[Paper Review] X-ray analog pixel array detector for single synchrotron bunch time-resolved imaging

Lucas J. Koerner, Sol M. Grüner|arXiv (Cornell University)|Jul 26, 2010

Advanced X-ray Imaging Techniques12 references4 citations

TL;DR

This paper presents an analog integrating pixel array detector with in-pixel storage and 150 ns temporal resolution, enabling single-bunch time-resolved x-ray imaging at synchrotrons. It achieves Poisson-limited accuracy at fluxes up to 3.7×10³ x-rays/pixel/train and resolves beam intensity and position fluctuations at 163 kHz, demonstrating suitability for ultrafast Bragg spot dynamics and beam diagnostics.

ABSTRACT

Dynamic x-ray studies may reach temporal resolutions limited by only the x-ray pulse duration if the detector is fast enough to segregate synchrotron pulses. An analog integrating pixel array detector with in-pixel storage and temporal resolution of around 150 ns, sufficient to isolate pulses, is presented. Analog integration minimizes count-rate limitations and in-pixel storage captures successive pulses. Fundamental tests of noise and linearity as well as high-speed laser measurements are shown. The detector resolved individual bunch trains at the Cornell High Energy Synchrotron Source (CHESS) at levels of up to 3.7x10^3 x-rays/pixel/train. When applied to turn-by-turn x-ray beam characterization single-shot intensity measurements were made with a repeatability of 0.4% and horizontal oscillations of the positron cloud were detected. This device is appropriate for time-resolved Bragg spot single crystal experiments.

Motivation & Objective

• Develop a fast, high-dynamic-range x-ray detector capable of resolving individual synchrotron bunches for time-resolved experiments.
• Overcome limitations of existing detectors—such as count-rate saturation in photon-counting PADs and slow readout in integrating detectors—by enabling in-pixel storage and analog integration.
• Enable single-shot, high-accuracy measurements of x-ray intensity and position fluctuations at sub-microsecond timescales for beam diagnostics and pump-probe studies.
• Demonstrate the detector’s capability for time-resolved Bragg spot imaging in single-crystal experiments with non-repetitive dynamics.
• Achieve Poisson-limited measurement accuracy at high flux levels and short exposure times using in-pixel frame storage and analog integration.

Proposed method

• Design and fabricate a 16×16 pixel CMOS ASIC with 150 µm pitch using a 0.25 µm process, incorporating radiation-hardened and differential amplifier architectures for stability under high flux.
• Hybridize the ASIC to high-resistivity silicon (500 µm, n-type) detector layers with gold pad metallization for efficient x-ray absorption and charge collection.
• Implement in-pixel storage using eight capacitive storage elements (C_S1–C_S8) and four front-end integration capacitors (C_F1–C_F4) to enable accumulation over multiple temporally separated exposure windows.
• Use FPGA-based control with 10 ns resolution and 30 ns minimum exposure time to gate x-ray signal acquisition and manage frame buffering up to 8,100 frames before data transfer.
• Apply analog integration with adjustable conversion gain (up to 6.5×) and flash-mode operation to capture multiple images per readout cycle without intermediate readout.
• Characterize detector performance using pulsed laser and x-ray synchrotron beam experiments, including noise, linearity, and FDEV (fractional Allan deviation) analysis to assess stability and statistical accuracy.

Experimental results

Research questions

• RQ1 Can an analog integrating pixel array detector achieve sufficient temporal resolution (≤150 ns) to isolate individual synchrotron bunches at 153–176 ns spacing?
• RQ2 To what extent does in-pixel storage enable high-dynamic-range, single-shot measurements of x-ray intensity with Poisson-limited accuracy at high fluxes?
• RQ3 Can the detector resolve fast beam intensity and position fluctuations, such as betatron oscillations, at timescales down to 2.56 µs?
• RQ4 How does the detector’s performance in terms of noise and linearity compare to theoretical limits, particularly Poisson statistics, under high-flux conditions?
• RQ5 Can the detector be used effectively for time-resolved Bragg spot imaging in single-crystal experiments involving non-repetitive sample dynamics?

Key findings

• The detector achieves a temporal resolution of 150 ns, sufficient to isolate synchrotron bunches with 153–176 ns spacing at APS and ESRF.
• The detector maintains Poisson-limited accuracy, with a fractional deviation bounded by Poisson statistics (P = 0.0038²), confirming that measurement uncertainty is dominated by photon statistics at high flux.
• The detector resolved individual bunch trains at flux levels up to 3.7×10³ x-rays/pixel/train with a single-shot intensity repeatability of 0.4%.
• Horizontal beam position fluctuations at 163 kHz were detected, consistent with betatron oscillations of the positron cloud, with the measured frequency matching the expected aliasing of 227.7 kHz at 390.1 kHz revolution frequency.
• The detector’s 600 µs readout time is faster than typical pump laser repetition rates, enabling efficient single-shot data acquisition for pump-probe experiments.
• The in-pixel storage capability allows capture of both laser-on and laser-off images in a single readout, reducing drift-related errors in time-resolved measurements.