## 3 Results
### MD Simulation  
In order to further investigate and explain the differences seen in the loops' morphology and defect clustering at different irradiation temperatures, Molecular Dynamics (MD) simulations were carried out in \(\alpha\)-Zr. The system is first held at a given temperature for 10 ps to achieve thermal equilibrium, then one PKA is introduced into the system, and the production-run lasts for 60 ps. Periodic boundary conditions and a various-time-step approach are applied in the current calculations. Wigner-Seize method and common-neighbor analysis are employed to analyze the defects [25]. The results of MD simulation show the temperature dependence of defect cluster distributions at the time of defect nucleation at very low doses, which is then reflected in the DY and size data at higher doses, which can be observed experimentally. The Zr potential selected for the current simulations was the Embedded Atom Method (EAM) potential developed by [PERSON] and [PERSON] [25]. The size of the simulation block was 72 \(\times\) 41 \(\times\) 45 unit cells, with a total of 531,360 atoms inside. This size of the simulation box is sufficient for the selected PKA energy. In displaced cascade simulations, the PKA (Primary Knock-on Atom) energy is given as 20 keV, and the simulation temperature is set as 600 K or 800 K (close to experimental temperatures). The simulation system is first held at either 600 K or 800 K for 10 ps to achieve thermal equilibrium, then one PKA is introduced into the system, and the production runs of duration out to 60 ps are followed. For each condition, 20 simulations with different PKA positions and recoil directions are carried out. Periodic boundary condition and various time-step approaches are applied in the MD simulations.  
Figure 6 illustrates the interstitial cluster size distributions at 600 K and 800 K. For the interstitial clusters with no more than 20 interstitial atoms, the cluster distributions are similar at 600 K and 800 K. Moreover, large interstitial clusters containing more than 40 interstitials are produced at both temperatures. The main difference between the two temperatures is that with the increase of the temperature from 600 K to 800 K, the number of interstitial clusters with 20 to 40 interstitials per cascade decreases with increased temperature.  
A similar tendency of cluster distributions is also observed for vacancy clusters, as shown in Figure 7.  
Figure 6: Interstitial cluster distribution at 600 K (white column) and 800 K (gray column).  
The length of a defect cluster with 21 to 50 interstitial atoms or vacancies ranges from 1 to 3 nm. TEM analysis at very low ion doses can indeed resolve such small vacancy or interstitial clusters. Furthermore, MD simulations show that a higher number of defect clusters are produced at 600 K than at 800 K. In terms of production bias, the MD investigation conducted here showed that more vacancy clusters are produced than interstitial clusters at 600 K. Most of the vacancy clusters consist of three \(\{\bar{1}\bar{1}00\}\) faces as a triangular prism structure and lie on the basal plane and the interstitial cluster extends along the \(<\)0002\(>\) direction with an irregular structure [19; 21]. The total number of vacancy and interstitial clusters produced at 600 K and 800 K were counted, and are summarized in Table 1.