Director Standard Reference Materials : Homogeneity Characterization of NBS Spectrometric Standards IV : Preparation and Microprobe Characterization of W-20 % Mo Alloy Fabricated by Powder Metallurgical Methods
A significant problem of the National Bureau of Standards Standard Reference Materials program is the provision of standards suitable in homogeneity for use with microanalytical techniques such as the spark source mass spectrograph and the electron probe microanalyzer. An interim approach to the problem has been the extended homogeneity characterization of selected existing standards. This paper describes the preparation and evaluation of the first NBS standard tested specifically from the beginning for application to electron probe microanalyzers. The standard designated SRM 480 is a tungsten-20 weight percent molybdenum alloy prepared by a powder metallurgy process. Based on the results of about 1500 determinations for both tungsten and molybdenum by electron probe microanalysis, the material was found to be of high homogeneity at about the micrometer level of spatial resolution. The coefficient of variation for molybdenum was 2.5% and that for tungsten 1.5%. Correction of relative intensity ratios to obtain concentrations is discussed in terms of input parameter uncertainties such as mass absorption coefficients, and electron backscatter factors. The result of studies for atomic number correction and effects on operating voltage on the microprobe absorption correction., will be given. It is concluded that SRM 480 should be a valuable addition to any microprobe laboratory doing quantitative analysts.
Correction for Non-Linearity of Proportional Counter Systems in Electron Probe X-Ray Microanalysis
While the theoretical basis for the correction of non-linearity of detector systems is well known, methods for the determination of dead-time effects must be adapted to electron probe microanalyzer systems. Two such methods, one employing both X-ray and current measurements and the other employing simultaneous X-ray measurements on two spectrometers, are described. The effect of pulse-height shrinkage at high counting rates on the linearity of the detector system is discussed. When the proposed corrections for the dead-time of X-ray detector systems employing proportional counters are applied to the X-ray intensity measurements obtained with the electron probe microanalyzer, count rates as high as 50,000 counts/sec can be used.
INVESTIGATION OF THE ENERGY DISTRIBUTION OF BACK-SCATTERED ELECTRONS IN THIN METAL LAYERS
In a continuation of an earlier investigation on the back-scattering of electrons ln thick layers, the energy distribution of back-scattercd electrons in thin gold, nickel, and aluminum layers (0.1 to 0.6 mg/cm/sup 2/) was determined at an initial electron energy of 25 kv. The electrons impinged perpendicularly on the scattering foil. The Lenard opposing field method was usced, which was however improved by superposing a small alternating current voltage with an opposing voltage and by amplifying the alternating current component of the initial current with a resonance amplifier. The curve of the energy dlstribution of the back-scattered electrons was obtained directly. The curve shows a continuous pattern which extended from the primary energy up to very small energies. With decreasing layer thickness the numbcr of electrons back-scattercd with the greater energy loss decreased, the number of the fastest did not change. The curve maximum was strongly marked at small layer thicknesses and was displaced in aluminum and nickel to higher energies. In gold its position remained independent of the layer thickness in the range investigated. (tr-auth)