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Single phase polycrystalline hexagonal boron nitride (BN) or mixed with boron carbide (BxC) embedded in an insulating polymeric matrix acting as a binder and forming a composite material as well as pure submicron size polycrystalline BN has been tested as a thermal neutron converter in a multilayer thermal neutron detector design. Metal sheet electrodes were covered with 20–50 μm thick layers of composite materials and assembled in a multi-layer sandwich configuration. High voltage was applied to the metal electrodes to create an interspacing electric field. The spacing volume could be filled with air, nitrogen or argon. Thermal neutrons were captured in converter layers due to the presence of the 10B isotope. The resulting nuclear reaction produced α-particles and 7Li ions which ionized the gas in the spacing volume. Electron-ion pairs were collected by the field to create an electrical signal proportional to the intensity of the neutron source. The detection efficiency of the multilayer neutron detectors is found to increase with the number of active converter layers. Pixel structures of such neutron detectors necessary for imaging applications and incorporation of internal moderator materials for field measurements of fast neutron flux intensities are discussed as well.

Single phase polycrystalline hexagonal boron nitride (BN) or mixed with boron carbide (B x C) and/or polycrystalline cubic lithium fluoride (LiF) can be embedded in an insulating polymeric matrix. This matrix acts as a binder forming a composite material. Such materials were tested as surface neutron converters of gaseous detectors. Signals were induced by ionized air near electrodes. The detectors show a rather good spectral response for ionized air produced by alpha particles emitted from a 241 Am (5.5 MeV) isotope source (∼10 5 cm −2 s −1 ) as well as low gamma sensitivity. For detection of thermal neutrons from a low flux source (∼200 cm −2 s −1 ), neutron counting is possible by subtracting the integrated noise from the integrated total signal counted above some threshold; here again the signals are produced by the ionized air and cannot be detected in vacuum.

Lead iodide (PbI2) polycrystalline thin films with [0 0 1] and [1 1 0] orientations have been grown using a physical vapor-deposition method on substrates coated with ITO or gold electrodes. The films orientation as function of the deposition conditions, such as substrate temperature and coating material, was characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Four different types of structures of PbI2 films deposited on ITO were identified and named A, B, C and D, where each structure is stable in its own temperature range. Changes of structure during the growth process due to the temperature range change were demonstrated. Moreover, the dependence of the temperature range and film structure on substrate coating materials was found. The electrical properties and detector response correlation with film structure have been summarized.

Composite polycrystalline semiconductor detectors bound with different binders, both inorganic molten glasses, such as B 2 O 3 , PbO/B 2 O 3 , Bi 2 O 3 /PbO, and organic polymeric binders, such as isotactic polypropylene (iPP), polystyrene or nylon-6, and coated with different metal electrodes were tested at room temperature for α -particles and very weak thermal neutron sources. The detector materials tested were natural occurring hexagonal BN and cubic LiF, where both are not containing enriched isotopes of 10 B or 6 Li. The radiation sources were 5.5 MeV α 's from 241 Am, 5.3 MeV from 210 Po and also 4.8 MeV from 226 Ra. Some of these detectors were also tested with thermal neutrons from very weak 227 Ac 9 Be, 241 Am– 10 Be sources and also from a weak 238 Pu+ 9 Be and somewhat stronger 252 Cf sources. The neutrons were thermalized with paraffin. Despite very low signal to noise ratio of only ∼2, the neutrons could be counted by subtracting the noise from the signal.

Mercuric iodide (HgI2) and lead iodide (PbI2) materials have been investigated for several years as direct converter layers for digital x-ray imaging applications. A difficult challenge of both lead iodide and mercuric iodide is the higher than desired leakage currents. These currents are influenced by different factors such as applied electrical field, layer thickness, layer density, electrode structure, material purity and by the deposition parameters. Minimizing the leakage current must also be achieved without adversely affecting charge transport, which plays a large role in gain and is influenced by these parameters. Other challenges relate to increasing film thickness without degrading electrical properties. This paper compares some imagers as the result of optimization process. We deposited the above materials on flat panel thin film transistor (TFT) arrays with 127 um pixel pitch. The imagers were evaluated for both radiographic and fluoroscopic imaging. Modulation Transfer Function (MTF) was measured as a function of the spatial frequency. The MTF data were compared to values published in the literature for indirect detector (CsI). Image lag characteristics of mercuric iodide appear adequate for fluoroscopic rates. The structure and x-ray diffraction data of the two materials were compared to explain the difference in image lag between them

Polycrystalline films of HgI2, which are used as nuclear radiation detectors, are fabricated by two different methods, either by Physical Vapor Deposition (PVD) or glued with a polymeric binder called also "Particle in Binder" (PIB). Both types of PVD or PIB Detectors are deposited directly on Thin Film Transistor TFT technology imagers and can be used for X-ray imaging. The present review is concentrated on PVD- Polycrystalline-HgI2. However, the main review body is a summary of the work performed by RTR, HU, and Varian. Works by other groups are mentioned briefly. The PVD process is explained by reviewing X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM) data. The charge transport properties of the PVD-Polycrystalline HgI2 detectors as they improved with R&D efforts are shown. The response of these PVD detectors to 241Am and 57Co gamma rays and their X-ray imaging properties such as dark currents, sensitivity, imaging resolution and Detector Quantum Efficiency (DQE) are reviewed

Polycrystalline HgI/sub 2/ layers prepared by different modifications of physical vapor deposition (PVD) exhibit different microstructure. Under some fabrication procedures, the samples exhibit a columnar structure, with columns highly oriented in the [001] direction (c-axis) normal to the layer surface. Differences in manufacturing procedures manifest themselves in different average column length, different porosity, and different average material density. The most nonporous, dense, thick HgI/sub 2/ layers are obtained by activating the preferential growth along the c-axis perpendicularly to the substrate plane. The microstructure correlates to the material electrical conduction properties: dark current, mobility, and trapping time. For a sufficiently pure starting material, and grain length approaching the layer thickness, the layer may exhibit electron mobility as high as /spl mu//sub n/=87 cm/sup 2//V/spl middot/s, electron trapping time as long as /spl tau//sub n/=18 /spl mu/s, hole mobility /spl mu//sub p/=4.1 cm/sup 2//V/spl middot/s, and hole trapping time of /spl tau//sub p/=3.5 /spl mu/s. These values are quite close to those of a single crystal. Nuclear detectors fabricated using such layers exhibit energy resolution of gamma absorption, as demonstrated for the 59.6 keV emission of /sup 241/Am.

Polycrystalline films of HgI/sub 2/ Prepared by Physical Vapor Deposition (PVD) exhibited electrical charge transport properties similar to those of single crystals. Transient charge transport (TCT) measurements were used to evaluate their electrical properties. The mobility /spl mu/, trapping time /spl tau/, and surface recombination velocity s of electrons or holes were determined by analyses of transient voltages developed across the sample in response to a drift of the corresponding charge carriers created by alpha particle absorption near one of the electrodes. Typical electron-, and hole mobilities were /spl mu//sub n/ = 88 cm/sup 2//V/spl middot/s and /spl mu//sub p/ = 4.1 cm/sup 2//V/spl middot/s, respectively; Trapping times were /spl tau//sub n/ >/spl tilde/16 /spl mu/s and /spl tau//sub p/

A review of growth methods used to produce Cd 1-x Zn x Te (CZT) (0.0

Characterization studies of Cd1−xZnxTe (0

Polycrystalline HgI 2 thick film detectors are among the leading semiconductor materials to be used as direct converters in X-ray digital radiography. Their properties along with a survey of the properties of alternative materials, such as PbI 2 or A-Se, will be given. The preparation of HgI 2 detector plates, both by direct evaporation (Physical vapor deposition, (PVD)) and by binding the individual crystallites with polymeric glue, forming screen-printed (SP) detector plates, will be described. The microstructure of the PVD thick films showing a columnar morphology, as determined by SEM measurements, will be shown. The X-ray response to radiological X-ray generator of 85 kVp using the current integration mode will be reported for both PVD and SP films. Finally, some X-ray images taken at Xerox-Parc using HgI 2 polycrystalline detectors will be shown.

Characterizations of Cd1−xZnxTe (0.04

This paper reviews some of the progress obtained in the understanding of defects in detector grade cadmium zinc telluride material (CZT). Several techniques have been utilized to elucidate some of the issues related to compositional uniformity, effects of precipitates, grain boundaries, and surface defects related to mechanical and chemical treatments. In few cases, special mapping capabilities had to be developed to allow correlations with detector performance.

Vertical high-pressure Bridgman (VHPB) Cd 1- x Zn x Te (0.04 x 241 Am radioactive source was found. Electron lifetimes below 1 μs were measured in crystals having significant numbers of micro-defects, compared to lifetimes of 5–15 μs found in spectrometer grade materials produced in the US. Furthermore, the zinc composition along the growth axis showed better homogeneity in comparison with the US material.

Sandia National Laboratories (SNL) is leading an effort to evaluate vertical high pressure Bridgman (VHPB) Cd1−xZnxTe (CZT) crystals grown in the former Soviet Union (FSU) (Ukraine and Russia), in order to study the parameters limiting the crystal quality and the radiation detector performance. The stoichiometry of the CZT crystals, with 0.04

Critical current density, J/sub c/, of Ag/Bi(Pb)SCCO tapes in as-deformed and as-sintered states were studied at different stages of tape processing. Intermediate pressing and rolling result in significant I/sub c/ degradation. Non-zero I/sub c/ remains after the applications of pressing load up to 11 and 8 GPa following the third and first sintering, respectively. Under rolling conditions I/sub c/ remains non-zero up to maximum tape thickness reduction of 20%. Bending tests show that I/sub c/ reduction is the result of deformation-induced micro-crack formation which are easily healed during the first hour of the next sintering. Intermediate deformation after the first sintering cycle has been shown to accelerate significantly the 2212 to 2223 phase transformation during the first hours of the second sintering step. I/sub c/ growth during the first hour of the second sintering cycle is not connected with that phase transformation and is presumed to be the result of pinning ability and structure connectivity enhancement.

The ability of HgI2 powders, used as precursors in mercuric iodide crystal growth, to produce high-quality detectors may be predicted by non-destructive methods like photoluminescence. In fact, it is possible to correlate the presence and the intensity ratio of specific bands in the photoluminescence spectrum of a HgI2 crystal to its impurity content and stoichiometry. These quantities determine the detector grade that may be achieved using that starting material. Nine different HgI2 precursors, obtained by different purification methods, have been characterized. The lowest impurity content is achieved via poly-ethylene treatment, which gives also a powder of relatively good stoichiometric quality.

Potentially low cost and large area polycrystalline mercuric iodide room-temperature radiation detectors, with thickness of 100–600 μm have been successfully tested with dedicated low-noise, low-power mixed signal VLSI electronics which can be used for compact, imaging solutions. The detectors are fabricated by depositing HgI 2 directly on an insulating substrate having electrodes in the form of microstrips and pixels with an upper continuous electrode. The deposition is made either by direct evaporation or by screen printing HgI 2 mixed with glue such as Poly-Vinyl-Butiral. The properties of these first-generation detectors are quite uniform from one detector to another. Also for each single detector the response is quite uniform and no charge loss in the inter-electrode space have been detected. Because of the low cost and of the polycrystallinity, detectors can be potentially fabricated in any size and shape, using standard ceramic technology equipment, which is an attractive feature where low cost and large area applications are needed. The detectors which act as radiation counters have been tested with a beta source as well as in a high-energy beam of 100 GeV muons at CERN, connected to VLSI, low noise electronics. Charge collection efficiency and uniformity have been studied. The charge is efficiently collected even in the space between strips indicating that fill factors of 100% could be reached in imaging applications with direct detection of radiation. Single photon counting capability is reached with VLSI electronics. These results show the potential of this material for applications demanding position sensitive, radiation resistant, room-temperature operating radiation detectors, where position resolution is essential, as it can be found in some applications in high-energy physics, nuclear medicine and astrophysics.

I–V characteristics, transport critical current density Jc at 77 K in moderate magnetic fields and irreversibility fields were measured in Ag/BiSCCO-2223 tapes at various stages of the OPIT process. Intermediate deformation dramatically reduces Ic in as-deformed state while retaining a finite-size percolative channel for transport current after used pressing load up to 11 GPa. The channel has a similar fraction of strong links (45%) and vortex pinning characteristics as those in a non-deformed state. Combination of intermediate deformation with subsequent sintering improves pinning characteristics of the tapes, reasonably increases vortex-glass–vortex-liquid crossover field Hg and significantly accelerates Ic growth during subsequent thermal processing.

We describe the current state of nuclear radiation detectors produced from single crystals of Cd1−xZnxTe(CZT), with 0.04

The material showing the greatest promise today for production of large-volume gamma-ray spectrometers operable at room temperature is cadmium zinc telluride (CZT). Unfortunately, because of deficiencies in the quality of the present material, high-resolution CZT spectrometers have thus far been limited to relatively small dimensions, which makes them inefficient at detecting high photon energies and ineffective for weak radiation signals except in near proximity. To exploit CZT fully, it will be necessary to make substantial improvements in the material quality. Improving the material involves advances in the crystallinity, purity, carrier lifetimes, and control of the electrical compensation mechanism. A more detailed understanding of the underlying material problems limiting the performance of CZT gamma-ray detectors is required; otherwise, problems with supply, delivery times, and unit cost of large-volume (>5 cm3 active volume) CZT spectrometers are expected to continue. A variety of analytical and numerical techniques have been employed to quantify crystallinity, strain, impurities, compositional and stoichiometric variations, bulk and surface defect states, carrier mobilities and lifetimes, electric field distributions, and surface passivation. Data from these measurements were correlated with spatial maps of the gamma-ray and alpha particle spectroscopic response, and feedback on the effectiveness of crystal growth and detector fabrication procedures has been generated. The results of several of these analytical techniques will be presented in this paper.

CVD diamond radiation sensors are being developed for possible use in trackers in the LHC experiments. The diamond promises to be radiation hard well beyond particle fluences that can be tolerated by Si sensors. Recent results from the RD 42 collaboration on charge collection distance and on radiation hardness of CVD diamond samples will be reported. Measurements with diamond tracking devices, both strip detectors and pixel detectors, will be discussed. Results from beam tests using a diamond strip detector which was read out with fast, 25 ns shaping time, radiation-hard pipeline electronics will be presented.

Mercuric iodide ceramic radiation detectors, which can act as nuclear particle counters, have been fabricated with single continuos electrical contacts and with linear strip contacts. They have been tested with different kinds of γ and β sources as well as in a high energy beam at CERN. The detectors were also successfully tested for radiation hardness with irradiation of 5*10 14 neutrons/cm 2 . The ratio of detected photons over the number of absorbed photons has been measured with T sources of different energies, and it ranges from 20% at 44 keV up to about 30% at 660 keV. An absolute efficiency of 70% has been measured for a 350 gm thick detector for β particles emitted by a 90 Sr source. Charge collection efficiency, defined as the amount of charge induced on the electrodes by a Minimum Ionizing Particle (MIP) traversing the detector, has been measured in two samples. The average collected charge fits well with a linear curve with slope of 35 electrons/(kV/cm) per 100 pro. This result is well described by a dynamic device simulation, where the free carrier mean lifetime is used as a free parameter, adjusted to a value of 1.5 ns, i.e. about 1/100 of the corresponding lifetime in single crystal HgI2 detectors. The response to MIP has also been studied with a high energy (100 GeV) muon beam in CERN. A preliminary beam profile is presented while a more detailed analysis is still in progress and will be presented elsewhere. These results together with the low cost of the material make ceramic HgI 2 detectors excellent candidates for large area particle tracking and imaging applications, even in a radiation harsh environment.

Polycrystalline mercuric iodide nuclear radiation detectors have been produced in a novel technology. Unlike the normal single-crystal technology, there is no intrinsic limit to the surface on which these detectors can be produced. Detectors with areas up to about 1.5 cm/sup 2/, thicknesses from 30 to 600 /spl mu/m, and with single electrodes as well as microstrip and pixel contacts have been fabricated and successfully tested with photons in the range of 40-660 keV, /spl beta/ particle's emitted from a Sr-Y source, and high energy (100 GeV) muons. Results on both charge collection and counting efficiency are reported as well as some very preliminary imaging results. The experimental results on charge collection have been compared with simulation, and a combined /spl mu//spl tau/ product 10/sup -7/ cm/sup 2//V for electrons has been estimated.

Single crystals of Cd/sub 0.8/Zn/sub 0.2/Te grown at the Institute of Solid State Physics, Chernogolovka, Russia, by the high-pressure vertical Bridgman method (HPVB) were mapped using X-ray fluorescence (XRF), X-ray diffraction (XRD), photoluminescence (PL), and leakage current measurements, most of the Russian samples which we refer to as p-type CZT were more uniform in Zn composition than U.S. commercially produced material. The Russian material had a poorer crystallinity and, in the best case, could only count nuclear radiation. Differences in the material properties between Russian (p-type) and U.S. (n-type) material will be described.

Texture formation in the ceramic filament of Ag/BiSCCO tapes was studied after cold rolling, uniaxial cold pressing and sintering by XRD /spl theta/-2/spl theta/ and /spl omega/-scan methods. More effective texturing of the outer layers as compared to that of the core interior was observed at the early rolling steps up to about 65% reduction in area. Texture inhomogeneity over the filament cross section is related to the inhomogeneous distribution of plastic deformation. Work hardening due to plastic deformation of non-sintered powder results in the broadening of X-ray peaks and an increase in microhardness. The recovery of these parameters occurs after annealing at the temperatures of 350-600/spl deg/C. The /spl omega/-scan studies revealed an improvement of c-axis texture by intermediate rolling compared to that in the as-sintered and pressed states. Deformation-induced texturing of the 2212-precursor is the main factor responsible for the texture of the final 2223-phase. The influence of Ag during the sintering stage is limited only to the outer layers of the filament; in the case of low preliminary deformation internal layers remained practically non-oriented.

We examined the effects of polishing and etching on the structural and electrical properties of various high pressure Bridgman Cd1−xZnxTe (CZT) crystals using high resolution x-ray diffraction (HRXRD) and alpha particle mapping. Two etching solutions investigated are: (1) standard bromine-methanol solution, and (2) standard solution mixed with lactic acid. HRXRD, and in particular, triple axis x-ray diffraction (TAXRD) showed the effectiveness of the bromine-methanol etch in removing residual strain and damage from the CZT crystal. TAXRD mapping of a ∼5 cm2 CZT crystal after etching resulted in a reduction of the average rocking curve full width at half maximum to 15 arc-sec (compared to 23 arc-sec for the “as-received”). Alpha particle mapping of the electron risetime and the pulse height spectrum, along with leakage current measurements, showed varying effects of the different etching solutions on the surface properties (and hence their influence on the electrical and detector properties). These preliminary results show the importance and the sensitivity of the overall detector properties on the surface preparation conditions of CZT crystals used as radiation detectors.

Single crystals of p-type semi-insulating Cd1 − xZnxTe (CZT) with x = 0.2, 0.3 and 0.4 were grown using the high-pressure vertical Bridgman method. The crystals showed a high degree of axial homogeneity over most of the length of the ingot. The crystals were characterized by measuring the specific resistivity, X-ray spectral analysis, triaxial rocking curves, photoluminescence, etch-pit density and photocurrent kinetics. Despite the high homogeneity the photocurrent kinetics are worse than n-type CZT (x = 0.1 and 0.2) also grown by high-pressure vertical Bridgman.

Various methods have been used to synthesize Ba(THD)2 and its molecular structure has been studied using nuclear magnetic resonance. Compared with Raman and infrared spectroscopy it was found that NMR is very useful to determine quantitatively the decomposition through aging of Ba(THD)2. The transport kinetics of Ba(THD)2 has been studied under experimental conditions of a OMCVD reactor. It has been found that the freshly prepared Ba(THD)2 by the reaction of Ba metal with THD ligand in anhydrous conditions with subsequent crystallization from methanol transported quantitatively without decomposition. A simple model shows that the transport kinetics corresponds to a diffusion controlled process.

Variable angle spectroscopic ellipsometry (VASE) and atomic force microscopy (AFM) measurements have been employed to characterize the surface aging of HgI 2 crystals — a material for room-temperature radiation detectors. Four different initial HgI 2 surface conditions were selected, i.e. an as-grown surface, a cleaved surface, a surface chemically etched by 10% KI solution and a parylene coated surface. A model including top surface roughness and subsurface defects of HgI 2 crystal, related to surface aging, was established and characterized, as a function of real time, by the VASE analysis. The VASE measurements indicated that high surface aging rates were related to high initial effective two-dimensional (2D) surface-defect densities. The AFM profiles revealed increasing physical surface roughness as surface aging took place. The cleaved HgI 2 crystal surface presented a smooth surface and the lowest surface aging rate, while the as-grown HgI 2 surface also presented a very low surface aging. A parylene coating on the HgI 2 surface can greatly slow down the surface aging.

We have employed both low-temperature photoluminescence (PL) spectroscopy and high-spatial-resolution, room-temperature PL mapping to determine composition variation in Cd1 − xZnxTe grown by high-pressure Bridgman. Composition variations Δx of approximately 5–10% are observed between tip and heel of an approximately 13 cm long boule, while fluctuations of 1–2% are observed within a single, detector-scale sample. We also show that there are great discrepancies in the calculated zinc concentration, depending on which expression for Eg(x) is chosen from the literature. We have performed high-resolution, triaxial X-ray analysis on selected samples to determine which of the published relations is most accurate for our material. We have also examined the relationship between the low-temperature PL spectrum and detector performance, and found that the line width of the donor-bound-exciton peak can be used as a predictor of performance.

The crystalline perfection of solid state radiation detectors was examined using triple axis x-ray diffraction. Triple axis techniques provide a means to analyze the origin of diffraction peak broadening: the effects of strain (due to deviations in alloy composition or stoichiometry) and lattice tilts (mosaic structure) can be separated. Cd1 − xZnxTe (x ≈ 0.1), HgI2, and GaAs detector materials were studied. In the cases of Cd1 − xZnxTe and HgI2 the crystalline properties of detectors with different spectral responses to γ-radiation were determined. Increased mosaicity was universally found to be related to deteriorated detector properties. For Cd1 − xZnxTe, detectors with poor performance possessed greater levels of diffuse scatter due to lattice tilts than did high quality detectors. For GaAs, low angle grain boundaries were attributed to impaired detector performance. Additionally, in large HgI2 detectors, deviations from stoichiometry were also related to reduced performance. Interestingly, HgI2 detectors which possessed a sharp spectral response to γ-radiation but also showed polarization were of comparable crystallinity to those detectors which did not exhibit polarization effects. This initial analysis suggests that polarization is related to native point defects or chemical impurities which do not significantly alter the crystallinity of the material. Overall, within a given class of materials, improved detector performance (better spectral response) always correlated with better material quality.

GaAs is a wide band gap (1.42 eV) semiconductor that has shown promise as a room temperature operated γ-ray detector. A practical γ-ray detector would be large in volume, hence the resistivity of the material must be high to ensure large depletion volumes and low leakage currents. Commercially available semi-insulating (SI) bulk GaAs is compensated by a balance between native defect deep donors (EL2) and residual dopant impurities. The high concentrations of electrically active deep and shallow levels are believed to contribute to electric field distortions observed in γ-ray detectors fabricated from SI bulk GaAs. Hence, the controlled reduction of native defects and contaminant impurities may yield improved bulk GaAs for γ-ray detectors. Custom grown vertical zone melt (VZM) bulk GaAs is presently under investigation as an alternative material for room temperature operated γ-ray detectors. The VZM technique allows for zone refinement (ZR) and zone leveling (ZL) of the GaAs ingots. Custom growth of the material allows for controlled changes in the bulk crystal, including deliberate reductions in EL2 and impurity concentrations. Comparisons are made to commercial vertical gradient freeze (VGF) and liquid encapsulated Czochralski (LEC) bulk GaAs.

The microstructure and phase distribution of palladium thin films sputter deposited onto α-HgI2 for use as electrical contacts in radiation detectors are investigated using electron microscopy. Our results show a limited reaction to form palladium mercuride (PdHg). It is shown that the formation of PdHg via several reaction pathways is thermodynamically feasible.

The transient charge technique, TCT, was used to measure the trapping levels of pure and doped HgI2 detectors over the temperature range of 170–300 K. The dopants were excess of Hg and I2, naphthalene, butane, heavy paraffins, polyethylene and polyethylene glycol. The charge carriers were induced by α particles as well as by a UV nitrogen (337 nm) pulsed laser beam. The ICT determined trapping levels are compared with levels reported in literature from other solid state measurements.

We report on the results of a number of investigations into the material properties of lead iodide and their relation to x- and gamma-ray spectrometers. The effectiveness of zone refining as determined by inductively coupled plasma optical emission spectroscopy is demonstrated. We show that zone refining is effective in producing lead iodide with a higher degree of purity in terms of extrinsic dopants and we determine the segregation coefficients for a number of these impurities. Low temperature photoluminescence also indicates an improvement in the material properties of the purified lead iodide. The chemical etching characteristics, including etch rates, of lead iodide are presented for a number of etching solutions. Triple axis x-ray diffraction measurements have been employed to determine the structural perfection of the lead iodide after diamond sawing and etching and recovery of the crystallinity of the material is seen after Nal etching.

The primary problems with mercuric iodide for use in room-temperature radiation detectors are the low yield of high quality devices and the problems with detector stability. It is believed that one of the prime causes of these failures is the presence of certain detrimental impurities. Previous experiments have demonstrated that these impurities can be introduced during processing and that they can strongly affect detector performance. Significant effort has been expended to develop and utilize various mercuric iodide purification schemes. While these techniques are effective to some degree, the level of impurities in detector materials is still higher than that found in more established semiconductors. In this work we developed and investigated a new purification scheme that is based on the mobility of many of the impurities in mercuric iodide under an applied electric field. In this “electrodrift” process undesirable metallic impurities are removed from a portion of a bulk mercuric iodide charge to produce higher purity material for detector crystal growth. The lower level of impurities results in fewer charge-trapping centers and therefore there is the potential for significant improvements in the carrier lifetimes and detector stability. This paper reports on some preliminary results of the effectiveness of this method.

The leading materials which operate as room temperature nuclear radiation detectors are HgI2, CdTe, and Cd1−xZnxTe (0.05>x>0.25). However, additional materials have also been developed, such as semi-insulating GaAs and PbI2. A comparison of the charge transport properties of all these materials will be made, followed by a discussion of each of the materials separately. Crystal growth methods of spectrometer-grade materials will be mentioned, and defects which limit their performance will be discussed. Nuclear spectra measurements with detectors fabricated from these materials, for low X-ray energies as well as for high-energy gamma-rays, will be shown. Polarization effects which occur in some detectors such as HgI2 will also be discussed. Correlation between crystalline perfection and detector performance will be shown. Results of quantitative chemical analysis of various detector materials and problems encountered in determining accurate values ofx in Cd1−xZnxTe and its homogeneity in the bulk will be presented. Finally, the present state of the art and developments for the near future will be discussed.

Red mercuric iodide (α-HgI 2 ) has been studied extensively for use as a room-temperature radiation-detector material. Although it has several properties which make it an extremely attractive material for such applications, it has not seen large-scale deployment, particularly in the commercial market, because of several continuing problems. These problems arise, in part, from some of the intrinsic properties of the material. However, the primary limiting factors at this point are related to the phenomena of incomplete charge collection and device polarization. This paper examines one purported cause for both the carrier trapping and the device instability. In particular, silver is an elemental impurity that is known to exist in typical HgI 2 . Leakage-current experiments demonstrate how this electrically active impurity is mobile in bulk HgI 2 . Detector characterization shows how the presence of the silver degrades the performance and affects the spectral properties of the device over time. These results are combined with related work on impurities in HgI 2 to develop a picture of the effects of silver on detectors and the processing requirements for detector fabrication.

Raman and infrared (IR) spectra of Ba, Sr and Ca 2,2,6,6-tetramethyl-3,5-heptanedionates (THD), volatile precursors used for organometallic chemical vapor deposition (OMCVD) of high- T c superconducting (HTCS) thin films, have been studied for freshly supplied commercial materials or those prepared in our laboratory and aged for several months in vacuum and in air. Some of the spectral lines present in freshly prepared material disappeared in precursors aged in air but were unchanged if aged in vacuum. Only freshly prepared or vacuum-stored Ba(THD) 2 could be 100% transported by sublimating at 210°C. Air aged material transported only 60% under the same conditions of evaporation. Mixing 20% aged Ba(THD) 2 with 80% naphthalene achieved 100% transport of Ba(THD) 2 .

Vertical zone melt (VZM) bulk GaAs boules have been zone refined (ZR) and zone leveled (ZL) to reduce EL2 deep donor levels and impurity concentrations with the intent of improving properties for gamma ray detectors. ZR and ZL GaAs boules had background impurity levels and deep donor EL2 concentrations near or below detectable limits. The crystal mosaic of the material at locations near the seed end was slightly superior to commercial liquid encapsulated Czochralski (LEC) material, and nearly equivalent to commercial vertical gradient freeze (VGF) material. The crystal mosaic in ZL material degraded towards the tail end. The homogeneity of the electrical properties for the ZL and ZR VZM material was inferior compared to commercially available bulk GaAs material. Post growth annealing may help to homogenize some electrical properties of the material. The charge collection efficiency of the ZR GaAs detectors was only 30% maximum, and only 25% maximum for the ZL GaAs detectors. Resulting gamma ray spectra was poor from detectors fabricated with the ZL or ZR VZM material. Detectors fabricated from material that was both ZR and ZL did not demonstrate gamma ray resolution, and operated mainly as counters. The poor spectroscopic performance is presently attributed to the inhomogeneity of the electrical properties of the ZR and ZL GaAs materials. Comparisons are made with detectors fabricated from VGF SI bulk GaAs.

Two sol-gel (citrates and acetates- derived) methods for the 2223 BiSCCO precursor powders and film preparation were studied. The acceleration of diffusion processes due to intimate cation mixing and homogeneity on atomic scale provides the effective shortening of the sintering time for the 2223-phase formation. In the case of fluorine-doped Bi(Pb)SCCO, single phase 2223 sample is formed after 48 h sintering at 840/spl deg/C. The multi-core composite Ag/BiSCCO highly c-axis textured tapes and multifilament wires were fabricated by slurry casting and powder-in-tube methods, respectively. >

The effect of various doping elements such as (V+Y), As, Sb, Pb, Ag and Ge in range of 3–5 atom% on the phase stability of the high temperature superconducting (HTS) Bi - Sr - Ca - Cu - O system was investigated by preparing (1) small single crystals from flux solvents, and (2) thick films by doctor blade casting. It was found that the Bi2Sr2CaCu2Ox (2212) is the predominant phase in all doped samples Examples of HTS critical temperature Tc results measured by microwave absorption are 94 K for (V+Y), 78 K for As, 74 K for Ge. For the Sb doped compound a two phase BSCCO material was obtained with two Tc of 90 and 78°K in the proportion of 20 to 80% respectively.

The near-stoichiometric region of the HgI 2 compound has been studied to update the phase diagram of the iodine - mercury system. Stoichiometry evaluation has been performed using Raman spectroscopy, to determine the Hg excess as Hg 2 I 2 , and photometric analysis for determination of the iodine excess. The improved sensitivity of the methods has enabled to determine minute amounts of either mercury or iodine excess, as low as few ppm. HgI 2 crystals doped by either mercury or iodine have been grown from the vapor phase at temperatures ranging from 58 to 200°C, and the results have been used for the completion of the phase diagram. The maximum amount of Hg dissolved in HgI 2 is 720 ppm at 120°C, but at higher crystal growth temperatures the concentration is reduced due to the thermal decomposition of Hg 2 I 2 . Free iodine solubility in HgI 2 at growth temperatures ranging from 70° to 120°C is 130 - 140 ppm, whereas above 130°C (the α–β phase transition temperature) iodine solubility increases up to 210 ppm.

X-ray diffraction (XRD) and transient charge technique (TCT) were applied to study the lattice parameters and the electronic properties of mercuric iodide (HgI2) crystals doped with hydrocarbons (HC) and excess of iodine or mercury. The lattice spacing along the c-axis is strongly influenced by HCs doping, and expansion of up to 4% has been measured. Smaller changes of lattice parameters have been measured on crystals with iodine or mercury excess. It is suggested that the incorporation of HCs into HgI2 lattice occurs mainly by intercalation, where excess of mercury or iodine are incorporated as interstitials or as vacancy-type defects which induce more traps for charge carriers as reflected by shorter trapping times.

The excess of iodine in mercury(II) iodide crystals was determined by spectrophotometry. Dynamic dissolution of mercury(II) iodide samples in carbon tetrachloride allows the detection of minute amounts (

A sensitive method for the determination of excess of mercury in high-purity mercury(II) iodide using Raman spectrometry was developed. The method allows the determination of micro amounts of mercury(I) iodide present in mercury(II) iodide with a sensitivity limit of 10 μ mol mol −1 . Samples with concentrations in the range 18–7200 μ mol mol −1 were analysed and the results were used to construct a calibration graph. The analysis is based on comparison of the relative intensities of the Raman lines at 114 cm −1 of Hg 2 I 2 and that of 140 cm −1 of β-HgI 2 above 130°C. The Raman spectrometric method is at least ten times more sensitive than other methods and seems to be the best technique available for stoichiometric studies of HgI 2 .

Stoichiometry of vapor-grown HgI 2 (mercuric iodide) crystals has been studied quantitatively using Raman spectroscopy and liquid solution spectrophotometry. Evaluation of mercury excess is based on differentiation between the Raman spectra of HgI 2 and Hg 2 I 2 (mercurous iodide). Iodine excess is determined by dynamic dissolution of HgI 2 in CC1 4 (tetrachloromethane) and measurements of the characteristic I 2 absorption intensity at 517 nm. The threshold for stoichiometric analysis has been lowered to about 10 ppm, which is an order of magnitude better than that achieved by other methods. Crystals grown from starting materials subjected to the standard purification by repeated sublimations, melting and additional sublimation have been found nearly stoichiometric regardless of the synthesis method of the compound. It is shown that both mercury and iodine rich crystals can be grown by controlling the composition of the starting material or the growth atmosphere. Large excess of either mercury or iodine ( > 100 mole ppm) causes a noticeable deterioration in the energy resolution of HgI 2 nuclear radiation detectors, which is discussed in terms of trapping of radiation-induced charge carriers by nonstoichiometric defects. The kinetics of trapping have been studied using the transient charge technique.

The present review describes the preparation of thin films of high temperature superconducting (HTSC) compounds such as YBaCuO, BiSrCaCuO and TlBaCaCuO using both physical and chemical transport methods on various substrates and buffer layers. Physical transport methods reviewed are MBE, laser ablation, electron beam evaporation and magnetron sputtering, whereas the chemical methods are mainly OMCVD and spray pyrolysis. The critical superconducting temperatures ( T c ) and spray pyrolysis. The critical superconducting temperatures ( T c ) and currents ( J c ) are given for each HTSC compound and its substrate. Results obtained by the author's laboratory on laser ablation and OMCVD aof YBaCuO and OMCVD and spray pyrolysis of YBaCuO and BiSrCaCuO are also presented. The advantages and disadvantages are discussed for each of the HTSC compounds, the substrates, and the physical or chemical deposition methods. The status of possible applications is briefly mentioned as well as some present and future trends in research.

A new approach to purification of mercuric iodide (HgI 2 ), based on oxidation and “cracking” of hydrocarbons, is described. Oxidation of a number of materials at different stages of standard purification (repeated sublimations and/or melting) has been performed at 50°C in an oxygen flow. High temperature (900°C) “cracking” has been applied to elemental iodine used for vapor phase synthesis of HgI 2 , and it was found very efficient in removal for hydrocarbons. A number of direct and indirect analytical methods have been employed (and compared) to determine the residual concentration of carbon in the starting materials and single crystals. The carbon content is reduced to a level of 10 ppm in oxidized materials. An apparent improvement in charge transport properties of low carbon HgI 2 crystals has been observed. Both electron and hole lifetimes increase in crystals grown from oxidized raw materials, and the energy resolution of HgI 2 gamma-ray detectors is improved correspondingly.