In this work, ion irradiations in-situ of a transmission electron microscope are performed on single-crystal
germanium specimens with either xenon, krypton, argon, neon or helium. Using analysis of selected area dif-
fraction patterns and a custom implementation of the Stopping and Range of Ions in Matter (SRIM) within MATLAB
(which allows both the 3D reconstruction of the collision cascades and the calculation of the density of va-
cancies) the mechanisms behind amorphization are revealed. An intriguing finding regarding the threshold
displacements per atom (dpa) required for amorphization results from this study: even though the heavier ions
generate more displacements than lighter ions, it is observed that the threshold dpa for amorphization is lower
for the krypton-irradiated specimens than for the xenon-irradiated ones. The 3D reconstructions of the collision
cascades show that this counter-intuitive observation is the consequence of a heterogeneous amorphization
mechanism. Furthermore, it is also shown that such a heterogeneous process occurs even for helium ions, which,
on average induce only three recoils per ion in the specimen. It is revealed that at relatively high dpa, the
stochastic nature of the collision cascade ensures complete amorphization via the accumulation of large clusters
of defects and even amorphous zones generated by single-helium-ion strikes.