Abstract

It can be more easily detected in quantitative fashion by observing the 511-k.e.v. annihilation quanta produced when the positron and a free electron mutually annihilate. To simplify the determination of the counting geometry, it is necessary to ensure that all positrons are stopped in the immediate vicinity of the source.Niteogen -14(d, «)Oxygen-15. The nuclide oxygen-15 has an inconveniently short life of 2.1 minutes. In order to get a reproducible counting rate of oxygen-15 from a given nitrogen sample, the deuteron beam must not vary appreciably in intensity. As this is sometimes difficult to achieve with a'Van de Graaff, another reliable means of obtaining reproducibility has been used. The charge on the capacitor in the beam current integrator is caused to decay at the same rate (through a leak resistor) as the nuclide under investigation. Thus a

Detailed statistical analyses have been made of results obtained from a series ofinterlaboratory measurements on the vapor pressures ofcadmium and silver. Standard Reference Materials 746 (cadmium) and 748 (silver) which were used for the measurements have been certified over the respective pressure ranges 10-11 -10-4 atm and 10-12 -10-3 atm. The temperature ranges corresponding to these pressures are 350-594 K for cadmium and 800-1600 K for silver. The heats of sublimation at 298 K and the associated two standard error Iimits for cadmium and silver are 26660 ± 150 caljmol and 68010 ± 300 caljmol, respectively. Estimates of uncertainty have been calculated for the certified temperature/pressure values as weil as for the uncertainties expected from a typical single laboratory's measurements. The statistical analysis has also been made for both the second and third law methods, and for the withinand between-laboratory components of error. The uncertainty Iimits are observed as functions of both the heat of Sublimation and the temperature.At the XXVIth Council Meeting ofiUPAC in Washington, DC, on 21 and 23 J uly 1971, the use of metallic cadmium and silver as standards for testing apparatus and procedures for vapor pressure measurement, proposed by the Commission on High Temperatures and Refractory Materials, was approved. of the Task Force on Vapor Pressure of the Commission on High Temperatures and Refractory Materials. The materials cadmium, silver, gold, platinum and tungsten are being certified by the US National Bureau of Standards for vapor pressures as a function of temperature. The certifications cover the 10~ 12 -10-3 atm range; for the complete series ofmaterials, the temperatures corresponding to these pressures will vary from 350 to 3 000 K. Gold has previously been certified over the temperature range 1300--2100 K 1 • This report describes the current certification of cadmium (350--594 K), and of silver (800--1600 K)*.These vapor pressure standard reference materials will allow workers in the field to detect systematic errors and to evaluate their results quantitatively. The materials should be most useful for checking low vapor pressure measurement methods, such as the Knudsen, torque Knudsen, Langmuir, and mass spectrometric methods.Experience in vapor pressure measurements, particularly at high temperatures, has shown that large systematic errors are common, even among experienced investigators. This report gives estimates of the uncertainties of the certified temperature/pressure values as weil as estimates of the uncertainties of a 'typicar single laboratory's measurements.The current certification of cadmium and silver, when taken in conjunction with the prior certification of gold, gives an indication of the temperature dependence of the uncertainties of vapor pressure measurements. These uncertainties are statistical in nature and reflect results for typical experienced investigators. The use of vapor pressure standard reference materials should aid in the detection and elimination of errors a...

No abstract

No abstract