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

The calculation of the velocity dependence and magnitude of the kinetic secondary electron yield for atomic and polyatomic ions, denoted by gamma-bar/sub a/ and gamma-bar/sub m/, 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 gamma-bar/sub m/=Σgamma-bar/sub a/ is verified, which is necessary to the model calculation of gamma-bar/sub m/. The fundamental assumption of this model is that gamma-bar/sub a/ is proportional (even at low ion velocities) to the electronic stopping power (dE/dx)/sub e/ of that projectile in the target. Example calculations are performed for the velocity dependence of gamma-bar/sub a/ for hydrogen, carbon, and fluorine striking copper, and from these results the predicted gamma-bar/sub m/ 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 utility of the model is shown by illustrating that the measurement of gamma-bar can provide, not only a relative measurement of (dE/dx)/sub e/ for different ion atomic numbers on the same target, but can also provide a means of determining the nuclear stopping power (dE/dx)/sub n/ at low ion velocities where range measurements are difficult to perform. Values of (dE/dx)/sub 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)/sub n/ values are a factor of 2 larger than theoretical. The relationship of the stopping power values derived from gamma-bar measurements with this model are compared to the values derived from ion range measurements