Fabrication of quencher-free liquid scintillator-based, high-activity 222Rn calibration sources for the Borexino detector

A reliable and consistently reproducible technique to fabricate 222Rn-loaded radioactive sources (∼0.5-1 kBq just after fabrication) based on liquid scintillator (LS), with negligible amounts of LS quencher contaminants, was implemented. This work demonstrates the process that will be used during the Borexino detectors upcoming calibration campaign, where one or several ∼100 Bq such sources will be deployed at different positions in its fiducial volume, currently showing unprecedented levels of radiopurity. These sources need to fulfill stringent requirements of 222Rn activity, transparency to the radiations of interest and complete removability from the detector to ensure their impact on Borexino's radiopurity is negligible. Moreover, the need for a clean, undistorted spectral signal for the calibrations imposes a tight requirement to minimize scintillator quenching agents (”quenchers”) to null or extremely low levels.