Femtosecond radiation experiment detector for X-ray free-electron laser (XFEL) coherent X-ray imaging

A pixel array detector (PAD) is being developed at Cornell University for the collection of diffuse diffraction data in anticipation of coherent x-ray imaging experiments that will be conducted at the Linac Coherent Light Source (LCLS) at the Stanford Linear Accelerator Center (SLAC). The detector is designed to collect x-rays scattered from femtosecond pulses produced by the LCLS x-ray laser at framing rates up to 120 Hz. Because x-rays will arrive on femtosecond time scales, the detector must be able to deal with instantaneous count-rates in excess of 10¹⁷ photons per second per pixel. A low-noise integrating front-end allows the detector to simultaneously distinguish single photon events in low-flux regions of the diffraction pattern while recording up to several thousand x-rays per pixel in more intense regions. The detector features a per-pixel programmable twolevel gain control that can be used to create an arbitrary 2-D, two-level gain pattern across the detector; massively parallel 14bit in-pixel digitization; and frame rates in excess of 120 Hz. The first full-scale detector will be 758 × 758 pixels with a pixel size of 110 × 110 microns made by tiling CMOS ASICs that are bumpbonded to high-resistivity silicon diodes. X-ray testing data of the first 185 × 194 pixel bump-bonded ASICs is presented. The measurements presented include confirmation of single photon sensitivity, pixel response profiles indicating a nearly single-pixel point spread function, radiation damage measurements and noise performance.