CERN Accelerating science

Synchrotron Applications

Synchrotron Storage Rings and Free-Electron Lasers

X-rays

Immediately after the discovery of X-rays in 1895 by Wilhelm Röntgen, scientists throughout the world realized their enormous potential for imaging otherwise opaque materials and structures. In 1912 Max von Laue showed that X-rays interacted with crystals to generate very sharp interference patterns, and shortly after that, father and son Bragg formulated the laws which permitted to retrieve atomic structures from crystal diffraction patterns. This was the birth of X-ray crystallography, which, no doubt, has had an immense impact on our understanding of the universe.

Synchrotron Storage Ring sources

In 1947 scientists at General Electric build an electron storage ring, where bunches of electrons traveling in a circle were gradually accelerated while synchronously the magnetic field forcing the electrons in a circular path was ramped: the first dedicated synchrotron was born. Since then many new generations of storage rings with ever increasing energy and brilliance were constructed. In the 80’s of the last century a major breakthrough in brilliance was achieved with the invention of so-called insertion devices; periodic magnetic structures inserted between two bending magnets, which force the electrons to follow an undulating path. By careful tuning of the magnetic structure an interference is achieved, which amplifies the X-ray intensity at certain energies (wavelengths) in a strongly focused forward direction. These insertion devices are nowadays the most used X-ray source at synchrotron storage rings.

The evolution of the brilliance of X-ray sources over time is depicted in Figure 1.

Figure 1. Development of the average brilliance over the years

Free-Electron Lasers

In storage rings, the electrons inside a bunch are radiating incoherently, since the average distance between electrons is relatively large, which is required in order to reach acceptable storage life-times. In Free-Electron Lasers (FEL), the electron bunch is extremely compressed, with the result that electrons start radiating coherently when passing through an undulator. This results in an increase in peak brilliance by 9 to 10 orders of magnitude compared to even the most powerful X-ray storage ring sources. The extreme compression of the electron bunch can only be maintained for a short time, which is why FELs are single- pass linear machines.

The peak brilliance of FELs compared to Synchrotron Storage Rings is given in Figure 2.

Figure 2. Comparison of the peak brilliance for different synchrotron sources.

Science at Synchrotron Storage Rings and Free-Electron Lasers

With over 50 Synchrotron Storage Rings and hundreds of experimental stations throughout the world, it is clear that the breath of the science preformed at Storage Rings and FELS is too large to cover here. All fields which use X-rays as an analytical tool are represented, including, physics, biology, chemistry, materials science, cultural heritage and paleontology. A more complete listing can be found at www.lightsources.org, and [ref 1].

Detectors for Storage Rings and Free-Electron Lasers

Due the exponential increase over time of the source brilliance, combined with the breath of science at synchrotron sources, detectors developments have always been extremely challenging. Over the last 10 years photon-counting hybrid-pixel array detectors have become the workhorse at almost all synchrotrons, because of their low-noise and high frame-rates as compared to CCD-based systems. One of the remaining main challenges, especially for the upcoming fourth-generation storage rings, is to increase the maximum count-rate, or flux per pixel. This might be solved by the detection techniques developed for Free-Electron Lasers. Single pulse intensities at FELs are so high that complete images can be recorded in a single shot. This means that certain pixels can see up to 105 photons in less than 100 femto-seconds, while others see one or none. For this Integrating Adaptive Gain pixel detectors are developed, like the AGIPD system (for the European XFEL), depicted in figure 3. A more comprehensive overview of detectors developments for Storage Rings and Free-Electron Lasers can be found at [ref 2].

Figure 3. A 1-Mega pixel AGIPD detector for the European XFEL

[ref 1] “Synchrotron Light Sources and Free-Electron Lasers; Accelerator Physics, Instrumentation and Science Applications”; Editors: Jaeschke, E.J., Khan, S., Schneider, J.R., Hastings, J.B. ; ISBN 978-3-319-14393-4.

[ref 2] “X-ray imaging detectors for synchrotron and XFEL sources”; T. Hatsui & H. Graafsma; IUCrJ; 2015 May 1; 2(Pt 3): 371–383; doi:  10.1107/S205225251500010X.

Requirements for future X-ray detectors for Synchrotron applications

Due to the large diversity in photon sources, ranging from Storage Ring bending magnets to high energy Free-Electron-Lasers in addition to the large diversity of the science performed, it is impossible give a comprehensive list of requirements. However, particular areas of current research and developments include:

Efficient imagers for the hard X-rays  25 - 100 keV
Soft X-ray imagers, with single photon sensitivity as well as a large dynamic range  250 eV - 2 keV
Mega-pixel imagers with sub-micron spatial resolution for the soft to tender X-rays  250 eV - 3 keV
Mega-pixel imagers with sub-micron spatial resolution for the harder X-rays

 ≥ 20 keV

Large dynamic range single-shot imagers (this is particularly important for Free-Electron Lasers, but will also find wide applications at Storage Rings) Ranging from single photons to 10^5 photons per image
Very high frame-rate mega-pixel imagers (especially for future hard X-ray Free-Electron Lasers) Running at 10^5 frames-per-second