Observation of lower hardness factor for displacement damage induced by high-energy electrons

Experimental measurements of electron displacement damage at the CLEAR facility at CERN were performed on several diodes previously calibrated with neutrons and protons. The idea behind the study was to assess whether the facility can be used as an alternative for displacement damage qualification to neutron and proton irradiations. In the last case, the foreseen advantages are a fast test, thanks to the high electron fluxes available, and of a lower activation of the test samples that enables accessing them for post-test characterization straight after the test. The experimental measurements on two different types of diodes, both previously calibrated against neutrons and protons, resulted in the observation of ~2 lower damage than expected. Monte-Carlo (MC) simulations are used to understand whether the lower hardness factor observed is due to (i) electron being less damaging than previously thought or (ii) specific beam conditions encountered during this test. The non-ionising energy loss (NIEL) and hardness factor determined from the MC tools when the energy threshold for primary knocked-on atoms is 21 eV are typically lower than those previously established. However, they are still higher than the experimental results. Calculations with higher energy threshold show that the electron NIEL and hardness factor would further decrease. However, this would require setting threshold in excess of 100 eV.