The measured normal-incidence reflectances of W/B4C multilayers mirrors are 2% in the 34–50-Å wavelength region and are consistent with interfacial roughness in the range of 3–4 Å.
J. F. Seely, G. Gutman, J. Wood, G. S. Herman, M. P. Kowalski, J. C. Rife, and W. R. Hunter, "Normal-incidence reflectance of W/B4 C multilayer mirrors in the 34–50-Å wavelength region," Appl. Opt. 32, 3541-3543 (1993)