![]() ![]() The electron impact source used was similar to the thermal emission source normally used in our laboratory except that the shield was replaced with an electron impact section consisting of a tungsten filament, impact chamber or “cage” and an electron trap. Source and collector slits were arranged so that complete resolution of each of the masses of interest was obtained. The magnetic field was controlled and changed with the use of a gaussmeter-controller. Mass measurements were made by magnetic field switching. The digital system and the range switching systems were under computer control. In general both systems were used redundantly. The output of the measuring system was fed into both an expanded scale recorder and a digital system consisting of a voltage to frequency converter and a precision scaler-timer. The measuring circuit consisted of two state-of-the-art vibrating reed electrometers with provisions for automatic range switching. The collector was a deep bucket Faraday Cage type equipped with a 50 percent transmission grid shadowing a series of electron suppression grids. The mass spectrometer was equipped with a “Z” focusing thin lens source (see below). Isotope ratio measurements were made on a 60° extended flight path 15 cm (6 inch) mass spectrometer. Based on the isotopic ratios measured by White and Cameron and others the value 28.086☐.001 was accepted but the error limits were expanded to ☐.003 based on the report of the variations of silicon isotopesby Allenby. measuring the ratios SiCl 4:4 Ag and SiBr 4:4 Ag. Prior to 1948 the accepted atomic weight of silicon was 28.06 based on the work of Baxter et al. The atomic weight can then be calculated from the absolute isotopic abundances and the atomic masses reported by Wapstra and Gove. These synthetic isotopic standards, prepared from chemically pure and very nearly isotopically pure separated isotope, provide a bias or fractionation correction (calculated isotope ratio-observed isotope ratio) which when applied to the observed isotope ratio of the reference sample being calibrated allow an absolute ratio to be calculated for this sample. To obtain absolute isotopic ratios from the observed or relative measurements made on a mass spectrometer it is necessary to calibrate the instrument using samples of accurately known isotopic ratios of the element under study. To achieve these objectives a project was begun to determine the absolute silicon isotope abundance ratios and, hence, the atomic weight of a reference sample of silicon, with an intermediate goal of ≤ 10 parts-per-million (ppm) uncertainty in the atomic weight. įinally, also unanswered was the question of whether the atomic weight of the silicon crystal had been distorted during the zone refining purification process. Since silicon in nature consists of a mixture of three stable and nonradioactive isotopes ( 28Si, 29Si, 30Si), the uncertainty of the relative proportions of these masses immediately became the limiting error in the precise characterization of the silicon crystal.Īs a secondary objective, the atomic weight of silicon could also be combined with the crystal parameter measurements by the IBS to permit a new and direct redetermination of Avogadro’s Constant. The meter and second have been redefined as multiples of measurable natural phenomena.Īs a milestone in achieving this goal, a high purity silicon crystal of high lattice perfection was selected as a candidate for the measurement of its unit cell dimensions, density and atomic weight, with desired measurement uncertainties for each of these at the parts-per-million level or less. Mass is the remaining triumvir of the m-k-s measurement system whose definition is expressed in terms of an artifact physical unit - the platinum-iridium l-kg mass that resides in Paris. Interest in the atomic weight of silicon was stimulated by the long-term project of the NBS Institute for Basic Standards to replace the kilogram as a standard of mass. ![]() This present work extends the study to silicon and demonstrates an expansion of the technique to include electron impact mass spectrometry. Previous elements studied include silver, 1 chlorine, copper, bromine, chromium, magnesium, lead, boron, rubidium, rhenium, and potassium. The Analytical Spectrometry Section of the National Bureau of Standards is conducting a long term program of absolute isotopic abundance ratios and atomic weight determinations using thermal ionization mass spectrometry. ![]()
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