Optimization of curved x-ray multilayer mirrors for total reflection x-ray fluorescence spectrometry

Various x-ray optical configurations combining focusing and monochromatizing mirrors for application in total reflection x-ray fluorescence (TXRF) spectrometry were numerically modelled for the case of extended x-ray sources. The influence of the system parameters, such as mirror shape, the extension of the source, the angular divergence and the nature of the multilayer (uniform or graded ML), on the detection limits (DL) of TXRF were studied for two excitation energies of practical interest: 9.67 keV (W L beta) and 17.48 keV (Mo K alpha). No significant dependence of the DL on the mirror shape was observed. For smaller anode sizes the use of curved x-ray mirrors leads to a more significant improvement in the DL compared with a flat mirror. The use of graded ML mirrors does not bring significant advantages to TXRF because of the very short section of the ML used and the corresponding very low gradient in the d-spacing over this section. A combination of focusing x-ray mirrors with hat ML monochromators is of advantage only for anode-sample distances longer than 20 cm, Optimum DLs can be achieved with a circular Pt mirror and a flat double ML for 9.67 keV and a circular ML mirror for 17.48 keV. (C) 1998 John Wiley & Sons, Ltd.