XSPRESS - A NEW-GENERATION OF DETECTOR SYSTEM FOR EXAFS STUDIES

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 v ery high detector counting rates with good resolution and this has res ulted in the development of high performance 13 element germanium dete ctor systems used presently on the SRS at Daresbury (Cramer et al., 19 88). However, these systems are still the limiting factor in such tech niques largely due to intrinsic rate limitations in the analogue shapi ng amplifiers used to process the signal from the detector. This limit ation is due to amplifier dead time which causes an effect known as pu lse pile-up. The length of this dead time is dictated by the shaping t ime 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 amplitud es and the edge height become compressed. The outcome of this is that QEXAFS is limited in speed and Ultra Dilute EXAFS is limited in concen tration. In order to overcome these limits a completely new detector s ystem has been developed for the new Wiggler II ultra dilute spectrosc opy station 16.5 at the SRS. This detector system combines state-of-th e-art germanium array detector technology with innovative digital sign al processing electronics. The detector has been developed through a c ollaborative agreement with EG&G ORTEC and packs 30 low-energy germani um crystals into an extremely small area yet still achieves energy res olution figures at least as good as smaller arrays. The small area all ows the detector to be placed extremely close to the sample thus maxim ising 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 givi ng unprecedented throughput linearity and rate/resolution performance. This combination of detector and electronics has allowed EXAFS data t o be successfully collected at count rates an order of magnitude per c hannel greater than existing systems whilst still retaining the resolu tion required for EXAFS. This will eventually yield a count rate impro vement of at least two orders of magnitude for the whole detector syst em.