The Online Scintillator Internal Radioactivity Investigation System is an 18-ton pre-detector of
JUNO, currently under commissioning in south-west China. During the 6-month filling phase
of the JUNO main detector, it will be responsible for the monitoring of the radiopurity of the
liquid scintillator filled into the JUNO central detector. Fast 214/212Bi/214/212Po coincidences
serve as a main measurement channel for OSIRIS’ high sensitivity to 238U/232Th contaminations
in the liquid scintillator. In addition, contamination measurements of 85Kr and 14C are also
foreseen. OSIRIS is located 700m underground in the JUNO laboratory near the central detector.
Its cylindrical central vessel is surrounded by 64 JUNO 20-inch PMTs and embedded into a water
Cherenkov muon veto. Calibration of the detector will be done by an automated calibration unit
featuring radioactive sources and a fast pulsed LED, as well as by a pico-second laser calibration
system responsible for time- and charge calibration. After OSIRIS’ main purpose of monitoring
the liquid scintillator has been fulfilled, a consecutive physics phase addressing solar neutrinos
and 0𝜈𝛽𝛽 decay is foreseen.