Getting a picture of the nanoworld

Markus Kuster pushes photography to the extreme. In his private photo library, you can find stunning high-shutter-speed pictures of water drops, high-dynamic-range photographs showing breath-taking details of scenes in difficult light, and spectacular panoramic views.

Physicist Markus Kuster, coordinator of detector development at European XFEL

This enthusiasm for photography comes in quite handy for his job at European XFEL: As the leader of Work Package 75, Kuster coordinates the detector development for the new facility. Although the main development project includes different types of detectors, the largest subprojects develop sophisticated 2D cameras for X rays.

These detectors help to determine numbers of photons, the ingredients of light. This they have to do extremely well. (For the experts, they are integrating detectors.)

The photons will be scattered off samples, such as biomolecules, that are hit by the ultrabright X-ray flashes of the European XFEL. By monitoring detectors behind this point of interaction, scientists using the facility will try to get a better picture of the nanoworld.

You can find photon detectors everywhere. To see one, just go to your local electronic superstore. Any digital camera—be it a smart phone or a professional, digital, single-lens reflex (SLR) camera—incorporates such detectors, which get better every year.

So why not just buy them off the shelf?

“Because our detectors will have features you can’t buy at the moment,” Kuster replies. “They have to be extremely fast, cover an enormously broad dynamic range, be sensitive to single photons in the X-ray wavelength regime, and resist very intense X-ray radiation.”

Diffraction pattern of the European XFEL logo—In this picture, you see a simulated diffraction pattern of the European XFEL logo. If you want this as wallpaper for your computer desktop, you can download a copy from: http://www.xfel.eu/organization/corporate_design/wallpapers

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Take their swiftness. A typical SLR 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 3 000 to 6 000 best shots out of a possible 27 000 shots. That is an extremely ambitious goal, something that has not been achieved before.

Or take the 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.

And then there is radiation damage. 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.

“The requirements are demanding, but we are on the right track,” physicist Kuster says. His photo library shows that he has the right passion for the job. The high artistic value of his photos also demonstrates what every photography handbook tells you: the best technology alone does not result in good pictures; good pictures require a gifted and dedicated photographer.

At the European XFEL, the photographers will be the scientists. It is already clear from talks at meetings and conferences all over the world that these users will make extremely innovative use of the European XFEL and its detectors.

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:

• AGIPD
  Adaptive Gain Integrating Pixel Detector
  Consortium: DESY, Hamburg University, Bonn University and the Paul Scherrer Institute in Villigen, SLS
• DSSC
  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
• LPD
  Large Pixel Detector
  Consortium: UK group led by the Rutherford Appleton Laboratory/STFC with contributions from Glasgow University

Many scientific experiments at the European XFEL will utilize 2D pixel detectors. These detectors enable the counting of photons that are scattered off samples when they are illuminated by high-intensity X-ray flashes.

In contrast to digital SLR cameras—which make use of sophisticated systems of lenses to project images of an object, say a bridal pair, onto the detector—the cameras at the European XFEL will produce strange-looking pictures that physicists call “diffraction patterns”. In the case of a coherent light source like the European XFEL, you don’t need lenses and, therefore, are not limited by their resolution.

Diffraction is an effect caused by waves interfering with themselves when they hit an object. An example of diffraction is your ability to hear around corners—even when there is no direct link between you and the source.

If an X-ray flash hits, say, a biomolecule, photons of the flash can collide with electrons of the molecule. In this case, a new wave is created that spreads spherically around the point of collision. In the case of many electrons and many photons, you get an uncountable number of new waves that interfere with each other: some waves get cancelled out, others get enhanced. If you do the math of this interference, you can calculate the resulting diffraction patterns.

But scientists at the European XFEL will tend to calculate the other way round. They record the diffraction patterns and then use computer power to calculate the shape of the object—or, to be more precise, its electron-density, which gives very useful insight in the structure of the object.

As you can see, using an X-ray laser is less direct than using an SLR. That is the reason why scientists at the European XFEL will likely prefer to call their future work as “resolving structures” instead of just “taking photos.”

Author: Dirk Rathje