The field of ultrafast x-ray science is flourishing, driven by emerging synchrotron sources (e.g., time-slice storage rings, energy recovery linacs, free electron lasers) capable of fine time resolution. New hybrid x-ray detectors are under development in order to exploit these new capabilities.
This paper describes the development of a 2160 x 2560 CMOS image sensor (CIS) system with a 6.5 µm pitch optimized for time-resolved x-ray scattering studies. The system is single photon quantum limited from 8 keV to 20 keV. It has a wide dynamic range and can operate at 100 Hz full-frame and at higher frequencies using a region-of-interest (ROI) readout. Fundamental metrics of linearity, dynamic range, spatial resolution, conversion gain, sensitivity and Detective Quantum Efficiency are estimated. Experimental time-resolved data are also presented.
A Mixed-Mode Pixel Array Detector has been developed to measure protein crystallographic diffraction patterns. X-rays
are stopped in a 500 μm thick layer of silicon diodes, and collected charge is processed by an attached ASIC. Goals
of the project are high flux (108 x-rays/s/pixel) capability and fast readout (< 0.5 ms dead time between frames). "Mixed-Mode" refers to a readout method whereby integrated signal accumulating in each pixel is compared against a threshold
value. When the threshold is reached, a digital count is added to an 18-bit in-pixel counter and a set quantity of charge is
removed from integrator. At the end of the x-ray exposure, analog signal left in the integrator is separately processed.
Thus, one obtains mixed digital and analog data where the counter bits are a high order word and the analog residual
provides higher precision. Typically, each count is equivalent to 100 10 keV x-rays, for a well-depth >107 10 keV x-rays/
pixel. The analog residual is digitized to 9-bit precision allowing measurement of the residual charge to better than
a quarter of the charge from single 10 keV x-rays. Measurements are presented on x-ray tests at the Cornell High Energy
Synchrotron Source (CHESS). Dynamic range, linearity, point-spread function and noise properties are shown. Status
will be is reported on five different approaches for ASIC-diode hybridization. Progress toward bonding of a 128 x 512
pixel device is also presented.
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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