Electron Scattering Cross Sections Experimental Apparatus: : Vacuum chamber and attached instruments

2.4 Vacuum chamber and attached instruments

Figure 2.4 shows the cross sectional view of the vacuum chamber and the items enclosed. The 45.72cm (18 inch) diameter soft iron chamber was mounted vertically. The electrostatic analyzer (ESA) and Faraday cup were housed inside the chamber. Details about these instruments will be discussed later in this chapter. The vacuum in the chamber was maintained at upper 10^-7 Torr or better using a Leybold-Heraeus model TURBOVAC 360SCV turbomolecular pump located beneath the chamber. Pressure inside the chamber was measured using Duniway Stockroom model I-075-K glass tabulated ion gauge attached to a Varian model 843 analog readout. It was found to be about 6 x 10^-7 Torr or better without the gas present in the gas cell. During the intensity measurement with gas present inside the gas cell, the pressure in this region was maintained at 10^-6 Torr or better. The AMETEK DYCOR M206 residual gas analyzer (RGA) was used to check the purity of the target gas and to monitor the atmospheric contaminants in the system. The ion source head of the RGA was connected to the computer via an AMETEK DYCOR model LC200 interface. The RGA head was connected to the chamber through a port. The analyzer head consists of ionizer, mass

Figure 2.4 An illustration of the interior of the chamber.

filter, and ion detector. Inside the analyzer head, electrons produced from a hot filament collide with gas molecules in the ionization region. These ions are directed through a mass filter which allowed passing only the specific mass/charge ratio. The ions were emitting from the filter then collected by the detector which measured the proportional current. The computer generated real time graphical result for the analysis.

To identify the molecules present in the chamber by inspecting the mass spectrum produced by RGA, the pattern must be identified and recognized by prior knowledge. First it was necessary to study the fragmentation patterns of already existing gases inside the chamber using RGA. Fragmentation patterns were generated by the ionizer since more than one kind of ions are produced from a single gas. For example, the fragments of Silane (SiH₄) molecule are SiH₄⁺, SiH₃⁺, SiH₂⁺, SiH⁺ ions and their multiply-charged ions. The RGA will produce an array of peaks representing each of these ions. The magnitudes of the peaks represent the numbers of ions produced, and hence the concentration of molecules of the indicated substance. The sum of the peaks is a measurement of the pressure of that molecular substance. In this work RGA was primarily used to detect the contamination and check the purity of the target gas by obtaining the pressure.

The chamber has a removable lid, which fitted to the chamber by vacuum tight seals with an O-ring. The lid contains conducting probes which are used to make electrical connections between external equipment and the ESA and Faraday cup. Inside the chamber, 0.5mm thick m metal shielding was placed to shield from the earth’s magnetic field and other stray magnetic fields.