  The Jiangmen Underground Neutrino Observatory (JUNO) is a neutrino detection
experiment characterized by an acrylic sphere, measuring 35.4 m in diameter,
containing 20,000 tons of liquid scintillator. This sphere is encompassed by as
many as 17,600 photomultiplier tubes (PMTs) with a 20-inch diameter, achieving
an overall coverage of 77.9%. With these impressive capabilities, the JUNO
experiment aims to achieve an exceptional effective energy resolution of 3% at
1 MeV. The Online Scintillator Internal Radioactivity Investigation System
(OSIRIS) serves as a precursor detector, around 100 m aside in horizontal of
the site of the JUNO detector underground, tasked with monitoring and examining
the purity of the liquid scintillator prior to its transfer to the JUNO central
detector. The OSIRIS, also as a pre-detector of JUNO, is constructed with a
cylindrical acrylic vessel designed to hold 18 tons of liquid scintillator. It
is situated within a 9-m height cylindrical tank filled with 550 tons of pure
water. This detector was specifically engineered to search for the fast
coincidence decays of $^{214}$Bi -$^{214}$Po and $^{212}$Bi -$^{212}$Po in the
decay chains of $^{238}$U and $^{232}$Th, respectively.OSIRIS has been designed
to reach a sensitivity of 10$^{-16}$ g/g for U/Th to test scintillator
radiopurity to the level required for the detection of solar neutrinos.
Additionally, 64 20-inch microchannel plate PMTs (MCP PMTs) were positioned
around the liquid scintillator vessel to observe incoming interactions, along
with an additional 12 20-inch MCP PMTs for the water Cherenkov muon veto
system. The OSIRIS pre-detector has been fully constructed and integrated,
providing a plenty of preliminary results from the air runs. The next step in
the process involves commencing the filling soon, aiming to involve an in-depth
examination of the impurities within the liquid scintillator.
