Continuous readout and auto-summation
One of the hallmark features of EIGER is its continuous readout that enables kilohertz frame rates with duty cycles greater than 99%. Every pixel of an EIGER ASIC features a digital counter for noise-free counting of the observed photons. A readout buffer accompanies this digital counter in each pixel. After acquisition of a frame, the state of the counter is transferred to the readout buffer virtually instantly. A subsequent frame can start after only 3 microseconds, while the previous frame is being read out from the readout buffer. The global, continuous readout of EIGER with counter and buffer in every pixel maximizes duty cycle and data collection efficiency without requiring a rolling shutter.
EIGER’s auto-summation mode is a further benefit of continuous readout with high duty cycle. While a single frame is limited to the 12 bits of the digital counter, auto-summation extends the data depth up to 32 bits, or more than 4.2 billion counts per pixel, depending on the number of summed frames in an image. At short exposure times and frame rates in the kilohertz range, all counts are captured in the digital counter of a pixel and directly read out as an image. If long exposure times are requested, frames are still acquired at high rates on the pixel level; effectively avoiding any overflows. The detector system sums the frames to images on the fly; extending the bit depth of the data by the number of summed frames. EIGER’s auto-summation mode maintains a duty cycle greater than 99.7% at all times due to continuous readout with 3 µs dead time.
No readout noise or dark current
EIGER Hybrid Photon Counting detectors are completely free of readout noise and dark current. The figure shows a one hour dark image of a single EIGER module. The vast majority of the pixels has zero counts because EIGER detectors do not exhibit any dark current and, in addition to this, the readout does not add any noise to the data. General background radiation only gives rise to a single count per event; keeping noise from background radiation to a minimum. Average background on an EIGER module is only 0.07 counts per hour and pixel.
Outstanding count-rate performance
EIGER combines a smart design for fast single-photon counting with small pixel size. This achieves exceptional count-rate performance per area. Count rates of more than 500 million counts per second and square millimeter or 3 million counts per second and pixel can be accurately measured with EIGER. The left hand plot shows observed count rate as a function of incoming rate for different X-ray energies, measured with a threshold of half the X-ray energy.
While EIGER benefits from its small pixel size, PILATUS3 takes advantage of DECTRIS instant retrigger and the properties of a non-paralyzable counter. This way, both detector families achieve similar, excellent count rate performance per area. The right hand plot shows count rate performance of EIGER and PILATUS3 measured at an X-ray energy of 10 keV with a threshold of half the X-ray energy.
HDF5 and NeXus
EIGER data is stored in HDF5 (Hierarchical Data Format) files. HDF is designed to store and organize large amounts of numerical data. It is ideally suited for the demands of high performance detectors, large research facilities and users who need to handle vast amounts of data.
EIGER detectors acquire up to 180,000 frames and about 360 gigabyte of raw data per minute. Any file format or strategy that aims at storing only one image per file is therefore not suitable for EIGER. One of the main benefits of HDF5 is providing a transparent and efficient way of storing and handling a large number of images in a small number of files. Furthermore, HDF5 offers versatile means of storing experiment metadata in the same file structure as the detector data. Many analytical packages such as MATLAB, IDL, and R support HDF5. These and numerous other advantages make HDF5 the preferred file format for many synchrotrons and large research facilities.
The NeXus data format uses HDF5 as a container and provides design principles, definitions, and data structures for storing of scientific data. All EIGER detector data and metadata is stored in NeXus classes in the HDF5 files written by the detector software. Beamline scientists and synchrotron users benefit from an expandable set of NeXus application definitions. Using NeXus definitions such as NXmx or NXtomo, all metadata describing a macromolecular crystallography or tomography experiment can be stored in the same files structure as the detector data.
Major crystallographic integration software packages are planning to support EIGER HDF5 data. The XDS package uses H5ToXds for processing of EIGER HDF5 data. H5ToXds is available for download below.