Ernie Fontes

Tests of a prototype pixel array detector for microsecond time-resolved X-ray diffraction

X-ray test results from a prototype 92 × 100 pixel array detector (PAD) for use in rapid time-resolved X-ray diffraction studies are described. This integrating detector is capable of taking up to eight full-frame images at microsecond frame times. It consists of a silicon layer, which absorbs the X-rays, bump-bonded to a layer of CMOS electronics in which each pixel has its own processing, storage and readout electronics. Tests indicate signal performance characteristics are comparable with phosphor-based CCD X-ray detectors, with greatly improved time resolution, comparable linearity and enhanced point spread. This prototype is a test module en route to a larger detector suitable for dedicated operation. Areas of needed improvement are discussed.

Pixel array detectors for time resolved radiography (invited)

Intense x-ray sources coupled with efficient, high-speed x-ray imagers are opening new possibilities of high-speed time resolved experiments. The silicon pixel array detector (PAD) is an extremely flexible technology which is currently being developed as a fast imager. We describe the architecture of the Cornell PAD, which is capable of operating with submicrosecond frame times. This 100×92 pixel prototype PAD consists of a pixelated silicon diode layer, for direct conversion of the x rays to charge carriers, and a corresponding pixellated complementary metal–oxide–semiconductor electronics layer, for processing and storage of the generated charge. Each pixel diode is solder bump bonded to its own pixel electronics consisting of a charge integration amplifier, an array of eight storage capacitors and an output amplifier. This architecture allows eight complete frames to be stored in rapid succession, with a minimum integration time of 150 ns per frame and an interframe deadtime of 600 ns. We describe the ...

Shock waves generated by high-pressure fuel sprays directly imaged by x-radiography.

Synchrotron x-radiography and a novel fast x-ray detector are used to visualize the detailed, time-resolved structure of the fluid jets generated by a high pressure diesel-fuel injection. An understanding of the structure of the high-pressure spray is important in optimizing the injection process to increase fuel efficiency and reduce pollutants. It is shown that x-radiography can provide a quantitative measure of the mass distribution of the fuel. Such analysis has been impossible with optical imaging due to the multiple-scattering of visible light by small atomized fuel droplets surrounding the jet. In addition, direct visualization of the jet-induced shock wave proves that the fuel jets become supersonic under appropriate injection conditions. The radiographic images also allow quantitative analysis of the thermodynamic properties of the shock wave.

Student Initiative Drives Education at CHESS

Excitement fills the air as a group of young children piece together components to a motorized three-wheel car as part of a summer program offered to area youth. Students, released from the structure and expectations of the classroom during vacation, are diligently attending to the task at hand, focused on creating a final product that resembles the presented prototype. Madison, a fifth grader wearing glasses with lenses so scratched it is hard to see through them, struggles to connect her battery to the short leads of exposed wires soldered onto a hobby motor (Figure 1). “Can you help me?” she asks at multiple times throughout the afternoon. Her frustration, building through the session, is palpable as she finally whimpers, “Its not right!” After some reassuring words and a little redirection, Madison places her car on the floor and watches with delight as it races across the room, stopping only when it runs directly into the wall. “It worked!” she shrieks and runs across the room to fetch the damaged vehicle. With a wheel hanging off and the leads once again disconnected, she exclaims, “Its okay; I can fix it,” and heads straight to her station to repair and improve her design.

Science at the Hard X-ray Diffraction Limit (XDL2011), Part 2

Diffraction limited, high-repetition rate, hard X-ray sources such as an Energy Recovery Linac (ERL) or an Ultimate Storage Ring (USR) have the potential to open new avenues of scientific inquiry and uncover discoveries not possible with existing facilities. These sources have properties never realized before: intense, highly-coherent, diffraction-limited X-ray beams pulsing at MHz to GHz repetition rates with pulse widths from 50 femtoseconds to tens of picoseconds. Hoping to understand new science horizons, CHESS, SSRL, DESY, and the Photon Factory at KEK joined forces to organize six topical workshops on “Science at the Hard X-ray Diffraction Limit” – hence the name XDL2011. With support from the U.S. National Science Foundation and U.S. Department of Energy, Cornell hosted this event aiming to maximize discussion of new ideas and coax invited speakers and participants to brainstorm new experiments that they would like to do, but are impractical with existing sources. To help encourage and educate future X-ray scientists and facility users, the NSF also provided funds to support travel costs for a diverse group of student and post-doctoral participants.

Development of a pixel array detector for time resolved x-ray imaging

We are developing an integrating Pixel Array Detector (PAD) for microsecond time-resolved x-ray diffraction. The current detector prototype has a format of 92×100 pixels, each 150μm square, and covers an active area of 15×13.8 mm2. The detector was tested at the CHESS D1 beamline and imaging capabilities within the microsecond time resolution regime were successfully demonstrated. This prototype is a test module en route to a larger detector of 1000×1000 pixels suitable for dedicated operation. Areas in need of improvement (radiation-hardness, large area coverage) will be discussed.