Doping of a Borexino-like Liquid Scintillator with Tellurium-Diols

TL;DR

The work demonstrates a feasible method to dope Borexino-style liquid scintillators with tellurium via Te-diols using a water-free, room-temperature synthesis. Optical characterization shows emission spectra and transparency are largely preserved across Te loadings, while light yield decreases with higher Te content (e.g., ~$8{,}400$ photons/MeV at $1\%$ Te). Time-profile measurements reveal significantly faster alpha-induced scintillation with increasing Te, indicating a trade-off between timing performance and light output. This Te-doping approach supports the feasibility of Te-based $0\nu\beta\beta$ searches in next-generation LS detectors and informs practical loading levels and detector design considerations for experiments like SNO$^+$ and KamLAND-Zen.

Abstract

One of the most promising approaches for future neutrinoless double beta decay searches is to incorporate a candidate isotope into the liquid scintillator of a next-generation neutrino detector. In this study, a sample of the high-performance 1,2,4-Trimethylbenzene-based liquid scintillator from the Borexino detector was loaded with different concentrations of Te-diols. Therefore,a novel and completely water-free synthesis in a non-acidic organic environment at room temperature was used. Key parameters of the loaded samples were analyzed and compared with those of the pure Borexino liquid. Both the emission spectrum and transmission remained nearly unchanged,even at high doping levels. The reduction in light yield was moderate, with approximately 8,400 photons emitted for a 1 MeV energy deposition by an electron at 1$\%$ tellurium loading. The time profile of the light emission induced by alpha particles was also investigated, revealing that the scintillation response becomes significantly faster with increasing tellurium concentration.

Figures (6)

• Figure 1: Effective emission spectra for the unloaded Borexino scintillator and a sample loaded with 1% Te. Within the measurement accuracy, no differences in spectral shape were detected. Both spectra are completely dominated by the fluor PPO.
• Figure 2: Comparative plot of the spectral absorbance of undoped and 1% Te-doped LS. The bands shown, represent the statistical uncertainty from ten individual measurements in each case. Due to the relatively short cuvette length, it should be noted that the systematic uncertainties are significant but are not shown for the sake of clarity.
• Figure 3: Schematic drawing of the LY setup. A small $1\times1$ inch highly reflective PTFE liquid scintillator cell with thin UV-transparent glass windows is filled with the LS sample. Two PMTs are coupled to the cell. The LS is irradiated by mono-energetic gamma-quanta from a $^{137}$Cs source. To fix the scattering angle and by that the energy deposition of the gammas in the LS, the setup is triggered in coincidence with a LaBr$_3$(Ce) detector. For further details on the setup and the measurement technique see also SteigerSlow2024.
• Figure 4: The plot shows the relative light yield of the sample vs. its Te-concentration. It should be mentioned, that light yields above 10$^4$ Photons for electron-like energy depositions are still realized with loadings well exceeding 0.25%. For loadings exceeding 1.0% the chemical quenching by then Te-complex molecules severely reduces the light emission in this simple LS based on a single solvent (PC) and a single fluor (PPO).
• Figure 5: Left: Cuvette filled with liquid scintillator before being flushed with nitrogen. At the top center of the cuvette it is possible to see the $^{244}$Cm source screwed to the cup. Right: Time profile experimental. It is possible to see: the cuvette on the bottom left, with inserted an optical fiber for the PMT characterization, the HL-PMT on the top left very close to the cell and on the right a neutral optical filter, behind which the LL-PMT is mounted.