01.09.2016
Fig. 1: Instrumentation for combined X-ray fluorescence and photoelectron spectroscopy on surfaces of silicon spheres
In order to become independent of the embodied standard of the international prototype of the kilogram which is kept in Paris, the SI unit of the kilogram is to be redefined by the determination of a numerical value of Planck's constant. For this purpose, the number of silicon atoms in a monocrystalline silicon sphere (weight: one kilogram) is determined at PTB within the scope of the Avogadro project. This requires the determination of the molar mass, the density and the crystal lattice constant at the highest metrological level
In the case of a sphere, the ratio of surface to volume is smallest. Nevertheless, surface effects, such as forming oxide and contamination layers, have – also in this geometry – an influence on the mass and the volume and therewith on the density of the sphere. To characterize the surface with smallest uncertainties, PTB has established a combined X-ray fluorescence and photoelectron spectroscopy instrumentation. By combining the two complementary measurement procedures in one facility, both the chemical composition and the mass deposition at the surface of the sphere(s) are to be determined. For the design and the installation of the instrumentation, the knowhow which was already available in the field of radiometry and spectrometry with synchrotron radiation in Berlin-Adlershof could be used.
Meanwhile, the facility for surface characterization in Braunschweig in the vicinity of the mass comparator is used, where the short ways also allow an easier transport of the spheres under vacuum conditions.
Contact:
M. Kolbe, 7.13, e-mail: 
Michael.Kolbe(at)ptb.de