Electron backscatter in an important process in models of high-voltage X-ray sources and collectors for high-power microwave tubes. A GamBet user reported observing backscatter ratios below the expected values. He had made two changes in our standard example of electron backscatter from a gold foil:
- Reduction of the incident electron energy by an order of magnitude.
- Initiation of electron trajectories outside the material in an adjacent void.
The original example was set up energies in the range ≥1 MeV. At 0.1 MeV, the user observed reported a backscatter ratio of 0.32 instead of the experimental value of 0.51.
The figure illustrates the source of the problem. At 0.1 MeV, the penetration depth for electrons in gold is less than 10 μm. The particle in the Void region moves in virtual interaction steps determined by the parameter DsMax. If the particle crosses a material boundary in a step, GamBet interpolates the particle position to the entrance boundary and then makes a small adjustment (DElemG) to ensure that the particle is inside the material. In the past, GamBet set DElemG equal to a small fraction of the average element size. A problem could occur if the scale length of the solution volume was much larger than that of the interaction region. In this case, particles could be placed a significant depth inside the material, reducing the backscatter ratio.
We modified GamBet to alleviate the issue. The program now calculates DElemG by determining the minimum element size and then reducing it by a larger factor. Users should encounter no problems as long as the element width at the surface is inordinately large (i.e., >50 times the interaction depth).
