Pixelated Detector With Photon Address Event Driven Time Stamping and Correlation

Abstract

To allow measurement of the equilibrium density fluctuations on a nanometer-length scale using small angle X-ray scattering, an asynchronous event-driven-address time-stamped (EDATS) pixelated array detector, operational over correlation time spans ranging from less than 10-6 to greater than 104 seconds, was designed, fabricated, and tested. Its performance was validated for use in X-ray photon correlation spectroscopy (XPCS) experiments. The EDATS pixelated detector arrays each include a custom readout integrated circuit (ROIC), the Voxtel model VX-807, hybridized to a silicon photodiode array optimized for 500 eV to 2000 eV X-ray photons. The 492 × 492-element array is composed of each 40-micrometer × 40-micrometer pixel unit cell. The unit cell circuits of the VX-807 ROIC include a low-noise capacitive transimpedence amplifier (CTIA) followed by a C-RC shaping amplifier. These analog front-end (AFE) circuits are used to convert the absorbed X-ray photon signal to a voltage pulse, which may trigger a user-programmable threshold comparator. The detected X-ray photon events are broadcast to token-based address arbitration circuits, at the periphery of the X-ray sensitive pixel array, which recover the address of the photon event and communicate the address to a time-to-digital converter (TDC), where the address is time stamped, with a maximum temporal jitter of 85 nanoseconds. Separate digital arbitration blocks are used in each of the four quadrants of the ROIC, allowing an overall count rate of 100 MHz to be achieved. The VX-807 ROICs were hybridized to pin photodiode arrays fabricated on six-inch silicon-on-insulator (SOI) wafers, with a fully depleted 20-micrometer-thick X-ray-sensitive silicon layer. The hybridized EDATS sensor chip assemblies (SCAs) were then tested and their performance was verified.