Title: Fabrication of quencher-free liquid scintillator-based, high-activity $^{222}$Rn calibration sources for the Borexino detector,
Abstract: A reliable and consistently reproducible technique to fabricate
$^{222}$Rn-loaded radioactive sources ($\sim$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 detector's upcoming calibration campaign, with one or
several $\sim$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 $^{222}$Rn 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 quenching agents
("quenchers") to null or extremely low levels.