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Titanium dioxide (TiO2) films have been deposited on SnO2 coated glass substrates by screen-printing. Film morphology and structure have been characterized by scanning electron microscopy, x-ray diffraction and BET analysis. Dye-sensitized TiO2 photoelectrochemical cells have been assembled and characterized. Cells sensitized with anthocyanin and a ruthenium complex have been investigated. A 0.77 cm2 ruthenium dye sensitized cell with 6.1% power conversion efficiency under Air Mass (AM1.5) conditions was obtained. Results obtained with a pure anthocyanin dye and dye extracted from blackberries were compared. Finally, a natural gel was found to improve the stability of anthocyanin sensitized cells.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The fabrication and testing of screen printed dye-sensitized large solar cell (15 cm × 15 cm) based on nanocrystalline TiO2 is described. It is the largest photo-electrochemical (PEC) cell that is based on the dye sensitization of thin (8-18 μm) films of TiO2 nanoparticles in contact with a non-aqueous liquid electrolyte. The cell has the potential to be a low cost, commercial, environmentally friendly, photovoltaic option. Surface as well as electrical characterization, of the nanostructured PEC cells have been performed. The efficiency of these large commercial cells are compared to the laboratory-made small PEC cells. Key words: Photoelectrochemical, solar cells, nanocrystalline, dye-sensitized

We report on the performance of T1Br nuclear detectors fabricated on crystals grown from highly purified material. The detectors are characterized in terms of their energy and timing resolution. In addition, parameters important to detector performance are presented. Finally, the effect of material processing on device performance is addressed.

A HgI/sub 2/ device structure was designed and tested which allows HgI/sub 2/ to be used to make low energy beta particle detectors. The devices detected tritium beta particles with an efficiency of about 25%. A protective encapsulant has been developed which should protect the devices for up to 20 years and will attenuate only a small fraction of the beta particles. It is noted that the devices hold significant promise to provide a practical alternative to liquid scintillation counters and gas flow-through proportional counters. >

Large area (≈1 cm 2 ) silicon avalanche photodiodes (SiAPDs) have been fabricated and their performance as optical detectors for use with scintillating crystals has been measured. Light sensitivity is measured for hexagonal SiAPDs of 1.57 cm 2 total package area, and the energy and timing resolution is measured for these devices coupled to CsI(Tl) scintillators operating as gamma spectrometers.

The principal problem limiting the use of HgI2 X-ray detectors is instability. We developed an encapsulation procedure potentially capable of stabilizing devices for 20 years. We also developed an accelerated test procedure to accurately simulate many years of aging.

Single crystals of InI (Eg = 2.01 eV at 300K) have been grown by vertical Bridgman technique using zone refined (ZR) starting materials. The quality of the grown crystal has been evaluated by X-ray diffraction (XRD), Electron probe microanalysis (EPMA) and Photoelectron spectroscopy (XPS). Chemically etched crystal wafer has been used to fabricate optical and nuclear detectors. The results are presented in this paper.

We have designed and built a large-area 1cm × 1cm position-sensitive solid-state photomultiplier (PS-SSPM) for use in detector design for medical imaging applications. Our new large-area PS-SSPM concept implements resistive network between the micro-pixels, which are photodiodes operated in Geiger mode, called Geiger Photodiodes (GPDs), to provide continuous position sensitivity. Here we present imaging and timing performance of the large-area PS-SSPM for different temperatures and operating biases to find the optimum operating parameters for the device in imaging applications. A detector module was built by coupling a polished 8 × 8 LYSO array, with 1 × 1 × 20 mm3 elements, to a 1 × 1 cm2 PS-SSPM. Flood images recorded at room temperature show good crystal separation as all 64 elements were separated from each other. Cooling the device at 10 °C showed significant improvement. The device optimum bias voltage was ~ 4.5V over breakdown voltage. The coincidence timing resolution was improved significantly by increasing the operating bias, as well as by lowering the temperature to 0 °C. Results show excellent imaging performance and good timing response with a large-area PS-SSPM device.

Anticoincidence detectors are required for a variety of satellite instruments, including high energy gamma-ray telescopes, in order to differentiate ambient background radiation from signals of interest. Presently, most anticoincidence systems use scintillators coupled to photomultiplier tubes. We have demonstrated that it is now possible to use very high gain solid state avalanche photodiodes (APDs) as photodetectors for this application. A single APD coupled to a 30 cm multiplied by 30 cm multiplied by 0.95 cm plastic scintillator tile demonstrated 100% detection efficiency for minimum ionizing particles, with a low false positive rate. Multiple APDs enhance the signal to noise ratio in addition to providing redundancy. Relative to PMTs, APDs are compact, low power, and mechanically robust devices. Ground test data of APDs for anticoincidence shields is presented.

Thick segmented scintillating converters coupled to optical imaging detectors offer the advantage of large area, high stopping power sensors for high energy x-ray digital imaging. The recent advent of high resolution and solid state optical sensors such as amorphous silicon arrays and CCD optical imaging detectors makes it feasible to build large, cost effective imaging arrays. This technology, however, shifts the sensor cost burden to the segmented scintillators needed for imaging. The required labor intensive fabrication of high resolution, large area hard x- ray converters results in high cost and questionable manufacturability on a large scale. We report on recent research of a new segmented x-ray imaging converter. This converter is fabricated using vacuum injection and crystal growth methods to induce defect free, high density scintillating fibers into a collimator matrix. This method has the potential to fabricate large area, thick segmented scintillators. Spatial resolution calculations of these scintillator injected collimators show that the optical light spreading is significantly reduced compared to single crystalline scintillators and sub-millimeter resolution x- ray images acquired with the segmented converter coupled to a cooled CCD camera provided the resolution to characterize the converter efficiency and noise. The proposed concept overcomes the above mentioned limitations by producing a cost-effective technique of fabricating large area x-ray scintillator converters with high stopping power and high spatial resolution. This technology will readily benefit diverse fields such as particle physics, astronomy, medicine, as well as industrial nuclear and non-destructive testing.

Boron nitride phosphide (BNxP1-x) films were grown on single crystal GaAs, using chemical vapor deposition. The films were smooth, well adhered to the substrate and exhibited resistivities on the order of 1011 ohm-cm. Photoconductive detectors fabricated from these films showed quantum efficiencies of 33% and 40% at 254 nm and 365 nm respectively, with a drop of an order of magnitude at wavelengths greater than 400 nm. These measurements demonstrate the potential of BNxP1-x as a material for visible- blind UV detectors.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Avalanche photodiodes (APDs) are solid state devices having an internal signal gain which gives them a better signal-to-noise ratio than standard photodiodes. Although they have been studied for years, recent advances in the fabrication techniques have allowed the construction of multielement arrays (up to 10 X 10) with high performance capability. This progress has resulted in increased potential for exploiting the advantages of APDs in a variety of important applications including measurements requiring fast response such as nuclear and high energy physics research, industrial nondestructive testing, medical instrumentation, and biomedical research using low energy particles. Recent experimental data characterizing APDs and APD arrays used as x-ray, particle, and low level light detectors are presented.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Thallium bromoiodide, a tuneable band gap semiconductor system, was investigated as a photodetector for scintillation spectrometers. Extensive zone refining of starting materials, based on numerical simulations, considerably enhanced the electrical resistivity to 1011 Ωcm. In addition, accelerated crucible rotation technique (ACRT) crystal growth and after-growth annealing have improved the charge carrier mobility-lifetime product. However, a relatively low signal-to-noise ratio due to a high dielectric constant and relatively low quantum efficiency continues to be an obstacle to achieving high performance, large area T1BrxI1−xphotodetectors.

This paper reports our recent work on the crystal processing, structural and optical characterization of HgBrxI2−x nuclear radiation detectors. To understand the electrical and optical properties of the detectors, we measured the energy gap of HgBrxI2−x as a function of the Br/I ratio. The energy band of this ternary semiconductor compound can be modulated from 2. 1eV (HgI2) to 3.4eV (HgBr2) by adjusting its chemical composition. This energy scope covers a wavelength spectrum between 365nm and 596nm, much of the visible spectrum. Nuclear and photoconductive detectors were fabricated from HgBrxI2−x single crystals and the responses of these devices were investigated with different radiation sources (241Am, 137Cs).

The use of cadmium telluride (CdTe) semiconductor nuclear detectors is continuing to expand into new areas because of their unique properties which include room temperature operation and high detection efficiency. In addition, they remain the material of choice in many critical applications such as nuclear medicine and power plant monitoring because of their reputation for reliability and long term stability1. CdTe is by far the most developed of the compound semiconductors used in nuclear detector applications and it offers a number of significant benefits to researchers, clinicians and engineers who have special requirements relating to size, sensitivity and operating temperature.Recently, there have been improvements in the growth of the crystalline material and in the fabrication procedures which have resulted in better performance and in the ability to produce arrays. This article describes the physical and electronic properties of CdTe nuclear detectors, discusses how the crystal growth and device fabrication procedures can affect these properties, and compares the performance to CdZnTe detectors.

The tracking of high energy, secondary products in the Superconducting Super Collider (SSC) presents challenging sensitivity and timing requirements for the scintillating fiber converters and the optical radiation detectors planned for use as signal readouts. Silicon Avalanche Photodiodes (APD) are very promising detectors which may ultimately meet these requirements.We have designed and manufactured 1 mm2 APDs, to be operated in Geiger mode as single photon detectors for the SSC. The progress on decreasing the APD response time by platinum doping, device thinning and external avalanche quenching is reported.

In many instances, the ability of scientists and engineers to make nuclear radiation measurements is only limited by the properties of available radiation detectors. Because of this, research into new semiconductor materials for radiation detection has been, and continues to be, a very active field. This article reviews recent research on several promising “new” materials.

Photoconductive detectors have been developed from a new wide bandgap (E g equals 2.01 eV) semiconductor, InI, and these photodetectors are intended for use in scintillation spectroscopy of nuclear radiation. InI single crystals were produced using the Bridgman process and these crystals were characterized by measuring their optical transmission spectrum, micro-hardness, electrical resistivity, and charge transport properties. Photodetectors were fabricated from InI crystal slices by evaporating thin ( 60%) in the 300 nm to 600 nm wavelength region and their uniform photo-response over the active detector area. Finally, these photodetectors were coupled to CsI(Tl) scintillator and were successfully tested as spectrometers at room temperature by irradiating the scintillator with 5.5 MeV (alpha) particles ( 241 Am source) and 662 keV (gamma) rays ( 137 Cs source).

The requirements of a semiconductor material intended to operate in a gamma-ray detector at room temperatures are discussed, and the status of the search for alternative materials is reviewed. The important material characteristics of a semiconductor gamma-ray detector material are high average atomic number, material's uniformity, resistivity, and electron and holes transport properties. Materials under investigation include GaAs, InP, TlBr, and PbI2. Theoretically, it is considered to be feasible to built a large volume semiconductor gamma-ray detector capable of good energy resolution at room temperature. But it is very unlikely that a semiconductor detector with germanium-like performance will be available in the next five years.

Recent advances in photomultiplier tube technology have led to the availability of position sensitive photomultiplier tubes (PSPMTs). These tubes make it possible to build a new generation of imaging instruments for gamma rays and other types of ionizing radiation. We have investigated the use of these tubes for the construction of several prototype instruments. The first application investigated measures the quantity and distribution of radioactive compounds on filter papers used in microbiology research. The intent of this instrument is to replace film autoradiography with an electronic imaging system which can analyze samples 75 to 110 times faster than film. The second application involved the development of an intraoperative imaging probe to help surgeons identify cancerous tissue and ensure its complete removal. This instrument will replace a non-imaging probe now in use at many hospitals. A third prototype instrument under evaluation is an imaging nuclear survey system which obtains both a video and gamma ray image for the purpose of locating and quantifying radioactive materials. This system would be used at nuclear power plants and radioactive materials preparation facilities. A modification of this system could be built into robots used for inspecting and repairing power plants.

Silicon drift photodiodes have been developed over the last five years and are a derivative of silicon drift chambers. These devices, while lacking the position sensitivity of the silicon drift chamber, retain the qualities of low capacitance and large area. These properties make them attractive for use in applications requiring low noise high efficiency photodetection, such as for scintillation light detectors in nuclear spectroscopy. These devices might also find other uses in photonics; as replacements for silicon p-i-n photodiodes in other applications demanding low noise operation. We report on our progress in fabricating silicon drift photodiodes for use as scintillator photodetectors, specifically optimized for detecting the 550 nm emission from CsI(Tl). The design we chose to build was a square photodiode 1 cm 2 in area, having the general features of that described by Avset et al. Although some technical problems have temporarily delayed us from producing working drift photodiodes, we have made some diagnostic measurements on our devices and have made observations that may be of general interest.

A study was undertaken to determine if laser-induced breakdown spectroscopy (LIBS) can be a practical and competitive alternative to x-ray fluorescence (XRF) methods for lead-in-paint inspection. Experiments in the laboratory confirmed that LIBS is suitable for detecting lead in paint at the hazard levels defined by federal agencies. Although we compared speed, function, and cost, fundamental differences between the XRF and LIBS measurements limited our ability to make a quantitative performance comparison. While the LIBS method can achieve the required sensitivity and offers a way to obtain unique information during inspection, the current component costs will likely restrict interest in the method to niche applications.

The preparation of radiation detectors from semiconducting lead iodide is discussed, and the performance of these devices as X-ray and gamma-ray spectrometers is evaluated. Detectors prepared from melt-grown crystals exhibit good energy resolution for low-energy ( >

Semiconductor radiation detectors have been fabricated from melt grown crystals of lead iodide (PbI2) and the performance of these detectors as room temperature X-ray spectrometers has been measured. These detectors exhibit good energy resolution (915 eV FWHM for the 5.9 keV peak of 55Fe at 20°C). Preliminary results indicate they are more stable than HgI2 detectors and capable of operating at temperatures over 100°C.

Low noise, large area silicon avalanche photodiodes have now been made to replace photomultiplier tubes in certain scintillator detector applications in which overall size and ruggedness are important. Previous devices have been limited by poor optical sensitivity and very small active size. These sensors when coupled to a 1-inch NaI(T1) crystal have produced pulse height spectra with resolution for Cs 137 gamma radiation as low as 9.5% FWHM. This achievement is the result of improvements in both silicon material quality and device design. Based on these results the future prospects of such detectors are very encouraging.

Recent research on the M-π-n CdTe device structure has resulted in the development of two new CdTe nuclear sensors. One of these operates at much higher biases than standard CdTe devices and exhibits good energy resolution and fast response. The other operates in the photovoltaic mode and requires no applied bias. It operates over a wide dynamic range and can measure radiation fluxes as high as 1012 photons cm−2 s−1. These two new sensors should significantly increase the range of applications for CdTe nuclear sensors.

Semiconductor radiation detectors have been constructed from indium phosphide to investigate the feasibility of using them in a solar neutrino detector. The 115 In (76% by mass) in an InP detector is expected to undergo the reverse beta decay reaction induced by the solar neutrino flux. This reaction produces energetic photons and electrons which might be used to identify and measure the energy of low energy neutrinos. InP detectors up to 1.0 cm 3 in volume capable of detecting individual electrons and photons have been constructed.

Crystals of highly purified thallium bromide were used to fabricate radiation detectors. These detectors were rested with isotopic photon sources and successfully detected gamma and X-rays in the 0.01- to 1 MeV energy range. The detectors were tested at and below room temperature, and their performance approaches that of other semiconductor detector materials which have been under development. The performance of TlBr detectors is due to the similarity in the transport characteristics of electrons and holes in TlBr and the relatively large number of charge carriers produced per unit of energy of detected particle. The performance of these detectors is currently limited by shot noise induced by the leakage current in the TlBr. >

An important need of the nuclear industry is an uncooled, high resolution nuclear spectrometer which would be useful in many applications including post accident monitoring, pipe inspection and health physics. Cadmium telluride detectors with high atomic number and ability to operate at room temperature, have for years been recognized as having the potential to fill the requirements of such an instrument. Previous work indicated that excellent high energy spectroscopy could be achieved using CdTe detectors with rise time discrimination. However, the resulting loss of efficiency was often unacceptable. Recently, a program h as been conducted to develop a CdTe spectrometer intermediate in resolution between NaI and HPGe systems using a PIN diode structure. This allows a much larger operating voltage and therefore a much increased photopeak fraction. By combining this device structure with the pulse discrimination technique, both high resolution and good sensitivity can be achieved. In addition the low noise and leakage current of the PIN devices, makes practical the assembly of an array with a single preamp to further increase the sensitivity.

Avalanche diodes which are sensitive enough to detect low energy x-rays and beta particles have been fabricated and tested. They operate at room temperature and their structure is based on large area avalanche photodiodes. X-ray response has been measured over the energy range of 4 keV to 26 keV. At 5.9 keV a FWHM of 10% was obtained. In addition, multi-element sensors have been fabricated. Beta particles were detected from a variety of isotopic sources including C1-36, Sr-90, C-14 and H-3. The tritium beta particles, with 5 keV average energy, were detected with a 20% counting efficiency from a calibrated source. Tritium beta particles from biological samples were also measured. The high sensitivity to low energy x-rays and beta particles makes this detector useful for many industrial, military and commercial applications, and in particular for biomedical research.

Research and development efforts on CdTe have been ongoing for many years. The improved sensor performance which has resulted from these efforts has increased the number of critical applications for which CdTe sensors are the best choice. Applications range from clinical diagnosis and life saving monitoring systems in nuclear medicine to quality assurance systems in manufacturing and safety systems in the nuclear power industry. New applications for CdTe have been increasing rapidly over the last few years and several of the most important are discussed.

We have demonstrated the potential of large area avalanche photodiodes to complement photomultiplier tubes and conventional photodiodes in numerous applications. In particular, they are compatible with BGO scintillator crystals in important applications such as positron emission tomography and high energy physics research. The photodiodes described here have overcome the limitations in size and sensitivity which had previously prevented the use of avalanche devices in these applications. As scintillation sensors in radiation detectors, one inch diameter avalanche photodiodes have been used to obtain Cs-137, 662 keV spectra with 8.5% FWHM resolution coupled to NaI at room temperature and 24% FWHM coupled to BGO at 0°C. As low energy x-ray sensors, small devices have a 10% FWHM for 5.9 keV Mn, k x-rays. The devices are also extremely sensitive to beta particles. These attributes make them useful for detecting ionizing radiation in a wide variety of applications.

A CdTe device structure was investigated for suitability in nuclear medicine instrumentation. These new detectors are room temperature, ?-ray sensors which have good charge collection and response times of 10-20 ns. Devices 0.8 mm to 5 mm thick have been characterized with regard to energy resolution and coincidence time interval resolution.

The authors fabricated particle detectors made from InP:Fe, InP:Zn, and InP:Cu and evaluated the performance of these devices. The InP detectors were suggested for gamma-ray detection with varying degrees of success. Gamma-ray pulse-height spectra were acquired and stored for detectors of each type. It was found that the InP:Fe detector performance is currently limited by the trapping of charge carriers by the iron dopant in the material InP:Zn detectors are limited by the low mobilities ( approximately=3 cm/sup 2/ V/sup -1/ s/sup -1/) existing in the material at the operating temperature of 14 K. The InP:Cu detectors fabricated are limited by defects induced in the material by the thermal cycling of the copper diffusion. >

Research has been performed on the use of chemical spray pyrolysis for depositing films of YBCO. The effects of reagent concentration, deposition rate, substrate temperature and substrate type were investigated. Films from less than 2 to more than 10 microns thick were deposited on single crystal strontium titanate, polycrystalline alumina and type 304 stainless steel.

Recent developments on HgI2 detectors fabricated from platelets grown by the Polymer Controller Growth (PCG) technique have resulted in a better understanding of this remarkable process and provide increased hope for the future of room temperature operable, high resolution x-ray detectors. The benefits of PCG are higher purity using reagent grade materials, better control of stoichiometry, rapid growth, and simplified fabrication. This latter benefit is even more significant when the extreme fragility of HgI2 crystals is considered. The problems of device resolution crystal stability and uniformity, and control ot platelet growth were investigated under a NASA funded program. HgI2 devices fabricated from PCG platelets gave room temperature energy resolutions under 400 eV (FWHM) for the 5.9 eV Mn x-ray. Models for the polymer assisted transport are discussed along with an analysis of spectra obtained from PCG grown devices. Reproducibility has been demonstrated both for platelet growth and device performance.