Molecular structure, electric property, and scintillation and quenching of liquid scintillators

TL;DR

The paper analyzes how molecular structure and electric properties govern scintillation and quenching in two-component liquid scintillators. It emphasizes that polar groups and dielectric screening modulate anion–cation recombination and electric dipole–dipole energy transfer, with the Förster transfer rate scaling as $k \propto 1/\varepsilon^2$ and energy transfer strongly influenced by the medium’s dielectric constant. By examining TeBD, a tellurium-loading candidate for SNO+, it demonstrates that polar hydroxyl groups and a high dielectric constant ($\varepsilon_r = 17 \pm 2$) can cause substantial quenching, partially explaining reduced light yield relative to LAB-based systems. The work highlights a design principle for high-light-yield, isotope-loaded liquid scintillators: minimize polar functionality and dielectric constant to enhance recombination control and energy transfer efficiency, guiding future chemical developments in scintillator formulation.

Abstract

Liquid scintillators are widely used in particle and nuclear physics. Understanding the scintillation and quenching mechanisms is a fundamental issue in designing a high-light-yield liquid scintillator. In this work, the basic scintillation process for two-component liquid scintillators is reviewed, highlighting the processes of excitation, ionization, anion-cation recombination, and electric dipole-dipole energy transfer. A molecule's polar group, polarization characteristics, and the corresponding material's dielectric constant are found to be correlated with a liquid scintillator's scintillation efficiency. Polar groups and high relative dielectric constant (permittivity) can cause severe quenching and should be avoided. The tellurium loading scheme in the liquid scintillator of the SNO+ experiment, TeBD, is discussed. The hydroxyl groups introduce polar structures in the TeBD, and the relative dielectric constant of our reproduced sample is measured to be $17\pm2$. These discussions explain part of the quenching of the TeBD liquid scintillator.

Figures (6)

• Figure 1: A simplified schematic diagram of the scintillation process. D is for the primary component, donor or solvent, and A is for the secondary component, acceptor or solute. More explanations are in the text.
• Figure 2: Free ion yield versus relative dielectric constant for water, four aromatic molecules, and some organic molecules containing the element oxygen, i.e., ether, alcohol, ketone, and ester. FreeIonYield.
• Figure 3: Positive ion mass spectrum for TeBD.
• Figure 4: Negative ion mass spectrum for TeBD.
• Figure 5: Molecule structure of TeBD at negative mode mass 411, which is the dominant synthesis product of this work.