Borexino “Water Tank” filling operations have begun. Ultrapure water in the WT will serve both as a shield against the external natural radioactivity background and as a Cherenkov detector for muon veto.
The picture below was taken at the beginning of the water tank filling procedure, before closing and sealing the access to the WT. The first few cm of water covering the tank floor are visible.
In the foreground you can also see one of the PMTs installed in the WT for the muon-veto detector (a few others can be seen in the background). Behind that you can glimpse some of the pipelines for inner vessels fluid handling. Also visible are some of the fiber-optic cables (the thin, orange ones) for the laser calibration system, some of the signal+HV cables connected to the PMTs (the thicker, black ones), some of the large stainless steel pillars which sustains the SSS and part of the white Tyvek covering used to increase Cherenkov light detection efficiency.