We report the assembly of an imaging phototube which, together with a thin NaI(Tl) x-ray scintillator, forms a photon counting area detector for x-ray crystallography. The tube contains a bialkali photocathode, a triple (Z) stack of 100 mm X 100 mm MCPs, and a 133 mm X 133 mm two dimensional delay line readout. Some aspects of the processing setup that are unique to the very large tube are described. Design considerations with respect to detective efficiency are discussed. Measured gain characteristics of 100 mm X 100 mm Z stacks are presented, showing fairly narrow pulse height distributions which are improved with UV scrubbing. The MCPs, manufactured from low resistivity glass, are subject to thermal instability, and an account is given of thermal runaway which occurred for a stack of MCPs composed of plates with an average room temperature resistivity of approximately 400 M(Omega) -cm2.
Reverse Geometry X-rayR (RGXR) systems, by reversing the conventional configuration of x-ray source, object, and x-ray detector, obtain (a) high contrast sensitivity due to minimal scatter detection, with (b) large-area imaging capability. Additional advantages include real-time first generation digital image quality, computerized control of scanning area and high resolution object magnification, increased freedom of source-detector alignment, and real-time stereoscopy.
General Imaging Corp. has developed an Electronic X-ray Imaging Technology (EXIT) which includes a design employed to fabricate large area solid-state x-ray imaging arrays. These arrays are vertically integrated multi-chip modules which have the potential to replace both film and real-time imaging modalities in industrial and medical radiography. The EXIT large- area sensing array is configured to resemble a screened film cassette in size and shape so that adaptation to existing radiographic imaging equipment is facilitated.
We have designed and evaluated a novel design of line array x-ray detector for use with digital radiography (DR) and computed tomography (CT) systems. The Radiographic Line Scan (RLS) detector is less than half the cost of discrete multi-channel line array detectors, yet provides the potential for resolution to less than 25 micrometers at energies of 420 kV. The RLS detector consists of a scintillator fiber-optically coupled to a thermo-electrically cooled line array CCD. Gadolinium oxysulfide screen material has been used as the scintillator, in thicknesses up to 250 micrometers . Scintillating glass, which is formed into a fiber optic bundle, has also been used in thicknesses up to 2 mm. The large 2.5 mm by 25 micrometers CCD cells provide high dynamic range while preserving high resolution; the 2.5 mm dimension is oriented in the x-ray absorption direction while the 25 micrometers dimension is oriented in the resolution direction. Servo controlled thermo-electric cooling of the CCD to a fixed temperature provides reduction of dark current and stabilization of the output. Greater dynamic range is achieved by reducing the dark current, while output stabilization reduces the need for frequent calibration of the detector. Measured performance characteristics are presented along with DR and CT images produced using the RLS detector.
A commercial prototype intraoral radiography system has been developed that can provide digital x-ray images for diagnosis. The system consists of an intraoral detector head, an intermediate drive electronics package, a main drive electronics package, and a PC-based digital image management system. The system has the potential to replace the use of dental film in intraoral radiographic examinations. High-resolution images are acquired, then displayed on a CRT within seconds of image acquisition.
We have designed and fabricated an x-ray linear scanner based on monolithic CdZnTe arrays. The arrays are 1.0 inch long with 16 detectors each. To increase the x-ray stopping power of the arrays, they are operated with the x-ray photons incident normal to the E-field direction. A photon-counting pulse mode read-out is utilized to produce high dynamic range images at low incident flux rates. Our read-out electronics are composed of independent 16-channel modules including charge pre-amplification, pulse shaping discriminated scaler, and buffered I/O. The linear packing density for the electronics is 16 channels per inch, commensurate with the array pitch. Results from measurements of spatial response generated by scanning a fine beam of gamma rays across the arrays, and x-ray images produced in a linear-scan mode are presented.
The paper presents a low capacitance, low reverse current, suited for x-ray dosimetry at room temperature. The detector is a `drift chamber' type, with a transversal field collection of charge carriers. Based on the low noise performance of the detector and the first stage of electronics, a pocket size x-ray dose ratemeter is proposed, for the 10...60 keV energy range.
This paper describes the x-ray image conversion panel (XICP) converting x-ray images to visible images. It is shown that the XICP consists primarily of a photoconductive (PC) layer and an electro-luminescent (EL) layer sandwiched together between two electrodes. Some results such as resolution, response speed, and output brightness are given.
Incorporation of polycrystalline phosphor screens into efficient, high precision x-ray detectors requires an understanding of the subtleties of x-ray capture and subsequent light transmission in the screen, as well as an awareness of how phosphors affect detector calibration. We discuss the preparation of such phosphor screens and their characterization with respect to efficiency, stopping power, spatial resolution, decay time, and spectral output.
We are developing NaI(Tl) phosphors to use on large-area, photon-counting imaging photomultiplier tubes. An x-ray absorption uniformity of 99% is expected over an area of 133 cm2 at a film thickness of 38 +/- 1.5 micrometers . The spatial resolution of the scintillator films, up to 61 micrometers thick, is consistent with a total detector resolution of less than 92 micrometers , given a phototube resolution of 65 micrometers fwhm. The fwhm for 8 keV spots is presented as a function of angle up to 40 degrees off-incidence and a simple model for the broadening of the fwhm due to parallax is presented. Improvements of 20% in the light- collection efficiency were observed for fiber optic disks coated with potassium silicate before vapor-depositing the NaI(Tl). We also present absolute scintillation energy efficiency measurements by comparing our films to the scintillation of single crystal NaI(Tl).
Preliminary investigations were carried out to evaluate the resolution of Cd1-xZnxTe detectors at temperatures achievable with commercially available, low power Peltier refrigerators. Detectors were in the form of cubes, 2 mm on a side. They were tested using a preamplifier with optical feedback. The input FET was cooled along with the detectors. Resolutions of 409 eV and 326 eV were observed at the 5.9 keV line of Fe-55, at temperatures of -10 degree(s)C and -20 degree(s)C, respectively. Results indicate that a straightforward extension of this work will lead to resolutions well below 200 eV.
We describe a modular detector designed primarily for macro-molecular x-ray crystallography experiments which use a rotating-anode x-ray source. The module consists of a fiber optic demagnifying taper with a phosphor x-ray converter deposited on the large end of the taper and a CCD bonded to the small end. Two or more modules can be used together to increase the total detector area. The detector components are chosen to optimize data-collection efficiency while keeping the overall cost relatively low and the reliability high. Performance measurements for a prototype detector of this design are presented.
The performance of a detector can be characterized by its efficiency for measuring individual x-rays or for measuring Bragg peak intensities. The performance for detecting individual x- rays or for measuring Bragg peak intensities. The performance for detecting individual x-rays is well modeled by the DQE. The performance for measuring Bragg peak intensities in the presence of an x-ray background can be modeled by an expanded definition of the DQE which allows inclusion of experimental constraints, the XDCE. These constraints include the observation that by increasing the crystal-to-detector distance and using a larger detector, Bragg peaks can be better resolved and the x-ray background reduced. Calculation of the XDCE for a detector consisting of a fiberoptic taper with a phosphor x-ray convertor deposited on the large end and a CCD bonded to the small end demonstrate the need to make the detector area relatively large, possibly at the expense of a decrease in the DQE.
This paper reviews the status of real-time imaging systems which are used in radiation-therapy for radiotherapy localization and verification. Imaging systems under review include (1) metal- fluorescent screens, optically coupled to video cameras; (2) metal-phosphor screen in direct contact with two-dimensional photo-diode array (flat panel detector); (3) two-dimensional liquid ionization chamber; and (4) linear diode arrays. These systems permit frequent verification during the treatment and have been shown to be very useful. Unfortunately the image quality achieved, while impressive considering the short time the devices have been on the market, is significantly inferior to that which is available from the metal/film combination (port film).
High energy photon backscatter uses pair production to probe deep beneath surfaces with single side accessibility or to image thick, radiographically opaque objects. At the higher photon energies needed to penetrate thick and/or highly attenuating objects, Compton backscatter becomes strongly forward peaked with relatively little backscatter flux. Furthermore, the downward energy shift of the backscattered photon makes it more susceptible to attenuation on its outbound path. Above 1.022 MeV, pair production is possible; at about 10 MeV, pari production crosses over Compton scatter as the dominant x-ray interaction mechanism. The backscattered photons can be hard x rays from the bremsstrahlung of the electrons and positrons or 0.511 MeV photons from the annihilation of the positron. Monte Carlo computer simulations of such a backscatter system were done to characterize the output signals and to optimize a high energy detector design. This paper touches on the physics of high energy backscatter imaging and describes at some length the detector design for tomographic and radiographic imaging.
Operating as a photoconductor, the sensitivity and the impulse response of semi-insulating materials greatly depend on the excitation duration compared to electron and hole lifetimes. The characteristic of ohmic contact for these compounds is briefly discussed. Before developing picosecond measurements with integrated autocorrelation system, this paper explains high energy industrial tomographic application with large CdTe detectors (25 X 15 X 0.9 mm3) where spatial resolution, contrast, and wide dynamic are the main criteria. The excitation is typically microsecond(s) range. X-ray flash radiography with 10 ns burst is in an intermediate time domain where excitation is similar to electron life-time in cadmium telluride. In a laser fusion experiment the excitation is in the range of 50 ps and we develop for such high band devices photoconductive structures able to study very short x-ray emission. Thin polycrystalline MOCVD CdTe films with picosecond response are an alternative material suitable to perform optical correlation measurements of single shot pulses with a very large bandwidth (approximately 50 GHz).
This paper discusses a type of x-ray camera designed to generate standard RS-170 video output that does not use x-ray or optical image intensifiers. Instead, it employs a very sensitive, very high resolution CCD sensor which views an x-ray-to-light conversion screen directly through a high speed imaging lens. This new solid state TV camera, which is described later, has very low readout noise plus unusually high gain which enables it to generate real-time video with incident flux levels typical of many inspection applications. Perhaps more important is an ability to integrate for multiple frame intervals on the chip followed by the output of a standard, RS-170 format video frame containing two balanced interlaced fields. In this integrating mode excellent quality images of low contrast objects can be obtained with only a few tenths of a second integration intervals. The basic elements of this type of camera are described and applications discussed where this approach appears to have important advantages over other methods in common use.
Recognizing the critical need to advance new composites for the aeronautics and aerospace industries, we are focussing on advanced test methods that are vital to successful modeling and manufacturing of future generations of high temperature and durable composite materials. These newly developed composites are necessary to reduce propulsion cost and weight, to improve performance and reliability, and to address longer-term national strategic thrusts for sustaining global preeminence in high speed air transport and in high performance military aircraft.
Recently x-ray cone beam computed tomography (CT) has become of interest for nondestructive testing (NDT) of advanced materials. Such a technique takes advantage of the cone beam geometry, to reduce the acquisition time and increase the resolution. Performances of CT systems rely mainly on geometric precision and measurement quality. Inaccurate geometry or incorrect data produce artifacts and blurring which limit the spatial resolution. A precise geometric calibration procedure is required and some corrections must be applied to the raw attenuation data in order to obtain accurate measurements. An x-ray cone beam CT system has been developed at the LETI. This machine was designed to control small parts limited to a few centimeters, with a high spatial resolution close to 30 microns. This paper introduces the machine setup and describes the calibration computing resources involved in the system. Then, we discuss the performances on experimental data.
The damage fields in an edge-cracked sheet specimen subjected to a constant crosshead speed were investigated using the real-time x-ray technique. The specimen was made from polybutadiene rubber embedded with hard particles. The x-ray data were analyzed to delineate the damage field near the crack tip and to generate contour plots of the damage intensity. The experimental data were analyzed and the results are discussed.
We previously reported on the development, design, and clinical evaluation of a CCD-based, high performance, filmless imaging system for stereotactic needle biopsy procedures in mammography. The MammoVision system has a limited imaging area of 50 mm X 50 mm, since it is designed specifically for breast biopsy applications. We are currently developing a new filmless imaging system designed to cover the 18 cm X 24 cm imaging area required for screening and diagnostic mammography. The diagnostic mammography system is based on four 1100 X 330 pixel format, full-frame, scientific grade, front illuminated, MPP mode CCDs, with 24 micrometers X 24 micrometers square pixels Each CCD is coupled to an x-ray intensifying screen via a 1.7:1 fiber optic reducer. The detector assembly (180 mm long and 13.5 mm wide) is scanned across the patient's breast synchronously with the x-ray source, with the CCDs operated in time-delay integration (TDI) mode. The total scan time is 4.0 seconds.
A requirement for production line characterization of the physical parameters of the new production reactor (NPR) target particles leads to the investigation of the use of digital, electronic micro-x-radiography and micro-x-ray computed tomography for this purpose. The target particles consist of concentric shells of various materials about a spherical nucleus of lithium aluminate. The outer diameter of the particle is 991 microns (0.039 in.) and the thinnest shell thickness is 25.4 microns (0.001 in.). The densities of the materials range from 0.7 gm/cm3 to 3.2 gm/cm3. The principal requirement was to measure accurately the diameter of the nucleus and the thickness of the four spherical shells. In order to accomplish the objective, an area, x-ray imager was designed with a calculated resolving power of 2 microns. The imager consisted of a cooled CCD, classical lens optics and an x-ray to visible light converter capable of very high spatial resolution. The system modulation transfer function was experimentally determined, and indicated a limiting resolution of about 100 1p/mm. This imager was used with a conventional x-ray source to acquire x-ray images that resolved the various shells of the pellet in a traditional 2-D manner. The same imager was used to obtain the projections required for computed tomography images. The images obtained from both techniques successfully provided the resolution to characterize the target particles in terms of spatial dimensions and material density.
Relatively low-cost high-resolution computed tomography can be implemented using a detector based on a self scanned linear array. Images from samples larger than the detector length can be obtained by translating the sample and acquiring the sinogram in more than one step. The reconstruction is performed in software using a personal computer.
The high resolution camera (HRC) is a set of microchannel plate based detectors designed to fly aboard the Advanced X-Ray Astrophysics Facility (AXAF), one of the `Great Observatories' under the auspices of NASA. The HRC is designed in two configurations, one for direct imaging and the other for reading out the spectrum of transmission gratings for high resolution spectroscopy. The current design calls for different photocathodes: the imaging detector will have a CsI coating, and the spectroscopic detector will have KBr. Since the instrument is expected to perform over an extended period in space, we are interested in the long-term behavior of these coatings in vacuum. In this paper, we examine the current ROSAT experience from flight calibration observations with the HRI (the predecessor of HRC), and we outline plans for a test of the long-term stability of these coatings in vacuum and under nitrogen storage.
The low energy grating readout for the AXAF-I mission will be implemented with microchannel plates in conjunction with a novel semi-solid substrate strip charge detector. One axis of the charge detector consists of a conventional wire grid. The other axis consists of charge pickup strips formed on a ceramic substrate. This configuration allows the construction of a three segment detector with the approximate curvature of the Rowland circle. A prototype detector has been built and tested. Spatial resolution is commensurate with conventional crossgrid detectors (< 25 micrometers FWHM).
Avalanche photodiodes (APDs) are solid state devices having an internal signal gain which gives them a better signal-to-noise ratio than standard photodiodes. Although they have been studied for years, recent advances in the fabrication techniques have allowed the construction of multielement arrays (up to 10 X 10) with high performance capability. This progress has resulted in increased potential for exploiting the advantages of APDs in a variety of important applications including measurements requiring fast response such as nuclear and high energy physics research, industrial nondestructive testing, medical instrumentation, and biomedical research using low energy particles. Recent experimental data characterizing APDs and APD arrays used as x-ray, particle, and low level light detectors are presented.