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Research paper : A new density standard replaced from water (K. Fujii)−191 Synthesiology - English edition Vol.1 No.3 (2009) optical frequency measurement, so the frequency of laser source used for the interferometer must be calibrated using the method traceable to the definition of second (s). However, since it is difficult to make absolute measurement of optical frequency each time measurements are made, CIPM set recommended wavelengths of frequency-stabilized lasers where the frequency was absolutely measured beforehand by the method traceable to the definition of second. In the diameter measurement of silicon spheres, we maintain the traceability to the definition of meter by using a laser diode as the light source, whose frequency is calibrated using recommended wavelength of I2 stabilized He-Ne laser,.For accurate diameter measurement by optical interferometry, it is important to evaluate the thickness of oxide layers on the sphere surface and to evaluate the phase shift when the incident light beam is reflected on the sphere surface. Particularly, to redefine the SI basic unit, kilogram, by determining the Avogadro constant, the diameter of the silicon sphere must be measured with subnanometer accuracy. The surface of the silicon sphere is normally covered by oxide layers with a thickness of about 3 nm, so it is necessary to implement a surface analysis technology. Traditionally, ellipsometry has been used for silicon spheres, but recently, more accurate and reliable surface measurement techniques are employed using new surface analysis methods such as the x-ray reflectometry (XRR) and x-ray photoelectron spectroscopy (XPS). Since silicon single-crystals have certain coefficient of thermal expansion, it is necessary to measure the sphere temperature with an uncertainty of about 1 mK. Therefore, improvements are made by introducing active radiation shield to realize stable temperature condition in a vacuum chamber.Table 1 shows the elemental technology for the absolute measurement of the density of silicon single-crystals. To establish the solid density standard traceable to the definition of SI, several measurements standards, such as optical frequency, temperature, surface analysis, and mass are required. The new density standard was built by combining these standards.3.3 Development of measurement technology for density comparisonTo provide traceable hydrometer, density standard liquid, and vibrating tube densimeter to users, new measurement technologies are necessary to compare the densities. Therefore, AIST developed a new hydrostatic weighing apparatus, hydrometer calibration system, and magnetic suspension densimeter as shown below.(37)−Fig. 1 Laser interferometer for measuring the diameters of silicon spheres.Table 1. Elemental technology developed for absolute measurement of density of silicon single-crystals.Evaluation of adsorption coefficient at silicon sphere surfaceIntroduction of sinker system for precise air buoyancy correctionEstablishment of volume derivation method by geometric considerationDevelopment of automatic control mechanism of orientation of sphere in vacuum and its computer control (1994)Achieved temperature measurement with an uncertainty of 1 mK by introducing radiation shield and its active temperature control (2008)Temperature control of vacuum chamber by circulation of temperature-controlled water: achieved temperature measurement with an uncertainty of 5 mK by evaluating temperature distribution by thermocouples and temperature measurement based on ITS-90 (1994)Combination of XRR and XPS (2007)Spectroscopic ellipsometry of sphere surface (from 1996)Interference fringe measurement by the dark fringe method: improved performance to the quantum noise limited interferometry (development in progress)Complete automated measurement of diameters by the phase shifting method: improved diameter measurement precision to 1 nm (2007)Measurement and control of optical frequency at 20 GHz bandModulation and analysis of interference fringe by mechanical scanning of etalon: achieved diameter measurement precision of 3 nm (1994)Elemental technology developed to achieve goalMass evaluation of silicon sphere in vacuumMathematical determination of volume of imperfect sphereDiameter measurement from multiple orientation in vacuumPrecision measurement of temperature of silicon sphere in vacuumEvaluation of thickness of oxide layers on the surface of silicon spheresDiameter measurement at subnanometre precisionWideband control of optical frequency and complete automated measurement of diameters by introducing laser diodeNanometre measurement of diameters using gas lasers with fixed frequency (goal when it was impractical to control optical frequency in a wide range)Development goalMass measurementMathematical derivation of volumeOrientation control of sphereMeasurement and control of temperatureSurface analysisMeasurement and control of optical frequencyElemental of technology

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