The International Committee for Weights and Measures (CIPM) approved, in its Recommendation 1 of 2005, preparative steps towards new definitions of the kilogram, the ampere, the kelvin, and the mole in terms of fundamental constants. Within the Consultative Committee for Thermometry (CCT), a task group (TG-SI) H. Ugur is the President, Consultative Committee for Thermometry (CCT).
We determined accurate values of ratios among the average molar masses M Ar of 9 argon samples using two completely-independent techniques: (1) mass spectrometry and (2) measured ratios of acoustic resonance frequencies. The two techniques yielded mutually consistent ratios (RMS deviation of 0.16 × 10 −6 M Ar from the expected correlation) for the 9 samples of highly-purified, commercially-purchased argon with values of M Ar spanning a range of 2 × 10 −6 M Ar . Among the 9 argon samples, two were traceable to recent, accurate, argon-based measurements of the Boltzmann constant k B using primary acoustic gas thermometers (AGT). Additionally we determined our absolute values of M Ar traceable to two, completely-independent, isotopic-reference standards; one standard was prepared gravimetrically at KRISS in 2006; the other standard was isotopically-enriched 40 Ar that was used during NIST's 1988 measurement of k B and was sent to NIM for this research. The absolute values of M Ar determined using the KRISS standard have the relative standard uncertainty u r (M Ar ) = 0.70 × 10 −6 (Uncertainties here are one standard uncertainty.); they agree with values of M Ar determined at NIM using an AGT within the uncertainty of the comparison u r (M Ar ) = 0.93 × 10 −6 . If our measurements of M Ar are accepted, the difference between two, recent, argon-based, AGT measurements of k B decreases from (2.77 ± 1.43) × 10 −6 k B to (0.16 ± 1.28) × 10 −6 k B . This decrease enables the calculation of a meaningful, weighted average value of k B with a uncertainty u r (k B ) ≈ 0.6 × 10 −6 .
In 2013, a team from NPL, Cranfield University and SUERC published an estimate of the Boltzmann constant based on precision measurements of the speed of sound in argon. A key component of our results was an estimate of the molar mass of the argon gas used in our measurements. To achieve this we made precision comparison measurements of the isotope ratios found in our experimental argon against the ratios of argon isotopes found in atmospheric air. We then used a previous measurement of the atmospheric argon isotope ratios to calibrate the relative sensitivity of the mass spectrometer to different argon isotopes. The previous measurement of the atmospheric argon isotope ratios was carried out at KRISS using a mass spectrometer calibrated using argon samples of known isotopic composition, which had been prepared gravimetrically.We report here a new measurement made at KRISS in October 2014, which directly compared a sample of our experimental gas against the same gravimetrically-prepared argon samples. We consider that this direct comparison has to take precedence over our previous more indirect comparison. This measurement implies a molar mass which is 2.73(60) parts in 10 6 lighter than our 2013 estimate, a shift which is seven times our 2013 estimate of the uncertainty in the molar mass.In this paper we review the procedures used in our 2013 estimate of molar mass; describe the 2014 measurement; highlight some questions raised by the large change in our estimate of molar mass; and describe how we intend to address the inconsistencies between them. We also consider the effect of a new estimate of the low pressure thermal conductivity of argon at 273.16 K. Finally we report our new best estimate of the Boltzmann constant with revised uncertainty, taking account of the new estimates for the molar mass and the thermal conductivity of the argon.
Highly compact, filter-free multispectral photodetectors have important applications in biological imaging, face recognition, and remote sensing. In this work, we demonstrate room-temperature wavelength-selective multipixel photodetectors based on GaAs 0.94 Sb 0.06 nanowire arrays grown by metalorganic vapor phase epitaxy, providing more than 10 light detection channels covering both visible and near-infrared ranges without using any optical filters. The nanowire array geometry-related tunable spectral photoresponse has been demonstrated both theoretically and experimentally and shown to be originated from the strong and tunable resonance modes that are supported in the GaAsSb array nanowires. High responsivity and detectivity (up to 44.9 A/W and 1.2 × 10 12 cm √Hz/W at 1 V, respectively) were obtained from the array photodetectors, enabling highresolution RGB color imaging by applying such a nanowire array based single pixel imager. The results indicate that our filter-free wavelength-selective GaAsSb nanowire array photodetectors are promising candidates for the development of future high-quality multispectral imagers.
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