High-resolution X-ray Detectors Research Articles

Simulation of 1.5-mm-thick and 15-cm-diameter gated silicon drift X-ray detector operated with a single high-voltage source

High-resolution silicon X-ray detectors with a large active area are required for effectively detecting traces of hazardous elements in food and soil through the measurement of the energies and counts of X-ray fluorescence photons radially emitted from these elements. The thicknesses and areas of commercial silicon drift detectors (SDDs) are up to 0.5 mm and 1.5 cm2, respectively. We describe 1.5-mm-thick gated SDDs (GSDDs) that can detect photons with energies up to 50 keV. We simulated the electric potential distributions in GSDDs with a Si thickness of 1.5 mm and areas from 0.18 to 168 cm2 at a single high reverse bias. The area of a GSDD could be enlarged simply by increasing all the gate widths by the same multiple, and the capacitance of the GSDD remained small and its X-ray count rate remained high.

A New Columnar CsI(Tl) Scintillator for iQID detectors.

A 1650 μm thick columnar CsI(Tl) scintillator for upgrading iQID detectors, which is a high-resolution photon-counting gamma-ray and x-ray detector recently developed at the Center for Gamma-Ray Imaging (CGRI), has been studied in terms of sensitivity, spatial resolution and depth-of-interaction effects. To facilitate these studies, a new frame-parsing algorithm for processing raw event data is also proposed that has more degrees of freedom in data processing and can discriminate against a special kind of noise present in some low-cost intensifiers. The results show that in comparison with a 450 μm-thickness columnar CsI(Tl) scintillator, the 1650 μm thick CsI(Tl) scintillator provides more than twice the sensitivity at the expense of some spatial resolution degradation. The depth-of-interaction study also shows that event size and amplitude vary with scintillator thickness, which can assist in future detector simulations and 3D-interaction-position estimation.

Research in digital mammography and tomosynthesis at the University of Toronto.

There have been major advances in the field of breast cancer imaging since the early 1970s, both in technological improvements and in the use of the methods of medical physics and image analysis to optimize image quality. The introduction of digital mammography in 2000 provided a marked improvement in imaging of dense breasts. In addition, it became possible to produce tomographic and functional images on modified digital mammography systems. Digital imaging also greatly facilitated the extraction of quantitative information from images. My laboratory has been fortunate in being able to participate in some of these exciting developments. I will highlight some of the areas of our research interest which include modeling of the image formation process, development of high-resolution X-ray detectors for digital mammography and investigating new methods for analyzing image quality. I will also describe our more recent work on developing new applications of digital mammography including tomosynthesis, contrast-enhanced mammography, and measurement of breast density. Finally, I will point to a new area for our research--the application of the techniques of medical imaging to making pathology more quantitative to contribute to use of biomarkers for better characterizing breast cancer and directing therapeutic decisions.

SU-E-T-143: Effect of X-Ray and Cone Beam CT Reconstruction Parameters On Estimation of Bone Volume of Mice Used in Aging Research

Purpose: To investigate the variations in bone volume calculations in mice involved in aging research when changing cone beam micro-CT x-ray and reconstruction parameters. Methods: Mouse spines were placed on an indexed turn table that rotated 0.5° per projection and imaged by a self-built micro CT machine containing a CCD-based high-resolution x-ray detector. After the full 360° rotation data set of object images was obtained, a standard filtered back-projection cone beam reconstruction was performed. Four different kVp's between 40–70 kVp in 10kVp increments were selected. For each kVp two mAs settings were used. Each acquisition was reconstructed using two voxel sizes (12 and 25μm) and two step angles, 0.5° and 1°, respectively. A LabView program was written to determine the total bone volume contained in the mouse's total spine volume (bone plus gaps) as a measure of spine health. First, the user selected the desired 512×512 reconstruction to view the whole spine volume which was then used to select a gray-level threshold that allowed for viewing of the bone structure, then another threshold to include gaps. The program returned bone volume, bone × gap volume, and their ratio, BVF. Results: The calculated bone volume fractions were compared as a functionmore » of tube potential. Cases with 25μm slice thickness showed trials with lower kVp's had greater image contrast, which resulted in higher calculated bone volume fractions. Cases with 12μm reconstructed slice thickness were significantly noisier, and showed no clear maximum BVF. Conclusion: Using the projection images and reconstructions acquired from the micro CT, it can be shown that the micro-CT x-ray and reconstruction parameters significantly affect the total bone volume calculations. When comparing mice cohorts treated with different therapies researchers need to be aware of such details and use volumes which were acquired and processed in identical conditions.« less

Structural and Electronic Properties of Gold Contacts on CdZnTe with Different Surface Finishes for Radiation Detector Applications

State-of-the-art room-temperature, high-resolution x-ray and gamma-ray semiconductor detectors can be fabricated from CdZnTe (CZT) crystals. The structural and electronic properties of the CZT surface, especially the contact interfaces, can have a substantial effect on radiation detector performance, for example leakage current, signal-to-noise ratio, and energy resolution, especially for soft x-rays and large pixilated arrays. Atomically smooth and defect-free surfaces are desirable for high-performance CZT-based detectors; chemo-mechanical polishing (CMP) is typically performed to produce such surfaces. The electrical behavior of the metal/CZT interface varies substantially with surface preparation before contact deposition, and with choice of metal and deposition technique. We report a systematic study of the structural and electronic properties of gold (Au) contacts on CZT prepared with different surface finishes. We observed subsurface damage under Au contacts on CMP-finished CZT and abrupt interfaces for Au on chemically-polished (CP) CZT. Schottky barrier formation was observed for Au contacts, irrespective of surface finish, and less charge trapping and low surface resistance were observed for CP-finished surfaces. Pre-deposition surface treatment produced interfaces free from oxide layers.

112-Pixel Arrays of High-Efficiency STJ X-Ray Detectors

We are developing the next generation of high-resolution high-speed X-ray detectors based on superconducting tunnel junctions (STJs). They consist of 112-pixel arrays of 208 \(\mu \)m \(\times \) 208 \(\mu \)m Ta–Al–\({\text {AlO}}_x\)–Al–Ta tunnel junctions whose Ta absorber increases the detection efficiency compared to earlier Nb-based STJs. To read out these medium size detector arrays we have also developed a compact and scalable 32-channel preamplifier with an input voltage noise \(<\)1 nV/\(\surd \)Hz and a dc voltage bias for stable STJ operation between Fiske mode resonances. The pixels have a uniform response across the array, an energy resolution between 7.5 and 9.5 eV FWHM at 525 eV, and can be operated at several 1,000 counts/s per pixel.

Scintillating Screens for Micro-Imaging Based on the Ce-Tb Doped LuAP Single Crystal Films

Single Crystal Film (SCF) scintillators are key-components of today's high-resolution 2D X-ray detectors used at synchrotrons for applications such as X-ray Radiography, Computed Tomography (CT), laminography, full-field XANES, and topography. Due to its high density and effective atomic number, LuAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (LuAP) perovskite has a great potential to replace or complement the currently used Eu-doped Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sub> (GGG:Eu) and Tb-doped Lu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> (LSO:Tb) SCFs. In this article we present the growth of LuAP single crystalline films on undoped YAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (YAP) by the vertical dipping isothermal Liquid Phase Epitaxy (LPE) growth method. For the comparison of the light output of Ce and Tb doped LuAP SCF screens both cathodoluminescence (CL) and X-ray excited luminescence (RL) measurements were used. For investigations of Ce-Tb energy transfer in the afore mentioned oxide hosts, in addition to using traditional spectroscopic methods, we made time-resolved luminescent spectroscopy of Ce-Tb doped LuAP SCFs under excitation by pulsed synchrotron radiation at the Superlumi station (HASYLAB at DESY). Recently we have demonstrated the strong quenching influence of Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> flux related impurity on the Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> emission in the Ce-doped LuAP SCF screens. However our present investigation shows that Tb-doped LuAP SCFs are efficient X-ray scintillators, whose light output can be significantly increased by co-doping with Ce <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions. Namely, we demonstrate the efficient Ce→ Tb energy transfer in LuAP:Tb,Ce SCFs. Finally, as-grown LuAP:Tb and LuAP:Tb,Ce films were evaluated for high-resolution X-ray imaging using a spatial resolution target at the European Synchrotron Radiation Facility (ESRF), and displayed promising results.

Direct measurement of the x-ray refractive index by Fresnel diffraction at a transparent edge

We demonstrate the feasibility of measuring x-ray refractive indices by transparent edge diffraction without recourse to the Kramers-Kronig relations. The method requires a coherent x-ray source, a transparent sample with a straight edge, and a high resolution x-ray detector. Here, we use the aluminum Kα radiation originating from a laser-produced plasma to coherently illuminate the edge of thin aluminum and beryllium foils. The resulting diffraction patterns are recorded with an x-ray CCD camera. From least-squares fits of Fresnel diffraction modeling to the measured data we determine the refractive index of Al and Be at the wavelength of the Al Kα radiation (0.834 nm, 1.49 keV).

ZnO nanowires in polycarbonate membrane as a high resolution X-ray detector (a Geant4 simulation)

The capability of polycarbonate cladded ZnO nanowires as scintillating optical fibers, in the presence of the 10keV X-ray is simulated using the Geant4 Monte Carlo simulation code. The simulated results show that using millions of scintillating fibers in a 1mm2 area, which are packed together in a polycarbonate membrane, have additional light guiding role that reduces the scattering of the X-ray generated optical photons inside the detector. This new low-cost technique can provide a higher spatial resolution X-ray imager with sharper edges at the object boundaries in comparison to a single crystal scintillating macro detectors.

Simulation of Thick Gated Silicon Drift X-ray Detector Operated by a Single High-Voltage Source

High-resolution X-ray detectors can be used to detect traces of hazardous or radioactive elements in food, soil, and the human body by measuring the energies and counts of emitted X-ray fluorescence photons. We have simulated the electric potential distributions in gated silicon drift detectors (GSDDs) with an active area of 18 mm2 and a Si thickness between 0.625 and 1.5 mm. A GSDD gate pattern was designed for each Si thickness and for various oxide charge densities in the SiO2 passivating layer near the SiO2/Si interface. The simulated GSDDs required approximately half the reverse bias voltage required by Si pin detectors. Our detector design could improve the absorption of Cd or Cs X-ray fluorescence photons and would reduce the cost of X-ray detection systems.

TU-A-218-04: Phantom Studies of a Newly Developed Solid State X-Ray Image Intensifier (SSXII) for X-Ray Image Guided Neurovascular Interventions

Purpose: X-ray image-guided interventional treatment of vascular pathology such as aneurysms involves precise delivery of devices such as stents and hence may require very high-resolution imaging capability. We report the first fluoroscopy and angiography phantom studies using a new high-resolution x-ray detector that has been developed to address this need. Methods: The Solid State X-ray Image Intensifier (SSXII) detector is based on Electron-Multiplying Charge Coupled Device (EMCCD) technology (E2V Technologies, Chelmsford, England). An EMCCD with 13- μm pixels and fiber optic window is coupled to a fiber-optic taper with a taper ratio of 3.3 that views a 300-μm thick CsI x-ray scintillator, thus making an effective pixel size of 42.9 μm and a Nyquist frequency of 11.8 lp/mm for this detector. A small aneurysm insert that was part of a flow loop was placed in the base of the brain region of an anthropomorphic head phantom (RS-230, Supertech, Elkhart, IN) to simulate pathology in the Circle of Willis of a patient and interventional procedures were done with similar attenuation and anatomical characteristics that would be experienced in the clinic. A guidewire was threaded into the aneurysm under fluoroscopic image guidance with the SSXII. Digital Subtraction Angiography (DSA) was performed using a standard contrast injection (iodine 350 mg/ml) into the flow loop and angiographic sequences were recorded with the new detector. Results: The operation of the new SSXII detector in both fluoroscopy and angiography exposure ranges was demonstrated for the first time on human phantoms. The resolution of this detector is substantially higher than current-state-of-the-art flat panel detectors with 150 to 200 μm pixels. Conclusions: The newly developed high-resolution EMCCD-based detector can potentially be used for both fluoroscopy and angiography to address the high-resolution imaging needs of x-ray image-guided neuro- vascular interventions. Supported in part by: NIH Grants R01-EB008425, R01-EB002873 and an equipment grant from Toshiba Medical Systems Corp.

X-ray phase contrast for CO2 microangiography

We demonstrate a laboratory method for imaging small blood vessels using x-ray propagation-based phase-contrast imaging and carbon dioxide (CO2) gas as a contrast agent. The limited radiation dose in combination with CO2 being clinically acceptable makes the method promising for small-diameter vascular visualization. We investigate the possibilities and limitations of the method for small-animal angiography and compare it with conventional absorption-based x-ray angiography. Photon noise in absorption-contrast imaging prevents visualization of blood vessels narrower than 50 µm at the highest radiation doses compatible with living animals, whereas our simulations and experiments indicate the possibility of visualizing 20 µm vessels at radiation doses as low as 100 mGy. Experimental computed tomography of excised rat kidney shows blood vessels of diameters down to 60 µm with improved image quality compared to absorption-based methods. With our present prototype x-ray source, the acquisition time for a tomographic dataset is approximately 1 h, which is long compared to the 1–20 min common for absorption-contrast micro-CT systems. Further development of the liquid-metal-jet microfocus x-ray sources used here and high-resolution x-ray detectors shows promise to reduce exposure times and make this high-resolution method practical for imaging of living animals.

Kinetic Inductance Detectors for X-Ray Spectroscopy

Abstract The lack of efficient x-ray detectors is often the main factor limiting the effective use of ever more powerful synchrotron light sources. Spectroscopic X-ray detectors are used for a wide variety of synchrotron experiments including X-ray micro/nano-probes and X-ray absorption spectroscopy for biology and geophysical applications. The current state-of-art spectroscopic X-ray detectors are semiconductor devices, and their energy resolutions are approaching their theoretical limit of about 100 eV at 6 keV. We describe a detector research and development program to develop the next-generation of high-resolution spectroscopic X-ray detectors using superconducting Kinetic Inductance Detectors (KIDs). With a required energy per charge carrier four orders of magnitude smaller than that of Si, superconducting detectors offer up to two orders of magnitude increase in energy resolution. In addition, KIDs can be optimized for detection of photons ranging in energy from hard X-ray to IR.

LANSCE Nuclear Science Facilities and Activities

Nuclear science activities at the Los Alamos Neutron Science Center (LANSCE) encompass measurements spanning the neutron energy range from thermal to 600 MeV. The neutron sources use spallation of the LANSCE 800 MeV pulsed proton beam with the time-of-flight technique to measure properties of neutron-induced reactions as a function of energy over this large energy range. Current experiments are conducted at the Lujan Center moderated neutron source, the unmoderated WNR target, and with a lead-slowing-down spectrometer. Instruments in use include the DANCE array of BaF{sub 2} scintillators for neutron capture studies, the FIGARO array of liquid scintillator neutron detectors, the GEANIE array of high-resolution HPGe x-ray and gamma-ray detectors, and a number of fission chambers, and other detectors. The LANL capabilities for production and handling of radioactive materials coupled with the neutron sources and detectors at LANSCE are enabling new and challenging measurements for a variety of applications including nuclear energy and nuclear astrophysics. An overview of recent research and examples of results is presented.

Use of the Microangiographic Fluoroscope for Coiling of Intracranial Aneurysms

Neurointervention is an ever-evolving specialty with tools including microcatheters, microwires, and coils that allow treatment of pathological conditions in increasingly smaller intracranial arteries, requiring increasing accuracy. As endovascular tools evolve, so too should the imaging. To detail the use of microangiography performed with a novel fluoroscope during coiling of intracranial aneurysms in 2 separate patients and discuss the benefits and potential limitations of the technology. The microangiographic fluoroscope (MAF) is an ultra high-resolution x-ray detector with superior resolution over a small field of view. The MAF can be incorporated into a standard angiographic C-arm system for use during endovascular procedures. The MAF was useful for improved visualization during endovascular coiling of 2 unruptured intracranial aneurysms, without adding significant time to the procedure. No significant residual aneurysm filling was identified post-coiling, and no complications occurred. The MAF is a high-resolution detector developed for use in neurointerventional cases in which superior image quality over a small field of view is required. It has been used with success for coiling of 2 unruptured aneurysms at our institution. It shows promise as an important tool in improving the accuracy with which neurointerventionists can perform certain intracranial procedures.

Measurement of persistence in YAG:Ce(3+)scintillator with pulsed synchrotron X-rays.

The decay time of YAG:Ce(3+) phosphor was studied using a CMOS camera with a frame rate of 1302000 s(-1) and pulsed X-rays from SPring-8. A high-resolution X-ray detector with YAG:Ce(3+) was used with the camera to view the focused beam from the helical undulator. Mismatch between the ring circulation time and the frame time gave rise to a periodic variation of beam intensity in successive frames. Analysis of data obtained with two bunch modes showed that the decay time of YAG:Ce(3+) was 60 ns. The variation of the beam positions in isolated bunches was small enough to be neglected in experiments using the focused beam. The results also show the possibility of an X-ray diffraction study at high time and space resolution.

A modular high-resolution photon-counting x-ray detector

A modular high-resolution photon-counting x-ray detector

Electronic characterization of several 100 μm thick epitaxial GaAs layers

Non intentionally doped thick epitaxial GaAs layers, grown by chemical vapour phase epitaxy using a high growth rate, are characterized by different electrical techniques applied on a junction (forward and reverse Current–Voltage characteristics, Current versus time and temperature, Deep Level Transient Spectroscopy, Capacitance–Voltage measurements) in order to detect the defects they contain. Experimental data are coupled with theoretical modelling to determine the electrical characteristics of the layers: type and concentration of the carriers, energy levels and concentrations of the defects, associated minority carrier lifetime. The results obtained indicate that i) the concentration of the residual impurities is in the order of 1013 to 1014 cm−3, ii) the layers contain two shallow defects (at 0.12 and 0.21 eV below the conduction band Ec) slightly compensated by a deep defect (at Ec-0.5 eV) and iii) the deep defect is in low concentration (1012 cm−3), resulting in a minority carrier lifetime still limited by the radiative recombination process. Therefore, epilayers obtained using high growth rates exhibit electronic properties very similar to the ones obtained using conventional epitaxial techniques. The good and uniform electronic properties of these layers coupled with the low yield of the fluorescence of GaAs is interesting for high resolution X-ray detection.

Energy calibration of superconducting transition edge sensors for x-ray detection using pulse analysis

Transition edge sensors (TESs) have been developed to be used as high-resolution x-ray detectors. They show excellent energy resolution and can be used in many applications. TESs are a special kind of calorimeters that can determine small temperature changes after x-ray absorption. Such a temperature change causes a strong resistance change (superconducting to normal-conducting phase transition) that can be measured. The energy calibration of a TES based spectrometer is problematic due to the nonlinear behavior of the detector response. In this article, a method is introduced to calibrate the energy scale of TES spectra. This is accomplished by calculating the energy dependence of the response of the detector operated in electrothermal feedback mode. Using this method a calibration accuracy of a few eV for an x-ray energy of 6keV can be achieved. Examples of energy dispersive x-ray spectroscopy (EDS) measurements demonstrate the high quality of this method for everyday use of TES EDS detectors in material analysis. However, because the method relies only on a few very general assumptions, it should also be useful for other kinds of TES detectors.

Study of avalanche photodiodes for soft X-ray detection below 20 keV

The performance of the large area reach-through avalanche photodiode (APD), manufactured by Hamamatsu Photonics, K.K. as a high resolution X-ray detector is presented. The mentioned APD has an area of 3 mm ∅ , a fast time response for signal carrier collection and its thick depletion layer of 130 μ m shows a potential to be used as an effective X-ray absorber below 20 keV. Having a capacitance of ∼ 10 pF and a low dark current of 5 nA for a gain of 15, at room temperature, this APD had demonstrated one of the best energy resolutions within this kind of devices: 6.4% (FWHM) for 5.9 keV photons with a minimum detectable energy of 0.3 keV, measured at - 20 ∘ C . The experiments for the timing property were made in a synchrotron beam facility using an 8 keV X-ray beam; the reached count rate was above 10 8 counts/s, corresponding to a very short dead time of 4.5 ns/pulse. In order to test the radiation hardness of the APD, the device was irradiated at a Ring Cyclotron Facility with a 53.5 MeV proton beam. The total dose was of 11.3 krad and no fatal damage was found in the APD, although the dark current of the APD had shown an increase of one order of magnitude. Finally, the obtained results allow us to affirm that the reach-through APD has the potential to become an excellent X-ray detector, especially in the space mission application.