G. Belev

The influence of deposition conditions and alloying on the electronic properties of amorphous selenium

Electronic properties of a-Se as a function of the source (boat) temperature and as a function of As (up to 0.7%) and Cl (up to 40 wt ppm) concentrations have been experimentally studied by carrying out conventional and interrupted field time-of-flight (IFTOF) transient photoconductivity measurements that provide accurate determinations of the drift mobility and the deep trapping time (lifetime). No variation in electron and hole lifetimes and mobilities for pure a-Se was observed with the source temperature, that is, no dependence was observed either on the deposition rate or on the vapor composition. The addition of As reduces the hole lifetime but does not change the hole mobility. At the same time, As addition increases the electron lifetime while reducing the electron mobility. The electron range μτ, however, increases with the As content, which means that the overall concentration of deep electron traps must be substantially reduced by the addition of As. Cl addition in the ppm range increases the hole lifetime but reduces the electron lifetime. The drift mobility of both carriers remains the same. We interpret the results in terms of a shallow-trap-controlled charge transport in which deep traps are due to potential under-and overcoordinated charged defects that can exist in the structure.

Properties of a-SbxSe1−x photoconductors

We have examined the thermal stability, and carrier drift mobilities and lifetimes (and hence electron and hole ranges) of a-SbxSe1−x photoconductors. We have found that the most thermally stable a-SbxSe1−x alloys can be obtained for x between 1 and 1.5 at.% which therefore limits the amount of Sb that can be added to improve the X-ray absorption and hence the X-ray photoconductivity of these alloys. On the other hand, we have found that the photoconducting films fabricated from a-SbxSe1−x alloys exhibit better electron and hole ranges when compared with the starting pure a-Se material. The X-ray sensitivity of a-Se:1 at.% Sb is greater than that of the pure a-Se.

Optically erasable samarium-doped fluorophosphate glasses for high-dose measurements in microbeam radiation therapy

Previous work has demonstrated that fluorophosphate (FP) glasses doped with trivalent samarium (Sm3+) can be used as a dosimetric detector in microbeam radiation therapy (MRT) to measure high radiation doses and large dose variations with a resolution in the micrometer range. The present work addresses the use of intense optical radiation at 405 nm to erase the recorded dose information in Sm3+-doped FP glass plates and examines the underlying physics. We have evaluated both the conversion and optical erasure of Sm3+-doped FP glasses using synchrotron-generated high-dose x-rays at the Canadian Light Source. The Sm-ion valency conversion is accompanied by the appearance of x-ray induced optical absorbance due to the trapping of holes and electrons into phosphorus-oxygen hole (POHC) and electron (POEC) capture centers. Nearly complete Sm2+ to Sm3+ reconversion (erasure) may be achieved by intense optical illumination. Combined analysis of absorbance and electron spin resonance measurements indicates that th...

Progress in the science and technology of direct conversion a-Se X-ray sensors

Abstract The present paper provides a critical review and discussion of some of the recent advances in the science and technology of stabilized a-Se (alloyed with 0.2–0.5% As and doped with small amounts of Cl) based flat panel X-ray sensors. Although a-Se is now a well-established X-ray photoconductor used in flat panel direct conversion X-ray sensors that have produced excellent images, there are many scientific aspects of the work that have not been resolved. We identify and discuss several further experimental and theoretical studies that must be undertaken to advance the science of X-ray photoconductors to a point that the new knowledge can be incorporated into enhanced device design.

Rare-Earth Doped Fluorochlorozirconate (Fcz) Glasses And Glass-Ceramics: Selected Thermal Properties And X-Ray Luminescence In Samarium Doped Fcz

Recently, a new class of rare-earth (RE) doped fluorochlorozirconate (FCZ) glass-ceramics has been developed and studied with the goal of using these materials in indirect conversion x-ray imaging applications. We report selected thermal and x-ray induced luminescence (XL) properties of rare-earth (EuF2 and SmF3) doped fluorochlorozirconate glasses and glass-ceramics, and highlight some of their interesting properties. Sm-doped FCZ glasses exhibit XL even in the unannealed state, while annealing at a temperature of 250°C in a reducing atmosphere (5% H2 and 95% Ar) significantly enhances the XL efficiency.

High-resolution (20 cycles/mm) digital x-ray mammography using amorphous selenium directly coupled to CCD readout devices

A simple x-ray detector that utilizes amorphous selenium (a-Se) directly deposited on a specially-designed CCD (charge coupled device) with a 25 micrometer del (detector element) pitch is described. This simple detector has been used to test the feasibility of creating digital mammography detectors. To enable the use of electron transport CCDs with a-Se, we have developed a-Se hole blocking layers to permit the transfer of electrons to the CCD while suppressing hole leakage current in the presence of the high negative bias (~1000 V) required to make the a-Se x-ray sensitive. We report measurements of the charge transfer efficiency (CTE), dark signal, x-ray sensitivity, x-ray signal linearity, and x-ray MTF (modulation transfer function) of the simple detector. As the thickness of the a-Se hole blocking layer was increased, the MTF decreased. For a thin (1 micrometer) blocking layer the MTF at a spatial frequency of 20 cycles/mm was 0.4.

X-ray sensing materials stability: influence of ambient storage temperature on essential thermal properties of undoped vitreous selenium

Amorphous selenium (a-Se) is currently used in x-ray image detectors as an x-ray photoconductor. Normally a-Se films used in device applications are fabricated by the evaporation of vitreous bulk material loaded into boats in a typical vacuum deposition system. The resistance against crystallization is an important factor in both film and bulk forms of a-Se. Previous work has indicted that the resistance to crystallization is surprisingly more pronounced around 35 ?C [1]. In this work we have therefore examined the essential thermal properties of vitreous selenium (99.999%) samples that have been stored at different temperatures. The thermal characterization experiments involved a series of DSC (Differential Scanning Calorimetry) measurements in which have monitored the glass transition and melting endotherms, and the crystallization exotherm in heating-cooling-heating scans. In DSC experiments, a sample would be heated to a temperature above the melting temperature, equilibrated, then cooled at a fixed rate down to 20 ?C, then equilibrated and finally scanned again under a heating schedule. The samples were isothermally stored at temperatures corresponding to 18, 35 and 55 ?C. The thermal analysis results show that there are distinct differences in the thermal properties. We have examined the stability in terms of the difference in the crystallization onset temperature (Tc) and the onset of glass transition temperature (Tg). We also examined the Hruby coefficient (Kgl) of these samples, that is Kgl = (Tc ? Tg)/(Tm ? Tc) where Tc is the crystallization onset temperature and Tm is the melting onset temperature. We have found Kgl to depend on the storage temperature. Surprisingly, we observed that the Hruby coefficient is actually larger at 35 ?C compared to the values at 18 and 55 ?C.

Thermally Induced Nanostructures in Samarium-Doped Glass Ceramics for X-Ray Sensor Applications

There is much interest in various glass-ceramics doped with rare-earth (RE) metals for x-ray storage phosphor and/or x-ray scintillator applications for potential use in high resolution x-ray imaging. The phosphor and scintillator properties of these glass ceramics depend on the formation of RE embedded nanocrystals in their structure. The heat treatment and annealing of the starting RE-doped glasses is critically important to the formation and control of the glass ceramic nanocrystals. We have studied the thermal and photoluminescence properties of Sm-doped fluorochlorozirconate glass ceramics. We selected useful host compositions and appropriate heat treatment and annealing procedures needed to grow the required RE-doped nanocrystals in a glass matrix for sensor applications.

Samarium Doped Borophosphate Glasses and Glass-Ceramics for X-ray Radiation Sensing

Samarium fluoride (SmF3) and samarium oxide (Sm2O3) doped Zn- and Sr-borophosphate glasses with mixed oxide-fluoride matrix structures were prepared. The synthesized materials were studied by differential scanning calorimetry (DSC) and temperature-modulated DSC (TMDSC) to determine their glass transformation properties before and after various thermal treatments. X-ray diffraction (XRD) analysis was performed to characterize the structure, that is, whether the synthesized materials have a glass or polycrystalline structure, or is composed of a heterogeneous mixture. We observed that under certain conditions several post-treated materials are partially nanostructured, i.e. the formation of nanocrystals takes place within a glass matrix. The samarium doped materials were irradiated by different types of LEDs and laser light in the UV-NIR wavelength range to examine the photoluminescence (PL) induced and also to study the changes in the PL signal before and after exposure to high-dose X-ray synchrotron irradiation due to a possible reduction of Sm3+ to Sm2+. The different PL signatures of the two Sm-ions allow such materials to be used as x-ray dosimeters with optical readout. The X-ray induced luminescence (XL) spectra were also recorded. We discuss the experimental results in terms of established models for the behavior of rare-earths in glasses, crystalline solids and in nanocrystalline glass-ceramics.