Innovation Award on Synchrotron Radiation 2019 for the development of XFEL detectors using the adaptive gain principle

The Innovation Award on Synchrotron Radiation 2019 was given to the researchers Prof. Heinz Graafsma from Desy and Dr. Aldo Mozzanica and Dr. Bernd Schmitt both from the Paul Scherrer Institute. The three physicists were honored for their contributions to the development of detectors for XFEL applications based on the dynamic gain switching principle enabling simultaneously single photon resolution and a large dynamic range. The laudation was held by Prof. Edgar Weckert from Desy. The Synchrotron Radiation Innovation Award is sponsored by SPECS GmbH and BESTEC GmbH.

One of the challenges at XFEL sources is the large dynamic range present in the images produced in a single pulse. An XFEL can deliver in a single pulse the same number of photons available in 1 second at most synchrotron source beamlines, the intensity in any given pixel of any given image can be anywhere between 0 and more than 10⁵ photons. This excludes any photon-counting scheme and required the development of large dynamic range integrating detectors capable of dealing with this enormous range of photon intensities.

As the name implies, an integrating detector accumulates the total charge generated in a pixel during the exposure time, which makes the detector able to cope with many photons arriving simultaneously. However, one of the drawbacks of integrating detectors has always been their inability to obtain at the same time single photon sensitivity and a large dynamic range. A reduced gain, increases the number of photons that can be detected but also increases the noise, excluding single photon sensitivity. If one prioritizes, on the other hand, to have single photon sensitivity, the amplifier in the front-end needs to be extremely sensitive, in other words needs to have a high gain, which in return means saturation already at low intensities, limiting the maximum number of photons that can be recorded. The major breakthrough came with the implementation of the adaptive-gain concept, in which every pixel starts in a high-gain setting, where it is sensitive to individual photons, and fully automatically decreases its gain when and if the signal increases.

The first implementation of this dynamic gain switching concept for photon science was done at PSI for the first Gotthard prototype in 2005 and later on in AGIPD, Gotthard I and Gotthard II and Jungfrau. The dynamic gain switching concept has proven to be very successful. It gives single photon sensitivity down to 2 keV (e.g. for Jungfrau) and also covers a large dynamic range up to 10⁴ 12 keV photons (or 6x10⁴ 2keV photons) with a noise which is over the entire dynamic range less than the Poisson statistical fluctuations. This ensures that the data quality is limited by statistics and not the electronic noise of the detector giving a data quality as good as the well-established single photon counting detectors.

In addition the charge integrating concept overcomes some of the limitations of single photon counting detectors in terms of count-rate, detection of low x-ray energies and pixel size. Therefore, there is currently also substantial interest in Jungfrau for applications at synchrotrons. Jungfrau has proven to give benefits over single photon counting detectors specifically in applications with a high photon rate like protein crystallography.