Various Optimizations of TES Arrays for X-Ray Astrophysics

We are developing arrays of superconducting transition-edge sensors (TES) for imaging X-ray spectroscopy telescopes such as the XMS on Constellation-X. While our primary focus has been on arrays that meet the XMS requirements (of which, foremost, are an energy resolution of 2.5 eV at 6 keV and a band pass from ∼0.3 keV to 12 keV), we are also beginning to investigate other optimizations that might be used to extend the XMS capabilities. In one of these optimizations, improved resolution below 1 keV is achieved by reducing the heat capacity. These low-energy pixels can be added to an array with broadband response either as a separate array or interspersed, depending on other factors that include telescope design and science requirements. To explore optimizations for higher count rates, we are also optimizing the design and operating temperature of pixels that are coupled to a solid substrate. We present analysis of the preliminary performance of such variations. This research was supported in part by appointments (A.-D. Brown and S.J. Smith) to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA.     

Characterization of a High-Performance Ti/Au TES Microcalorimeter with a Central Cu Absorber

We are developing X-ray microcalorimeters based on Ti/Au transition-edge sensors (TES). Among sensors we have fabricated, one with a Cu absorber at the center of the TES shows a particularly good X-ray energy resolution: 1.56 eV at 250 eV and 2.5 eV at 5.9 keV. In this paper, a detailed study of its impedance and noise is presented. The noise is not explained by a sum of known sources. The magnitude of unexplained noise is largest when the sensitivity of the TES on temperature (α) and on current (β) are the highest. The observed relation between the noise level and sensitivity suggests a source of thermal fluctuations inside the TES or between the TES and the absorber. We also found that β is linearly correlated to the product of α and current, which limits the effective sensitivity that is expressed as α/(1+β).      

Characterizing the Superconducting-to-Normal Transition in Mo/Au Transition-Edge Sensor Bilayers

We are developing arrays of Mo/Au bilayer transition-edge sensors (TES’s) for future X-ray astronomy missions such as NASA’s Constellation-X. The physical properties of the superconducting-to-normal transition in our TES bilayers, while often reproducible and characterized, are not well understood. The addition of normal metal features on top of the bilayer are found to change the shape and temperature of the transition, and they typically reduce the unexplained ‘excess’ noise. In order to understand and potentially optimize the properties of the transition, we have been studying the temperature, widths and current dependence of these transitions. We report on the characterization of devices both deposited on silicon substrates and suspended on thin silicon nitride membranes. This includes key device parameters such as the logarithmic resistance sensitivity with temperature α, and the logarithmic resistance sensitivity with current β, and their correlation with excess noise.      

A versatile, PC-based gamma ray monitor

This paper describes a new computer-based gamma ray monitor for laboratory and field measurements. The monitor consists of a universal peripheral device, and GM counter gamma ray probe, connected via USB to the PC. The peripheral device is generic; in addition to the GM probe, it accepts sensors for spectrometric measurements as well as sensors for the measurement of other environmental parameters. Owing to its low-power consumption, it is used with laptop or palmtop computer, as a portable field measurement instrument. The proposed solution has advantages over conventional instrument: (1) open architecture; (2) user-defined functions and (3) easy network connection.     

Preparation of titania covered multi-walled carbon nanotube thin films

The aim of this work was to investigate the effects of relative humidity on the formation of titania layers on the surface of multi-walled carbon nanotubes under regulated conditions in a sealed system. Reactive precursor compounds such as titanium (IV) oxychloride hydrochloric acid and titanium (IV) bromide were used as precursor to cover the surface of multi-walled carbon nanotubes (MWCNTs) under solvent conditions. The mixtures of MWCNTs and titania compounds were not stirred or sonicated. The effect of relative humidity was influenced using the mixture of sulphuric acid and water in desiccators. As-prepared titan-dioxide (TiO₂) layers were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TG), X-ray diffraction (XRD) and Raman spectroscopy. Our results revealed that TiO₂ layers with different thicknesses can be obtained using this simple sealed system. These TiO₂ covered multi-walled carbon nanotube films can be ideal candidates for different kinds of applications (e.g. sensors, virus filtration or catalysts).     

Numerical study of methods of linearization of the output characteristics of radio-frequency sensors of the level of dielectric media

Different constructions of the sensing elements of radio-frequency level sensors based on sections of long lines are numerically investigated by means of the Matlab program for the purpose of minimizing the measurement error caused by the nonlinearity of the output characteristic. It is shown that the nonlinearity is lowest for U-shaped designs. The nonlinearity factor KN may be regulated by varying the capacitance of a correcting capacitor connected to the input of the sensing element and (or) the length of one of the sections of the long line. In selecting an optimal capacitance of the correcting capacitor KN ≤ 0.15% for control media with dielectric permittivity in the range 2.2–30.     

Organic–inorganic hybrid mesoporous monoliths for selective discrimination and sensitive removal of toxic mercury ions

Abstract  

The selective optical sensing is attracting strong interest due to the use of “low-tech” spectroscopic instrumentation to detect relevant chemical species in biological and environmental processes. Our development has focused on tailoring specific solid mesoporous monoliths to be used as highly sensitive solid sensors for simple and simultaneous naked-eye detection and removal processes of extremely toxic heavy metal ions such as mercury ions in aquatic samples. The methods are emerging to design optical disc-like sensors by the immobilisation two different organic groups; however, the first organic moiety can enhance the polarity of the inorganic mesoporous disc-like monoliths “additional agents” and the second one can act as a recognition center “probe”. The latter one such as tetraphenylporphine tetrasulfonic acid (TPPS) probe led to facile handling of signal read-out with visual detection of ultra-trace concentrations of mercury ions at the same frequency as the human eye. The facile signaling was quantitatively evident using simple spectrophotometric techniques to indicate the TPPS–Hg(II) ion binding events. Control sensing assays of Hg(II) ions such as contact-time “signal response time”, thickness of support-based sensor, reaction temperature, and pH were established for achieving enhanced signal response and color intensities. Based on our results, these new classes of optical cage sensors exhibited long-term stability of recognition and signaling functionalities of Hg(II) ions that in general provided extraordinary sensitivity, selectivity, reusability, and fast kinetic detection and quantification of Hg(II) ions in our environment.     

Optical sensors for the determination of concentrated hydroxide. Characterization of the sensor materials and evaluation of the sensor performance

Optical sensors for the determination of highly concentrated bases such as NaOH (1-10 M) and materials to make these sensors have been characterized by X-ray photoelectron spectroscopy, 29Si solid-state NMR, FT-IR, and measurements of film porosity, surface area, and thickness. The bonding character and composition of the Si-Zr mixed oxides-organic polymer composites were evaluated. These studies suggest that there are Si-O-Zr matrixes in the mixed oxides, and that the Si-O-Zr matrixes contribute to the durability of the base sensors in highly alkaline solutions. The performance of these base sensors has been studied in detail as well. These sensors were stable for approximately 120 days, exhibited short response times (typically <10 s), and were fully reversible with minimal hysteresis effects in NaOH-ROH-H2O solutions (R = Me, Et, and i-Pr).     

Rutile structured SnO2 nanowires synthesized with metal catalyst by thermal evaporation method

One-dimensional (1-D) nanostructures such as tubes, rods, wires, and belts have attracted considerable research activities owing to their strong application potential as components for nanosize electronic or optoelectronic devices utilizing superior optical and electrical properties. Characterizing the mechanical properties of nanostructure is of great importance for their applications in electronics, optoelectronics, sensors, actuators. Wide-bandgap SnO2 semiconducting material (Eg = 3.6 eV at room temperature) is one of the attractive candidates for optoelectronic devices operating at room temperature, gas sensors, and transparent conducting electrodes. The synthesis and gas sensing properties of semiconducting SnO2 nanomaterials have became one of important research issues since the first synthesis of SnO2 nanobelts. Considering the important application of SnO2 in sensors, these structures are not only ideal systems for fundamental understanding at the nanoscale level, but they also have potential applications as nanoscale sensors, resonator, and transducers. The structured SnO2 nanorods have been grown on silicon substrates with Au catalytic layer by thermal evporation process over 800 degrees C. The resulting sample is characterized and analyzed by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDS). The morphology and structural properties of SnO2 nanowires were measured by scanning electron microscopy and high-resolution transmission electron microscopy. The mean diameter of the SnO2 nanorods grown on Au coated silicon (100) substrate is approximately 80 nm. In addition, X-ray diffraction measurements show that SnO2 nanorods have a rutile structure. The formation of SnO2 nanowires has been attributed to the vapor-liquid-solid (VLS) growth mechanisms depending on the processing conditions. We investigated the growth behavior of the SnO2 nanowires by variation of the growth conditions such as gas partial pressure and temperature.     

Effect of thermal treatment on ZnO substrate for epitaxial growth

ZnO is a highly efficient photon emitter, and has optical and piezoelectric properties that are attractive for a variety of applications in sensors and potentially optoelectronic devices such as emitters. Due to its identical stacking order and close lattice match to GaN, it is also being developed as a substrate material for GaN epitaxy. However, the surface finish of the ZnO is such that much of the damage induced by sawing and follow up mechanical polishing remains. We developed a thermal treatment method to eliminate surface damage on the 0 face of ZnO (0 0 0 1) to prepare it for epitaxial growth. Atomic force microscopy images of ZnO (0 0 0 1) annealed at 1050 °C for 3 h etc. show that residual scratches from mechanical polishing are removed and atomically flat, terrace-like surfaces are attained. In addition, low-temperature photoluminescence and high-resolution X-ray diffraction measurements have been employed to investigate the effect of annealing on ZnO substrates.     

Absorber Materials for Transition-Edge Sensor X-ray Microcalorimeters

Arrays of superconducting transition-edge sensors (TES) can provide high spatial and energy resolution necessary for X-ray astronomy. High quantum efficiency and uniformity of response can be achieved with a suitable absorber material, in which absorber X-ray stopping power, heat capacity, and thermal conductivity are relevant parameters. Here we compare these parameters for bismuth and gold. We have fabricated electroplated gold, electroplated gold/electroplated bismuth, and evaporated gold/evaporated bismuth 8×8 absorber arrays and find that a correlation exists between the residual resistance ratio (RRR) and thin film microstructure. This finding indicates that we can tailor absorber material conductivity via microstructure alteration, so as to permit absorber thermalization on timescales suitable for high energy resolution X-ray microcalorimetry. We show that by incorporating absorbers possessing large grain size, including electroplated gold and electroplated gold/electroplated bismuth, into our current Mo/Au TES, devices with tunable heat capacity and energy resolution of 2.4 eV (gold) and 2.1 eV (gold/bismuth) FWHM at 5.9 keV have been fabricated. A.-D. Brown’s and S. Smith’s research was supported in part by appointments to the NASA Postdoctoral Program at NASA Goddard Space Flight Center administered by Oak Ridge Associated Universities through a contract with NASA.     

Numerical and analytical method for the design of piezoelectric modal sensors/actuators for shell‐type structures

In this paper we treat the problem of designing distributed piezoelectric modal sensors/actuators for cylindrically curved panels. The design problem is tackled as an optimization problem where the design variable is a function (the polarization profile of the electrode) that takes on three values only: ?1 (negative polarization), 0 (zero polarization or no piezoelectric material), 1 (positive polarization), and the objective function is connected with the frequency response of the transducer. For the model described here, we analytically prove that the electrode patterns that make it possible to ideally isolate particular vibration modes must entirely cover the piezoelectric lamina with either positive or negative polarization. Further, we propose an accurate numerical method for systematically designing these polarization patterns and a novel algorithm for parameterizing and visualizing them in 3d. Copyright © 2009 John Wiley & Sons, Ltd.     

Effect of chemical environments on palladium phthalocyanine thin film sensors for humidity analysis

Chemiresistors based on palladium phthalocyanine (PdPc) thin films were investigated as humidity sensors. The samples were thermally evaporated onto gold electrodes with a thickness about 100 nm. Optical and electrical characteristics of PdPc thin films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and electrical measurements. The SEM image demonstrates PdPc (30–60 nm) nanosized particles, and XRD pattern shows that thin films are in α-phase at room temperature. Electrical measurements also confirm that PdPc exhibit semiconducting and photoconducting behaviors, and thermal activation energies of thin films were calculated. After that, the sensitivity and reversibility of devices were investigated on exposure to 20–90% RH in various chemical environments at 293 and 323 K. The response time (35–45 s) and recovery time (75–105 s) of sensors were measured at 293 K with respect to different chemical environments. At last, the stability of devices versus different RH% and chemical environments were tested. The sensors show very good stability on exposure to RH for a period of 2 months but their stability has been reduced in ethanol, acetone, and ammonia environments.     

Nanosized Metal-Oxide Semiconducting SrTi1 ±xO3 − δ Oxygen Gas Sensors for Low-Temperature Application

The X-ray diffraction and transmission electron microscope results show that nanosized-SrTi_1plusmnxO_3-delta material series (27 nm) with perovskite structure can be synthesized using the high-energy ball milling technique. The thick-film screen-printed nanosized-SrTi_1plusmnxO_3-delta-based sensor series with annealing temperature of 400 degC are found to have good oxygen-sensing property at near human-body temperature for the first time for such a low temperature. The effect of the deviating stoichiometry of the nanosized-SrTi_1plusmnxO_3-delta -based sensors on their sensing properties was also investigated. The optimal relative resistance (R_nitrogen/R_20%oxygen ) value of 6.35 was obtained by a nanosized-SrTiO_3-delta -based sensor at 40 degC operating temperature. Their near human-body operating temperature is much lower than that of the conventional low-temperature semiconducting oxygen gas sensors (300degC-500degC) and SrTiO₃ oxygen sensors (>700degC). This can extend the application of the semiconducting oxygen gas sensors from the conventional high and medium temperature to the lower operating temperature areas such as the medical, environmental, and domestic fields, etc     

Reversible potentiometric oxygen sensors based on polymeric and metallic film electrodes.

Various materials and sensor configurations that exhibit reversible potentiometric responses to the partial pressure of oxygen at room temperature in neutral pH solution are examined. In one arrangement, platinum electrodes are coated with plasticized poly(vinyl chloride) films doped with a cobalt(II) tetraethylene pentamine complex. For such sensors, potentiometric oxygen response is attributed to a mixed potential originating from the underlying platinum electrode surface as well as a change in redox potential of the Co(II)-tetren-doped film as the complex binds oxygen reversibly. The response due to the platinum surface is prolonged by the presence of the Co(II)-tetren/PVC film. Alternately, thin films of metallic copper, electrochemically deposited on platinum and/or sputtered or vapor deposited on a single crystal silicon substrate, may be used for reversible oxygen sensing. The long-term reversibility and potentiometric stability of such copper film-based sensors is enhanced (up to 1 month) by preventing the formation of cuprous oxide on the surfaces via the application of an external nonpolarizing cathodic current through the working electrode or by specifically using sputtered copper films that have [100] preferred crystal structures as determined by X-ray diffraction. The implications of these findings in relation to fabricating analytically useful potentiometric oxygen sensors are discussed.     

Fourier transform photocurrent spectroscopy applied to a broad variety of electronically active thin films (silicon, carbon, organics)

Steady state and low frequency photocurrent spectroscopies have proved as a valuable tool for investigation of many different semiconductors, used for example as an absorber in photovoltaic solar cells or in the large area sensors. Fourier transform photocurrent spectroscopy (FTPS), described here, exhibits advantages as a high sensitivity (we demonstrate dynamical range up to 9 orders of magnitude of the optical absorption coefficient, connected with the absorption process leading to free carriers; or sensitivity for dopant detection better than 1 part-per-billion), fast acquisition of data (it can be of the order of seconds) or high resolution (under more lengthy acquisition of data). Results on amorphous silicon, microcrystalline silicon, diamond layers, nanocrystalline diamond and very thin organic films, as poly(2-methoxy-5-(3′,7′-dimethyl-octyloxy))-p-phenylene-vinylene (MDMO-PPV), regioregular poly(3-hexylthiophene (P3HT) and their blends with (6,6)-phenyl-C61-butyric-acid (PCBM) are reported, together with the results measured on various thin film silicon or polymer solar cells.     

Evaluation of unpleasant odor with a portable electronic nose

An intelligent multi-sensor system with an entire autonomy, a low weight and a small size has been developed for in situ applications such as environmental pollutant gas detection or olfactory estimation. This portable electronic nose works with commercial metal oxide gas sensors and a microcontroller connected to a compact flash memory as intelligent unit. In this work we present the conception of this electronic nose, its laboratory validation, and a real application which concerns an outdoor air monitoring of a duck breeding. This application is developed in order to quantify continuously discomfort odor spread, causing neighbor complaints. The odor measurements were made using our electronic nose (Nepo) and results are compared to the simultaneous results obtained from other olfactometric techniques.     

Transition-Edge Sensors for Particle Induced X-ray Emission Measurements

In this paper we present a new measurement setup, where a transition-edge sensor detector array is used to detect X-rays in particle induced X-ray emission (PIXE) measurements with a 2 MeV proton beam. Transition-edge sensors offer orders of magnitude improvement in energy resolution compared to conventional silicon or germanium detectors, making it possible to recognize spectral lines in materials analysis that have previously been impossible to resolve, and to get chemical information from the elements. Our sensors are cooled to the operation temperature ( \(\sim \) 65 mK) with a cryogen-free adiabatic demagnetization refrigerator, which houses a specially designed X-ray snout that has a vacuum tight window to couple in the radiation. For the best pixel, the measured instrumental energy resolution was 3.06 eV full width at half maximum at 5.9 keV. We discuss the current status of the project, benefits of transition-edge sensors when used in PIXE spectroscopy, and the results from the first measurements.     

Creep of electrical resistance under uniaxial pressures for carbon black–silicone rubber composite

A composite comprised of dispersed conductive particles in an insulating polymer matrix is an excellent sensing material and could be used in flexible pressure sensors and tactile sensors. In this study, we investigated the variation of electrical resistance as a function of pressure for carbon black–silicone rubber composite. Samples were fabricated with different carbon black volume fractions. From experimental results, it was found that the composite has not only piezoresistivity but also electrical resistance creep behavior, which illustrates the relationship between electrical resistance and time. To describe and predict the above two phenomena, a mathematical model was established for particles filled polymer composites. When the piezoresistive composite was applied as a pressure-sensing unit, errors were seen due to “resistance creep” behavior. Based on this study, a method to inhibit such errors were investigated, developed, and realized.     

3 dimensional polymorphous silicon based metal-insulator-semiconductor position sensitive detectors

In this work we investigate the properties of a polymorphous silicon (pm-Si:H) metal-insulator-semiconductor (MIS) structure used in 3D position sensitive detectors (PSD). For the first time a 3D sensor made-up by pm-Si:H/SiO₂/Au layers is presented. MIS structures present several advantages over p-i-n structures, such as easier fabrication, fast response time and higher resolution. The 1D MIS PSD that constitute the array were extensively studied aiming its application in 3D pattern recognition. The results obtained show that MIS PSD can achieve non-linearities below 2% and sensitivities of 3.2 μA/cm over 6 mm length sensors. The miniaturization of the sensors length to arrays of 6 and 16 mm, respectively showed average non-linearities of about 1.9% for the 16 mm sensor which proved to be the best solution for this MIS structure.