Determination of the real transmission of an x-ray lens for micro-focus XRF at the SEM by coupling measurement with calculation of scatter spectra

The accurate knowledge of the x-ray excitation spectrum is a prerequisite for the x-ray fluorescence analysis (XRFA), particularly when a standardless algorithm is applied. For micro-focus XRFA the excitation spectrum is the x-ray tube spectrum multiplied with the transmission of the x-ray lens. The tube spectrum is usually calculated by fundamental equations. The transmission function is conventionally determined by measuring two scatter spectra, with and without lens, and dividing them by each other. The measurement without lens is costly, requiring a calibrated collimator to be interchanged with the lens. The alternative to the measurement of the scatter spectrum without lens is its calculation. A rather simple scatter model can be applied for the calculation of the transmission when scatter spectra are calculated for two different tube voltages and measured with lens at the same voltages. The procedure was verified with an iMOXS x-ray source attached to the scanning electron microscope (SEM). The SEM was equipped with an efficiency calibrated energy dispersive x-ray (EDX) spectrometer to enable the comparison between measured and calculated spectra. The transmission obtained was in good agreement with that resulted from two measured scatter spectra. The procedure is especially recommended after an exchange of the x-ray tube. This will likely modify the alignment of the lens relative to the x-ray spot on the tube target. Because the transmission of the lens is quite sensitive to this alignment, the real transmission has to be calculated. Additionally, the procedure is sensitive to the effect of possible degradation of the tube target or window. Copyright (C) 2009 John Wiley & Sons, Ltd.