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A synchrotron radiation based x-ray microprobe analytical technique, x-ray beam induced current ͑XBIC͒, is suggested and demonstrated at the Advanced Light Source at the Lawrence Berkeley National Laboratory. The principle of XBIC is similar to that of electron/laser beam induced current with the difference that minority carriers are generated by a focused x-ray beam. XBIC can be combined with any other x-ray microprobe tool, such as the x-ray fluorescence microprobe ͑-XRF͒, to complement chemical information with data on the recombination activity of impurities and defects. Since the XBIC signal, which carries information about the recombination activity of defects in the sample, and the-XRF signal, which contains data on their chemical nature, can be collected simultaneously, this combination offers a unique analytical capability of in situ analysis of the recombination activity of defects and their chemical origin with a high sensitivity and a micron-scale spatial resolution. Examples of an application of this technique to multicrystalline silicon for solar cells are demonstrated.

The risks and costs of the present method of visualizing the coronary arteries have limited the use of coronary angiography in long-term serial studies needed to establish the natural history of coronary atherosclerosis and its response to interventions. A less invasive method, in which the contrast agent is administered intravenously, has been developed using synchrotron radiation as the illuminating source. The present report describes the initial results in human subjects. The findings indicate that transvenous coronary angiograms can be acquired in this manner. Further refmements in the x-ray imaging system are expected to result in increased x-ray fluence and improved image quality.Coronary artery disease is the leading cause of death in the United States (1). Although a number of factors that increase the risk of the development of the disorder are known, the natural history of the underlying atherosclerotic process and its response to preventive measures have been difficult to assess (2). Large-scale studies have been conducted on the effects of efforts to reduce the severity of predisposing factors (3, 4). The ambiguity of the results may be due in part to the use of such end points as definite coronary artery disease death or nonfatal myocardial infarction rather than the direct evaluation ofthe status of the coronary arteries (4). Serial studies of the coronary circulation, as determined by angiography, have been done; but the risks of the procedure limited the number of subjects and the number of examinations (5). A less-invasive imaging system, suitable for longterm sequential studies, is needed (6).The hazards of coronary angiography are principally the result of the arterial catheterization procedure and the injection of undiluted contrast agent directly into the orifice of the coronary arteries. This approach is necessary because of the low sensitivity of conventional x-ray imaging systems to iodine-containing contrast agents. This problem stems from the limitations of the x-ray tube, which emits radiation into a cone of large solid angle and over a broad energy spectrum. Intravenous injections, far safer than intraarterial injections, can be used by employing synchrotron radiation (emitted from an electron storage ring) as the x-ray source. The Lorentz transformation that determines the angular distribution of synchrotron radiation creates the opportunity (7): the extreme collimation of the radiation into a forward-directed cone of very small solid angle allows for monochromatization by Bragg diffraction and results in the exceedingly high brightness needed for brief, motion-freezing exposure times. METHODS AND PATIENTSThe transvenous angiographic method is based on the principle of iodine dichromography (8). Two monochromatic x-ray beams closely bracket the k-edge of iodine, 33,170 electron volts or 33.17 keV. The logarithmic subtraction of the images produced by these beams results in an image that enhances signals arising from attenuation by iodine and suppresses signals arising from a...

In this study, we have utilized characterization methods to identify the nature of metal impurity precipitates in low performance regions of multicrystalline silicon solar cells. Specifically, we have utilized synchrotron-based x-ray fluorescence and x-ray absorption spectromicroscopy to study the elemental and chemical nature of these impurity precipitates, respectively. We have detected nanometer-scale precipitates of Fe, Cr, Ni, Cu, and Au in multicrystalline silicon materials from a variety of solar cell manufacturers. Additionally, we have obtained a direct correlation between the impurity precipitates and regions of low light-induced current, providing direct proof that metal impurities play a significant role in the performance of multicrystalline silicon solar cells. Furthermore, we have identified the chemical state of iron precipitates in the low-performance regions. These results indicate that the iron precipitates are in the form of oxide or silicate compound. These compounds are highly stable and cannot be removed with standard silicon processing, indicating remediation efforts via impurity removal need to be improved. Future improvements to multicrystalline silicon solar cell performance can be best obtained by inhibiting oxygen and metal impurity introduction as well as modifying thermal treatments during crystal growth to avoid oxide or silicate formation