Paul R. Bennett

Lead iodide X-ray detection systems

Recent progress in the development of room-temperature lead iodide (Phi,) X-ray detectors is reported. Progress has been made in the areas of detector fabrication and preamplifier electronics, and this has resulted in improved detection performance. An energy resolution of 415 eV (FWHM) has been reported for 5.9 keV X-rays (“Fe source) with 1 mm’ detector at room temperature. A better estimation of the Fano factor in PbIz has been carried out and the upper limit of the Fano factor is calculated to be 0.19. Larger lead iodide detectors (up to 2.5 mm*) have been fabricated and their spectroscopic performance has been evaluated. The timing characteristics of lead iodide detectors have been investigated. A compact, portable lead iodide probe assembly has been designed and built for X-ray spectroscopic applications. Finally, optical and charge particle detection properties of lead iodide detectors have also been characterized.

Characterization of polycrystalline TlBr films for radiographic detectors

Vapor deposited films of thallium bromide are evaluated as potential photoconductive layers in new large-area radiographic detectors. The attractiveness of the material lies in its inherent high effective atomic number and high density. Polycrystalline films up to 200 /spl mu/m have been grown and show a columnar structure with grains reaching 100 /spl mu/m in diameter. Current-voltage (IV) tests indicate a bulk resistivity of 10/sup 9/-10/sup 10/ /spl Omega//spl middot/cm, limited by ionic conduction. The instability of current with time is also observed, but it can be minimized with cooling. The films demonstrate high gain at relatively low field strengths as compared to other photoconductive layers. Benefits and drawbacks of TIBr are compared to other materials, and possible solutions are discussed.

Electronic noise in lead iodide X-ray detectors

Lead iodide (PbI2) is a wide bandgap semiconductor (Eg = 2.32 eV) and has been studied over the past several years as a semiconductor material for use in solid state X-ray and gamma-ray detectors. Small lead iodide detectors have been found to operate with low noise and good energy resolution (500 eV FWHM for 5.9 keV 55Fe X-rays). In the interest of reducing the electronic noise in lead iodide detection systems we have characterized the measured noise in the lead iodide detectors, and have compared the measurements with known noise models. It has been assumed that the noise sources in the lead iodide detectors would take form of a combination of series thermal noise, detector shot noise, and 1ƒ noise. Detectors with differing areas and thicknesses were analyzed by measuring their noise as a function of the amplifier integration time. A computer fitting program was used to obtain the magnitude of each noise source. Based on this analysis, 1ƒ noise was found to be the dominant noise source in most detectors at larger integration times of 4 μs to 12 μs, in which range the detectors are normally operated. The 1ƒ noise was also found to be proportional to the input capacitance, indicating that it is dominated by the series1ƒ noise. The 1ƒ noise magnitude appears to be dependent on the detector fabrication procedures, and may be reduced in future detectors by using more suitable fabrication procedures.

High-resolution direct-detection x-ray imagers

We report on a-Si direct detection x-ray image sensors with polycrystalline PbI2, and more recently with HgI2. The arrays have 100 micron pixel size and, we study those aspects of the detectors that mainly determine the DQE, such as sensitivity, effective fill factor, dark current noise, noise power spectrum, and x-ray absorption. Line spread function data show that in the PbI2 arrays, most of the signal in the gap between pixels is collected, which is important for high,DQE. The leakage current noise agrees with the expected shot noise value with only a small enhancement at high bias voltages. The noise power spectrum under x-ray exposure is reported and compared to the spatial resolution information. The MTF is close to the ideal sinc function, but is reduced by the contribution of K-fluorescence in the PbI2 film for which we provide new experimental evidence. The role of noise power aliasing in the DQE and the effect of slight image spreading are discussed. Initial studies of HgI2 as the photoconductor material show very promising results with high x-ray sensitivity and low leakage current.

Comparative study of Pbl2 and Hgl2 as direct detector materials for high-resolution x-ray image sensors

X-ray imaging properties are reported for HgI2 and PbI2, as candidate materials for future direct detection x- ray image sensors, including the first results from screen- printed HgI2 arrays. The leakage current of PbI2 is reduced by using new deposition conditions, but is still larger than HgI2. Both HgI2 and PbI2 have high spatial resolution but new data shows that the residual image spreading of PbI2 is not due to k-edge fluorescence and its possible origin is discussed. HgI2 has substantially higher sensitivity than PbI2 at comparable bias voltages, and we discuss the various loss mechanisms. Unlike PbI2, HgI2 shows a substantial spatially non-uniform response that is believed to originate from the large grain size, which is comparable to the pixel size. We obtain zero spatial frequency DQE values of 0.7 - 0.8 with PbI(subscript 24/ under low energy exposure conditions. A model for signal generation in terms of the semiconducting properties of the materials is presented.

Bismuth iodide crystals as a detector material: some optical and electrical properties

This paper describes the preliminary results obtained from our study of optical and electrical properties of BiI3 crystals. The bismuth iodine polycrystals were grown using commercial starting material by vertical Bridgman method. For our measurements we used only single crystal samples that were cut out from grown crystals perpendicular to C6-axis.

PbI2 for high-resolution digital x-ray imaging

In this paper, we discuss recent progress that has been made in the development of high resolution X-ray imaging detectors using photoconducting films of lead iodide (PbI2). PbI2 is a wide bandgap semiconductor with high X- ray stopping efficiency. We have been investigating thick films of lead iodide which can be prepared in large areas in a cost effective manner. These films can be coupled to readout technologies such as amorphous silicon flat panel arrays and vidicon tubes to produce X-ray imaging detectors for applications such as mammography, fluoroscopy, X-ray diffraction and non-destructive evaluation. Recent results obtained when these PbI2 films are coupled to 512 X 512 flat panel a-Si:H array are reported. This includes dark current, signal and resolution measurements. Properties of lead iodide films which are relevant to imager performance are also discussed.

CHARACTERIZATION OF INDIUM IODIDE DETECTORS FOR SCINTILLATION STUDIES

Abstract Indium iodide (InI) is a wide bandgap semiconductor ( E g = 2.0 eV) and has been investigated as an optical detector material for use in γ-ray scintillation spectroscopy. Single crystals of InI have been grown by the Bridgman process using zone-refined starting material and optical detectors have been fabricated from such crystals. The performance of these detectors has been investigated by measuring their quantum efficiency, direct X-ray detection characteristics, and electrical resistivity. The InI photodetectors have been coupled to Csl(Tl) scintillators and room-temperature energy resolutions of 7.5% (FWHM) and 9.8% (FWHM) were recorded for 662 keV and 511 keV γ-rays, respectively. Successful γ-ray detection has also been accomplished with InI photodetectors coupled to LSO (Lu 2 SiO 5 :Ce) scintillators, and a resolution of 14% (FWHM) has been recorded for 511 keV γ-rays. Finally, analysis of the electronic noise behavior of the InI detectors has been performed.

Lead iodide films for x-ray imaging

This paper discusses the x-ray detection and imaging characteristics of anew semiconductor material, lead iodide, when prepared in form of a vapor deposited film for use in digital imaging. Lead iodide is a wide bandgap semiconductor and provides direct conversion of x-ray energy into electrical charges. This provides higher signal amplitude than conventional systems using scintillation or phosphor screens since only about 5 eV is required to form a charge pair in lead iodide as opposed to more than 30 eV in case of phosphors to produce optical photons. FUrthermore due to very little lateral diffusion of charge pairs, high spatial resolution can be obtained with such direct conversion films. Finally, due to low dark current in these films, the electronic noise in the films is also very low. In this paper we discuss the lead iodide film preparation procedure, its electronic properties such as resistivity and charge transport, its signal amplitude, and its x-ray imaging performance.

Improved properties of PbI2 x-ray imagers with tighter process control and using positive bias voltage

Vapor deposited lead iodide films show a wide range of physical attributes dependant upon fabrication conditions. High density is most readily achieved with films less than 100 μm. Thicker films, with lessening density, often show lower response (gain) as charge collection becomes less efficient. Lack of consistency in density throughout a deposition invariably leads to non-uniform electronic properties, which is challenging to both model and predict. To overcome this, tighter control of deposition parameters is required during the slow growth process (<10 μm/hour). Lead iodide films are characterized in forms of planar devices deposited onto conductive glass and active pixel arrays deposited onto a-Si TFT arrays1. Electronic properties (e.g. leakage current, gain) show little variation that can be traced to substrate choice. Films generally provide less than 100 pA/mm2 leakage current as they show saturation in gain (at approximate fields of 1 V/μm). We recently modified our readout electronics to accept positive bias. Using positive bias on the top electrode provides better charge collection for the lower mobility electrons and (despite process variability) better quality films can provide sensitivities greater than 6 μC/R*cm2, with only partial x-ray absorption, and show less than 20 pA/mm2 dark current.