showed that when Medipix3 is operated in CSM mode, it generates a single count per detected event and consequently the charge sharing effect between adjacent pixels is eliminated. However in CSM mode, it was also observed that an incorrect allocation of X-rays counts in the pixels occurred due to an unexpectedly high pixel-to-pixel threshold variation. The present experiment helped to better understand the CSM operating mode and to redesign the Medipix3 to overcome this pixel-to-pixel mismatch.Index Terms-Medipix3, synchrotron radiation, X-ray detectors, X-ray imaging.

This contribution reports on the development of a new high rate readout system for the Medipix3 hybrid pixel ASIC developed by the Detector Group at Diamond Light Source. It details the current functionality of the system and initial results from tests on Diamond's B16 beamline.The Merlin system is based on a National Instruments PXI/FlexRIO system running a Xilinx Virtex5 FPGA. It is capable of recording Medipix3 256 by 256 by 12 bit data frames at over 1 kHz in bursts of 1200 frames and running at over 100 Hz continuously to disk or over a TCP/IP link. It is compatible with the standard Medipix3 single chipboards developed at CERN and is capable of driving them over cable lengths of up to 10 m depending on the data rate required. In addition to a standalone graphical interface, a system of remote TCP/IP control and data transfer has been developed to allow easy integration with third party control systems and scripting languages.Two Merlin systems are being deployed on the B16 and I16 beamlines at Diamond and the system has been integrated with the EPICS/GDA control systems used. Results from trigger synchronisation, fast burst and high rate tests made on B16 in March are reported and demonstrate an encouraging reliability and timing accuracy. In addition to normal high resolution imaging applications of Medipix3, the results indicate the system could profitably be used in 'pump and probe' style experiments, where a very accurate, high frame rate is especially beneficial. In addition to these two systems, Merlin is being used by the Detector Group to test the Excalibur 16 chip hybrid modules, and by the LHCb VELO Pixel Upgrade group in their forthcoming testbeams.Additionally the contribution looks forward to further developments and improvements in the system, including full rate quad chip readout capability, multi-FPGA support, long distance optical communication and further functionality enhancements built on the capabilities of the Medipix3 chips.

X-ray photon-counting detectors consisting of a silicon pixel array sensor bumpbonded to a CMOS electronic readout chip offer several advantages over traditional X-ray detection technologies used for synchrotron applications. They offer high frame rate, dynamic range, count rate capability and signal-to-noise ratio. A survey of the requirements for future synchrotron detectors carried out at the Diamond Light Source synchrotron highlighted the needs for detectors with a pixel size of the order of 50µm. Reducing the pixel size leads to an increase of charge-sharing events between adjacent pixels and, therefore, to a degradation of the energy resolution and image quality of the detector. This effect was observed with MEDIPIX2, a photon-counting readout chip with a pixel size of 55µm. The lastest generation of the MEDIPIX family, MEDIPIX3, is designed to overcome this charge-sharing effect in an implemented readout operating mode referred to as Charge Summing Mode. MEDIPIX3 has the same pixel size as MEDIPIX2, but it is implemented in an 8-metal 0.13µm CMOS technology which enables increased functionality per pixel. The present work focuses on the study of the charge-sharing effect when the MEDIPIX3 is operated in Charge Summing Mode compared to the conventional readout mode, referred to as Single Pixel Mode. Tests of a standard silicon photodiode array bump-bonded to MEDIPIX3 were performed in beamline B16 at the Diamond Light Source synchrotron. A monochromatic micro-focused beam of 2.9µm x 2.2µm size at 15keV was used to scan a cluster of nine pixels in order to study the charge collection and X-ray count allocation process for each readout mode, Single Pixel Mode and Charge Summing Mode. The study showed that charge-shared events were eliminated when Medipix3 was operated in Charge Summing Mode.

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