Femtosecond Radiation Experiment Detector for X-Ray Free-Electron Laser (XFEL) Coherent X-Ray Imaging

Abstract

A pixel array detector (PAD) module has been 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 SLAC National Accelerator Laboratory. The detector is designed to collect X-rays scattered from monochromatic 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 two-level gain control that can be used to create an arbitrary 2-D, two-level gain pattern across the detector; massively parallel 14-bit in-pixel digitization; and frame rates in excess of 120 Hz. The first full-scale detector will be 1516 x 1516 pixels with a pixel size of 110 X 110 microns made by tiling CMOS ASICs (Application Specific Integrated Circuits) that are bump-bonded to high-resistivity silicon diodes. X-ray testing data of the first 185 X 194 pixel bump-bonded ASICs is presented. These are tiled to make the final detector. 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.