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

Time-resolved ultrafast phenomena with hard X-ray radiation are among the groundbreaking research fields under- lying scientific applications like pump-and-probe spectroscopy. Research on detectors and studies of new multichannel acquisi- tion techniques are continually driven by increasingly stringent requirements. This motivates our proposal of an innovative detector designed for a fully digital 3D (x-y-time) imager for hard X-rays. From the detector’s perspective, the aspects in need of improvement include time resolution, spatial resolution (limited by the multipixel approach in most of the solutions), and quantum efficiency, particularly poor for silicon-based detectors in the case of hard X-rays. In this context, a Separate Absorption and Multiplication Avalanche PhotoDiode (SAM-APD) based on III-V semiconductors will be employed. Specifically, GaAs-based alloys feature a higher atomic number and mobility, making them much more efficient and faster than silicon in absorbing hard X-rays. Considering acquisition systems, the multichannel approach adopted by modern applications and the reduction of power and area budgets dedicated to individual channels shift the read-out electronics from the classical pixelated voltage-mode approach, involving analog waveform acquisition and digital processing, to time-based acquisition where both spatial (i.e., x and y) and timing information are tied to the time of the detection event. Temporal and spatial resolutions less than tens of picoseconds and hundred micrometers will be achieved by capacitive coupling of a large area (a few mm in diameter) GaAs SAM-APD to two Cross Delay-Lines (CDLs) connected to a 4- channel 15-ps precision FPGA-based Time-to-Digital-Converter (TDC). This presents a powerful alternative to pixelation from both a technological and a processing standpoint; notably, it requires no aggressive lithography and only four channels, as opposed to one per pixel.