Real Time Monitoring of Defect Evolution and Relaxation by In-Situ Positron Annihilation Spectroscopy

In-situ measurements during irradiation are critical to monitor material response to irradiation in real time; several in-situ techniques with remarkable capabilities have been developed to address that and examine material structure, properties, and performance under extreme radiation environments. In-situ TEM has been particularly powerful in monitoring the microstructural changes and growth of cavities in real time during irradiation. However, revealing the mechanisms governing early formation of defects and their evolution in extreme environments require measuring defects on all length scale from atomic- to meso-scale. Positron annihilation spectroscopy (PAS) is uniquely sensitive to atomic scale defects revealing their density and structure even in the very early stages of damage and has been shown to be an effective tool to probe vacancies and stresses in nuclear and structural materials [1,2].<br/>Here we report the first in-situ PAS (iPAS) measurements during high energy ion irradiation. The measurements reveal that vacancies are formed and their number increases during collision cascades without change in structure or clustering. However, vacancies are shown to coarsen, and their structure substantially change during relaxation after ceasing irradiation. The trend is shown to dominate in the low irradiation regime and the defect density increases linearly with irradiation time up to 0.1 dpa. In higher regime above 1 dpa, vacancies are formed and coarsen during collision cascades. Further, iPAS during annealing reveals the various recovery stages of induced radiation and the defect type involved in each stage. Lastly, I will discuss how iPAS may shed the light on the mechanism behind some interesting phenomena such as radiation induced vacancy injection in muti-phase materials that we recently observed in Fe / Fe₂O₃ bilayers.<br/><br/>This work was funded as part of FUTURE (Fundamental Understanding of Transport Under Reactor Extremes), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.<br/>[1] Selim, F. A. "Positron annihilation spectroscopy of defects in nuclear and irradiated materials-a review." <i>Materials Characterization</i> 174 (2021): 110952.<br/>[2] Selim, F. A., D. P. Wells, J. F. Harmon, J. Kwofie, G. Erikson, and T. Roney. "New positron annihilation spectroscopy techniques for thick materials." <i>Radiation Physics and Chemistry</i> 68, no. 3-4 (2003): 427-430.