High-Z metal oxide nanoparticles for the enrichment of liquid scintillators in rare events searches

Liquid scintillators (LSs) represent promising detection materials in rare-event physics, particularly in large scale neutrino physics experiments, owing to their exceptional degree of purity, high volumetric scalability and low costs of production and implementation. However, commercially available LSs are primarily composed of an aromatic organic matrix, as toluene, xylene or linear alkyl benzene (LAB), characterized by a low effective atomic number (Zeff). As a result, their use in γ-ray detectors with volumes of a few cubic centimeters is hindered by the low photoelectric absorption cross-sections, which results in a reduced probability of full energy deposition. To address this issue, a promising approach is the loading of high-Z organometallic complexes or nanoparticles (NPs), to increase the density and Zeff of the medium, thus improving the resolution for X- and γ-ray spectroscopy [1, 2]. Nevertheless, the preparation of highly loaded LSs with NPs without compromising transparency and without deteriorating their stability and radio-physical properties remains challenging, mainly due to NPs aggregation. Pushed by these issues, this work focuses on the development of high-Z metal oxide NPs with controlled size and surface features to engineer highly loaded LSs with increased stability and optical transparency. The final goal is their application in the detection of high-energy photons. In this context, tungsten oxide (WOX) systems were selected thanks to the high electronic density and Zeff of tungsten, which could potentially enhance the probability of full energy deposition within the liquid medium. Specifically, to achieve fine control over NP morphology and optical response, WOX materials were synthesized through a microemulsion-mediated route using WCl6 as precursor, with CTAB and butanol serving as surfactant and co-surfactant, respectively. As an alternative approach, tungsten-based NPs were supported onto pre-synthesized Stöber silica NPs (WOX/SiO2) through a wet impregnation procedure with Na2WO4 to achieve W loadings up to 40 wt% over SiO2 NPs. This strategy exploits the silica support to optimize tungsten dispersion into the liquid matrix, while taking advantage of the well-established protocols for the surface functionalization of silica with organic ligands to ensure a higher stability to the resulting colloidal dispersion. Indeed, the functionalization of WOX/SiO2 system with different silane functionalizing agents (i.e. 3-aminopropyltriethoxysilane, APTES) or organic units (polyethylene glycol, PEG) was investigated to improve their dispersion in LSs. The structural and optical characterization of the materials revealed that the precursor molar ratio (WCl6/NH3) is a determinant parameter on the composition, size and optical response of bare WOX NPS. Indeed, NPs of ~ 30 nm were obtained after calcination at 500 °C by keeping the WCl6/NH3 ratio below 1:6. Conversely, increasing NH3 content to a 1:14 ratio resulted in significantly larger particles, ranging from 90 to 120 nm. This growth is likely attributed to an uncontrolled condensation pathway that leads to rapid grain growth. Diffuse reflectance spectroscopy also showed an absorption onset that shifted from 2.6 eV to 3.4 eV depending on the synthetic parameters. Moreover, a uniform coverage of silica NPs (~ 30 nm) with tungsten was achieved in WOX/SiO2, as demonstrated by TEM/EDX maps. Finally, the successful functionalization of WOX/SiO2 with either APTES or PEG units was confirmed by TGA, FTIR and elemental analyses. Dispersions in the LS were then prepared both with bare NPs synthesized with a high WCl6/NH3 ratio and with WOX/SiO2 NPs, using commercial Ultima Gold AB, Revvity. Under soft X-ray excitation, the suspensions show a promising increase in the radioluminescence intensity with respect to the unloaded LS with increasing loading fraction. Besides, the introduction of the surface functionalizing agents improved the stability of the colloidal dispersions as proved by DLS and zeta-potential experiments. Measurements of the scintillation response through excitation with monochromatic γ-ray emissions (22Na, 57Co, and 241Am) are currently ongoing to reconstruct the spectra of energy deposited in the loaded LS.