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

TL;DR

This paper presents a new method to produce high-activity, quencher-free liquid scintillator sources loaded with $^{222}$Rn for precise calibration of the Borexino detector, ensuring high radiopurity and minimal spectral distortion.

Contribution

The work introduces a reproducible fabrication process for $^{222}$Rn sources in liquid scintillator with negligible quencher contaminants, suitable for detector calibration.

Findings

Sources achieve 0.5-1 kBq activity post-fabrication.

Sources exhibit unprecedented radiopurity levels.

Calibration sources meet strict transparency and removability requirements.

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.