An active filter for inert gas lines in a hollow cathode light source

Abstract

The hollow cathode discharge tube has proved an excellent source for emission spectroscopy of metals, but in the red and near-infrared regions the limit of sensitivity for the detection of weak lines is set by scattered light or photon noise from the strong lines of the carrier gas, usually neon or argon. The paper describes an 'active filter' consisting of a long, low-current, positive column discharge in which an appreciable population of excited atoms is maintained in the lower levels of the lines in question, resulting in selective absorption of the hollow cathode radiation. Reduction factors of about 8 were achieved for lines starting from the two metastable levels. From subsidiary experiments on the reversal temperatures and equivalent widths of the filter discharge lines and on the widths and shifts of the hollow cathode lines, a simple radiative transfer model was constructed so that the effects of different discharge parameters on the reduction factors could be predicted. Of the various ways of increasing these, the most promising is to broaden the filter discharge lines by, for example, applying a magnetic field to the discharge.