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Separation of the 235U and 239Pu Prompt Energy Spectra in NEOS-II

The NEOS searches for sterile neutrinos by detecting reactor antineutrinos at a very short baseline in Korea. The NEOS detector (1-ton Gd-LS) is deployed at the tendon gallery of the Hanbit reactor unit 5 (2.8 GW thermal power), 23.7 m away from the reactor core. In NEOS-I, we measured the prompt energy spectrum from inverse beta decay using 180 days of reactor-on data and observed the “5 MeV excess”. To understand the origin of the “5 MeV excess”, NEOS-II has taken 368 (120) days of reactor-on (-off) data from September 2018 to October 2020, covering a whole burnup cycle of the reactor. We tested a method for the extraction of the prompt energy spectra for 235U and 239Pu from the whole burnup cycle using pseudo data.