Pietro Zambon

The recent interest in 100 keV cryo-electron microscopy has created a demand for specialized detectors that maximize information per unit dose while remaining cost-effective. We present a hybrid-pixel electron counting detector system tailored for cryo-electron microscopy applications using 100 keV electron energy. The demonstrator uses a 500 μm-thick, chromium-compensated gallium arsenide (high-Z) sensor with a 36 μm pixel pitch arranged in a seamless 1266 × 1057 matrix. Its low-noise front-end electronics achieve a threshold energy as low as 2.5 keV and include an in-pixel hit digitization mechanism. The matrix is read out at a speed of 7.2 kfps and has a counter depth of 1 bit, allowing for an incoming rate of 28 e/s/pix at 5% coincidence loss. The imaging performance is evaluated in standard counting and super-resolution acquisition modes. Thanks to a custom-developed super-resolution algorithm, the detective quantum efficiency at zero-frequency amounts to 0.96 and at the physical Nyquist frequency to 0.56, resulting in an effective pixel size of 27.5 μm. Experimental data are complemented and critically compared with Monte Carlo simulations and analytical models.

We devised a novel non-paralyzable counting strategy, an evolution of the instant retrigger technology, that has the potential to extend by several times the count rate capabilities of current X-ray and electron detectors. Based on the same working principle – counting the time-over-threshold of piled-up signals in multiples of a predefined and selectable retrigger time – the dynamic retrigger features a variable retrigger time which is adjusted dynamically upon consecutive re-evaluations of the piled-up signal, in an accelerating fashion. The rationale is to boost the recorded count rate in regimes of heavy signal pile-up, where even the standard retrigger suffers from count losses. In this work, we provide the statistical foundation for the dynamic retrigger concept and operation, based on the relationship between the rate of incoming events and the length of piled-up signals. Monte Carlo simulations are used to qualify the analytical model, to compute the resulting count rate behavior and to provide heuristic optimizations to the sequence of retrigger times obtained by theoretical means. We show that the dynamic retrigger strategy improves the count rate performance, with respect to the standard retrigger, by a factor 4.75 for ideal rectangular signals and by a factor 3.05 for a more realistic signal shape. Furthermore, the model is validated through a semi-empirical approach using experimental pulse parameters from a HAADF detector illuminated with 300 keV electrons. This hardware-grounded validation confirms a count rate enhancement of a factor 1.84, demonstrating the model’s robustness under real-world operating conditions.

In this article we present SELUN, a novel X-ray photon counting hybrid pixel detector developed at DECTRIS Ltd for coherent diffraction imaging techniques at synchrotron facilities. Its notable features are a pixel size of 100 µm × 100 µm arranged in a matrix of 192 × 192 elements, the possibility of use of both silicon and high- Z sensors to guarantee high quantum efficiency across a wide range of incoming X-ray energies, fast front-end electronics equipped with instant retrigger technology working in non-paralyzable counting mode, and high frame rates capability up to 120 kfps thanks to two on-chip data-compression mechanisms. Optimized towards speed to cope with the enhanced brilliance of fourth-generation synchrotron sources, it shows remarkable count rate saturation values ranging from about 30 to 60 Mcts s ⁻¹ pixel ⁻¹ , depending on the sensor material and value of the incoming X-ray energy, and energy-resolution figures of 664 eV r.m.s. for a silicon sensor and 1.22 keV r.m.s. for a cadmium zinc telluride (CZT) sensor.

We have designed, fabricated and tested a lens with a chromatic aberration coefficient (Cc) of 1.0 mm, a 4.0 mm pole-gap and 2.0 mm bore that is wide enough to accommodate an anti-contamination system and an objective aperture. This lens extends the temporal-coherence envelope of the electron microscope beyond 2 Å, using a low-cost Schottky FEG. We hope that this lens design can be used to improve all 100 keV electron microscopes designed for single-particle electron cryomicroscopy (cryoEM).

Coincidence loss can have detrimental effects on the image quality provided by pixelated counting detectors, especially in dose-sensitive applications like cryoEM where the information extracted from the recorded signal needs to be maximized. In this work, we investigate the impact of coincidence loss phenomena on the recorded statistics in counting detectors producing sparse binary images. First, we derive exact analytical expressions for the mean and the variance of the recorded counts as a function of the incoming event rate. Second, we address the problem of the mean and variance of the recorded events (i.e., pixel clusters identified as individual incoming events), which also acts as a function of the incoming event rate. In this frame, we review previous studies from different disciplines on approximated two-dimensional models, and we critically reinterpret them in our context and evaluate the suitability of their adoption in the present case. The knowledge of the first two momenta of the recorded statistics allows inferring about the signal-to-noise ratio (SNR) and the detective quantum efficiency at zero frequency (DQE₀). Analytical results are validated through comparison with numerical data obtained with a custom-made Monte Carlo code. We chose a realistic case study for cryoEM application consisting of a 25-µm-thick MAPS detector featuring a pixel size of 10 µm and illuminated with electrons of 300 keV energy over a wide range of incoming rate.

This study presents the performance assessment of a novel hybrid pixel detector. The electron counting application-specific integrated circuit (ASIC), named KITE, was optimized for 4D STEM applications and bump-bonded to a silicon sensor that is suitable for electron energies in the range 30–200 keV. The KITE ASIC was developed for high frame rates, currently up to 120 kfps, and high count rates, owing to fast signal shaping times and instant retrigger technology, which allows operation in a unique non-paralyzable counting mode. Investigated quantities include the spectral response, the identification of optimal threshold energies, an estimation of the threshold trimming accuracy, the analysis of the single-event multiplicity distributions, the count rate capabilities, and imaging performance metrics MTF and DQE. To highlight the capabilities of the detector, the best value of 10% of linearity loss was achieved at 84 Mcts/s/pix, the best value of count rate “cutoff” at nearly 150 Mcts/s/pix, and DQE (0) between 0.75 and 0.82 up to the electron energy of 160 keV.

We developed an analytical model to evaluate the effect of signal pileup on the recorded energy spectrum in X-ray photon-counting detectors affected by dead time and equipped with retrigger capability. The retrigger function allows the system to work in a specific non-paralyzable counting mode by counting the time-over-threshold of piled-up signals in multiples of a predefined and selectable retrigger time. The model, designed for rectangle-like-shaped signals, allows for arbitrary input energy spectra and can significantly help understand and optimize the behavior of counting detectors with spectral capabilities and retrigger mechanisms in applications involving polychromatic beams, e.g., spectral X-ray imaging and computed tomography (CT), in a time-efficient way. Dedicated numerical simulations were used to validate the model under several conditions of incoming flux and threshold energy, with excellent results.