W.I. Helsby

The new materials processing beamline at the SRS Daresbury, MPW6.2

A new beamline (MPW6.2) has been designed and built for the study of materials during processing where three synchrotron techniques, SAXS, WAXS and XAS, are available simultaneously. It has been demonstrated that Rietveld refinable data can be collected from silicon SRM 640b over a 60 degrees range in a time scale of 1 s. The data have been refined to a chi(2) of 2.4, the peaks fitting best to a Pearson VII function or with fundamental parameters. The peak halfwidths have been found to be approximately constant at 0.06 degrees over a 120 degrees angular range indicating that the instrumental resolution function has matched its design specification. A quantitative comparison of data sets collected on the same isotactic polypropylene system on MPW6.2 and DUBBLE at the ESRF shows a 17% improvement in angular resolution and a 1.8 improvement in peak-to-background ratio with the RAPID2 system; the ESRF data vary more smoothly across detector channels. The time-dependent wide-angle XRD was tested by comparing a hydration reaction of gypsum-bassanite-anhydrite with energy-dispersive data collected on the same system on the same time scale. Three sample data sets from the reaction were selected for analysis and gave an average chi(2) of 3.8. The Rietveld-refined lattice parameters are a good match with published values and the corresponding errors show a mean value of 3.3 x 10(-4). The data have also been analysed by the Pawley decomposition phase-modelling technique demonstrating the ability of the station to quickly and accurately identify new phases. The combined SAXS/WAXS capability of the station was tested with the crystallization and spinodal decomposition of a very dilute polymer system. Our measurements show that the crystallization of a high-density co-polymer (E76B38) as low as 0.5% by weight can be observed in solution in hexane. The WAXS and SAXS data sets were collected on the same time scale. The SAXS detector was calibrated using a collagen sample that gave 30 orders of diffraction in 1 s of data collection. The combined XRD and XAS measurement capability of the station was tested by observing the collapse and re-crystallization of zinc-exchanged zeolite A (zeolite Zn/Na-A). Previous studies of this material on station 9.3 at the SRS were compared with those from the new station. A time improvement of 38 was observed with better quality counting statistics. The improved angular resolution from the WAXS detector enabled new peaks to be identified.

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. The 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. This paper described the design and performance of the detector and read-out system and presents some recent beamline results.

W16.1: A new fixed wavelength diffraction station at the SRS Daresbury

Station W16.1 is a fixed wavelength (1.4 A) x‐ray diffraction station recently constructed and commissioned at the SRS. It has been designed specifically for time‐resolved studies of noncrystalline and fibrous materials and optimized for low angle measurements. Wide angle diffraction will also be available with simultaneous small and wide angle scattering/diffraction a future facility. In order to perform dynamic (∼1 ms) low angle measurements on weakly scattering systems, the station design has had to incorporate several novel features so as to achieve the predicted 1×1013 photon/s at the specimen.

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.

A gas microstrip wide angle X-ray detector for application in synchrotron radiation experiments

Abstract The Gas Microstrip Detector has counting rate capabilities several orders of magnitude higher than conventional wire proportional counters while providing the same (or better) energy resolution for X-rays. In addition the geometric flexibility provided by the lithographic process combined with the self-supporting properties of the substrate offers many exciting possibilities for X-ray detectors, particularly for the demanding experiments carried out on Synchrotron Radiation Sources. Using experience obtained in designing detectors for Particle Physics we have developed a detector for Wide Angle X-ray Scattering studies. The detector has a fan geometry which makes possible a gas detector with high detection efficiency, sub-millimetre spatial resolution and good energy resolution over a wide range of X-ray energy. The detector is described together with results of experiments carried out at the Daresbury Laboratory Synchrotron Radiation Source.

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.

Real-time simultaneous wide- and small-angle fibre diffraction.

A combination of two independent imaging area-detector systems controlled by a single data-acquisition system, provides a powerful system for X-ray diffraction studies of time-resolved phenomena over a wide q range, in samples with intrinsic or induced structural orientation. With this system we have observed a transient, tensile-stress-induced, orthorhombic-to-monoclinic transition in high-density polyethylene.

The RAPID2 interpolating system

A second generation of the RAPID readout electronics has been designed, built and commissioned at Daresbury Laboratory. One of the first applications is for powder diffraction on dynamic systems on the new Multi-Pole Wiggler 6.2 beam-line at the SRS. The ADC per channel system is designed to interpolate to give typically 32 pixels per channel. The Wide-Angle X-ray Scatter detector uses 128 channels of electronics to provide 4096 pixels. The target spatial resolution has a FWHM of 4 pixels, i.e. 1024 FWHM across the detector. This corresponds to approximately 400 μm FWHM over a 384 mm long (60°) curved detector. This spatial resolution requirement is more stringent than the previous generation RAPID1 system. A description of RAPID2 detailing changes to the detector design and optimisation of the lookup RAM based interpolation algorithms to meet the resolution target will be given. Initial beam-line test results will be discussed.

The Daresbury fast multi-wire linear X-ray detector system

Abstract Multiwire proportional counters (MWPCs) have been used regularly on the Synchrotron Radiation Source (SRS) at Daresbury. They are photon counting, can cover large areas, and have fast readout times which makes them attractive to study dynamic processes such as muscle contraction. However, their operation at high rates has been a major limitation, due to two factors, space charge in the detector reducing the local count rate capability, and slow readout electronics which reduces the global count rate. The Daresbury biological X-ray detector group have developed a new type of detector called the wire microgap which significantly improves the local count rate capability of the detector. Coupled to this is a complex 1D electronic readout system which uses an amplifier and discriminator per wire and can handle charge spreading across two wires. The result is a 1D detector system 200 × 15 mm active area which can accept a global count rate of 200 MHz. This paper describes the design and performance of the detector and readout system and presents some results illustrating the fast framing capability of the detector which demonstrate a single shot timing sequence with a resolution of 50 μs. Also presented are recent beamline results.

A gamma tracking detector for nuclear medicine

The concept of tracking the interactions of a gamma ray through a solid state detector is being pursued by several groups in the nuclear physics community. Here the ability to rid the detectors of the escape suppression shields and decrease the Doppler broadening of the spectroscopic lines will enable new physics measurements. Our group has embarked upon a project to apply this technology to medical imaging. We report on the progress in developing a planar germanium detector with gamma ray tracking ability, for positron emission tomography (PET). The prototype detector is a planar high purity germanium unit with X-Y readout electrodes. Using digital signal processing the timing and depth of individual interactions are recorded for each gamma ray. Compton scattering information for some of the events can be used to reconstruct the angle of the gamma ray to the first interaction point. In a similar technique to the Compton camera, this information can be used to reduce random coincidence rates in the PET system and provide the possibility for highly efficient use of events in the tomography reconstruction.