Vol.1 No.3 2009

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Research paper : A new density standard replaced from water (K. Fujii)−189 Synthesiology - English edition Vol.1 No.3 (2009) Vibrating tube densimeter is a density measuring instrument with extremely high resolution, and the most stable instrument can measure the liquid density with a reproducibility of 10-6 to 10-7. Although vibrating tube densimeters had already been introduced experimentally in the brewing industry when the issue was first investigated, it was necessary to calibrate the relationship between the density and vibration frequency using density standard liquids. It was necessary to supply such density standard liquids with third-party certification at a relative standard uncertainty of about 0.001 %, but there was no system for providing traceable density standard liquids in Japan.2.2 Scientific needsSpeed of light in vacuum c, the Planck constant h, elementary charge e, and the Avogadro constant NA are fundamental physical constants that are used to described nature. If such fundamental physical constants are determined in conformity with the definition of SI, various fundamental physical constants can be derived from them. Because the values of fundamental physical constants are of primary importance for science and technology, the Task Group on Fundamental Constants that was established in the Committee on Data for Science and Technology (CODATA) of the International Council for Science (ICSU) summarized the recommended values of fundamental physical constants through a process of adjustment so that rigorous relationships among them are kept in a consistent way [20].The Avogadro constant is important not only for adjustment of the fundamental physical constants, but also to define the unit mole (mol) for the amount of substance. Moreover, if the Avogadro constant can be determined with a sufficiently small uncertainty, it will become possible to redefine the kilogram, the only SI basic unit still defined by material artifact, based on mass of an atom or fundamental physical constants [21][22]. Therefore, Conférence Générale des Poids et Mesures (CGPM) that was organized under Convention of Meter recommended experimental studies to redefine several SI basic units including kilogram using fundamental physical constants and to evaluate the mass stability of the International Prototype of the Kilogram (IPK) under cooperation of national metrology institutes throughout the world.Technology to measure the density of silicon single-crystals with a small uncertainty plays an important role in the determination of the Avogadro constant by the x-ray crystal density (XRCD) method. In the XRCD method, the Avogadro constant NA is derived by NA = 8M/(a3) from absolute measurements of the density , molar mass M, and lattice constant a of silicon single-crystals. In 2005, the NMIJ, the Physikalisch-Technische Bundesanstalt (PTB) of Germany, and the Institute of Reference Materials and Measurement (IRMM) of the Europe Joint Research Center collaborated to measure the Avogadro constant from silicon single-crystals with natural isotropic compositions, and achieved a relative standard uncertainty of 3×10-7, which is the highest accuracy achieved by the XRCD method[23]. In 2007, the International Avogadro Coordination (IAC) Committee organized by the CIPM, under cooperation of eight research institutes around the world including NMIJ, prepared a single-crystal from highly enriched silicon isotope 28Si for improving the uncertainty of the Avogadro constant to 2×10-8, and the research continues toward the redefinition of the kilogram. To achieve this goal, it is necessary to determine the density of silicon single-crystals with a relative standard uncertainty of 1×10-8.The Avogadro constant determined from silicon single-crystals is also used to verify theories in the AC Josephson and quantum Hall effects[20]. The electric potential difference generated by the AC Josephson effect is expressed as U = nf/KJ (n is integer, f is frequency of microwave irradiated onto the Josephson junction device, KJ = 2e/h is the Josephson constant), and the electrical resistance realized by the quantum Hall effect is expressed as R = RK/i (i is integer, RK = h/e2 is the von Klitzing constant), and these are important fundamental theories in establishing the electrical standards. However, since whether the electric potential difference and the electrical resistance are exactly quantized by 2e/h or h/e2cannot be proven by theories, work to verify the theory in the range of uncertainty of experiment is conducted by comparing and investigating with values such as h and e obtained from experiments that do not depend on the AC Josephson and quantum Hall effects. In the Task Group on Fundamental Constants of CODATA, the value for the Avogadro constant obtained by the XRCD method is used as an input quantity because the value for h is obtained from the Avogadro constant without relying on the AC Josephson or quantum Hall effects. From these investigations, it is now verified that the AC Josephson and quantum Hall effects are correct with an uncertainty of about 10-7 [20].2.3 Scenario for achieving the goalIt is necessary to clarify the R&D policy to achieve the goal by satisfying both the demands from society and science. The policies are summarized as follows:(1) To set the National Primary Standard (the highest order standard in Japan) for density that is traceable to the definition of the SI basic units.(2) The primary standard of density must have function that can meet the demands from society as well as future needs in science.(3) To be able to calibrate the users’ measuring instrument such as hydrometer, density standard liquid, and vibrating tube densimeter by unbroken chain linked to the primary standard of density.(4) To create a system where the density calibration service can be provided by registered calibration service (35)−

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