Electron Scattering Cross Sections Experimental Apparatus: : Gas cell and gas delivery system

2.3 Gas cell and gas delivery system

An aluminum gas cell was used to define the region where electrons scatter off gas molecules. Figure 2.2 is a cross sectional view of the gas cell. The gas cell is 24.5 cm long and has a 0.75 mm (0.03 inch) diameter entrance aperture and a 1.0 mm (0.04 inch) diameter exit aperture. The inside diameter of the gas cell is 2.54 cm (1 inch). Inside the gas cell there is a 1.5 mm thick m metal tube to shield the electron beam from the earth’s magnetic field and other stray magnetic fields. Gas pressure in the gas cell was measured with a MKS Baratron 626 A capacitance manometer attached to a digital MKS PDR-D readout display unit. The sensor in this capacitance manometer consists of a pressure inlet tube connected to a small chamber in the transducer body. One wall of this chamber is an elastic metal diaphragm; the front panel of this diaphragm is exposed to the gas whose pressure is to be measured. The diaphragm bends with changing pressure independently of the gas type or composition of the measured gas.

Fig. 2.2 Cross-sectional view of the gas cell.

In order to clean contaminated walls inside the gas cell to obtain the desired low pressure, the system was baked each time before a new gas was introduced, and after the area has been opened for maintenance. Baking was done by wrapping the gas cell with a heat tape made of resistive wire in a fiber glass coating, and sending a power of 600 Watts through the tape for 30-60 minutes. During the baking the temperature of the gas cell was measured with a mercury-glass thermometer placed in thermal contact with the gas cell body. The temperature was not allowed to exceed more than 80° C because the gas cell was attached to the system using four rubber O-rings seal flanges.

Fig. 2.3 Gas delivery system to the gas cell.

The gas delivery system is shown in Figure 2.3. Gas flow from the supply cylinder was controlled using a Matheson regulator to reduce the pressure from the high cylinder pressure to a safe working pressure at about 10 Torr above the atmospheric pressure. This regulator has a stainless steel diaphragm which does not absorb any contaminants, so it does not contribute to system contamination. Also, the low internal volume inside the regulator made the purging process quicker. The regulator is attached to a tee purge assembly. This purging assembly eliminates the escape of hazardous (PH₃ gas) or flammable (SiH₄ gas) gases into air when switching from one gas to another. It also minimizes the amount of gas waste during a cylinder change, while reducing the danger of exposing these pyrophoric gases to atmosphere. Beyond the tee, purge assembly flow is control by a fine needle valve, which limits the gas pressure in the cell to between 0 – 60 mTorr.