OSIRIS: The Online Scintillator Internal Radioactivity Investigation System of JUNO

The Online Scintillator Internal Radioactivity Investigation System (OSIRIS) is a 20 ton liquid scintillator (LS) detector. During the months-long filling of the 20 kton JUNO central detector, it will monitor the LS radiopurity, ensuring stringent radiopurity levels needed for the various physics goals of JUNO. This is achieved by exploiting fast Bi-Po coincidences in the $^{238}$U and $^{232}$Th decay chains. The setup is expected to reach sensitivities of 10$^{−15}$ g/g or better for both $^{238}$U and $^{232}$Th. OSIRIS will also measure the $^{14}$C and $^{210}$Po levels present in the scintillator. In this poster, the general design of OSIRIS and its different subsystems will be explained. In OSIRIS, a water-submerged acrylic vessel holding the LS is instrumented by an array of 76 self-triggering intelligent PMTs. Their timing and charge calibration will be performed with a Laser- and an LED-based system. The calibration of the energy and vertex reconstruction of OSIRIS utilizes height-adjustable radioactive sources within the LS