There is no secondary standard of length established in the region of 10
-10m. In the field of X-ray spectroscopy, x-unit, introduced by Siegbahn, was used as a relative scale of length. The x-unit was determined by the grating constant of calcite crystal. Since variation was observed among the lattice spacings of calcite crystals, wavelengths of certain X-ray lines, mainly CuKα
1and MoKα
1, were employed as empirical working standard. Discrepancy was found in the commonly used wavelength values of CuKα
1 and MoKα
1. Bearden introduced a new secondary standard A based on the peak wavelength of WKα
1 line ( (WKα
1) =0. 209 010 0A) . The absolute value of A, however, has no greater accuracy than the order of ppm. With X-ray/optical interferometer, Deslattes and co-workers have measured the grating constant of silicon α
0in terms of optical wavelength (a
0 (25°C) =0.543106 61 nm (±0.25 ppm) ) . They measured the densities and isotopic abundances for highly perfect silicon crystals. Finally they have determined the Avogadro constant
NA=6. 022 0943×10
23mol
-1. This new
NAreduced the uncertainty of previous direct measurements to one-thirtieth.
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