School of Science Department of Physics 67 Theoretical Study of Properties of Real Materials Supervisor: LIU Junwei / PHYS Student: QIAN Yuchen / PHYS Course: UROP 1100, Spring UROP 2100, Summer I will review how to extract critical behavior like critical temperature and exponents using finite-size scaling (FSS) theory. In particular, scaling behavior of these quantities and high-precision determination of critical exponents and error analysis will be demonstrated with the example of 2d Ising model. Basics on Dark Matter Supervisor: LIU Tao / PHYS Student: CHAU Yat Chi / PHYS Course: UROP 1100, Summer During the 2425 Summer term, I participated in UROP 1100 through the project ‘Basics on Dark Matter’ supervised by Professor Liu Tao, which involved a comprehensive study of Cosmology. This report presents a comprehensive exploration of the ΛCDM (Lambda Cold Dark Matter) model, examines the roles of dark energy (Λ), dark matter, and ordinary matter in shaping cosmic history. Search for Light Dark Matter with Positronium Supervisor: LUK Kam Biu / PHYS Student: LAM Chi-yin Sienna / PHYS-IRE Course: UROP 4100, Fall 22Na is an isotope of Sodium that can be used as a positron source to facilitate the formation of positronium. There is a 90.2% branching ratio for decay into a positron and an gamma ray photon, but the photon with the same energy can also be produced via the electron capture pathway, with a 9.7% branching ratio. Our experiment involves using positrons produced by 22Na to form positronium, so that we can study the decay products of positronium decays. Due to the 9.7% chance of a false positive event caused by the electron capture decay pathway, there is a motivation to detect a positron without relying on the gamma ray photon. We hope to suspend graphene quantum dots within the aerogel powder that is used to capture positrons for positronium formation. Positrons will excite the quantum dots, causing them to emit a photon that can then be detected with our silicon photomultipliers. This will indicate the presence of a positron, which can help to eliminate false positive events produced by the electron capture decay pathway of 22Na, allowing us to determine whether a positron has been emitted by the 22Na source.
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