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Performance Evaluation of the High Frame Rate Detector XSPA

Yasukazu Nakaye, Yasutaka Sakuma, Takuto Sakumura, Satoshi Mikusu and Kazuyuki Matsushita

Winter 2024 Volume 40, No. 1 , 01-07

In recent years, Hybrid Photon Counting (HPC) detectors have been widely used in the field of X-ray measurement. These detectors display no readout noise and provide a large dynamic range, high frame rate, small point spread function and no blurring. By combining these advantages of HPC detectors with high-speed data readout systems, it is possible to realize a high-performance X-ray detection system with single-photon detection and high detection efficiency. One of the most important elements of HPC detectors is their high-speed readout technology. With the latest HPC detector systems, frame rates of about 1000 fps can be easily realized, which is 1–2 orders of magnitude faster than the frame rate of CCDs, which have been mainstream until now. To meet these demands, the X-ray Seamless Pixel Array (XSPA) detector series was developed. XPSA technology is capable of continuous measurements at 56,000 fps and, with the XSPA Burst Mode, it is possible to achieve intermittent measurements at 970,000 fps. The shortest exposure time for a single frame is 48 nanoseconds. This is achieved by the high-performance UFXC32k chip jointly developed with AGH University and a data readout system that can handle high data rates in real time. In this report, we evaluated the basic performance of the XSPA detector.

Highlights

  • The XSPA hybrid photon counting detector achieves continuous acquisition at up to 56,000 frames per second and burst-mode acquisition at 970,000 frames per second, with a minimum exposure time of 48 nanoseconds.
  • A seamless pixel architecture eliminates oversized inter-chip pixels, providing uniform detector performance across the entire active area and improving statistical reliability for quantitative measurements.
  • Uniformity correction reduces pixel-to-pixel intensity variation from 1.54% to 0.37%, while maintaining an energy resolution of approximately 1.5 keV at 9.9 keV and count-rate linearity approaching 10 million counts per second per pixel.

Summary

Hybrid photon counting (HPC) detectors have become the preferred technology for many advanced X-ray techniques because they eliminate readout noise while offering high dynamic range, rapid acquisition, and excellent spatial resolution. The XSPA detector extends these capabilities by combining a high-speed readout ASIC with an optimized data acquisition architecture capable of continuous operation at 56,000 frames per second and burst-mode measurements approaching one million frames per second. These performance levels enable studies of extremely fast structural and dynamic processes that are difficult or impossible to capture with conventional CCD-based detectors.

A key design feature is the seamless pixel array architecture. Unlike many large-area HPC detectors that contain oversized inter-chip pixels with different counting characteristics, the XSPA uses uniformly sized pixels across the entire detector. This eliminates inconsistencies at chip boundaries, allowing every pixel to contribute equally to quantitative analysis without masking or correction of inter-chip regions. The detector can also accommodate multiple sensor materials, including silicon, gallium arsenide, cadmium telluride, and cadmium zinc telluride, providing flexibility across a wide range of X-ray energies.

Performance testing demonstrated excellent detector uniformity after flat-field correction, reducing pixel-to-pixel variation to approximately 0.37% and significantly improving the photon response non-uniformity. The detector exhibits an energy resolution of approximately 1.5 keV at 9.9 keV, comparable to other state-of-the-art photon-counting detectors. Count-rate measurements showed a detector time constant of 112 ns and linear performance up to roughly 9 × 10⁶ counts per second per pixel, making the system well suited for demanding synchrotron experiments such as X-ray photon correlation spectroscopy, pump-probe studies, and other time-resolved measurements requiring both high temporal resolution and quantitative accuracy.

Frequently asked questions

Conventional large-area hybrid photon counting detectors often contain oversized pixels at the boundaries between adjacent readout chips. These inter-chip pixels collect charge over a larger area, reducing positional accuracy and reaching saturation sooner than standard pixels. A seamless pixel array uses uniformly sized pixels across the entire detector, eliminating these inconsistencies so that every pixel provides equivalent spatial and counting performance without requiring masked regions.

Many modern synchrotron techniques investigate structural changes that occur on microsecond or nanosecond timescales. High frame rates allow detectors to capture rapid transient events, dynamic processes, and evolving microstructures without motion blur or missed intermediate states. This capability is particularly valuable for pump-probe experiments and X-ray photon correlation spectroscopy, where temporal resolution is as important as spatial resolution.

Individual pixels exhibit small variations in sensitivity due to manufacturing tolerances and threshold differences. Flat-field correction measures these variations using a uniform X-ray field and applies correction factors to each pixel. This significantly reduces fixed-pattern noise, improves pixel-to-pixel consistency, increases the signal-to-noise ratio, and allows detector performance to approach the theoretical Poisson noise limit over a much wider intensity range.

Energy resolution determines how precisely the detector can distinguish photons of different energies when setting counting thresholds. An energy resolution of about 1.5 keV at 9.9 keV enables accurate threshold selection for photon-counting measurements while remaining comparable to other leading hybrid photon counting detectors. This level of performance supports reliable discrimination of desired X-ray signals from background radiation.

In conventional readout schemes, counting pauses while data are transferred from the detector, creating brief gaps between frames. Zero dead time operation alternates between two counters within each pixel so that one counter continues recording photons while the other is being read out. This eliminates temporal gaps between successive images and enables continuous high-speed acquisition.

Count-rate linearity ensures that measured photon counts remain proportional to the actual X-ray intensity over a broad range of flux levels. Good linearity is essential for quantitative measurements because detector saturation or dead-time effects can otherwise distort measured intensities. A detector capable of maintaining linear response near 10 million counts per second per pixel can accurately measure intense synchrotron beams while preserving quantitative reliability.

Different semiconductor sensor materials provide optimal detection efficiency over different X-ray energy ranges. Silicon performs well for lower and moderate energies, while materials such as gallium arsenide, cadmium telluride, and cadmium zinc telluride offer higher absorption efficiency for harder X-rays. Supporting multiple sensor materials allows the same detector architecture to be optimized for a broad range of scientific and industrial X-ray experiments.

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