Study of charge-sharing in MEDIPIX3 using a micro-focused synchrotron beam

Abstract

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.

Precision scans of the Pixel cell response of double sided 3D Pixel detectors to pion and X-ray beams

“…Currently the telescope resolution of 2.3µm gives a more exact position of the particle's point of incidence than the 4.5µm FWHM of the micro-focused beam achieved in the first study here. Since completion of this experiment it has been demonstrated that the beam size can be reduced to a FWHM of 2.2µm and the background present greatly reduced by detuning the monochromator in order to suppress higher harmonics present in the beam [20].…”

Measuring Identification and Quantification Errors in Spectral CT Material Decomposition

“…Energy ranges are user-defined, therefore offering flexibility and customization for a wide range of medical and industrial applications. To reduce the effect of charge sharing [42], the charge summing feature is included in the Medipix3RX ASIC which allows the total charge for a single interaction to be summed and allocated to the closest pixel [43]. To do this, each pixel communicates with its four neighbours to locate the pixel with the highest charge for a co-incident event, the total charge is then allocated to this pixel [42].…”

“…To reduce the effect of charge sharing [42], the charge summing feature is included in the Medipix3RX ASIC which allows the total charge for a single interaction to be summed and allocated to the closest pixel [43]. To do this, each pixel communicates with its four neighbours to locate the pixel with the highest charge for a co-incident event, the total charge is then allocated to this pixel [42]. The threshold settings for the selection of energy bins have a major impact on spectral image quality in terms of image contrast and noise level.…”

Medipix3RX: Characterizing the Medipix3 Redesign With Synchrotron Radiation

“…It is activated in order to eliminate the effect due to charge being shared between adjacent pixels as a consequence of charge diffusion in the sensor material [19], which is more significant the smaller the pixel pitch and the thicker the sensor. In CSM, a pixel communicates with its neighbours in order to (1) reconstruct the charge from a single photon interaction that is spread in a cluster of pixels and (2) assign the reconstructed charge to a single pixel (the pixel with the largest charge deposition).…”

“…This is defined as charge sharing effect and is stronger for sensors with small pixel area to sensor thickness ratio. The resulting charge sharing effect is discussed in [19], [27]. In order to reduce the count loss due to the charge sharing effect, the threshold discriminator is set at half of the impinging X-ray energy.…”