First results from the 128x128 pixel mixed-mode Si x-ray detector chip

Abstract

A Mixed-Mode Pixel Array Detector has been developed to measure protein crystallographic diffraction patterns. X-rays are stopped in a 500 &mu;m 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.