Das thermische plasma als intensitäts normalstrahler im wellenlängenbereich von 1100 bis 3100 å

In this paper a standard intensity source is described consisting of a thermal argon plasma with small additives of hydrogen, nitrogen, and carbon dioxide. These gases are introduced in such a concentration to produce a series of optically thick spectral lines in the region 1100 Å to 3100 Å. The intensity in an optically thick line center is given by the Kirchhoff-Planck-function B (T) which is determined if the plasma temperature is known. The continuous character of the function allows for interpolation between the spectral lines.At longer wave lengths above 1800 Å only two carbon I-lines are found to be optically thick. Here the optically thin argon continuum is calibrated against the carbon I-lines and is used as an additional standard emission.By this procedure an absolute intensity scale is constructed at every wave length between 1100 Å and 3100 Å.The precision of the absolute intensities depends essentially on the precision of the temperature measurement for the plasma. Four different spectroscopic methods are used for this determination giving a temperature of 12540 K with an error of ±1 % to ±2 %. This means an error of ±10 % to ±20 % for the absolute intensity scale. The internal consistency of the measurements is better than ±2 %A plasma with these properties is produced in a wall-stabilized cascade arc burning through a central hole in a stack of refrigerated discs. By an adequate flow pattern of argon, hydrogen, nitrogen, and carbon dioxide an argon window is formed through which the radiation of the added gases leaves the arc axially without self-absorption.The arc is burning at a pressure of 1 atm, and a windowless light path to a vacuum spectrograph is formed by a 3 chamber differential pumping stage allowing continous observation.As a result the absolute intensities of the optically thick lines observed at high resolution are given. Moreover, the total spectral energy flux of the light source is presented for a lower resolution.In the range 2500 Å to 3100 Å the absolute scale overlaps the useful range of the carbon arc which is used as an intensity standard in the visible and near ultraviolet. A comparison of the two sources yields a compatibility within the experimental errors of a few percent.