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A Phillips ionization gage helium discharge has been used to provide a stable source of vacuum ultraviolet photons. Filtration of the ultraviolet beam with tandem tin films reduced the spectrum to substan'tially the 584-A line of helium. Absorption by target gases of the 584-A radiation was observed in a windowless cell with a sodium salicylate detector. Precise absorber gas pressure measurement was achieved with an MKS Baratron. Photoabsorption cross sections, which were determined in this experiment with an accuracy of about 2%, are in essential agreement with values recently reported by others.A continuously operated supersonic gas jet target has recently been developed for use in a hard vacuum by Brolley [1971]. This device is currently located at the Los Alamos meson physics facility, where it will serve as a target for beams of intermediate energy particles in nuclear physics bombardments. In the course of jet target experimentation, it was learned that the best method of measuring spatial density distributions of the target gas was to observe transmission of a narrow beam of vacuum ultraviolet photons. A typical gas manifests an absorption cross section of the order of 10 -•7 cm.. for photons whose energy appreciably exceeds its ionization potential. Moreover, regimes often can be found where this cross section does not vary rapidly with photon energy. From the Lambert-Beer law of exponential absorption, one may infer that a range of areal densities that can fruitfully be surveyed by this technique lies between 1016 and 1016 absorbers/cm". Procedures that may prove useful outside this

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