transmission/absorption through the post-detonation plume as it propagated through the beam path. The swept-ECQCL was operated continuously to record broadband absorption spectra at a 200 Hz rate over a spectral range from 2050 to 2230 cm−1 (4.48–4.88 μm). Fitting of measured spectra was used to determine time-resolved column densities of CO, CO2, H2O, and N2O. Analysis of visible video imagery was used to provide timing correlations and to estimate plume dimensions, from which gas mixing ratios were estimated. Measured emission factors and modified combustion efficiency show good agreement with previously reported values.

Since solids are only sometimes seen en masse in a pure bulk form, and for liquids other than water almost never, a capability to model reflectance spectra from analytes deposited on various substrates would be highly advantageous. If available, the real, n ( ν ) , and imaginary, k ( ν ) , components of the complex refractive index, n∼ = n + ik, can be used to simulate infrared spectra, accounting for reflection, refraction, and absorption phenomena as a function of wavelength. We focus on using the Pacific Northwest National Laboratory (PNNL) derived n / k vectors for solid and liquid analytes deposited as thin layers on different types of substrates including conductors, such as aluminum, and inorganic dielectrics, such as glass. The model is an adaptation of the Monte Carlo ray trace modeling program, TracePro, extended through the use of its macrolanguage. The model is tested using thin films of organic liquids including silicone oil and no. 2 diesel fuel, as well as organic solids such as caffeine and acetaminophen on aluminum and glass. The predicted spectra for the solid films were compared to experimental hemispherical reflectance data measured using a Fourier transform spectrometer with an integrating sphere. The thickness of the calculated layer is a parameter for predicting the (transflectance) spectra and is obtained using the areal density measured from gravimetric methods to generate the thin-layer samples. Comparison of the calculated spectra with experimental hemispherical reflectance data shows excellent agreement, indicating promise for the use of measured n / k data to synthesize reference spectral data. In particular, accounting for the inhomogeneity of the deposits greatly improved the match with experimental data. Finally, the theoretical modeling shows that for thicker layers (ca. 20 to 100 μm) of typical organics possessing moderately strong k values, the longwave infrared features are often saturated and better spectral contrast is obtained from the overtone/combination bands in the shortwave infrared.

The formation and emission of molecular species in a laser-produced plasma have been the subject of a number of recent studies. In laser-induced plasmas molecules are formed through a number of reaction pathways: combustion (oxidation) by direct interaction with the ambient atmosphere, recombination between species present within the plasma, and fragmentation of larger molecular clusters. However, even though extensive studies are available in the literature, still there exists a lack of knowledge about when and where the molecules are formed in a transient plasma system like a LPP. Typically, the molecular emissions are observed when the plasma has cooled and undergone significant expansion into the ambient atmosphere, and molecular emission persist for longer periods compared to the excited atomic and ionic emissions. Recent studies have shown that shock waves formed during LPP expansion hinder molecular formation through combustion at early times of its evolution.1 The emission intensity, delay, and persistence of the atomic, ionic, and molecular emissions are influenced by the plasma chemistry.

We present the rationale, methods, and results of modeling of thin film organic liquids on various substrates. These liquids may coat surfaces (substrates) either as a result of their production, dispersal via aerosols or spills. Identification of unknown coated surfaces using either reflectance or emittance spectroscopy cannot be accomplished simply through reference to reflectance signature libraries since neither the thickness of the liquid layer nor the substrate type is known beforehand and both contribute to the signature. Liquid spectral libraries offer the complex index of refraction (n,k) as a function of wavelength which by itself is useful only for thick (bulk) liquid layers via computation of reflectance and transmittance coefficients using the Fresnel equations. Thin liquid layers both reflect and refract incident light in combination with reflectance from the substrate. We show modeling of various organic liquids on substrates using commercial thin film design and modeling software, as well as Monte Carlo ray tracing software to demonstrate the variety of potential signatures encountered that depend on the thickness of the liquid layer as well as the characteristics of the substrate (metal or dielectric). These substrates give rise to transflectance behavior, while many dielectric substrates have rich absorption features that provide complex signatures that combine attributes of both the liquid and the substrate. Knowledge of the complex index of refraction of both target liquids and substrates is essential in order to synthesize spectra necessary in the application of target identification algorithms.

We present accurate measurements for the optical constants for a series of organic liquids, including organophosphorous compounds. Bulk liquids are rarely encountered in the environment, but more commonly are present as droplets of liquids or thin layers on various substrates. Providing reference spectra to account for the plethora of morphological conditions that may be encountered under different scenarios is a challenge. An alternative approach is to provide the complex optical constants, n and k, which can be used to model the optical phenomena in media and at interfaces, minimizing the need for a vast number of laboratory measurements. In this work, we present improved protocols for measuring the optical constants for a series of liquids that span from 7800 to 400 cm-1. The broad spectral range means that one needs to account for both strong and weak spectral features that are encountered, all of which can be useful for detection, depending on the scenario. To span this dynamic range, both long and short cells are required for accurate measurements. These protocols are presented along with experimental and modeling results for thin layers of silicone oil on aluminum.

Pacific Northwest National Laboratory has recently opened a shallow underground laboratory intended for measurement of low-concentration levels of radioactive isotopes in samples collected from the environment. The development of a low-background liquid scintillation counter is currently underway to further augment the measurement capabilities within this underground laboratory. Liquid scintillation counting is especially useful for measuring charged particle (e.g., $\beta$, $\alpha$) emitting isotopes with no (or very weak) gamma-ray yields. The combination of high-efficiency detection of charged particle emission in a liquid scintillation cocktail coupled with the low-background environment of an appropriately-designed shield located in a clean underground laboratory provides the opportunity for increased-sensitivity measurements of a range of isotopes. To take advantage of the 35 meters-water-equivalent overburden of the underground laboratory, a series of simulations have evaluated the scintillation counter's shield design requirements to assess the possible background rate achievable. This report presents the design and background evaluation for a shallow underground, low background liquid scintillation counter design for sample measurements.

Liquid scintillation counting (LSC) supports a range of environmental science measurements. At Pacific Northwest National Laboratory, we are constructing an LSC system with an expected background reduction of 10–100 relative to values reported in the literature. In this paper, a number of current measurement applications of LSC have been considered with an emphasis on determining which aspects of such measurements would gain the greatest benefit: improved minimum detectable activity (MDA), reduction in sample size, and reduction in total analysis time.

We demonstrate a sensor based on tunable diode laser absorption spectroscopy for the detection of hydrogen fluoride (HF) gas at ambient pressure. Absorption from the HF R(1) ro-vibrational peak at ν = 4038.962 cm−1 (2.476 µm) in the fundamental (Δν = 1) band is measured. A quantitative spectral fit based on HITRAN data is used to account for overlapping spectral peaks of HF and water vapor, with an rms residual noise of 5 × 10−4 absorbance units. The sensor is optimized for the detection of transient variations in HF concentration. We measure noise-equivalent concentrations for HF of 38 parts-per-trillion by volume (ppt) for 1-s integration times and 2.3 ppt for 10-min integration times.

Helicopter-mounted gamma-ray detectors can provide law enforcement officials the means to quickly and accurately detect, identify, and locate radiological threats over a wide geographical area. The ability to accurately distinguish radiological threat-generated gamma-ray signatures from background gamma radiation in real time is essential in order to realize this potential. This problem is non-trivial, especially in urban environments for which the background may change very rapidly during flight. This exacerbates the challenge of estimating background due to the poor counting statistics inherent in real-time airborne gamma-ray spectroscopy measurements. To address this challenge, we have developed a new technique for real-time estimation of background gamma radiation from aerial measurements without the need for human analyst intervention. The method can be calibrated using radiation transport simulations along with data from previous flights over areas for which the isotopic composition need not be known. Over the examined measured and simulated data sets, the method generated accurate background estimates even in the presence of a strong, 60 Co source. The potential to track large and abrupt changes in background spectral shape and magnitude was demonstrated. The method can be implemented fairly easily in most modern computing languages and environments.

We have developed methods to measure the directional-hemispherical (ρ) and diffuse (ρd) reflectances of powders, liquids, and disks of powders and solid materials using a commercially available, matte gold-coated integrating sphere and Fourier transform infrared spectrometer. To determine how well the sphere and protocols produce quantitative reflectance data, measurements were made of three diffuse and two specular standards prepared by the National Institute of Standards and Technology (NIST), LabSphere Infragold and Spectralon standards, hand-loaded sulfur and talc powder samples, and water. Relative to the NIST measurements of the NIST standards, our directional hemispherical reflectance values are within ±4% for four of the standards and within ±7% for a low reflectance diffuse standard. For the three diffuse reflectance NIST standards, our diffuse reflectance values are within ±5% of the NIST values. For the two specular NIST standards, our diffuse reflectance values are an order of magnitude larger than those of NIST, pointing to a systematic error in the manner in which diffuse reflectance measurements are made for specular samples using our methods and sphere. Sources of uncertainty are discussed in the paper.

For optical modeling and other purposes, we have created a library of 57 liquids for which we have measured the complex optical constants n and k. These liquids vary in their nature, ranging in properties that include chemical structure, optical band strength, volatility, and viscosity. By obtaining the optical constants, one can model most optical phenomena in media and at interfaces including reflection, refraction, and dispersion. Based on the works of others, we have developed improved protocols using multiple path lengths to determine the optical constants n/k for dozens of liquids, including inorganic, organic, and organophosphorus compounds. Detailed descriptions of the measurement and data reduction protocols are discussed; agreement of the derived optical constant n and k values with literature values are presented. We also present results using the n/k values as applied to an optical modeling scenario whereby the derived data are presented and tested for models of 1 µm and 100 µm layers for dimethyl methylphosphonate (DMMP) on both metal (aluminum) and dielectric (soda lime glass) substrates to show substantial differences between the reflected signal from highly reflective substrates and less-reflective substrates.

Hyperspectral imaging (HSI) continues to grow as a method for remote detection of vegetation, materials, minerals, and pure chemicals. We have used a longwave infrared (7.7 to 11.8 μm) imaging spectrometer in a static outdoor experiment to collect HSI data from 24 minerals and background materials to determine the efficacy with which HSI can remotely detect and distinguish both pure minerals and mineral mixtures at a 45-deg tilt angle relative to ground using two different backgrounds. Measurements were obtained separately for the minerals and materials mounted directly on both a bare plywood board and a board coated with aluminum foil: 19 powders (3 mixtures and 16 pure mineral powders) held in polyethylene bottle lids as well as five samples in rock form were taped directly to the boards. The primary goal of the experiment was to demonstrate that a longwave infrared library of solids and minerals collected as directional-hemispherical reflectance spectra in the laboratory could be used directly for HSI field identification along with simple algorithms for a rapid survey of the target materials. Prior to the experiment, all 24 mineral/inorganic samples were measured in the laboratory using a Fourier transform infrared spectrometer equipped with a gold-coated integrating sphere; the spectra were assimilated as part of a larger reference library of 21 pure minerals, 3 mixtures, and the polyethylene lid. Principal component analysis with mean-centering was used in an exploratory analysis of the HSI images and showed that, for the aluminum-coated board, the first principal component captured the difference between the signal that resembled a blackbody and the highly reflective aluminum background. In contrast, the second, third, and fourth principal components were able to discriminate the materials including phosphates, silicates, carbonates, and the mixtures. Results from generalized least squares target detection clearly showed that laboratory reference spectra of minerals could be utilized as targets with high fidelity for field detection.

Infrared laser absorption spectroscopy provides a powerful tool for probing physical and chemical properties of high-explosive detonations. A broadly tunable swept-wavelength external cavity quantum cascade laser operating in the mid-wave infrared (MWIR) spectral region is used to measure transmission through explosive fireballs generated from 14 g charges of 4 different explosive types detonated in an enclosed chamber. Analysis of time-resolved transmission and emission at a 2 μs sampling rate shows the evolution of fireball infrared opacity in the first 10 ms after detonation. Broadband high-resolution absorption spectra acquired over the spectral range of 2050–2300 cm−1 (4.35–4.88 μm) at a 100 Hz rate are used to measure properties of fireball evolution over longer time scales out to 100 s. Path-integrated concentrations of combustion products CO, CO2, H2O, and N2O are measured and show evolutions over multiple time scales and significant differences between explosive types. Spectral analysis is used to characterize gas temperature and to measure broadband attenuation from absorption and scattering of particulates. Analysis of the results provides information on the MWIR optical properties, gaseous detonation/combustion products, and particulates throughout the explosive process including initial detonation, fireball expansion and cooling, and diffusive mixing in the chamber.

The Ultra-Low Background Liquid Scintillation Counter developed by Pacific Northwest National Laboratory will expand the application of liquid scintillation counting by enabling lower detection limits and smaller sample volumes. By reducing the overall count rate of the background environment approximately 2 orders of magnitude below that of commercially available systems, backgrounds on the order of tens of counts per day over an energy range of ~3–3600 keV can be realized. Initial test results of the ULB LSC show promising results for ultra-low background detection with liquid scintillation counting.

We present a study of the spectral and angular dependence of scattered mid-infrared light from surfaces coated with explosives residues (TNT, RDX, and tetryl) detected at a 2 m standoff distance. An external cavity quantum cascade laser provided tunable illumination between 7 and 8 μm. Important differences were identified in the spectral features between specular reflection and diffuse scattering which will impact most practical testing scenarios and complicate material identification. We discuss some of the factors influencing the dependence of observed spectra on the experimental geometry.

Technologies for directing daylight deeper within a building space are highly sought after for energy efficiency applications in order to offset artificial lighting costs and to improve workplace productivity via the use of natural light. Vertical window coatings that can perform this task by redistributing sunlight deeper into a space are especially attractive as they are significantly more straightforward to incorporate into a wide variety of architectures as well as to retrofit into existing facades as compared to roof-based skylights or bulky horizontal daylight shelf-type options. The potential energy savings are even greater when one takes into account the fact that such technologies would mitigate harsh glare, allowing window shades to be open for longer portions of the day. However, low-cost and readily scalable techniques are essential for widespread adoption of these window coating technologies. Here, we describe a potentially low-cost method to create a window coating that could enhance daylight penetration, requiring only a thin film of polymeric material deposited on an elastomeric substrate. The resulting structure is a disordered, spontaneously buckled optical grating that spreads incident light without noticeable chromatic dispersion due to its stochastic patterning. The described method has the potential to improve energy efficiency while maintaining acceptable optical clarity.

We describe a high accuracy frequency stepping method for a tunable diode laser to improve a three dimensional (3D) imaging approach based upon interferometric speckle imaging. The approach, modeled after Takeda, exploits tuning an illumination laser in frequency as speckle interferograms of the object (specklegrams) are acquired at each frequency in a Michelson interferometer. The resulting 3D hypercube of specklegrams encode spatial information in the x-y plane of each image with laser tuning arrayed along its z-axis. The specklegrams are processed by Fast Fourier Transformation (FFT) along the z-axis of the hypercube and the center of the peak in the resulting power spectrum for each pixel encodes its surface height. Alternatively, Takeda’s method can be followed which uses the phase of the FFT, unwraps it, and determines the surface height encoded in the slope of a line fitted to the phase. Wraparound of modulations above the Nyquist limit results in ambiguity in the optical path difference (OPD) between test and reference surfaces. Wraparound also amplifies measurement noise caused by errors and jitter in frequency stepping the illumination laser. By locking the laser frequency to successive cavity modes of a reference confocal interferometer, tuning is precisely controlled resulting in dramatically improved imaging quality/. We present laboratory data of before and after results showing enhanced 3D imaging resulting from precise laser frequency control.

We present the design, modeling and performance of a proof-of-concept tamper indicating approach that exploits newlydeveloped subwavelength-patterned films. These films have a nanostructure-dependent resonant optical reflection that is wavelength, angle, and polarization dependent. As such, they can be tailored to fabricate overlay transparent films for tamper indication and authentication of sensitive or controlled materials not possible with currently-known technologies. An additional advantage is that the unique optical signature is dictated by the geometry and fabrication process of the nanostructures in the film, rather than on the material used. The essential structure unit in the subwavelength resonant coating is a nanoscale Open-Ring Resonator (ORR). This building block is fabricated by coating a dielectric nanoscale template with metal to form a hemispherical shell-like structure. This curved metallic shell structure has a cross-section with an intrinsic capacitance and inductance and is thus the optical equivalent to the well-known “LC” circuit where the capacitance and inductance are determined by the nanoshell dimensions. For structures with sub 100 nm scale, this resonance occurs in the visible electromagnetic spectrum, and in the IR for larger shells. Tampering of the film would be visible though misalignment of the angle-sensitive features in the film. It is additionally possible to add in intrinsic oxidation and strain sensitive matrix materials to further complicate tamper repair and counterfeiting. Cursory standoff readout would be relatively simple using a combination of a near-infrared (or visible) LED flashlight and polarizer or passively using room lighting illumination and a dispersive detector.

We present optical subwavelength nanostructure architecture suitable for standoff radiation dosimetry with remote optical readout in the visible or infrared spectral regions. To achieve this, films of subwavelength structures are fabricated over several square inches via the creation of a 2D non-close packed (NCP) array template of radiationsensitive polymeric nanoparticles, followed by magnetron sputtering of a metallic coating to form a 2D array of separated hemispherical nanoscale metallic shells. The nanoshells are highly reflective at resonance in the visible or infrared depending on design. These structures and their behavior are based on the open ring resonator (ORR) architecture and have their analog in resonant inductive-capacitive (LC) circuits, which display a resonance wavelength that is inversely proportional to the square root of the product of the inductance and capacitance. Therefore, any modification of the nanostructure material properties due to radiation alters the inductive or capacitive behavior of the subwavelength features, which in turn changes their optical properties resulting in a shift in the optical resonance. This shift in resonance may be remotely interrogated actively using either laser illumination or passively by hyperspectral or multispectral sensing with broadband illumination. These structures may be designed to be either anisotropic or isotropic, which can also offer polarization-sensitive interrogation. We present experimental measurements of a radiation induced shift in the optical resonance of a subwavelength film after exposure to an absorbed dose of gamma radiation from 2 Mrad up to 62 Mrad demonstrating the effect. Interestingly the resonance shift is non-monotonic for this material system and possible radiation damage mechanisms to the nanoparticles are discussed.

Liquid scintillation counters measure charged particle-emitting radioactive isotopes and are used for environmental studies, nuclear chemistry, and life science. Alpha and beta emissions arising from the material under study interact with the scintillation cocktail to produce light. The prototypical liquid scintillation counter employs low-level photon-counting detectors to measure the arrival of the scintillation. For reliable operation, the counting instrument must convey the scintillation light to the detectors efficiently and predictably. Current best practices employ the use of two or more detectors for coincidence processing to discriminate true scintillation events from background events due to instrumental effects such as photomultiplier tube dark rates, tube flashing, or other light emission not generated in the scintillation cocktail vial. In low-background liquid scintillation counters, additional attention is paid to shielding the scintillation cocktail from naturally occurring radioactive material present in the laboratory and within the instrument’s construction materials. Low-background design is generally at odds with optimal light collection. This study presents the evolution of a light collection design for liquid scintillation counting (LSC) in a low-background shield. The basic approach to achieve both good light collection and a low-background measurement is described. The baseline signals arising from the scintillation vial are modeled and methods to efficiently collect scintillation light are presented as part of the development of a customized low-background, high-sensitivity LSC system.

We describe the development of a custom-built two-axis flow coater for the deposition of polymeric nanosphere monolayers that could be used in the fabrication of large area nanoplasmonic films. The technique described here has the capability of depositing large areas (up to 7 in. × 10 in.) of self-assembled monolayers of polymeric nanospheres onto polyethylene terephthalate (PET) films. Here, three sets of films consisting of different diameters (ranging from 100 to 300 nm) of polymeric nanospheres were used to demonstrate the capabilities of this instrument. To improve the surface wettability of the PET substrates during wet-deposition, we enhanced the wettability by using a forced air blown-arc plasma treatment system. Both the local microstructure, as confirmed by scanning electron microscopy, describing monolayer and multilayer coverage, and the overall macroscopic uniformity of the resultant nanostructured film were optimized by controlling the relative stage to blade speed and nanosphere concentration. We also show using a smaller nanoparticle template that such monolayers can be used to form nanoplasmonic films. As this flow-coating approach is a scalable technique, large area films such as the ones described here have a variety of crucial emerging applications in areas such as energy, catalysis, and chemical sensing.

Neutron multiplicity counters are used in safeguards to provide rapid assay of samples which contain an unknown amount of plutonium in a potentially unknown configuration. Alternatives to the use of 3He for the detection of thermal neutrons are being investigated. With appropriate detector design, the neutron single, double, and triple coincidence events can be used to extract information of three unknown parameters such as the 240Pu-effective mass, the sample self-multiplication, and the (α,n) rate. A project at PNNL has investigated replacing 3He-based tubes with LiF/ZnS neutron-scintillator sheets and wavelength shifting plastic for light pipes. A demonstrator system was constructed and is being used to test potential data acquisition system options and neutron/gamma-ray discrimination algorithms for a larger-scale system. A full-scale system has been extensively modeled to better understand the impact of all the various components. The results of the current testing and modeling effort will inform the final design of a full scale system which is expected to match the performance of existing 3He-based neutron multiplicity counters. A review of the current effort and the most recent findings will be presented.

Accurate and calibrated directional-hemispherical reflectance spectra of solids are important for both in situ and remote sensing. Many solids are in the form of powders or granules and to measure their diffuse reflectance spectra in the laboratory, it is often necessary to place the samples behind a transparent medium such as glass for the ultraviolet (UV), visible, or near-infrared spectral regions. Using both experimental methods and a simple optical model, we demonstrate that glass (fused quartz in our case) leads to artifacts in the reflectance values. We report our observations that the measured reflectance values, for both hemispherical and diffuse reflectance, are distorted by the additional reflections arising at the air–quartz and sample–quartz interfaces. The values are dependent on the sample reflectance and are offset in intensity in the hemispherical case, leading to measured values up to ∼6% too high for a 2% reflectance surface, ∼3.8% too high for 10% reflecting surfaces, approximately correct for 40–60% diffuse-reflecting surfaces, and ∼1.5% too low for 99% reflecting Spectralon® surfaces. For the case of diffuse-only reflectance, the measured values are uniformly too low due to the polished glass, with differences of nearly 6% for a 99% reflecting matte surface. The deviations arise from the added reflections from the quartz surfaces, as verified by both theory and experiment, and depend on sphere design. Empirical correction factors were implemented into post-processing software to redress the artifact for hemispherical and diffuse reflectance data across the 300–2300 nm range.

Optical imaging spectroscopy is investigated as a method to estimate radiological background by spectral identification of soils, sediments, rocks, minerals and building materials derived from natural materials and assigning tabulated radiological emission values to these ma terials. Radiological airborne surveys are undertaken by local, state and federal agencies to identify the presence of radi ological materials out of regulatory comp liance. Detection performance in such surveys is determined by (among other factors) the uncertainty in the radiation background; increased knowledge of the expected radiation background will improve the ability to detect low-activity radiological materials. Radiological background due to naturally occurring radiological materials (NORM) can be estimated by reference to previous survey results, use of global 40 K, 238 U, and 232 Th (KUT) values, reference to existing USGS radiation background maps, or by a moving average of the data as it is acquired. Each of these methods has its drawbacks: previous survey results may not include recent changes, the global average provides only a zero-order estimate, the USGS background radiation map resolutions are coarse and are accurate only to 1 km – 25 km sampling intervals depending on locale, and a moving average may essentially low pass filter the data to obscure small changes in radiation counts. Imaging spectroscopy from airborne or spaceborne platforms can offer higher resolution identification of materials and background, as well as provide imaging context information. AVIRIS hyperspectral image data is analyzed using commercial exploitation software to determine the usefulness of imaging spectroscopy to identify qualitative radiological background emissions when compared to airborne radiological survey data. Keywords: imaging spectroscopy, background radiation estimation, naturally occurring radioactive material 1. INTRODUCTION Estimation of radiological background using imaging spectroscopy may provide improved mapping and classification of the existing natural radiological backgrou nd of an airborne radiological survey area by using imaging spectroscopy data from a variety of hyperspectral or multispectral imaging platforms that exhibit much higher spatial resolution (< 30 m) than that offered by radiolog ical sensing. While the use of imaging spec troscopy is well-accepted in the earth sciences for identification of mineral resources, natural resource cataloging and abundance mapping, it has not been shown (to the best of our knowledge) that the assignment of

We present the design of an immersion-type 3D headset suitable for virtual reality applications based upon digital micromirror devices (DMD). Current methods for presenting information for virtual reality are focused on either polarizationbased modulators such as liquid crystal on silicon (LCoS) devices, or miniature LCD or LED displays often using lenses to place the image at infinity. LCoS modulators are an area of active research and development, and reduce the amount of viewing light by 50% due to the use of polarization. Viewable LCD or LED screens may suffer low resolution, cause eye fatigue, and exhibit a “screen door” or pixelation effect due to the low pixel fill factor. Our approach leverages a mature technology based on silicon micro mirrors delivering 720p resolution displays in a small form-factor with high fill factor. Supporting chip sets allow rapid integration of these devices into wearable displays with high-definition resolution and low power consumption, and many of the design methods developed for DMD projector applications can be adapted to display use. Potential applications include night driving with natural depth perception, piloting of UAVs, fusion of multiple sensors for pilots, training, vision diagnostics and consumer gaming. Our design concept is described in which light from the DMD is imaged to infinity and the user’s own eye lens forms a real image on the user’s retina resulting in a virtual retinal display.

We present the design, modeling, construction, and results of a three-dimensional imager based upon multiplewavelength speckle interferometry. Speckle imaging used in non-destructive evaluation is well-known but requires a precisely acquired reference image and can measure excursions only within the 2π ambiguity range determined by the illumination wavelength. Our approach is based upon earlier efforts pioneered by Takeda, but with updated illumination, imaging, and processing tools, in which a surface under test is illuminated with tunable laser light in a Michelson interferometer configuration. A speckled image is acquired at each laser frequency step creating a data hypercube. Interference between the reference wavefront and light from the object causes the amplitude of the speckles to cycle with laser tuning. Fourier transforming the hypercube in the laser frequency dimension reveals periods that map heights of surface features. Height resolution is determined by the maximum tuning range of the laser, which for our 16-nm tuning range provides approximately 18 micron resolution without any efforts at interpolation. The largest height without wraparound depends on the smallest tuning steps, which for our laser is 15 cm for 0.002 nm (1 GHz) tuning steps. In this way, objects with large discontinuous steps or holes can be imaged without confusion. Also, due to the illumination beam being normal to the surface under test, shadowing is eliminated. To inform our design and better understand our system’s limitations, we have developed extensive numerical models based upon Monte Carlo ray tracing in which speckle patterns are produced after scattering from model surfaces by coherent summing of rays at the detector plane. Data acquired by the system as well as modeling results will be shown.

We report divergence, astigmatism, and beam propagation factor (M2) measurements of quantum cascade lasers (QCLs) with emission wavelengths of 8.77 mum. Emission profiles from the facet showed full-width at half-maximum divergence angles of 62deg and 32degplusmn2deg for the fast and slow axes, respectively. Diffraction-limited Ge aspheric microlenses were designed and fabricated to efficiently collect, collimate, and focus QCL emission. A confocal system comprised of these lenses was used to measure M2 yielding 1.8 and 1.2 for the fast and slow axes, respectively. Astigmatism at the exit facet was calculated to be about 3.4 mum, or less than half a wave. To the best of our knowledge, this is the first experimental measurement of astigmatism and M2 reported for midinfrared QCLs.

We describe the design and phenomenology imaging results of a fully polarimetric W-band millimeter wave (MMW) radiometer developed by Pacific Northwest National Laboratory for wide-area search. Operating from 92–94 GHz, the W-band radiometer employs a Dicke switching heterodyne design isolating the horizontal and vertical mm-wave components with 40 dB of polarization isolation. Design results are presented for both infinite conjugate off-axis parabolic and finite conjugate off-axis elliptical fore-optics using optical ray tracing and diffraction calculations. The received linear polarizations are down-converted to a microwave frequency band and recombined in a phase-shifting network to produce all six orthogonal polarization states of light simultaneously, which are used to calculate the Stokes parameters for display and analysis. The resulting system performance produces a heterodyne receiver noise equivalent delta temperature (NEDT) of less than 150m Kelvin. The radiometer provides novel imaging capability by producing all four of the Stokes parameters of light, which are used to create imagery based on the polarization states associated with unique scattering geometries and their interaction with the down welling MMW energy. The polarization states can be exploited in such a way that man-made objects can be located and highlighted in a cluttered scene using methods such as image comparison, color encoding of Stokes parameters, multivariate image analysis, and image fusion with visible and infrared imagery. We also present initial results using a differential imaging approach used to highlight polarization features and reduce common-mode noise. Persistent monitoring of a scene using the polarimetric passive mm-wave technique shows great promise for anomaly detection caused by human activity.

We present a novel architecture based upon a Dicke-switched heterodyne radiometer architecture employing two identical input sections consisting of horn and orthomode transducer to detect the difference between the horizontal ( H) and vertical (V) polarization states of two separate object patches imaged by the radiometer. We have constructed and described previously a fully polarimetric W-band passive millimeter wave imager constructed to study the phenomenology of anomaly detection using polarimetric image exploitation of the Stokes images. The heterodyne radiometer used a PIN diode switch between the input millimeter wave energy and that of a reference load in order to eliminate the effects of component drifts and to reduce the effects of 1/f noise. The differential approach differs from our previous work by comparing H and V polarization states detected by each of two input horns instead of a reference load to form signals ' H and ' V from adjacent paired object patches. This novel imaging approach reduces common mode noise and enhances detection of small changes between the H and V polarization states of two object patches, now given as difference terms of the fully polarimetric radiometer. We present the theory of operation, initial proof of concept experimental results, and extension of the differential radiometer to a system with a binocular fore optics that allow adjustment of the convergence or shear of the object patches as viewed by the differential polarimetric imager. Keywords: passive millimeter wave imager, differential millimete r wave imager, polarimetric millimeter wave imager