The replacement system of the JUNO liquid scintillator pilot experiment at Daya Bay

TL;DR

This paper details the design, construction, and operation of a liquid scintillator replacement system at Daya Bay, which was crucial for optimizing the scintillator for JUNO's neutrino detection goals.

Contribution

It introduces a novel LS replacement system that effectively refines the scintillator composition for JUNO, ensuring the required energy resolution.

Findings

Successful implementation of the LS replacement system

Optimized LS composition achieved for JUNO

Enhanced scintillator transparency and light yield

Abstract

The Jiangmen Underground Neutrino Observatory (JUNO), a multi-purpose neutrino experiment, will use 20 kt liquid scintillator (LS). To achieve the physics goal of determining the neutrino mass ordering, 3$\%$ energy resolution at 1 MeV is required. This puts strict requirements on the LS light yield and the transparency. Four LS purification steps have been designed and mid-scale plants have been built at Daya Bay. To examine the performance of the purified LS and find the optimized LS composition, the purified LS was injected to the antineutrino detector 1 in the experimental hall 1 (EH1-AD1) of the Daya Bay neutrino experiment. To pump out the original gadolinium loaded LS and fill the new LS, a LS replacement system has been built in EH1 in 2017. By replacing the Gd-LS with purified water, then replacing the water with purified LS, the replacement system successfully achieved the designed goal. Subsequently, the fluorescence and the wavelength shifter were added to higher concentrations via the replacement system. The data taken at various LS compositions helped JUNO determine the final LS cocktail. Details of the design, the construction, and the operation of the replacement system are reported in this paper.