B. R. Dobson

XSPRESS — X-ray signal processing electronics for solid state detectors

Abstract With recent improvements in synchrotron sources and X-ray optics great pressures have been placed on detector systems to produce higher count rates and better resolutions. Present high performance 13 element germanium detector systems can give reasonable count rates with good resolution (∼ 10 4 –10 5 Hz per channel and ∼ 250 eV FWHM @ 55 Fe with 0.5 μs shaping time). However, these systems are restricted by limitations in both the detector and in the analogue pulse processing after the detector. With respect to the detector, increasing the number of channels without degrading the energy resolution is a great challenge due to increased crosstalk and capacitance. The analogue pulse processing electronics are significantly limited by the dead time introduced by the shaping amplifier. This dead time causes pulse pile-up at higher rates which leads to non-linearity and poor resolution. This paper describes the XSPRESS system which has been developed at Daresbury Laboratory for the new Wiggler II beamline 16. This system overcomes previous limits in both signal processing and detector fabrication to give great improvements in system performance. The signal processing electronics departs from standard analogue processing techniques and employs sophisticated adaptive digital signal processing hardware to reduce the dead time associated with each event to a minimum. This VME based technology allows us to vastly increase the count rate for each channel yet still retain the ability to gain very good resolution. The detector has been developed through a collaborative agreement with EG & G Ortec and packs an unprecedented 30 germanium crystals into an extremely small area whilst still retaining the energy resolution of smaller arrays. This system has increased throughput rate by an order of magnitude per channel and when all channels are implemented, an increase of at least two orders of magnitude for the whole array should be seen. Data has been taken using this system on the SRS at Daresbury Laboratory and these results will be given along with a detailed explanation of the operation of this system.

XSTRIP: a silicon microstrip-based X-ray detector for ultra-fast X-ray spectroscopy studies

For a number of years, an exciting and important area of synchrotron radiation science has been X-ray absorption spectroscopy fine structure studies of dynamically changing samples on the sub-second time-scales. By utilizing this technique, precise measurement of detailed structural changes can be investigated during a chemical or phase change reaction without the need for repeated experiments or expensive stopped flow techniques. Until recently, instrumentation to facilitate these studies has been based on commercially available detectors developed predominantly for other applications. Whilst these systems have yielded quality science, they have been subject to a number of fundamental limitations, particularly their speed, linearity and dynamic range. We have developed a new detector, XSTRIP, to overcome some of these. This new instrument marries dedicated silicon microstrip technology with specialist low noise, custom developed, fast readout integrated circuits, to yield an instrument that will unlock whole new areas of science to researchers. This paper will discuss some of the drawbacks of historical systems, give details of the XSTRIP system and also present the operating parameters of the system. In addition, some of the initial scientific experimental results will also be presented.

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.

Integrated materials science facility on the SRS

The development of a new integrated materials science facility (station 9.3) on the wiggler line of the Daresbury Synchrotron Radiation Source is outlined. The facility combines instrumentation for data acquisition using both spectroscopic (XANES and EXAFS) and diffraction techniques and is optimized towards industrial applications such as catalysis and electrochemistry using in situ techniques.

Initial data from the 30-element ORTEC HPGe detector array and the XSPRESS pulse-processing electronics at the SRS, Daresbury Laboratory

Following the completion of the collaborative project between CLRC Daresbury Laboratory and EG&G ORTEC to develop the worlds first 30-element HPGe detector for fluorescence XAFS, it has now been tested and commissioned at the SRS. The system was commissioned with the XSPRESS digital pulse-processing electronics and this has demonstrated processed count rates in excess of 10 MHz. Initial data have been recorded and are presented.

Quick Fluorescence XAFS: A Technique for Recording Fluorescence X-ray Absorption Spectra during Chemical/Biochemical Reactions

The Quick EXAFS (QuEXAFS) technique (1,2) provides an alternative way of recording X-ray absorption fine structure (XAFS) data where scan time is minimised by continuous scanning of the monochromator. In contrast to the dispersive technique, QuEXAFS is capable of obtaining data in fluorescence mode as well as in transmission and is therefore suitable for dilute samples. The reduction in data collection time makes it feasible to follow some biochemical or chemical reactions at room temperature with the XAFS technique. We have recently commissioned a QuEXAFS experimental set up on station 9.3 on the 5T wiggler at Daresbury SRS. This station is equipped with a vertically focussing mirror, which in addition to providing extra flux also performs harmonic rejection due to the critical angle cutoff off the mirror reflectivity. The use of the mirror for some harmonic rejection is complemented by the use of a very stable two crystal water cooled monochromator. This provides additional harmonic rejection yet is stable enough to be used without a dynamic harmonic rejection servo, an important consideration for QuEXAFS scans. The monochromator Bragg angle is driven by a dc motor system encoded by a 0.1mdeg encoder. This system can be optimised for constant angular velocity scans and thus is well suited for QuEXAFS and avoids vibrations associated with stepper motors driven at speed. The high flux on the sample together with the use of a high count rate 13 element solid state fluorescence detector (3) have allowed us to obtain quality data on a 5mM solution in less than a minute. Studies have also been carried out on a dilute solution of a transferrin intermediate. We note that this is the first fluorescence QuEXAFS reported and is the lowest concentration for which a XAFS spectrum has been collected in less than a minute. Improvements iii the sample geometry and in the scanning protocol will allow faster data collection on more dilute systems.

A multielement solid‐state detector system for use on the SRS materials science Station 9.3

This article describes the design of a multielement solid‐state detector system for use on the SRS materials science Station 9.3. The detector system is discussed in detail and test data are presented. The current system consists of 13, 8‐mm diam, germanium diodes mounted in a single cryostat. The system operates with a 0.5‐μs shaping time and achieves better than 200‐eV resolution at 5.9 keV. Rate and resolution characteristics of the system are discussed with a view to future improvements in the system.

Dopant‐defect interactions in hydrogen‐free amorphous silicon

Abstract Substitutional (B, P, As and Ga) and interstitial (K) dopants have been incorporated into H‐free amorphous Si (a‐Si) films produced by ion beam amorphization of crystalline silicon material. X‐ray absorption fine‐structure, photothermal deflection spectroscopy and electronic transport measurements have been performed on these films to monitor the annealing‐induced ordering phenomena around the implanted dopant impurity sites. We find that, in thermally relaxed a‐Si, substitutional dopant impurities have a strong tendency to enter the Si random network in the form of threefold‐coordinated, electrically inactive, alloying sites. It is shown that the bonding constraints associated with these sites retard the structural relaxation process of the a‐Si films and the crystallization of the a‐Si network in the immediate neighbourhood of these sites. In agreement with previous work, we find that high‐defect‐density a‐Si films can be electrically doped with interstitial K impurities. In such interstitially...

FIVE NEW EXPERIMENTAL STATIONS AT THE SRS DARESBURY FROM A 6 T SUPERCONDUCTING WIGGLER MAGNET

Daresbury Laboratory is currently completing the construction and commissioning of five new experimental research stations utilizing high flux hard x‐ray radiation from a 6 T superconducting wiggler magnet. The broad areas of science covered by the new stations and the novel features are presented.

PROBING ACTIVE SITES IN SOLID CATALYSTS FOR THE LIQUID-PHASE EPOXIDATION OF ALKENES

Using X-ray absorption fine structure measurements with a synchrotron source, the local environment of the titanium-centred active site of a siliceous mesoporous catalyst for the epoxidation of cyclohexane by H2O2 has been determined prior to the onset and during the course of catalysis.