A model of secondary electron yields from atomic and polyatomic ion impacts on copper and tungsten surfaces based upon stopping-power calculations

Abstract

The calculation of the velocity dependence and magnitude of the kinetic secondary electron yield for atomic and polyatomic ions, denoted by γ̄a and γ̄m, respectively, is modeled on stopping-power equations and compared to experimental results from the dynode materials copper and tungsten. From the experimental data, the relationship γ̄m=Σγ̄a is verified, which is necessary to the model calculation of γ̄m. The fundamental assumption of this model is that γ̄a is proportional (even at low ion velocities) to the electronic stopping power (dE/dx)e of that projectile in the target. This fundamental assumption of γ̄a∝ (dE/dx)e has been previously used by Sternglass for calculation of γ̄a’s at ion velocities above 4×108 cm/sec. Example calculations are performed for the velocity dependence of γ̄a for hydrogen, carbon, and fluorine striking copper, and from these results the predicted γ̄m curves for polyatomic ions are in close agreement with experiment. Analogous calculations for a tungsten surface are outlined and the final results are compared with experiment. The model’s utility is shown by illustrating that the measurement of γ̄ can provide not only a relative measurement of (dE/dx)e for different ion atomic numbers on the same target, but can also provide a means of determining the nuclear stopping power (dE/dx)n at low ion velocities where range measurements are difficult to perform. Values of (dE/dx)n derived from measurements for fluorine and carbon striking copper are the same as values predicted by Lindhard and Scharff; for a tungsten target the experimental (dE/dx)n values are a factor of 2 larger than theoretical. The relationship of the stopping power values derived from γ̄ measurements with this model are compared to the values derived from ion range measurements. In addition, the model’s relationship to the secondary electron emission model of Parilis and Kishinevskii is discussed.