To leverage the unique qualities of the European XFEL, special detector systems are being developed that have to fulfil demanding requirements.
The basic idea behind a typical experiment at an X-ray free electron laser is easy: Illuminate a sample by intense X-ray flashes and count the photons, the ingredients of light, that are scattered off the sample in different directions.
From the resulting two-dimensional patterns of photons scientists can deduce the structure of the sample. For counting the photons they need detectors.
Each of the two-dimensional photon detectors that will be used at the European XFEL falls into one of three classes:
- Fast detectors for a broad energy range,
- Fast detectors for high photon energies,
- And slow detectors for low photon energies.
Fast detectors for a broad energy range
To leverage the unique qualities of the European XFEL, a detector programme was started in 2006. After the European XFEL project team had published a call for proposals, three consortia out of six were selected to develop pixel detector systems with the required properties.
The resulting projects have had to master the vast technological challenge of combining high sensitivity, angular resolution, radiation hardness, and storage capacity. Although each project follows a different approach to fulfil the requirements, all three designs are based on silicon as sensor material and come with readout electronics (preamplifiers and memory) in each cell to be extremely fast and sensitive. This makes them bigger than commercially available sensors for visible light.
In 2010 and 2011, all three projects passed a major review, and first performance measurements with prototypes were completed successfully.
The three projects are:
Adaptive Gain Integrating Pixel Detector
Consortium: DESY, Hamburg University, Bonn University and the Paul Scherrer Institute in Villigen, SLS
DEPFET (Depleted P-Channel Field Effect Transistor) Sensor with Signal Compression
Consortium: DESY, Max-Planck Halbleiterlabor München, Siegen University, Bergamo University, Heidelberg University, Politecnico di Milano
Large Pixel Detector
Consortium: UK group led by the Rutherford Appleton Laboratory/STFC with contributions from Glasgow University
Requirements for the 2D pixel detectors
A professional, digital, single-lens reflex (SLR) camera can take a few still images per second. In video mode, it takes up to 30. In dramatic contrast, the European XFEL will produce 27 000 flashes per second! What makes things even more complicated: The flashes are not distributed evenly, but come in 10 trains per second, each train comprised of 2 700 flashes at a distance of 220 billionths of a second only. The objective for the first detectors is to capture the 3000 to 6000 best shots out of a possible 27 000 shots. That is an extremely ambitious goal, something that has not been achieved before.
Dynamic range and sensitivity to single photons
Some areas of the detector will be bombarded by tens of thousands of photons, while others will be hit by only one. For data analysis, it will be crucial to know whether there was exactly one photon, or two, or maybe even none at all. To be good at counting very few and very many photons is called “having a high dynamic range”. Here, the detectors of the European XFEL will supersede commercially available detectors by many orders of magnitude. For instance, the detectors in your camera can probably distinguish 256 different shades of red, yellow, and blue. The detectors at the European XFEL, however, will be able to distinguish up to 10 000 different shades! This data has to be stored and processed somewhere in the detector before the information is transmitted to the data centre. This processing and storage takes space and requires big detectors.
X-ray photons have high energies. That’s what makes them penetrate matter easily. It’s the reason they are so useful for science. But if the photons deposit their energy in the electronic layer of a sensor, malfunctioning is just a matter of time. A special design has to address this problem.
Fast detectors for high photon energies
The program for these detectors will start in 2012.
Slow detectors for low photon energies
These detectors sport more conventional designs, but are still sophisticated devices of have to endure high radiation and be of high precision.