Paraffin Mirrors for Ultrasoft X-rays

Abstract

OUR search for a suitable dispersion technique to identify ultrasoft X-radiation (wavelength λ>20 Å) led us to consider the wavelength dependence of the critical angle of total reflexion. A critical angle of total reflexion exists only if the reflector is transparent to radiation. Thus, one reason for the unsatisfactory results reported by Franks and Braybrook¹ could be the high absorption of the ultra-soft X-rays in the beryllium reflector used by these authors. At first glance beryllium is the best choice for a low-absorption reflector, but commercial beryllium is contaminated by heavy metals. Even a small proportion of, say, iron or nickel, combined with the beryllium oxide that always forms on the surface where total reflexion occurs, can destroy the critical angle.