We report improvements to our previous (Zhang et al 2011 Int. J. Thermophys. 32 1297) determination of the Boltzmann constant k B using a single 80 mm long cylindrical cavity. In this work, the shape of the gas-filled resonant cavity is closer to that of a perfect cylinder and the thermometry has been improved. We used two different grades of argon, each with measured relative isotopic abundances, and we used two different methods of supporting the resonator. The measurements with each gas and with each configuration were repeated several times for a total of 14 runs. We improved the analysis of the acoustic data by accounting for certain second-order perturbations to the frequencies from the thermo-viscous boundary layer. The weighted average of the data yielded k B = 1.380 6476 × 10 −23 J K −1 with a relative standard uncertainty u r (k B) = 3.7 × 10 −6. This result differs, fractionally, by (−0.9 ± 3.7) × 10 −6 from the value recommended by CODATA in 2010. In this work, the largest component of the relative uncertainty resulted from inconsistent values of k B determined with the various acoustic modes; it is 2.9 × 10 −6. In our previous work, this component was 7.6 × 10 −6 .

A single, fixed-path-length cylindrical-cavity resonator was used to measure c 0 = (307.825 2 ± 0.001 2) m · s −1 , the zero-density limit of the speed of sound in pure argon at the temperature of the triple point of water. Three even and three odd longitudinal modes were used in this measurement. Based on the ratio M/γ 0 = (23.968 644 ± 0.000 033) g · mol −1 , determined from an impurity and isotopic analysis of the argon used in this measurement and the measured c 0 , the value k B = 1.380 650 6 × 10 −23 J · K −1 was obtained for the Boltzmann constant. This value of k B has a relative uncertainty u r (k B ) = 7.9 × 10 −6 and is fractionally, (0.12 ± 8.1) × 10 −6 larger than the value recommended by CODATA in 2006. (The uncertainty is one standard uncertainty.) Several, comparatively large imperfections of our prototype cavity affect the even longitudinal modes more than the odd modes. The models for these imperfections are approximate, but they suggest that an improved cavity will significantly reduce the uncertainty of c 0 .

We report the first observation of a sharp heat-capacity signature related to the superfluid transition in thin 4 He films adsorbed on porous Vycor and xerogel glasses. The transition temperatures of these films range from 0.14 to 1.1 K. This new feature is found in addition to the broad peak centered at a higher temperature reported in a number of earlier studies.PACS numbers: 67.40. Kh, 67.40.Hf, 67.40.Rp, The confinement of He in a multiply connected geometry such as porous glass has received renewed experimental 1,2 as well as theoretical interest. 3 " 6 Superfluid density (p s ) measurements 1,2 of 4 He films in porous Vycor glass (pore diameter -70 A) reveal a sharp transition at T c with an asymptotic power-law dependence similar to the bulk X transition, i.e.,This behavior was seen for films with T c > 70 mK up to full pores where T c = 1.955 K (Refs. 1 and 2). More recently, studies of 4 He in two other porous glasses have revealed different exponents: 0.89 for 10" 3

A quantitative understanding of the absorption and scattering properties of mixed soot and aerosol particles is necessary for evaluating the Earth's energy balance. Uncertainty in the net radiative forcing of atmospheric aerosols is relatively large and may be limited by oversimplified models that fail to predict these properties for bare and externally mixed soot particles. In this laboratory study of flame-generated soot, we combine photoacoustic spectroscopy, particle counting techniques, and differential mobility analysis to obtain high-precision measurements of the size-dependent absorption cross section of uncoated and coated soot particles. We investigate how the coating of soot by nonabsorbing films of dibutyl phthalate (chosen as a surrogate for sulfuric acid) affects the particles' morphology and optical properties. Absorption measurements were made with photoacoustic spectroscopy using a laser at λ = 405 nm. We report measurements and model calculations of the absolute cross section, mass absorption coefficient, and amplification of the absorption cross section. The results are interpreted and modeled in terms of a core-shell geometry and Lorenz-Mie theory of scattering and absorption. We discuss evidence of soot particle and collapse as a result of the coating process and we demonstrate the ability to resolve changes in the coating thickness as small as 2 nm.

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