Mark W. Tate

Nonbilayer phases of membrane lipids

Numerous liquid crystalline biomembrane lipids are known to exhibit non-lamellar phases characterized by curvature of their component lipid monolayers. An understanding of the phase stability of these systems begins with analysis of the energy of bending the monolayers, the interactions which lead to the bending energy, and the geometrical constraints which lead to competing energy terms which arise when the monolayers are bent and packed onto lattices with different structures. Diffraction and other techniques suitable for probing lipid phase structure are described. A phenomenological model is reviewed which successfully explains many of the qualitative features of lipid mesomorphic phase behavior. A key result of this model is that lipid bilayer compositions which are close to the non-lamellar phase boundaries of their phase diagrams are characterized by a frustrated elastic stress which may modulate the activity of imbedded membrane proteins and which may provide a rationale for the prevalence of non-lamellar-tending lipid species in biomembrane bilayers. Areas in need of future research are discussed.

Characterization of a prototype pixel array detector (PAD) for use in microsecond framing time-resolved X-ray diffraction studies

A 4 by 4 pixel array device has been designed and built as a prototype for a high-speed two-dimensional X-ray imaging detector. This detector is a two-tier device with an X-ray sensitive photo-diode array bump-bonded to a 1.2 micrometer CMOS analog integrated circuit. Each 150 micrometer square pixel of the detective layer is matched to a corresponding section of the electronics. Each pixel of the electronics is capable of integrating the signal from the diode for the desired framing time, storing the result in one of eight storage capacitors and sequentially outputting the stored value to an on chip buffering op amp. Tests of the electronics have demonstrated a full-well (per pixel, per frame) of over 10.500 12 keV X-rays with an electronics noise level corresponding to less than 2.8 12 keV X-rays. Speed tests indicate the ability to integrate to the full-well within 2 microseconds (an average pixel count-rate of 5.2 GHz). We present detailed characterizations of the performance of this initial device, including an analysis of noise, stability, linearity and point spread. In addition, we will discuss plans for large scale integration towards the goal of a 1024 by 1024 pixel detector.

A direct-coupled detector for synchrotron X-radiation using a large format CCD

A novel X-ray area detector based on a large-format charge-coupled device (CCD) imaging array has been constructed and characterized. It was found to exhibit high signal-to-noise ratio, wide dynamic range, and high spatial resolution. Tests with both conventional and synchrotron sources showed the detector to be highly suitable for X-ray diffraction studies. The prototype detector has been thermally cycled more than six times and transported to and from the Cornell High Energy Synchrotron Source with no apparent degradation in performance. Directions for future development of this type of detector are briefly discussed. >

Coupling format variations in x-ray detectors based on charge coupled devices

Three variations of charge coupled device (CCD) based x-ray detectors have been constructed and evaluated, two of which are used for routine data collection on small angle x-ray diffraction beamlines at Princeton. The first detector consists of a commercially available CCD camera system lens coupled to the output of a four-stage magnetically focused image intensifier. The second system, assembled to gather test data, consists of a CCD fiber-optically coupled to an x-ray conversion phosphor via a 5:1 reduction fiber-optic taper. The final detector configuration was comprised of this fiber-optic taper system with the addition of a single stage proximity-focused image intensifier. Rationale is given for choosing components to be used in the assembly of CCD-based detectors.

High-resolution macromolecular structure determination using CCD detectors and synchrotron radiation

BACKGROUNDnSynchrotron radiation sources have made impressive contributions to macromolecular crystallography. The delay in development of appropriate X-ray detectors has, however, been a significant limitation to their efficient use. New technologies, based on charge-coupled devices (CCDs), provide capabilities for faster, more accurate, automated data collection.nnnRESULTSnA CCD-based X-ray detector has been developed for use in macromolecular crystallography and has been in operation for about one and a half years at the Cornell High Energy Synchrotron Source. It has been used for a variety of crystallographic projects, including a number of high-resolution structural studies. The statistical quality of the data, the detectors ease and efficiency of use, and the growing number of structural results illustrate the practical utility of this new detector system.nnnCONCLUSIONSnThe new detector has enhanced capabilities for measuring diffraction patterns from crystals of macromolecules, especially at high resolution, when the X-ray intensities are weak. The survey of results described here ranges from virus crystallography to weakly diffracting small-molecule structure determination and demonstrates the potential of CCD detectors when combined with synchrotron radiation sources.

Study of afterglow in x-ray phosphors for use on fast-framing charge-coupled device detectors

There is a need in the x-ray imaging community for phosphors with short persistence (I /Ib<0.1 to 0.01% in less than 0.5 ms). Persistence limits a detector’s performance by defining the minimum xray exposure discernible for a given time after a previous exposure, by ghosting of previous images, and by limiting the dynamic range. We characterize the persistence and relative light output of 25 commercially available and specially synthesized phosphors, five of which are gadolinium oxysulfide (Gd2O2S) with five different activators. The phosphor’s ‘‘practical efficiency’’ is presented for use with either front- or rearilluminated CCDs. The persistence of these phosphors is characterized as a function of x-ray intensity, exposure time, and, when possible, impurity concentrations. Each phosphor’s usefulness for particular x-ray imaging experiments is also discussed.

Characterization of polycrystalline phosphors for area x-ray detectors

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.

Development of a fast pixel array detector for use in microsecond time-resolved x-ray diffraction

A large-area pixel x-ray detector is being developed to collect eight successive frames of wide dynamic 2D images at 200kHz rates. Such a detector, to conjunction with a synchrotron radiation x-ray source, will enable time-resolved x-ray studies of proteins and other materials on time scales which have previously been inaccessible. The detector will consist of an array of fully-depleted 150 micron square diodes connected to a CMOS integrated electronics layer with solder bump-bonding. During each framing period, the current resulting from the x-rays stopped in the diodes is integrated in the electronics layer, and then strored in one of eight storage capacitors underneath the pixel. After the last frame, the capacitors are read out at standard data transmission rates. The detector has been designed for well-depth of at least 10,000 x-rays (at 12 keV), and a noise level of one x-ray. Ultimately, we intend to construct a detector with over one million pixels (1024 by 1024). We present the result of our development effort and various features of the design. The electronics design is discussed, with special attention to the performance requirements. The choice and design of the detective diodes, as they relate to x-ray stopping power and charge collection, are presented. An analysis of various methods of bump bonding is also presented. Finally, we discuss the possible need for a radiation-blocking layer, to be placed between the electronics and the detective layer, and various methods we have pursued in the construction of such a layer.

Analog pixel array detectors.

X-ray pixel array detectors (PADs) are generally thought of as either digital photon counters (DPADs) or X-ray analog-integrating pixel array detectors (APADs). Experiences with APADs, which are especially well suited for X-ray imaging experiments where transient or high instantaneous flux events must be recorded, are reported. The design, characterization and experimental applications of several APAD designs developed at Cornell University are discussed. The simplest design is a ;flash architecture, wherein successive integrated X-ray images, as short as several hundred nanoseconds in duration, are stored in the detector chips for later off-chip digitization. Radiography experiments using a prototype flash APAD are summarized. Another design has been implemented that combines flash capability with the ability to continuously stream X-ray images at slower (e.g. milliseconds) rates. Progress is described towards radiation-hardened APADs that can be tiled to cover a large area. A mixed-mode PAD, design by combining many of the attractive features of both APADs and DPADs, is also described.

A Pixel-Array Detector for Time-Resolved X-ray Diffraction

An integrating pixel-array detector for recording time-resolved X-ray diffraction measurements on microsecond timescales has been designed and tested as a 4 x 4 pixel prototype. Operational characteristics and radiation tolerance are discussed. A 100 x 92 array with 151.2 micro m square pixels is currently under construction.