Xspress4 is an active development project at Diamond Light Source tasked with delivering a state-of-the-art Digital Pulse Processor (DPP) for use on Spectroscopy beamlines that will provide better resolution and lower background than current systems.
Spectral plot of Molybdenum fluorescence obtained from 580kHz incident X-rays on one pixel of ...
Spectral plot of Molybdenum fluorescence obtained from 580kHz incident X-rays on one pixel of the Canberra 64 pixel HPGe detector. The black line is the response from the current Xspress2 DPP, while the red line is the response obtained using the exact same raw data but processed using off-line software that models the Xspress4 DPP algorithm.
Plot of FWHM resolution against output count rate for Molybdenum Kα fluorescence from ...
Plot of FWHM resolution against output count rate for Molybdenum Kα fluorescence from 32 of 64 pixels of the Canberra 64 pixel HPGe detector. The resolution achieved with the current Xspress2 DPP is represented by the black circles, while the red dots are the resolutions obtained using the exact same raw data but processed using off-line simulation software modelling the Xspress4 DPP algorithm. Note the improvement in resolution with increasing count rate of Xspress4 over Xspress2.
Real time Xspress4 DPP firmware is presently in development. The plot shows that passing ...
Real time Xspress4 DPP firmware is presently in development. The plot shows that passing real digitised data through test firmware results in the level of improvement previously seen in software simulation. The yellow trace was obtained for 600kHz Mo data processed by Xspress2; the green is with cross talk correction algorithms implemented, but note the high very low energy tail due to incomplete cancellation and consequent nuisance events; the red trace reduces the very low energy tail by rejecting small events that are time coincident with cross talk signals.
Diamond presently uses the Xspress2 DPP to process the signals from multi-element monolithic germanium detectors for X-ray spectroscopy. Xspress2 was developed by STFC for Diamond based on the original Xspress DPP previously developed for SRS, the UK’s first synchrotron facility in operation at Daresbury until 2008. Diamond and STFC licenced the Xspress algorithm to Quantum Detector who developed Xspress3, a DPP with an up-to-date hardware architecture. Xspress3 is also used at Diamond to process signals from single-channel and multi-channel silicon drift detectors for X-ray spectroscopy applications.
The use of the Xspress architecture enables Diamond to achieve very high counting rate per channel/pixel. However experience gained with multi-element monolithic germanium detectors showed that the limitation in the energy resolution at high counting rate was in large part determined by the cross talk between neighbouring channels. An algorithm to remove the cross talk was conceived [patent application 1408853.8 “Analysis of Signals from Pixelated Detectors of Ionizing Radiation”] and then tested using raw detector signal data processed by an off-line algorithmic software model [SRI 2015 Conference Proceedings – “Overview of Multi-element Monolithic Germanium Detectors for XAFS Experiments at Diamond Light Source”]. Diamond therefore initiated the Xspress4 project to develop the new DPP hardware and firmware architectures required to fully implement cross talk correction in real time.
The firmware development comprises the porting of the Xspress algorithms to the new FPGA. In addition the firmware development will have to include the implementation of the algorithms to correct the cross talk between many channels in real time. This will entail the development of the firmware blocks to properly manage the flow of data from channel to channel occurring between processing channels within the same FPGA and between channels in different FPGAs (i.e. on different cards), with timing accuracy to the order of 10ns. A flexible, accurate and error-free inter-board communication method will have to be developed for this purpose.
Diamond Light Source is the UK's national synchrotron science facility, located at the Harwell Science and Innovation Campus in Oxfordshire.
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