Electron Scattering Cross Sections Experimental Apparatus: : Experimental errors

2.8 Experimental errors

The errors from this experiment results mainly from the measurement of the gas pressure, beam current, and electron-gas interaction length. Pressure inside the gas cell was measured by a MKS Baratron 626 A capacitance manometer. Possible errors in this pressure measurement are due to the zero drift in the scale and temperature difference between the gas cell and diaphragm region inside the device. According to the manufacture’s specifications, the combined errors from zero drift and temperature difference is 2% or less²⁸.

The primary electron beam intensity (I_o) and attenuated electron beam intensity (I) were measured from the combination of ESA, Faraday cup, and electrometer. By passing the electron beam through the ESA, inelastically scattered forward electrons were deflected and the electron beam intensity was measured on the Faraday cup. However, in this method elastically forward scattered electrons also contribute to the measured electron beam intensity. From the experimental results published by Fink, Jost and Herrmann²⁹ for elastic differential cross sections for electrons scattering for the angle between 3° and 130°, we estimated the contribution from the 0° elastic scattering by extrapolating their data. We found that the greatest error due to this contribution of is about 0.5% for experiments of this type. Another error in the current measurement is caused by the small drift in electron current. After taking the attenuation current measurement and closing the gas supply needle valve, the electron beam took few minutes to return to its original value 100 pA. In some instances, the electron beam failed to return to the exact 100.0 pA but to a value between 99.5 – 100.5 pA. Based on this information, the error due to the drift in the current was estimated to be 1% or less for 100 – 4000 eV electron energies.

A geometrical length of 24.5cm gas cell was used in this experimental work. From previous work¹⁸ done in this lab it has been shown that the effective length was essentially the geometrical length with 2% or less error. In reference [18], this error was determine by measuring the total scattering cross section of N₂ for 400 – 1200 eV electron energies for three different gas cell lengths, 16.0, 24.5 and 32.0cm.

2.9 Target gases

Research grade target gases of PH₃, from Liquid Air Corp., Denver, CH₄, NH₃, and SiH₄ from Matheson Co., Laporte, Texas within minimum purity of 99.9% or better were used during this experiment. The RGA attached to the vacuum chamber was used to monitor the purity of the target gas by eliminating air leaks or other gas contaminant from the gas transportation system.