Physical analysis of high intensity focusing x-ray optics: Doubly curved crystals vs. highly focusing polycapillaries

Choosing the best performing x-ray optic for a specific x-ray fluorescence application is tricky since x-ray fluorescence requirements can vary from extremely small x-ray beams for spatially resolved measurements to intense, highly monochromatic beams for locating trace amounts of impurities in samples. Additionally, due to the wide variety of commercially available x-ray optics to choose from, each providing different outputs, one has to decide which optic suits one's application requirements best. Two such optics, a doubly curved crystal (DCC) optic and a monolithic polycapillary focusing optic, were examined for use in micro-beam x-ray fluorescence and low level impurity detection. In these two measurements, intense output beams are needed. With the two optics examined, the average CuKalpha x-ray intensities were 6 x 10(7) photons/sec.mum(2) for the polycapillary optic and 8 x 105 photons/sec(.)mum(2) for the DCC optic from a 20W sealed-tube, 120 mum diameter Cu source. Thus, with an extremely low power source, the polycapillary output intensity was almost 100 times more than from the DCC optic. Because the spot sizes from the two optics were different, a better intensity comparison is insertion gain, which showed the polycapillary optic had an 8 times higher insertion gain than the DCC optic. In addition to intensity, lowering the minimum detectable limit (MDL) in x-ray fluorescence measurements requires highly monochromatic x-ray beams. Of the two optics examined, the DCC optic (with a 0.37 mm Ni filter) produced an x-ray beam that would detect about a 20% lower impurity concentration than the polycapillary optic (with a 0.44 mm Ni filter). In addition, the DCC optic's MDL can be improved, since this optic produces a highly monochromatic beam by diffraction, eliminating the need for a filter. On the other hand, the polycapillary optic transmits polychromatic x rays, requiring a filter to create a monochromatic beam from the polycapillary output, thereby reducing the characteristic line intensity. Thus we found the polycapillary optic would be the better optic choice for micro-beam x-ray fluorescence and the DCC would be better for low level impurity detection measurements.