The complex index of refraction, ñ = n + ik, has two components, n(ν) and k(ν), both a function of frequency, ν. The constant n is the real component, and k is the complex component, proportional to the absorption. In combination with other parameters, n and k can be used to model infrared spectra. However, obtaining reliable n/k values for solid materials is often difficult. In the past, the best results for n and k have been obtained from bulk, polished homogeneous materials free of defects; i.e. materials where the Fresnel equations are valid and there is no appreciable light scattering. Since it is often not possible to obtain such pure macroscopic samples, the alternative is to press the powder form of the material into a uniform disk. Recently, we have pressed such pellets from ammonium sulfate powder, and have measured the pellets’ n and k values via two independent methods: 1) ellipsometry, which measures the changes in amplitude and phase of light reflected from the material of interest as a function of wavelength and angle of incidence, and 2) single-angle reflectance using a specular reflectance device within a Fourier transform infrared spectrometer. This technique measures the change in amplitude of light reflected from the material of interest as a function of wavelength over a wide spectral domain. The optical constants are determined from the single-angle measurements using the Kramers-Kronig relationship, whereas an oscillator model is used to analyze the ellipsometric measurements. The n(ν) and k(ν) values determined by the two methods were compared to previous values determined from single crystal samples from which transmittance and reflectance measurements were made and converted to n(ν) and k(ν) using a simple dispersion model. [Toon et al., Journal of Geophysical Research, 81, 5733–5748, (1976)]. Comparison with the literature values shows good agreement, indicating that these are promising techniques to measure the optical constants of other materials.
We report the results of Differential Aperture X-ray Microscopy (DAXM) measurements near Te precipitates in CdZnTe
grown via low-pressure Bridgman. White-beam Laue patterns were acquired with 3-D spatial resolution (with 0.25 μm
resolution in the scanning directions and 1 μm resolution in depth) at depths of up to 35 μm deep normal to the surface.
We find very little crystal strain (< 10-3) or rotation (<0.05 degrees) near Te precipitates. We also examine local
deformations in the vicinity of a microhardness indent, and find that although significant rotations exist, the spatial
extent is limited to a few tens of microns. Furthermore, observed crystal strains are limited to 5 x 10-3 or less in regions
near the microhardness indent.
Dense and transparent cadmium tungstate (CWO) scintillation films have been first synthesized by sol-gel processing and their optical properties have been studied. Different precursors (tungsten oxychloride and tungstic acid), solvents (alcohol based and aqueous based) and thermal annealing processing conditions were investigated to achieve stable sols and resultant dense nanocrystalline CWO films. XRD showed CWO was the only detectable crystalline phase in the film derived by tungstic acid based sol and fast sintering at 500°C for 20 min, while the slow sintered films derived both from tungstic acid and tungsten oxychloride at 500°C for 1 hour with a heating ramp of 8°C/min resulted in porous films containing some extra tungsten oxide phases besides CWO. The fast sintered CWO film was uniform, fully dense, crack-free and of 0.5 μm in thickness. Optical transparency and photoluminescence of CWO films were characterized, and the results showed that high density and low porosity of CWO film by fast sintering led to higher transmittance and photoluminescence output. By controlling synthesis and sintering methods the nanocrystalline grains in CWO films can be of 15~52 nm in diameter. The relationships between sol-gel processing, precursor and solvent chemistry, nanostructures, densification and optical properties were discussed.
This paper reports experimental study on the development of cadmium tungstate scintillator material in the form of nanocrystal films through controlled sol-gel processing and pre-designed doping. We chose cadmium tungstate as a base material for doping and nanostructure development due to its excellent inherent photoluminescence property. In addition, our studies revealed that doping with Li+, B3+ and Bi3+ resulted in appreciably reduced grain size and porosity, leading to enhanced optical transmittance. Further analyses indicated that photoluminescence output changed significantly with dopants. The relationships between doping, defects and luminescence were discussed.
Thermoelectric Effect Spectroscopy and Thermally Stimulated Current measurements were used to investigate trapping levels in a semi-insulating CdTe and Cd1-xZnxTe crystals from multiple ingots grown by vertical Bridgman with over pressure control and high-pressure Bridgman methods. The crystals from different growth methods have different dislocation densities as well as Zn concentrations. The thermal ionization energies of these levels were extracted using both the variable heating rate and initial rise methods; the trapping cross sections were then calculated using the temperature maximum method. We report here that the shallow levels observed at E1=0.11+/- 0.02 and E2=0.17+/- 0.02 eV are intrinsic and the latter level is most likely related to the dislocation density.
Semi-insulating Cd1-xZnxTe (x = 0.1) with improved structural perfection has been grown using a gradient freeze technique and active control of the Cd partial pressure in the ampoule, both during crystal growth and cool-down of the ingots. The crystal growth was performed in low temperature gradients to minimize thermal stress and achieve material with low dislocation density. Low growth rates were also used to avoid constitutional super-cooling effects. The gradient-freeze technique allowed the growth of large single crystals extending across the entire cross-section of the ingots. The control of the Cd partial pressure allowed the solidification and cool-down of the ingots close to the stoichiometric composition. As a result, the formation and incorporation of large size (>= 1 micrometers diameter) Te inclusions was avoided during crystallization and ingots with high structural perfection were achieved. The improved structural perfection of the material was found to be associated with large spatial variation in the compensation conditions in the ingots and a resulting spatial variation of the bulk electrical resistivity of the material, ranging from 105 (Omega) cm to 1010 (Omega) cm. Samples cut from the high-resistivity sections of the ingots yield detectors exhibiting good spectral performance and an electron mobility-lifetime product of micrometers (tau) e=1.2x10-3 cm2/V.
The ability to collect broadband spectroscopic information about chemical analytes is highly desirable. We report on a technique that combines chemically selective coatings and optical spectroscopy. A 1-meter fiber 150 micrometers in diameter has approximately 5 cm2 surface area. This entire surface is used by incorporating selective moieties into the fiber cladding. The Large-Area Chemical Sensor concept for chemical sensing and measurement is based on a combination of three techniques. Specifically, it uses: (1) optical waveguides as the sensor substrate, (2) selectively adsorbing or absorbing materials to concentrate the target materials, and (3) spectroscopic interrogation for verification and quantification. The concept has been demonstrated for an iodine sensor by co-polymerizing methyl, phenyl siloxane into di-methyl siloxane. The phenyl group forms a charge-transfer complex with iodine which has an absorption at ca. 500 nm. Fused silica is the waveguide core. This system provides sensitivities in the 10-ppm range. The concept has been implemented into a prototype field iodine sensor unit. Work on the sensor concept continues with the goal of improving the sensitivity by allowing each photon multiple opportunities to interact with a target molecule.
In a conventional FTIR, the spectral content of the light is analyzed by a Michelson interferometer or other interferometer with a moving arm. The transmitted light intensity is measured by a wavelength-insensitive detector. The interferogram derived from a Michelson interferometer lies in the temporal domain. An alternative method for obtaining the interferogram involving no moving parts is demonstrated. For this, the interferogram lies in the spatial domain. This concept has many applications to sensor systems, for instance, forming a low-cost demodulator for Bragg-grating systems or field-portable spectrometers.
Neutron-sensitive scintillating glass fiber sensors provide several advantages over 3He and BF3 gas-tubes for plutonium detection and surveillance. Large active areas provide significant improvements in sensitivity versus cost. In addition, the glass sensors offer a wide dynamic counting range, fast response time, and low microphonic susceptibility relative to conventional sensors. We report the results of detection limits for neutron glass panels used for portal, freight and vehicle monitoring.
A radiation detector has been assembled that monitors human intrusion in rooms containing kilogram quantities of special nuclear material (SNM). The detector is fabricated from scintillating glass fibers that contain 6Li as the neutron absorber. The detector is designed to consume a minimum of power and to be placed in a standing position, thereby presenting a minimum profile and allowing placement in existing facilities. A small footprint is achieved by using intrinsically-thin fiber optics and by undermoderating the system. The detector operates by alarming on a rapid change in the thermal neutron count rate, which corresponds to albedo neutrons that are thermalized or absorbed in the hydrogen and carbon of human body tissues when someone enters the existing neutron flux found in SNM storage rooms.
Pacific Northwest National Laboratory is developing a fiber optic grating sensor demodulator using a low cost static Fourier-transform interferometer. The spectrometer uses a fiber optic source and a plane mirror to form an interferogram in the spatial domain that is recorded by a linear photodiode array detector. Using this instrument with an interferogram fringe spacing of 20 microns provides fiber grating strain resolution of about 700-microstrain.
A neutron spectrometer is a device that measures the spectrum of the kinetic energy of neutrons. There are numerous applications that can profitably use a compact neutron spectrometer. For instance, fast neutron resonance radiography requires sufficiently high resolution (several percent) to identify the absorption spectra of carbon, nitrogen and oxygen nuclei for incident neutrons in the thermal to 5 MeV range. In the nuclear arms-control arena, a device that can collect neutron spectral information without revealing design information would have considerable value for treaty verification. Conventional neutron spectrometers operate on a time-of-flight (TOF) basis. Neutrons of interest range in energy from thermal energy (0.025 eV) to a few MeV for special nuclear material and from ca. 100 KeV to 5 MeV for identification of explosives. A thermal neutron has a speed of ca. 2,000 mis; a 1 MeV neutron has a speed of ca. 13,000 km/sec. A TOF spectrometer has a series of choppers, each turning at different speeds, that pass only those neutrons in a given energy (velocity) range; the velocity cohort that is allowed to pass through the spectrometer and be counted is selected by varying the relative speeds ofrotation. Thus, the TOF spectrometer is, by necessity, large (meters to tens of meters). In addition, only a small fraction of all the incident neutrons are measured during any given time interval. That is, the TOF spectrometer makes very inefficient use of the neutron flux. We will describe a spectrometer that has been made practical by the development of neutron-sensitive scintillating fibers. 1-s This concept is "work-in-progress" but the results of a simple theoretical test are reported here.
This paper provides a brief summary of the scintillation process in organic and inorganic materials, the properties and uses of fiber lightguides made from these materials. Also presented are some recent data on a new application of scintillating fiber lightguides in nuclear medicine.
Pacific Northwest National Laboratory is developing a large- area, fiber-optic chemical sensor that combines chemically selective coatings and optical spectroscopy. This is a potentially hyphenated sensing technique because of the ability to collect broadband spectroscopic information in addition to sensing the quantity of the target species. Selective compound coating of optical waveguides enables the production of chemical sensors in large lot sizes. This paper describes the progress to date to produce iodine vapor selective fiber sensors that use through the fiber absorption spectroscopy. Spectra have been collected on uncalibrated I2/N2 gas mixtures using visible light.
Pacific Northwest Laboratory (PNL) has fabricated cerium-activated lithium silicate scintillating fibers via a hot-downdraw process. These fibers typically have a operational transmission length (e-1 length) of greater than 2 meters. This permits the fabrication of devices which were not possible to consider. Scintillating fibers permit conformable devices, large-area devices, and extremely small devices; in addition, as the thermal-neutron sensitive elements in a fast neutron detection system, scintillating fibers can be dispersed within moderator, improving neutron economy, over that possible with commercially available 3He or BF3 proportional counters. These fibers can be used for national-security applications, in medical applications, in the nuclear-power industry, and for personnel protection at experimental facilities. Data are presented for devices based on single fibers and devices made up of ribbons containing many fibers under high-and low-flux conditions.
Pacific Northwest Laboratory (PNL) has fabricated cerium-activated lithium silicate glass scintillating fiber waveguide neutron sensors via a hot-downdraw process. These fibers typically have a transmission length (e-1 length) of greater than 2 meters. The underlying physics of, the properties of, and selected devices incorporating these fibers are described. These fibers constitute an enabling technology for a wide variety of neutron sensors.
Pacific Northwest Laboratory is developing a large-area chemical sensor that combines chemically coatings and optical spectroscopy to detect target compounds. The chemically selective material is incorporated into the cladding of an optical fiber waveguide. The material is interrogated using optical spectroscopic techniques to determine the concentration of target compounds. The optical interrogation method includes two spectroscopies: visible-near infrared absorption spectroscopy and Raman spectroscopy. This work develops the physical and mathematical models of such a sensor and provides a set of tools with which to make design predictions for the large-area chemical sensors. The theoretical relationships derived herein allow the use of bulk absorption parameters and bulk Raman coefficients to predict sensor performance.