In many experiments with lower-energy X-ray beams such as protein crystallography, hybrid pixel detectors with silicon sensors are now the best technology available. However, these silicon sensors are poor at absorbing higher-energy photons. By replacing these silicon sensors with semiconductors with higher atomic number (“high-Z”) such as germanium, it should be possible to increase their quantum efficiency by more than an order of magnitude at energies above 35 keV, as shown in the adjacent plot. These detectors will then combine high quantum efficiency with effectively noise-free photon counting, a high frame rate and good resolution.
DESY are collaborating with Canberra (Lingolsheim) and Fraunhofer IZM (Berlin) to develop finely-pixelated germanium sensors and bond them to Medipix3 readout chips. Compared to other heavy semiconductors, germanium has the advantage of extremely high crystal quality, and is available in reasonably large wafers. So, we hope to develop these detectors and make them available to PETRA-III beamlines relatively quickly. The main disadvantage of germanium is that it needs to be cooled during operation. However, these photon-counting pixel detectors are relatively tolerant of leakage current, so liquid nitrogen temperatures will not be necessary.
The pixelated sensors are being developed by Canberra. By modifying existing fabrication techniques used for fine-pitch strip detectors, they have produced a set of Medipix3-compatible sensors with 55μm pixels. Fraunhofer IZM bump bond these sensors using an indium bump process. The choice of indium ensures that the sensors are not damaged during bonding, and that the sensors can then be cooled during operation without damaging the bonds. DESY is organizing the project, and the LAMBDA readout system is designed to operate at a low temperature. The first prototype sensors are functional (albeit with insensitive areas on each chip), and a second batch of sensors are in production which should correct provide better image quality.
Project Leader: David Pennicard