La/B 4 C multilayer mirrors for x-rays below 190 eV

We have fabricated La/B4C multilayer films by magnetron sputtering for the use as x-ray mirrors at energies below 190 eV, particularly for the detection of boron Ka x-rays at 183 eV, and compared them to Mo/B4C multilayers that are currently used in x-ray fluorescence spectrometers for this purpose. Transmission electron microscopy and synchrotron soft x-ray reflectometry at energies between 50 and 525 eV were used to study the structural quality and the x-ray optical performance of the multilayers. The results show a significant improvement of the reflectance at 183 eV with simultaneously improved suppression of other, undesired x-ray energies, indicating that La/B₄C has a high potential to replace Mo/B₄C in many x-ray optical applications below 190 eV. As an example, a comparison between La/B₄C and Mo/B₄C multilayers was performed by laboratory x-ray fluorescence measurements of the boron Ka emission using samples of B4C and borophosphosilicate glass. The improvements of the peak intensity and the lower limit of detection amounted to about 64% and 29%, respectively. The thermal stability of La/B₄C multilayers was also investigated.