When you need to characterize displacement damage in a device and you’re going to use protons, one of the first decisions is what energy to test at. This sounds like it should be straightforward — just pick something representative of the environment — but in practice there are several competing considerations.
Common choices and their rationale:
-
50-63 MeV (e.g., UC Davis, Indiana University Cyclotron): Popular because many proton SEE tests are done at these energies, so you can potentially get TID and displacement damage data during the same beam campaign as your SEE test. NIEL at ~60 MeV is well into the plateau region, so the damage should be representative of higher-energy protons too.
-
200 MeV (e.g., Indiana, MGH, various medical proton facilities): Better energy match to the trapped proton belt spectrum peak for LEO and MEO missions. Long range in silicon means very uniform dose deposition through even thick devices. Facilities designed for medical proton therapy often have excellent dosimetry and beam uniformity.
-
Low energy (< 20 MeV): Relevant if your shielding analysis shows the environment is dominated by protons that have been degraded through shielding. But NIEL scaling can be less reliable at these energies, and beam range/uniformity issues become more challenging.
The key question: If you’re relying on NIEL scaling to translate your test results to the mission environment, does the test energy even matter that much? In principle, if NIEL scaling is valid, any energy should give equivalent results when properly normalized. In practice, the answer depends on the device technology.
For silicon devices where NIEL scaling works well, 50-200 MeV protons are largely interchangeable. For compound semiconductors and some optocouplers, the choice matters more.
What energy do you typically test at, and how do you justify it? Has anyone seen cases where results at different proton energies didn’t scale as expected?