b'In situ positron annihilationPositron annihilation spectroscopy quantifies neutron damage spectroscopy fordefect generation and recombination, which can accelerate new characterizing irradiation- material qualification for use in extreme environments.induced defects O ne of the questions that remained unanswered in understanding defects in irradiated materials was how defects evolve at very low dose of neutron irradiation, especially for vacancies. We conducted in situ positron annihilation spectroscopy experiments to observe defect evolution in nuclear materials. This concept is innovative and relevant to research of materials in extreme environments. The initial focus was on method development on theory, hardware, and software through modeling, experiments, and optimization PROJECT NUMBER:algorithms. Two research papers were published and an open-source Python-based 21A1050-054FP software was published in support of the eventual large data sets. Through the method development effort, a Neutron Radiography Reactor East Radiography TOTAL APPROVED AMOUNT:Station model was developed in the Monte Carlo N-particle Program. Also, a $1,405,000 over 3 years thorough mechanical design interfacing the nuclear facility was completed based PRINCIPAL INVESTIGATOR:on the Monte Carlo N-particle Program model. All components were acquired or Chuting Tsai fabricated. The project initiated collaborations with universities and a local business and established talent pipelines that eventually led to the hiring of a staff member CO-INVESTIGATORS: supporting research activities. Additionally, a graduate students PhD dissertation Chase Taylor, INL topic was based on this project.Jagoda Urban-Klaehn, INLCOLLABORATORS:Idaho State UniversityPhysics Education and Consulting LLCThe Ohio State University(a)a) Final design of the in situ positron annihilation spectroscopy experiment in East Radiography Station, (b) simulated spectrum from one of the detectors.(b)32'