Time-Resolved 3D Imaging for Hard X-Ray Photon Detection Using Capacitively Coupled GaAs SAM-APD and Cross-Delay Lines

Time-resolved ultrafast phenomena with hard X-ray radiation are key research areas for applications like pump-and-probe spectroscopy. The demand for higher performance drives advancements in detector technologies and multichannel acquisition techniques. This motivates our proposal for an innovative fully digital 3D (x-y-time) imager for hard X-rays. Key challenges in detector design include improving time resolution, spatial resolution (limited by the multipixel approach), and quantum efficiency, which is low for silicon detectors in the hard X-ray range. We propose using a Separate Absorption and Multiplication Avalanche PhotoDiode (SAM-APD) based on III-V semiconductors. GaAs-based alloys, with higher atomic number and mobility, offer significantly better efficiency and speed for hard X-ray absorption compared to silicon. Regarding acquisition systems, the shift towards multichannel methods and the need to minimize power and area per channel has led to the transition from traditional pixelated voltage-mode electronics to time-based acquisition systems, where both spatial (x and y) and timing information are linked to the detection event's time. By coupling a large-area GaAs SAM-APD (several mm in diameter) to two Cross Delay Lines (CDLs) and a 4 -channel 15 -ps precision FPGA-based Time-to-Digital Converter (TDC), we aim to achieve temporal and spatial resolutions of tens of picoseconds and hundreds of micrometers. This approach offers a powerful alternative to pixelated systems, requiring neither aggressive lithography nor one readout channel per pixel, using only four channels.