The low energy crossover point for secondary electron emission from Lexan

The lowest primary electron energy for which the secondary electron emission coefficient is equal to one is referred to as the low energy crossover point. Previous measurements for a number of common polymers have indicated that this crossover point occurs between 30 eV and 60 eV depending on the particular polymer. This means that a small amount of charge on the surface of the sample can produce a surface potential which can be a significant fraction of the crossover point energy. The method of measurement used in our work assumes that the sample is initially uncharged and the surface potential is calculated from the integral over time of the replacement current. If the sample has an initial charge this method will predict the wrong crossover point energy. Our measurements show initial surface potentials ranging from 10 to 500 volts depending on the sample preparation technique. In addition, the surface potential is non uniform and may even alternate in polarity on adjacent regions of the surface. The secondary emission data collected on a number of identically prepared samples exhibit a random scatter due to this residual charge which causes a ±25% uncertainty in the average of the measured crossover point energy