Electron Scattering Cross Sections Experimental Apparatus: : Electrostatic analyzer and Faraday cup combination

2.5 Electrostatic analyzer and Faraday cup combination

The cross sectional view of the ESA is shown in Figure 2.5. The ESA consists of two concentric 160° spherical sector surfaces which are made from oxygen-free copper; an inner convex surface of radius 3.25 cm and an outer concave surface of 4.05 cm. Only the electrons within a predetermined energy range are transmitted between the two sectors of the ESA which has 3.65 cm mean radius. The two surfaces are accurately held in position on a common center and insulated from each other by sapphire spheres located in the flat sides and ends of the sector blocks themselves. The side and end plates are held in position by non-magnetic stainless steel screws. A detailed description of the ESA design, resolution and energy scales can be found in reference [26].

In this experiment, the ESA was operated in a constant energy transmission mode. If a DV potential difference is maintained between the two spherical sectors, the energy of the transmitted electrons, (E_tr) is given by

(2.0)

where radius r₁ = 4.05 cm and r₂ = 3.25 cm are the radii of the spherical sectors. According to the Equation 2.0, the ESA may be used in 50 eV constant energy transmission mode by applying 22.18 V potential difference between the spherical sectors. If the ESA entrance is biased to -950 V, 1000 eV energy electron can be passed through ESA under the 50 eV constant energy transmission mode.

In the present experiment two constant energy transmission modes were used depending upon the energy of the analyzing electrons. The 50 eV constant energy transmission was used for the electrons in the energy range 100 – 1500 eV while 100 eV constant energy transmission was used for the electrons at 1600 eV and higher energies.

Fig. 2.5 Cross-sectional view of the electrostatic analyzer.

Fig. 2.6 Electrical connections to the electrostatic analyzer for constant transmission energy.

It was necessary to use the greater constant energy transmission for electrons with greater energies to keep the efficiency of the ESA in an acceptable level. The electrical connection for constant transmission energy mode is shown in Figure 2.6. Eveready, model NEDA 710, d.c. batteries were used as the DV supply to the ESA. A series combination of two BERTAN model 230-03 high voltage power supplies, each capable of producing 0 – 3000 V, were used as the analyzer bias voltage power supply. A computer control interface was used to match the bias voltage power supply with the electron gun energy to pass the desired energy electrons through the electrostatic energy analyzer. A detailed description of the connections and computer control interface will be given later in this chapter.

The electrons emerging from the ESA were accelerated to their original energy applying the same voltage applied at entrance at the exit aperture. These electrons were collected on a Faraday cup near the exit aperture of the ESA. The intensity of the electrons was measured by a KEITHLY model 6517-A electrometer. This meter had the accuracy of ±0.01 pA. The electron beam intensity under normal conditions was 100 pA, and decreasing during the attenuation measurements.