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Front-end Electronics for X-ray Imaging

Single photon counting (MPEC pixel chips)

The development of hybrid pixel detectors for tracking detectors which originated in the 1990's lead to a new class of X-ray detectors which originated as a spin-off from the tracking detectors. The first series of these photon counting detectors simply replaced the digital part of a pixel cell with a counter which was able to record the number of detected X-ray photons.

To acquire not only count rate but also some energy information two or more independent threshold comparators and counters have been introduced - in particular to enable new methods for contrast enhancement in medical X-ray images (MPEC). Also new materials apart from Silicon have been evaluated (GaAs, CdTe, CZT) to improve the absorption efficiency for higher photon energies (60keV and above) as used for medical imaging.

 

Counting and Integrating X-ray imager (CIX)

For count rates exceeding the ~1MHz counting detectors start to become inefficient due to signal pile-up. To overcome these limit a new detection concept has been development in collaboration with an industry partner (Philips Medical) which makes use of simultaneous counting and integrating (CIX). In addition to the dynamic range enhancement spectral information can be extracted from the simultaneous acquired count and integrated charge data.

 

High dynamic range integrating photon detector (AGIPD)

Another field of application for X-ray detectors are synchrotron beam lines where single photon counting detectors are the-state-of-the-art. New X-ray sources like free electron X-ray lasers (X-FELs) however produce pulsed photon fluencies which cannot be processed by counting anymore. Since the energy spectrum of the incident X-ray photons in monochromatic the integrated charge signal yields the number of detected photons. The AGIPD project implements such a detector to be used at the European XFEL at DESY. The challenge of this application is the high dynamic range of the incoming signal (> 104) and the repetition rate of 4.5 MHz.

 

Low energy X-ray imaging with fully Depleted Monolithic Active Pixels Sensors (DMAPS)

The newly investigated CMOS Active Pixel Detectors  may also be of interest for low energy X-ray imagers. In particular the concept of a 'smart' hybrid pixel detector which combines a fully depleted active pixel sensor with a digital read-out chip would allow the implementation of photon sensor with high integration density as required for small pixels and/or advanced in-pixel signal processing and storage.

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