The gas density was measured by a small ionization gauge (5) (photo). The filament was made of thoriated iridium. It was calibrated against a micromembrane manometer (6).
For the purpose of measuring the gas density in the collision chamber our ionization gauge, the filament of which was made of thoriated iridium, is an appropriate tool, at a vapour pressure of 2 x 10-5.
The main error source is the calibration of the ionization manometer by the Atlas-micromembrane manometer. The accuracy of this calibration is certainly not better than 20% (specification of Atlas Mess- und Analysentechniek GMBH, Bremen).
We found that for the halogens the gauge loses its linearity above 2 x 10-5torr. This was also found for chlorine by Shaw .
The ionization gauge was calibrated at 5 x 10-4torr, but we do not know if the gauge is linear up to that pressure.
This gauge was calibrated at about 10-3torr by an Atlas-type micromembrane manometer, which in turn was calibrated against a McLeod manometer.
[concession: Although NO2 is also very aggressive] the gauge is linear for this gas at least up to 10-4torr.
Curron reports a linear behaviour of an ionization gauge up to 10-3torr. For the other gases the linearity was checked in our laboratory up to 10-3torr by total elastic cross-section measurements.