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GALAPAD-2

Development of large area hybrid pixel detectors based on GaAs:Cr sensors and Medipix3RX readout chips for synchrotron radiation and neutron sources

Hybrid Pixel Array detectors (HPAD) are revolutionizing the field of photon detection at synchrotron storage ring and free-electron sources and currently represent the state-of-the-art of X-ray detectors. They offer high count rate capability (typ. > 106 cps/pixel), direct detection of X-rays, single-photon-counting, zero readout noise, zero dark current, and a narrow point-spread function compared for example to CCD cameras. Current examples are the Pilatus, the Medipix and the XPAD detectors developed at various places in Europe.

 

With the availability of high X-ray energy beamlines the demand for high efficiency sensor materials has rapidly increased. The most promising materials for this energy range of X-rays are GaAs and Cd(Zn)Te. The medical imaging industry has mainly invested in Cd(Zn)Te, since for their main applications (human imaging) high energies, above 100 keV, are needed, which requires very high-Z sensors. In X-ray photon science there is a large and growing emphasis on the energy range up to 80 keV, for which GaAs is excellently suited.

 

This German-Russian collaboration project has two clear goals: to continue the successful project GALAPAD (2011-2013) for the development of the technology to produce GaAs-based hybrid pixel detectors, and the construction of a large area pixel detector with 82x28 mm2 active area.

 

An important milestone will be the development and production of a “Double Hexa” (6x2 chip) detector module with 800.000 pixels to meet the following requirements:

firstly, many X-ray scattering experiments need to cover a large angular range with minimal gaps. While GaAs sensor technology has been demonstrated at synchrotrons, and shows much higher speed and greater sensitivity than existing systems, building large GaAs systems would allow increased throughput in such hard X-ray experiments, and make it possible to perform time-resolved measurements on shorter timescales.

secondly Free-Electron Laser sources are rapidly developing, and one extremely promising direction is the production of higher photon-energy (up to 36 keV) pulses by using the higher harmonics of SASE-undulators. This is crucially important for example in extreme conditions experiments, for example at ultra-high pressures and magnetic fields. The development of a megapixel GaAs-based adaptive gain integrating pixel detector would constitute a major breakthrough in the field.

 

“spectroscopic” CT of a cylindrical phantom, enabling the identification of contrast agents Gd (blue) and I (red/tellow), as used e.g. in medical examinations

Radiography of a computer hard disc recorded with a GaAs Medipix3 detector (8×9 tiles, false color scale)