Refractive index in the JUNO liquid scintillator

TL;DR

This paper measures the refractive index of JUNO liquid scintillator across a broad wavelength range using two methods, providing precise data crucial for understanding detector response in neutrino experiments.

Contribution

It provides the first detailed measurement of the refractive index of JUNO liquid scintillator over 260-1064 nm with high precision using multiple techniques.

Findings

Refractive index measured with sub-percent accuracy

Wavelength dependence characterized across 260-1064 nm

Group velocity verified through interferometry

Abstract

In the field of rare event physics, it is common to have huge masses of organic liquid scintillator as detection medium. In particular, they are widely used to study neutrino properties or astrophysical neutrinos. Thanks to its safety properties (such as low toxicity and high flash point) and easy scalability, linear alkyl benzene is the most common solvent used to produce liquid scintillators for large mass experiments. The knowledge of the refractive index is a pivotal point to understand the detector response, as this quantity (and its wavelength dependence) affects the Cherenkov radiation and photon propagation in the medium. In this paper, we report the measurement of the refractive index of the JUNO liquid scintillator between 260-1064 nm performed with two different methods (an ellipsometer and a refractometer), with a sub percent level precision. In addition, we used an interferometer to measure the group velocity in the JUNO liquid scintillator and verify the expected value derived from the refractive index measurements.