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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 1 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 ~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.
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