Pixel array detector for the capture of femtosecond duration x-ray images

An imaging Pixel Array Detector (PAD) is being developed to record x-ray scattering images from single particles at the SLAC Linac Coherent Light Source (LCLS) x-ray free electron laser. The LCLS will deliver x-ray pulses of 5 - 200 femtosecond duration 120 times per second. Proposed experiments require that the scatter from each pulse be independently recorded. This necessitates a detector with a charge integrating front-end because the high instantaneous arrival rate of photons (> 1000 photons per pixel in femtoseconds) exceeds the processing speed capabilities of digital Counting detectors. Other capabilities of the PAD are a frame rate >120 Hz, a full-well depth in excess of 2000 8-keV photons, a detective quantum efficiency near unity, and the ability to readily differentiate between 0 and I photons per pixel. The detector will be a 4x4 array of subunit tiles. Each tile consists of two silicon chips solder-bump bonded together. A pixelated 500 micron thick, fully depleted silicon chip converts x-ray energy into charge carriers. The charge created is conveyed by solder connecting bumps to a CMOS ASIC in which each pixel has its own signal processing electronics. Each tile has similar to 190 x 190 pixels, resulting in a detector of > 760 x 760 pixels. Tests of prototype 16x16 readout pixel arrays show a read noise equivalent to 0.14 8-keV photons. Features of the detector include an in-pixel parallel 14-bit digitization scheme, and the capability to be configured with an adaptable, 2-level, 2D gain profile. The development of the read-out electronics and the effects of tiling on dead area are also discussed.