## 1 Introduction
It was concluded that Ni ion bombardment at 300 and 400 \({}^{\circ}\)C is a reasonable simulation of neutron damage. However, no damage structures were found after irradiation at 600 \({}^{\circ}\)C. An in situ heavy ion irradiation investigation was conducted by [PERSON] et al. [15] on pure Zr in order to observe prismatic loops experimentally, particularly at lower doses close to their nucleation threshold as a function of dose and irradiation temperature. It was reported that the small defect clusters (\(\sim\)2 nm) are perfect dislocation loops, with Burgers vector \(\frac{1}{3}\langle 11\overline{2}0\rangle\). The size of these loops increases and defect densities decrease with an increase of temperature. Similarly, a trend of increased loop size and decreased defect yield (DY) with an increase of irradiation temperature was observed by several authors after neutron and ion irradiations [14; 16; 17]. In an early study on neutron-irradiated Zr and Zr-based alloys by [PERSON] et al. [17], it was found that dislocation loops are not formed at higher temperature in Zr and Zr alloys, which indicates that at higher temperatures, vacancies and interstitials instead recombine after the cascade collapse. It was also found that the dislocation loops' size increases and DY decreases with an increase of irradiation temperature until the damage structure declines at 773 K. It was pointed out that in these neutron irradiated samples, there is an absence of radiation damage at 773 K, which is 0.42 of the absolute melting temperature of \(\alpha\)-Zr. Ion irradiation studies on pure Zr show similar effects on loop sizes and densities with increasing irradiation temperature, except there was still a formation of large dislocation loops at 773 K [15]. The presence of these loops at 773 K may indicate decreased V-SIA (vacancy/self-interstitial atom) recombination possibilities leaving a high concentration of point defects, which then cluster in the form of dislocation loops. We can expect that this would be affected by the presence of suitable alloying elements. It is vital to know and understand the effects of alloying element on the point defect clustering behaviour at higher temperatures. The effect of temperature on the production of point defects in displacement cascades and their clustering during the life time of cascades have been studied using Molecular Dynamic computer modeling (MD). MD simulations have been carried out in alpha Zr at lower temperatures (100 K) [18; 19; 20; 21] and at intermediate temperature (600 K) [20]. [PERSON] et al. [20] reported that the number of vacancies and interstitials deceases with increasing temperature for a given cascade energy, which has been attributed to the longer thermal spike time. These interstitials and vacancies form perfect dislocation loops with Burgers vector b =\(\frac{1}{3}\langle 11\overline{2}0\rangle\)Contrarily, [PERSON] et al. [21] have reported that the number of Frenkel pairs produced in Zr are independent of irradiation temperature within a temperature range of 100-600 K, which was attributed to the stability of both vacancy and self-interstitial atom (SIA) clusters at all irradiation temperatures. The study of defect distribution at temperatures higher than 600 K is important in order to understand the defect clustering behaviour and dislocation loop formation at higher temperatures as well as for a given Primary Knock-on Atom (PKA) energy. Initial nucleation of <a> loops in the form of small defect clusters was reported after MD computation, but has not been extensively studied experimentally. Ion irradiation can produce a high PKA energy for the study of prismatic loop formation at low doses, similar to the doses at which prismatic loops are seen during neutron irradiation, for comparison with the results of MD simulations. In addition, in situ ion irradiation within TEM coupled with a high energy irradiation facility such as a tandem accelerator allows direct observation of internal microstructure of materials while being irradiated [22].