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.
Before entering the collision chamber the beam intensity is measured by a surface-ionization detector (4), (photo). If desirable the beam shape can be measured after collision by a movable surface ionization detector (9).
The surface ionization detector consists of a 0.1mm hot iridium wire.
The stability of the wire The temperature of the wire was measured by a pyrometer and corrected for the emissivity Husmann. Although Ir has a high work function, about 5.4eV, Lacmann and Herschbach we sometimes had problems detecting Na and Li, especially in the low-energy range.
We found that we could overcome these problems by a procedure, similar to that used by Touw and Trischka. We treated the wire at a working temperature of 1325K in an oxygen bath of 1 x 10-5torr for 5 minutes. This treatment guarantees a stable high work function of the detecting wire for at least one day. which is essential for the detection of sodium and lithium.
Energy dependence of the efficiency It was checked that the efficiency of the detector was insensitive for small temperature changes around 1325K. This, however, does not guarantee an energy-independent efficiency. Reflection of atoms on the surface is possible at high energies (Politiek and Los) and hence the detection efficiency may be lower. Such an energy dependence was found by different authors Hollstein and Pauly, Hulpke and Schlier, Politiek and Los for potassium atoms above 3eV.
All the measurements presented here should be corrected by a factor (E) taking this effect into account. But, as the detection efficiency varies smoothly with the energy and because the energy range is not very large, it does not substantially affect the measurements.
The charged particles are accelerated with 4keV and focussed on a multiplier. What actual detector is typically used?
The noise level of the measuring amplifier is about 1 x 10-16A.