XSPRESS: A new generation of detector system for EXAFS studies

Abstract

Over the last few years improvements to synchrotron sources and X-ray optics have allowed sophisticated EXAFS techniques such as QEXAFS and Ultra Dilute EXAFS to be developed. Many of these techniques require very high detector counting rates with good resolution and this has resulted in the development of high performance 13 element germanium detector systems used presently on the SRS at Daresbury (Cramer et al., 1988). However, these systems are still the limiting factor in such techniques largely due to intrinsic rate limitations in the analogue shaping amplifiers used to process the signal from the detector. This limitation is due to amplifier dead time which causes an effect known as pulse pile-up. The length of this dead time is dictated by the shaping time which in turn is set by the energy resolution required to separate the fluorescence and scatter peaks. At present, this processing dead time limits us to around 30 KHz per channel with an energy resolution of around 250 eV at 5.9 KeV. Above this limit, both the EXAFS amplitudes and the edge height become compressed. The outcome of this is that QEXAFS is limited in speed and Ultra Dolute EXAFS is limited in concentration. In order to overcome these limits a completely new detector system has been developed for the new Wiggler II ultra dilute spectroscopy station 16.5 at the SRS. This detector system combines state-of-the-art germanium array detector technology with innovative digital signal processing electronics. The detector has been developed through a collaborative agreement with EG&G ORTEC and packs 30 low-energy germanium crystals into an extremely small area yet still achieves energy resolution figures at least as good as smaller arrays. The small area allows the detector to be placed extremely close to the sample thus maximising the fluorescence detected. The electronics has been developed at Daresbury and utilises advanced adaptive digital filtering techniques based in VME hardware to minimise the processing dead time, thus giving unprecedented throughput linearity and rate/resolution performance. This combination of detector and electronics has allowed EXAFS data to be successfully collected at count rates an order of magnitude per channel greater than existing systems whilst still retaining the resolution required for EXAFS. This will eventually yield a count rate improvement of at least two orders of magnitude for the whole detector system.