Velocity effect in swift heavy ion irradiation: how the low- and high-energy track formation thresholds meet

Variations of the threshold of swift heavy ion track formation with the ion energy and mass are studied here. We apply the hybrid multiscale approach combining TREKIS-3 code describing the electronic kinetics with the classical molecular dynamics tracing the evolution of the excited lattice. The results demonstrate that with a lowering of the energy loss curve (by choosing lighter ions for irradiation), the high-velocity threshold starts to shift down, whereas the low-velocity one remains constant until the thresholds meet. For certain ion parameters, a situation is possible in which both thresholds (low- and high-velocity) lie at the low-energy shoulder of the Bragg curve which results from the mismatch between the ion energy positions of the maximal track radius and the Bragg peak.