Journal of research of the National Bureau of Standards (1977)最新文献

An absolute value has been obtained for the isotopic abundance ratio of a reference sample of gallium (Standard Reference Material 994), using thermal ionization mass spectrometry. Samples of known isotopic composition, prepared from nearly isotopically pure separated gallium isotopes, were used to calibrate the mass spectrometers. The resulting absolute ⁶⁹Ga/⁷¹Ga ratio is 1.50676±0.00039, which yields atom percents of ⁶⁹Ga=60.1079±0.0062 and ⁷¹Ga=39.8921±0.0062. The atomic weight calculated from this isotopic composition is 69.72307±0.00013. The indicated uncertainties are overall limits of error based on two standard deviations of the mean and allowances for the effects of known sources of possible systematic error.

This paper contains descriptions of the construction and use over the temperature range -27 °C to 570 °C of a Merritt-Saunders (optical interferometric) linear thermal expansion apparatus. Measurements of thermal expansion are reported for platinum and for two platinum-rhodium alloys (nominally 12 wt% Rh and 20 wt% Rh). Detailed analyses are given of the measurement uncertainties involved in the experiment and of the representation of the data by polynomials in the sample temperatures. The data show precision at the 1-ppm level and good agreement with results already published.

A new method has been developed for determining rapidly changing translational temperatures in a gas that has been heated by such transient phenomena as the passage of a shock wave or the absorption of sub-microsecond pulses of radiation from an infrared laser. The method depends upon the use of trace amounts of Hg vapor and its absorption of radiation in the neighborhood of the 253.7 nm isotopic and hyperfine multiplet. As the Hg atoms sense changes in the translational temperature of the host gas, the absorption of 253.7 nm radiation also changes by virtue of the Doppler and Lorentz broadening of the multiplet lines. Emission spectra of a Hg discharge light source in the neighborhood of 253.7 nm were shown to be readily simulated by a two zone computer model even at large optical densities. The same lamp parameters that were used in these calculations could also be used to simulate the experimental pressure and temperature dependence of the total integrated absorption. This provided a means for obtaining the temperature calibration curves needed to monitor the changing translational temperature of a gas undergoing rapid heating or cooling.

The triple points of oxygen samples sealed in miniature pressure cells were investigated by means of adiabatic calorimetry. The triple point of a 99.999 percent pure commercial oxygen sample was found to be 0.94₀ mK higher than that of an "ultra-pure" sample prepared by thermal decomposition of potassium permanganate (KMnO₄). The higher value is attributed principally to argon impurity in the commercial oxygen. The results of eight sets of observations using six thermometers, calibrated on the International Practical Temperature Scale of 1968 as maintained at the National Bureau of Standards, and two sealed cells of the ultra-pure oxygen show a range of 0.17₈ mK. The capsule-type platinum resistance thermometers that have been used are shown to have outstanding stability and the multiple calibrations made on them at the National Bureau of Standards extending over six years are shown to be consistent to within 0.15 mK at 54.361 K. The results of measurements on an internationally circulated sealed cell of commercial oxygen show its temperature to be 0.58₁ mK higher than those of the ultra-pure oxygen.

A large capacity calorimeter was designed and constructed in order to determine the enthalpies of combustion of kilogram-size samples of municipal solid waste (MSW) in flowing oxygen near atmospheric pressure. The combustion of the organic fraction of the samples was complete to greater than 99.9+%. The percent coefficient of variation (100 × standard deviation/average), % CV, of calibration measurements using microcrystalline cellulose was 0.2%. The % CV of the measurements of the enthalpy of combustion of a processed MSW sample was 0.4%. The combined systematic errors due to departure from usual design standards and conventional operating procedures is estimated to be less than 0.4% of the calorific value.

The Consultative Committee on Electricity of the International Committee on Weights and Measures is considering adopting sometime in the future 1) a new value for the Josephson frequency-voltage ratio 2e/h (e is the elementary charge and h is the Planck constant) and 2) a value for the quantized Hall resistance R _H≡h/e ². Both values are to be chosen as consistent with their International System of Units (SI) values as possible and would be used by every national standards laboratory which employs the Josephson and quantum Hall effects to define and maintain their national or legal units of voltage and resistance. Based on current knowledge, this would lead to an increase in the U.S. Legal Volt of about nine parts-per-million (ppm) and an increase in the U.S. Legal Ohm of about 1.5 ppm. Comparable changes would be required in the voltage and resistance units of most other national, governmental, and industrial standards laboratories throughout the world. Many high-precision instruments would also have to be readjusted to make them consistent with the new units. It is the purpose of this paper to review in some detail the basis for these proposed and potentially significant changes.

This work describes the techniques used to prepare and analyze a reflectance wavelength standard composed of three rare-earth oxides. A mixture of dysprosium oxide (Dy₂O₃), erbium oxide (Er₂O₃), and holmium oxide (Ho₂O₃) provides a pressed powder specimen exhibiting a near infrared reflectance spectrum characterized by many discrete absorption minima in the wavelength range 700 to 2000 nm. The object of this activity was to develop a wavelength standard for improving the accuracy of reflectance measurements in the near infrared. The reflectance minima of the rare-earth oxide mixture was analyzed for the effects of varying spectral resolution and temperature. The uncertainties associated with the various parameters affecting the measurements and the determination of the location of the reflectance minima have been analyzed. The overall uncertainty in the location of these reflectance minima is believed not to exceed ± 1 nm.

The method used at the National Bureau of Standards for the calibration of strontium-90 + yttrium-90 beta-particle ophthalmic applicators in terms of absorbed dose to water, is described. The method involves measurement of ionization density at the applicator surface with an extrapolation chamber, correction for the difference in backscatter between the collection electrode and water, and application of the Bragg-Gray equation. The calibration obtained is an average over the active surface of the applicator. The overall uncertainty of the surface calibration is about ±15 percent.

An experiment has been completed which demonstrated quantum mechanical tunneling of electrons between two gold electrodes separated in vacuum. The tunneling current between the gold electrodes has been measured, for fixed voltages of 0.1 and 0.01 volts, as the electrode spacing was varied from a distance of approximately 2.0 nm down to a point where the electrodes touched. Current changes of over five orders of magnitude were found for electrode spacing changes of approximately 1.2 nm. For the first time, these data enable one to deduce the work function of the electrodes in a tunneling experiment from experimental parameters independent of the tunneling device. Also obtained were current-voltage characteristics for fixed electrode spacings in the direct tunneling region where electrode spacings were less than 2.0 nm. An analysis is given which attempts to deduce an absolute electrode spacing and tunneling area from the nonlinear properties of the I-V data and the current versus spacing data. The analysis suggests that van der Waals and electrostatic forces play a major role in determining the I-V characteristics and that the tunneling area may be as small as 10^-16 m². Along with a review of the theory of work functions and quantum mechanical tunneling, numerical calculations of the tunneling current based on the free-electron model of the electrodes and the barrier, an image-potential reduced barrier, and a WKB approximation for the tunneling probability have been performed and compared with Simmons' theory and with the experimental results.