Free Electron Lasers (FELs) produce light and X-Rays of a similar type to synchrotrons – highly coherent and of a high brilliance. However, they operate in a manner which is more similar to a laser, passing electrons through an undulator to create incredibly focussed, bright pulses. This gives them several advantages. The short, bright pulses allow the imaging of samples that require a lot of illumination but are destroyed when imaged and the wavelength of the X-ray can be tuned by adjusting the magnetic field of the undulator. This makes the devices very versatile.
FEL nearing completion in Hamburg is called the European X-Ray Free Electron
Laser (XFEL). A characteristic of this machine is that the X-ray pulses arrived
in bunches, separated by gaps with no X-Rays. A bunch arrives every 100ms and within that
bunch, the pulses are separated by only 222ns.
This structure places huge demands on the detectors to be used with
XFEL. A picture must be captured for every pulse i.e. once every 222ns. This is
equivalent to a frame rate of 4.5 million frames per second. At this speed, there is no time to read out all the pixels before the next pulse arrives, so the data must be stored in the pixel array and read out to the outside world during the 100ms “pause" before the next bunch arrives. At the same time, the pixel must be able to cope with a hue range of illumination levels, as this is the kind of image that the experiments will produce.
Detector and Electronics Division at RAL has developed a detector to cope
with this extremely challenging application. Called the Large
Detector (LPD), its design has required the full complement of skills in the
division. Specialist detector materials are needed to cope with the energy of
the incoming X-rays and a new readout chip (ASIC) had to be developed to handle
the huge demands of the system. Detector division staff were also heavily
involved in the design and management of the supporting electronics for the
detector – everything from FPGA firmware and circuit board design to wafer
probing of ASICs.
LPD has been successfully deployed at XFEL and is now available for user experiments. More information can be found in this article from BBC News and this European XFEL Press Release.