ITHACA, N.Y. -- A one-of-a-kind X-ray camera, capable of capturing a succession of microsecond images of events hidden to optical cameras, has been developed by researchers at Cornell University.
The first experiment using the novel camera has captured a moving image of shock waves from diesel fuel as it emerges at supersonic speeds from an automobile engine fuel injector. The X-ray imaging was able to penetrate the fog of aerosol droplets formed by the fuel as it cycles through the injector within a thousandth of a second. In a series of images, the camera depicted the shock wave created by the fuel, a phenomenon never before observed or measured, according to the camera's principal developer, Sol Gruner, professor of physics at Cornell.
Gruner and his collaborators, including Jin Wang of the Advanced Photon Source (APS) at Argonne National Laboratory, reported on the camera's revelations in a recent issue of the journal Science (Vol. 295, Feb. 15, 2002). Their article, "X-ray Imaging of Shock Waves Generated by High-Pressure Fuel Sprays," indicates that fuel injectors, now an automobile industry standard, could be made more efficient and less polluting than at present, according to Wang. "For example, just a 1 percent increase in efficiency and a few percent reduction in emissions would have an enormous economic impact," he says.
The development of the camera takes advantage of recent strides in semiconductor technology to build a bonded integrated circuit that incorporates a silicon layer, which converts X-rays into electrical signals, and a second layer of electronics. The chip used by the camera to capture the diesel fuel shock wave had a density of 9,200 pixels, or picture elements. The experiments were performed both at the Cornell High Energy Synchrotron Source (CHESS), which Gruner directs, and at APS, where Wang operates a beam line. Both of these sources produce high-intensity, high-energy X-ray beams for scientific research. The X-ray camera, the Cornell Pixel Array Detector (PAD), was funded by a $1.5 million Department of Energy (DOE) grant and in its final form will be used at Cornell and at the DOE's Argonne laboratory for a number of major experiments never before possible. "The applications we are targeting are unique," says Gruner. They include the X-ray imaging of fractures i! n materials at the instant they break. "This is a very important area of material science because it is very difficult to get information about what happens right at the point at which material breaks," says Gruner.
The fuel injector experiments reported in Science indicates the ability of the PAD to image phenomena that are transparent to X-rays but are hidden from optical cameras. As the fuel emerges through the fuel injector nozzle -- a mere 178 microns across -- at 345 meters a second, it forms a dense fog that causes light to scatter. Until now this has prevented researchers from knowing how finely dispersed the fuel is, how uniformly it is spread and when it starts to break up into an aerosol. Using an X-ray beam at CHESS, Wang, Gruner and collaborators were able for the first time to penetrate the droplets and calculate the manner in which the fuel shock wave affects distribution.
This calculation is particularly important for an automobile engine fuel injector, because uneven atomization of fuel can affect over all combustion efficiency and introduce unwanted particulate and other emissions, says Wang.
According to Gruner, "From a research perspective, the longer-range thinking is that we now have a mechanism for viewing phenomena that haven't been viewed in context before. There are, for example, many kinds of sprays used in industrial processing. Understanding the exact nature of these flows tells you a lot about how to optimize the system."
The complexity of the task facing the development of the PAD was finding a way to capture a sequence of X-ray images without any pause. The imaging devices widely used in research, silicon electronic light sensors called charge-coupled devices, or CCDs, capture images well enough, but each pixel first has to release its electronic signals before capturing another image, requiring a pause of about one-tenth of a second between exposures.
Instead, Gruner's group has built a detector on which the processing electronics, instead of being on the chip, are built into every pixel, even though each pixel is no more than 150 microns across. The X-ray signal enters the pixel, goes into an amplifier where it is turned into an electrical signal and is shuttled to a storage area. Thus each pixel is able to capture a "slice of time," store it and then capture the next "slice." In this way each pixel, operating in parallel, can capture eight images very rapidly before releasing its signals. For the shock wave experiment, eight X-ray images were taken, stored, downloaded and then eight more were captured. In this way a composite, moving image of the shock wave, a sum of many images, was able to capture the explosive release of the fuel.
The PAD chips require "solder bump-bonding" of two layers of silicon: the first, a specialized layer containing the X-ray conversion pixels; the second, the electronics processing layer. The bonding joins the two layers by laying down "solder bumps," each one joining a pixel to the processing layer. In the chip used for the shock wave experiment, that meant laying down 9,200 solder bumps. The chips are designed at Cornell and fabricated by commercial silicon foundries and companies specializing in state-of-the-art integrated circuit construction.
The latest chips designed by Gruner's group contain almost 45,000 pixels. Experiments are under way to bump-bond four chips together to build larger area detectors with as many as 726,000 pixels.
The PAD has obvious commercial possibilities, and Gruner and some of his collaborators have been working with a company that has a National Institutes of Health (NIH) grant to develop a related PAD for protein crystallography.
Gruner's co-authors on the Science paper who were involved in the development of the PAD include Mark Tate, Matthew Renzi and Alper Ercan of Cornell. Co-authors who participated in the fuel injector experiments include Ernest Fontes of Cornell; Jin Wang, Andrew MacPhee, Christopher Powell, Suresh Narayanan and Yong Yue of Argonne lab; and Jochen Walther and Johannes Schaller of Bosch of Germany. The work was supported by the DOE, the National Science Foundation and the NIH.
Related World Wide Web sites:
o Movie of fuel injection shock wave:
o Gruner Lab Home Page: http://bigbro.biophys.cornell.edu/
o Advanced Photon Source: http://www.aps.anl.gov/aps.php
Materials provided by Cornell University. Note: Content may be edited for style and length.
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