Galapad (GaAs LArge Pixel Array Detector)

Photon counting pixel detectors using heavier semiconductors, rather than silicon, could combine low noise and high speed with high detection efficiency with hard X-rays. The Galapad collaboration aims to develop gallium arsenide pixel detectors controlled by the LAMBDA readout system.

Image of an insect taken with a 256 by 256 pixel GaAs sensor (flat field corrected). Thanks to Simon Procz, Alex Fauler and Michael Fiederle (FMF / University of Freiburg)

The project Galapad is developed in the framework of a German-Russian partnership. Project Leaders are FMF Freiburg, KIT Karlsruhe and DESY Hamburg. The collaborating Russian Partners are JINR in Dubna and RID Ltd in Tomsk.

As part of this German-Russian partnership, DESY is working to develop gallium arsenide (GaAs) pixel sensors. Like other high-Z (high atomic number) semiconductors like germanium, GaAs can achieve an order of magnitude better quantum efficiency than silicon at high photon energies, making it a suitable sensor material for high-energy experiments. GaAs is an attractive material because large wafers (6”) are readily available. However, standard GaAs wafers are unsuitable for use in detectors because they contain high concentrations of defects. The aim of the project is to develop detector-grade GaAs, and integrate it into Medipix3 pixel detectors.

 The GaAs will be produced and pixelated by the Russian project partners, JINR (Dubna) and RID Ltd (Tomsk). RID’s approach is to start with n-doped material, where the defects will be compensated by the donor atoms, and then diffuse chromium into the material to overcompensate it and achieve a high resistivity. This approach has already shown promising results with small-area sensors. The sensors will then be bonded to Medipix readout chips by FMF (Freiburg), and readout electronics will be developed by DESY and KIT (Karlsruhe). The first prototype sensors show promising results. The ultimate goal of the project is to build these GaAs sensors into large-area Medipix3 modules for use on PETRA-III beamlines.

Project Leader: David Pennicard, Julian Becker