Comprehensive Analysis of Impurity Detection in Linear Alkyl Benzene–Based Liquid Scintillators Using Infrared Spectroscopy for Enhanced Neutrino Detection

Abstract We present a comprehensive analysis of infrared (IR) spectra for linear alkyl benzene (LAB)-based scintillation solutions, focusing on the detection and identification of impurities. Previous research primarily utilized ultraviolet-visible spectroscopy to investigate electronic structures and transitions, whereas our approach emphasizes vibrational transitions and IR spectral characteristics. We specifically examined the presence of common impurities, such as acetone, water, and three impurity compounds (IMP1, IMP2, and IMP3) identified by the JUNO Collaboration. Acetone, a common contaminant from cleaning procedures, was detected by its characteristic absorption peaks at 1200, 1360, and 1700 $\rm {cm}^{-1}$. Water, an inevitable by-product of Gd-loaded LAB using a neutralization reaction process, was identified through distinct O-H stretching and H-O-H bending vibrations at 3200–3600 $\rm {cm}^{-1}$ and 1600 $\rm {cm}^{-1}$, respectively. The IR spectra of IMP1, IMP2, and IMP3 were theoretically calculated, revealing unique absorption bands for key functional groups, including carbonyl (C=O), amide (C-N), sulfoxide (S=O), aryl chloride (Ar-Cl), azo (N=N), and ether (C-O-C) groups. The findings confirm the absence of these impurities in the LAB (+ 2,5-Diphenyloxazole [PPO] + 1,4-Bis(2-methylstyryl)benzene [bis-MSB]) sample, ensuring the high performance and accuracy of neutrino detectors. This study demonstrates the effectiveness of IR spectroscopy as a powerful analytical tool for quality assurance in liquid scintillation solutions, providing a robust framework for enhancing the reliability and precision of neutrino detection experiments.