A. Zuck

Thick films of X-ray polycrystalline mercuric iodide detectors

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.

Near single-crystal electrical properties of polycrystalline HgI/sub 2/ produced by physical vapor deposition

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/ </spl tilde/ 3.5 /spl mu/s, and surface recombination velocities s/sub n/ /spl cong/ 1.4/spl times/10/sup 5/ cm/s and s/sub p/ /spl cong/ 3.7/spl times/10/sup 3/ cm/s. All parameters depend to a large extent on the material deposition technology. The effect of carriers being first generated in near-surface traps and then gradually released is observed.

Mercuric iodide and lead iodide x-ray detectors for radiographic and fluoroscopic medical imaging

Mercuric iodide (HgI2) and lead iodide (PbI2) have been under development for several years as direct converter layers in digital x-ray imaging. Previous reports have covered the basic electrical and physical characteristics of these and several other materials. We earlier reported on 5cm x 5cm and 10cm x 10cm size imagers, direct digital radiography X-ray detectors, based on photoconductive polycrystalline mercuric iodide deposited on a flat panel thin film transistor (TFT) array, as having great potential for use in medical imaging, NDT, and security applications. This paper, presents results and comparison of both lead iodide and mercuric iodide imagers scaled up to 20cm x 25cm sizes. Both the mercuric iodide and lead iodide direct conversion layers are vacuum deposited onto TFT array by Physical Vapor Deposition (PVD). This process has been successfully scaled up to 20cm x 25cm -- the size required in common medical imaging applications. A TFT array with a pixel pitch of 127 microns was used for this imager. In addition to increasing detector size, more sophisticated, non-TFT based small area detectors were developed in order to improve analysis methods of the mercuric and lead iodide photoconductors. These small area detectors were evaluated in radiographic mode, continuous fluoroscopic mode and pulsed fluoroscopic mode. Mercuric iodide coating thickness ranging between 140 microns and 300 microns and lead iodide coating thickness ranging between 100 microns and 180 microns were tested using beams with energies between 40 kVp and 100 kVp, utilizing exposure ranges typical for both fluoroscopic and radiographic imaging. Diagnostic quality radiographic and fluoroscopic images have been generated at up to 15 frames per second. Mercuric iodide image lag appears adequate for fluoroscopic imaging. The longer image lag characteristics of lead iodide make it only suitable for radiographic imaging. For both material the MTF is determined primarily by the aperture and pitch of the TFT array (Nyquist frequency of ~3.93 mm-1 (127 micron pixel pitch).

Novel mercuric iodide polycrystalline nuclear particles counters

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/ particles 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.

Radiation-hard polycrystalline mercuric iodide semiconductor particle counters

Abstract Mercuric iodide polycrystalline radiation detectors, which can act as nuclear particle counters and for large area imaging devices, have been fabricated using three different methods. Response to X- and gamma rays, beta particles and to 100 GeV muons, as well as radiation hardness results are briefly described.

Evaluation of mercuric iodide ceramic semiconductor detectors

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.

CHARACTERIZATION STUDIES OF PURIFIED HGI2 PRECURSORS

Abstract The ability of HgI 2 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 HgI 2 crystal to its impurity content and stoichiometry. These quantities determine the detector grade that may be achieved using that starting material. Nine different HgI 2 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.

Microstructure and energy resolution of 59.6 keV /sup 241/Am gamma absorption in polycrystalline HgI/sub 2/ detectors

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.

Ceramic mercuric iodide semiconductor particle counters

Radiation detectors have been fabricated from very thick films (100-600 /spl mu/m) of mercuric iodide (HgI/sub 2/). These devices, which function as nuclear particle counters and not as spectrometers, have been prepared with single continuous electrical contacts, linear microstrips and square pixel contacts. The word ceramic is used to distinguish the detectors from single crystals which are usually studied for this application. The detectors have been tested with different kinds of gamma and beta sources as well as in a high energy beam of 100 GeV muons at CERN. The presented results show the potential of this material for applications demanding position sensitive, radiation resistant, room-temperature operating radiation detectors, where position rather than spectroscopic resolution is essential, as it can be found in some specific applications in high energy physics, nuclear medicine and astrophysics. Because of the low cost and of the polycrystallinity, detectors can be potentially fabricated in any size and shape, using standard ceramic technology shaping equipment, which is an attractive feature where low cost and large area applications are needed.

Progress in polycrystalline HgI/sub 2/ used for X-ray imaging detectors

Use of physical vapor deposition (PVD) of polycrystalline HgI/sub 2/ films on Si-TFT arrays brought about a breakthrough in the use of HgI/sub 2/ for large area pixellated X-ray imaging. Latest advances in the deposition process led to full-texture high-density films, with highly orientated crystallites, as evidenced for example by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The good structural data also yielded excellent electrical charge transport properties, which approached those of single crystals. Transient charge transport (TCT) with alpha-particle near-surface absorption was used to measure carrier mobility, trapping time, and surface recombination velocity for each sample. Typical electron and hole mobility of high quality polycrystalline HgI/sub 2/ films 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 cong/ 18 /spl mu/s and /spl tau//sub p/ /spl cong/ 3.5 /spl mu/s, and surface recombination velocities s/sub n/ /spl cong/ 1.4 /spl times/ 10/sup 5/ cm/s and s/sub p/ /spl cong/ 3.7 /spl times/ 10/sup 3/ cm/s. The performance of these detectors as spectrometers in a standard nuclear spectroscopy system was evaluated. We used a gamma source of /sup 241/Am with the characteristic 59.6 keV gamma photo-peak. The full width at half maximum (FWHM) of the detector photo peak depended on its charge transport properties. High quality polycrystalline HgI/sub 2/ film detectors yield a peak of approximately 38 keV FWHM, while lower quality ones yield a much broader peak of FWHM > 70 keV. Such widths are still inferior to those of a single crystal (typically /spl sim/5 keV), yet the results suggest that further improvement through optimization of manufacturing conditions is possible. The talk reviews our past efforts, recent new results, and plans for the future.