Phosphor-coupled CCDs are established as one of the most successful technologies for x-ray diffraction. This application demands that the CCD simultaneously achieve both the highest possible sensitivity and high readout speeds. Recently, wafer-scale, back illuminated devices have become available which offer significantly higher quantum efficiency than conventional devices (the Fairchild Imaging CCD 486 BI). However, since back thinning significantly changes the electrical properties of the CCD the high speed operation of wafer-scale, back-illuminated devices is not well understood. Here we describe the operating characteristics (including noise, linearity, full well capacity and CTE) of the back-illuminated CCD 486 at readout speeds up to 4 MHz.
We are developing pixel detectors for macromolecular crystallography, in which diffracted X-rays are directly absorbed by high-resistivity, crystalline silicon that has been micro-machined by inductively-coupled plasma etching. Arrays of 64 × 64 holes at 150 μm pitch are first formed by etching through the entire silicon bulk, then backfilled with polysilicon that is doped to create conducting p and n type columnar electrodes. When reverse biased, these electrodes generate electric fields that define the individual pixels. By forming conducting polysilicon on the sides of the sensors, which are cut-out of the silicon wafer by plasma etching, the entire surface of the detector may be made active. CMOS readout integrated circuits are conductively bump bonded behind each 3D detector, providing a direct connection to every pixel. A large array will be assembled with no insensitive bands along the edges by overlapping these sensors, each of area 0.96cm2. This detector will measure X-ray signal intensities of up to 105 events/pixel/sec without any pile-up loss, by using an integration method that retains the benefits of discrete photon counting. The detector sensitivity will be highly uniform, it will not exhibit any dark signal or spurious noise, and no geometric distortion will occur within each sensor.
We have prototyped and characterized a very large format X-ray detector for macromolecular crystallography. The X-ray field strength is converted to visible light in a phosphor film. Light from the phosphor is focused onto a CCD imager by a lens specially designed for this detector, that has a very high numerical aperture. The CCD is very large (61 mm, 4,096 × 4,096 pixels), and employs a very low-noise on-chip preamplifier.
Lens coupling between phosphor film and CCD avoids many of the optical imperfections of fiber optic coupling, but it remains a challenge to make a lens system with optical transfer efficiency matching or exceeding that of fiber optical systems. We have met this challenge by enhancing system gain in our detector through implementation of modern lens technologies and imaginative CCD design. At this point the system gain equals that of conventional CCD-based X-ray crystallography detectors, which couple the CCD to the phosphor through a fiber optic taper. Although many of our technical developments could also be used in fiber optic detectors, the overriding virtues of the lens-couple detector are simplicity, optical perfection, and cost.
During the 1990's, macromolecular crystallography became progressively more dependent on synchrotron X-ray sources for diffraction data collection. Detectors of this diffraction data at synchrotron beamlines have evolved over the decade, from film to image phosphor plates, and then to CCD systems. These changes have been driven by the data quality and quantity improvements each newer detector technology provided. The improvements have been significant. It is likely that newer detector technologies will be adopted at synchrotron beamlines for crystallographic diffraction data collection in the future, but these technologies will have to compete with existing CCD detector systems which are already excellent and are getting incrementally better in terms of size, speed, efficiency, and resolving power. Detector development for this application at synchrotrons must concentrate on making systems which are bigger and faster than CCDs and which can capture weak data more efficiently. And there is a need for excellent detectors which are less expensive than CCD systems.
After 3 years experience of operating very large area (210 mm X 210 mm) CCD-based detectors at the Advanced Photon Source, operational experience is reported. Four such detectors have been built, two for Structural Biology Center (APS-1 and SBC-2), one for Basic Energy Sciences Synchrotron Radiation Center (Gold-2) at Argonne National Laboratory's Advanced Photon Source and one for Osaka University by Oxford Instruments, for use at Spring 8 (PX-210). The detector is specifically designed as a high resolution and fast readout camera for macromolecular crystallography. Design trade-offs for speed and size are reviewed in light of operational experience and future requirements are considered. Operational data and examples of crystallography data are presented, together with plans for future development.
A large area, charge-couple-device (CCD) based fiberoptic taper array detector (APS-1) has been installed at the insertion-device beam line of the Structural Biology Center at the ANL Advanced Photon Source. The detector is used in protein crystallography diffraction experiments, where the objective is to measure the position and intensity of x-ray Bragg peaks in diffraction images. Large imaging area, very high spatial resolution, high x-ray sensitivity, good detective quantum efficiency, low noise, wide dynamic range, excellent stability and short readout time are all fundamental requirements in this application. The APS-1 detector converts the 2D x-ray patterns to a visible light images by a thin layer of x-ray sensitive phosphor. The phosphor coating is directly deposited on the large ends of nine fiberoptic tapers arranged in a 3 by 3 array. Nine, thermoelectrically cooled 1024 by 1024 pixel CCDs image the patterns, demagnified by the tapers. After geometrical and uniformity corrections, the nine areas give a continuous image of the detector face with virtually no gaps between the individual tapers. The 18 parallel analog signal- processing channels and analog-to-digital converters assure short readout time and low readout noise.
Hydrogenated amorphous silicon (a-Si:H) is an electronically readable semiconductor material which can be inexpensively deposited over large surface areas. It is a technology under active development within the Xerox Corporation's Palo Alto Research Center (PARC) for a variety of possible applications. Our calculations indicate that, properly designed and properly applied, this material has great promise as a two-dimensionally sensitive electronic sensor for x-ray area detectors, useful in protein crystallography. The Argonne and Brandeis groups are currently making CCD area detectors, which are excellent but are expensive to build and have a much smaller area then we would like. We therefore have begun to develop large, inexpensive area detectors for protein crystallography, based upon a-Si:H sensors manufactured by Xerox Corporation. Our calculations suggest we can make them large, efficient, fast, high resolution, and with high dynamic range. These detectors should be much less expensive to manufacture than CCD-based detectors, their active areas should be comparable to or larger than image phosphor plate detectors, and they will be electronically readable directly into computer systems with speeds of 1 second or faster.
We have designed, fabricated, and tested a modular CCD area detector system for macromolecular crystallography at synchrotron x-ray sources, code-named the `gold' detector system. The sensitive area of the detector is 150 mm X 150 mm, with 3,072 X 3,072 pixel sampling, resulting in roughly a 50 micrometers pixel raster. The x-ray image formed on the face of the detector is converted to visible light by a thin phosphor layer. This image is transferred optically to nine CCD sensors by nine square fiberoptic tapers (one for each CCD), arranged in a 3 X 3 array. Each taper demagnifies the image by a factor of approximately 2. Each CCD has a 1,024 X 1,024 pixel raster and is read out through two independent data channels. After each x-ray exposure period the x-ray shutter is closed and the electronic image is digitized (16-bit) and read out in 1.8s. Alteratively, the image may be binned 2 X 2 during readout, resulting in a 1,536 X 1,536 raster of 100 micrometers pixels; this image can be read out in 0.4s. The CCD sensors are operated at -40 degree(s)C to reduce electronic noise. The detector is operated under full computer control: all operational parameters (readout rates, CCD temperature, etc.) can be adjusted from the console. The image data (18 MByte/image) are transferred via a fast VME system to a control processor and ultimately to disk storage. During April 1994 we carried out a complete set of measurements at the Stanford Synchrotron Radiation Laboratory (SSRL) for a full characterization of the gold detector. Characterization includes quantitative evaluation of the instrument's conversion gain (signal level/x-ray photon); detective quantum efficiency (DQE); point-spread function; sensitivity as a function of x-ray energy; geometrical distortion of images; spatial uniformity; read noise; and dark image and dark image noise. Characterization parameters derived from these measurements show that this detector will be extraordinarily valuable for macromolecular crystallography.
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.
We have modeled the performance of x-ray crystallography detectors by combining the characteristics of the detector and the experimental conditions. From this model, we have derived a single expression, the experimental detective collection efficiency (XDCE), which predicts the detector's performance. This expression is evaluated for detectors constructed from a square array of identical modules, each module consisting of a fiberoptic taper with a phosphor x-ray converter deposited on the large end and a CCD bonded to the small end. Using this expression, we have developed a design for a modular detector. In order to explore parameters of this design, we have constructed a test detector module in which we can change the fiberoptic taper, phosphor converter and CCD. We have measured the DQE, spatial resolution, response linearity, and dynamic range for the test module for a 3:1 taper. From these measurements, we predict the performance of this type of detector for x-ray crystallography.
X-ray detector systems for protein crystallographic applications require a large active area and a high detective quantum efficiency (DQE). Using large-format scientific CCDs we are developing a detector based on multiple modules each consisting of a fiberoptic taper with a phosphor x-ray converter deposited on the large end and a CCD coupled to the small end. This approach contrasts with the more typical method of imaging a large area onto a single sensor which requires intensification stages. This system will provide a greater DQE lower spatial and response nonuniformities a smaller package size higher spatial resolution and uniformity and greater reliability. The construction of this system has necessitated development and cha racterization of x-ray-tolight phosphor converters and techniques for coupling fiberoptics to COD imagers. Refinement of phosphor deposition and reflective enhancement methods has led to an increase in phosphor conversion efficiencies over our previous methods of -80. A system to epoxy bond fiberoptic tapers to both front and back illuminated COD imagers with precise control over positioning and clamp force has been developed and tested with fiber faceplates. 1.
A detector with a 114 mm aperture, based on a charge-coupled device (CCD), has been designed for x-ray diffraction studies in protein crystallography. The detector was tested on a beamline of the National Synchrotron Light Source at Brookhaven National Laboratory with a beam intensity greater than 109 x-ray photons/s. A fiber-optic taper, an image intensifier, and a lens demagnify, intensify, and focus the image onto a CCD having 512 X 512 pixels. A detective quantum efficiency (DQE) of 0.36 was obtained by evaluating the statistical uncertainty in the detector output. The dynamic range of a 4 X 4 pixel resolution element, comparable in size to a diffraction peak, was 10 4. The point-spread function shows FWHM resolution of approximately 1 pixel, where a pixel on the detector face is 160 micrometers . A complete data set, consisting of forty-five 1 degree(s) rotation frames, was obtained in just 36 s of x-ray exposure to a crystal of chicken egg-white lysozyme. In a separate experiment, a lysozyme data set consisting of 495 0.1 degree(s) frames, was processed by the MADNES data reduction program, yielding symmetry R-factors for the data of 3.2- 3.5. Diffraction images from crystals of the myosin S1 head (a equals 275 angstroms) were also recorded. The Bragg spots, only 5 pixels apart, were resolved but were not sufficiently separated to process these data. Changes in the detector design which will improve the DQE and spatial resolution are outlined. The overall performance showed that this type of detector is well suited for x-ray scattering investigations with synchrotron sources.
31 July 2005 | San Diego, California, United States
Ultrafast X-Ray Detectors and Applications
6 August 2003 | San Diego, California, United States
X-Ray and Gamma-Ray Detectors and Applications IV
8 July 2002 | Seattle, WA, United States
X-Ray Detectors
7 July 2002 | Seattle, WA, United States
Detectors for Crystallography and Diffraction Studies at Synchrotron Sources
19 July 1999 | Denver, CO, United States
Course Instructor
SC408: Crystallographic X-Ray Detector Technology
This course compares and contrasts a variety of technologies that can be applied to the design and manufacture of detectors for the crystallographic analysis of biological macromolecules. Successful existing detector systems are studied to examine what attributes are required for a good detector in this field. The course examines where improvements can be made with existing technologies, and identifies new technologies showing promise for future development. In particular, design attributes that make a good detector for applications at synchrotron sources are contrasted to those attributes that make a good detector on a laboratory X-ray source.
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