Robert A. Lewis

Time‐resolved x‐ray diffraction station: X‐ray optics, detectors, and data acquisition

A new x‐ray beamline has recently been built, and is now operational, on dipole magnet 2 of the Synchrotron Radiation Source (SRS) at Daresbury. This beamline takes 32 mrad of horizontal aperture from a central tangent point. The time‐resolved x‐ray diffraction (TRXD) Station 2.1, takes 17 mrad of horizontal aperture and is the central point of this paper. Beamline 2 has been realized as part of a SERC–MRC agreement.

Breast cancer diagnosis using scattered X-rays

Small-angle X-ray diffraction data has been collected from 99 `core-cut breast tissue specimens representing a number of different pathologies. Data in the range 75–1390u2005A have been compared with controls from patients with no breast disease. Bessel functions and Bragg maxima resulting from the fibrillar structure of collagen have been identified. The Bragg maxima indexed onto a 649u2005A lattice. Systematic differences in the intensities and D-spacings between the collagen of malignant, benign and normal tissue groups have been clearly demonstrated and quantified. These differences appear to be due to a significantly lower structural order within the malignant tissues. Possible explanations for this are discussed and the potential for utilizing this observation in cancer diagnosis is considered.

Two-dimensional time-resolved X-ray diffraction studies of live isometrically contracting frog sartorius muscle.

SummaryResults were obtained from contracting frog muscles by collecting high quality time-resolved, two-dimensional, X-ray diffraction patterns at the British Synchrotron Radiation Source (SERC, Daresbury, Laboratory). The structural transitions associated with isometric tension generation were recorded under conditions in which the three-dimensional order characteristic of the rest state is either present or absent. In both cases, new layer lines appear during tension generation, subsequent to changes from activation events in the filaments. Compared with the ‘decorated’ actin layer lines of the rigor state, the spacings of the new layer lines are similar whereas their intensities differ substantially. We conclude that in contracting muscle an actomyosin complex is formed whose structure is not like that in rigor, although it is possible that the interacting sites are the same. Transition from rest to plateau of tension is accompanied by approximately 1.6% increase in the axial spacing of the myosin layer lines. This is explained as arising from the axial disposition of the interacting myosin heads in the actomyosin complex. Model calculations are presented which support this view. We argue that in a situation where an actomyosin complex is formed during contraction, one cannot describe the diffraction features as being either thick or thin filament based. Accordingly, the layer lines seen during tension generation are referred to as actomyosin layer lines. It is shown that these layer lines can be indexed as submultiples of a minimum axial repeat of approximately 218.7 nm. After lattice disorder effects are taken into account, the intensity increases on the 15th and 21st AM layer lines at spacings of approximately 14.58 and 10.4 nm respectively, show the same time course as tension rise. However, the time course of the intensity increase of the other actomyosin layer lines and of the spacing change (which is the same for both phenomena) shows a substantial lead over tension rise. These findings suggest that the actomyosin complex formed prior to tension rise is a non-tension-generating state and that this is followed by a transition of the complex to a tension-generating state. The intensity increase in the 15th actomyosin layer line, which parallels tension rise, can be accounted for assuming that in the tension-generating state the attached heads adopt (axially) a more perpendicular orientation with respect to the muscle axis than is seen at rest or in the non-tension-generating state. This suggests the existence of at least two structurally distinct interacting myosin head conformations. The results of comparing the meridional intensities between the myosin layer lines at rest and the actomyosin layer lines at the plateau of tension (measured to a resolution of approximately 2.6 nm) are interpreted to indicate that the majority of the myosin heads in the actomyosin complex do not perform random axial rotations with a mean value greater than approximately 3.0 nm. From this we conclude that the extent of axial order in the interacting heads must be at least as high as is that of resting heads.

The “RAPID” high rate large area X-ray detector system

The multiwire proportional counter (MWPC) is a well-established device for capturing X-ray images from synchrotron sources and is particularly well suited to dynamic experiments. Its advantages include, almost zero noise, high dynamic range limited only by the electronic memory depth, large area and time resolutions of microseconds. It does however have some limitations, notably in global and local count rate performance. n nThe RAPID two-dimensional detector system delivers a more than twentyfold increase in throughput over present systems. It comprises a “wire MicroGap” detector, which has much higher count rate performance than coventional MWPCs and a sophisticated multi-channel data acquisition system. The system has a global count rate capability of greater than 2 × 107 photons s−1 with a maximum local count rate of ∼106 photons mm−2s−1. A spatial resolution of ∼200 μm, over an active area of 12.8 × 12.8 cm, has been achieved which compares well with exiting read-out systems. Each electrode of the detector is instrumented with a preamplifier and ADC and the position of the event is determined independently in X and Y by centroiding the induced charge distribution. The X and Y coordinates are correlated using a unique time stamp. n nThis paper described the design and performance of the detector and read-out system and presents some recent beamline results.

Multiwire X-ray detector systems at the daresbury srs

Abstract Synchrotron radiation allows the study of structural dynamics by X-ray diffraction techniques. These experiments impose unusual demands on X-ray detector systems. High count rates, wide dynamic range, good spatial and temporal resolution and real time data processing are all required. A recently completed system fulfills many of these requirements having a spatial resolution of better than 0.3 × 0.3 mm over an area of 200 × 200 mm with a 600 kHz throughput and a dynamic range limited by counting statistics. Examples derived from the existing experimental programme are used to illustrate the performance of these devices.

Multiwire Gas Proportional Counters: Decrepit Antiques or ClassicPerformers?

An overview of the operational characteristics of multiwire gas proportional counters is given with particular reference to their use in X-ray diffraction. Their strengths and weaknesses are analysed and it is demonstrated that these devices are able to offer a combination of features that is unique. Some examples of synchrotron radiation experiments performed with gas detectors are used to illustrate their performance, and finally, the current status of development and prospects for the future development of gas detectors are reviewed.

High counting rate gaseous x‐ray detectors for synchrotron radiation applications (invited)

Multiwire gas proportional detectors have been used for many years on synchrotron radiation experiments and offer unrivaled dynamic range and detection efficiency but have been somewhat limited in count rate performance. We report here recent test results from two new rapid data acquisition systems developed at Daresbury and in addition the results of comparative tests on a new design of gas detector, the Microgap, and a gas Microstrip detector. Both designs appear capable of high rate ≳100 kHz/mm2 operation and combined with the new acquisition systems should begin to alleviate the count rate problems for some types of experiment.

The application of multiwire x-ray detectors to experiments using synchrotron radiation

Multiwire detectors have certain characteristics that can be used to advantage, particularly in small-angle diffraction or scattering experiments on biological systems. The size of the active region of the detector and the total number of pixels can be conveniently matched to the dimensions of the synchrotron beam and to the optics of the beam line camera. Furthermore, the large dynamic range inherent in single-photon counting systems can be exploited to ensure the collection of high-quality data even when the scattering factor varies over several orders of magnitude. n nLinear and two-dimensional detectors are in regular use at the Daresbury Synchrotron Radiation Source, all using the delay-line readout method and a standardised data acquisition system which includes facilities for time-resolved measurements. n nThe development of a new system (now at the prototype stage) to operate at the very high count rates available from synchrotron sources is outlined in this paper.

The RAPID detector system – first user data

Abstract The RAPID detector system, one of the fastest 2D imaging photon counting systems currently in operation, has successfully completed the final stage of its commissioning and is now a user facility on the Synchrotron Radiation Source (SRS). A description of the detector system and its operational performance under real experimental conditions are presented. A discussion is also made of refinements to the system that would further enhance its performance.

X-ray calibration of the SODART flight telescopes

The on- and off-axis imaging properties and effective area of the two SODART flight telescopes have been measured using the expanded beam x-ray facility at the Daresbury synchrotron. From on-axis measurements the encircled power distribution and the point spread function at three energies 6.627 keV, 8.837 keV and 11.046 keV have been measured using a one-dimensional position sensitive detector. We found that the point spread function can be presented well by a function which is a sum of a Gaussian divided by the radius and two exponential terms where the first has a 1/e value close to 2 arcmin and the other a 1/e value of ca. 15 arcmin. The data have been used to calculate the half power diameter (HPD) for three different SODART focal plane detectors, the high energy proportional counter (HEPC) with a field of view (FOV) of 65 arcmin, the low energy proportional counter (LEPC) with a FOV of 33 arcmin and the 19 element solid state detector array (SIXA) with a FOV of 18 arcmin. We found that the HPD decreases with increasing energy due to poorer figure of the outermost mirrors. The HPD falls in the range from 2.4 to 3.8 arcmin depending on energy and FOV. Data have also been obtained on the on- and off-axis effective area at all three energies and compared to that obtained from a raytracing of an ideal telescope configuration. We found that the measured on-axis effective area integrated over a FOV of 105 arcmin is ca. 65% of the area predicted from an ideal geometry. Finally the one- dimensional detector data has been used to obtain the radial dependence of the on-axis HPD and the on-axis effective area and the data from the two-dimensional position sensitive detector has been used to obtain the azimuthal dependence of the on-axis HPD and the on-axis effective area.