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 during an exposure. A readout buffer accompanies this digital counter in each pixel. After acquisition of an exposure, the state of the counter is transferred to the readout buffer virtually instantly. A subsequent exposure can commence after a short dead time of only 3 microseconds, while the previous exposure is being read out from the buffer. The continuous readout of EIGER with counter and buffer in every pixel maximizes duty cycle while allowing for an electronic global shutter. This ensures that the data for all pixels of an image were acquired simultaneously, in contrast to a rolling shutter that leads to skewed images.
EIGER’s auto-summation mode is a further benefit of continuous readout with high duty cycle. While a single exposure 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 exposures in an image. At frame rates of several hundred Hertz and higher, all counts are captured in the digital counter of a pixel. If lower frame rates are requested, exposures are still acquired at high rates on the pixel level, effectively avoiding any overflows. Exposures are summed to images with the requested frame rate on the fly; extending the bit depth of the data by the number of summed exposures. 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 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. Figure 2 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.