Organic Scintillators Containing High-Z Nanoparticles
Spectroscopic scintillation detectors for gamma rays are desirable for medical imaging and nuclear non-proliferation. Thanks to their high-Z nature, inorganic single crystals are commonly used, despite their high cost and limited size. Inexpensive, high performance spectroscopic scintillators are in demand and recently polymer-matrix nanocomposites have become one of the most promising candidates. Nanocomposite used here is mostly two-phase solid where one phase is inorganic nanoparticles with sizes below 20 nm and the other is polymer matrix. Nanocomposites loaded with high-Z, large band gap nanoparticles and luminescent quantum dots have been developed; however, transmittance at its emission wavelength decreased drastically as loading content and sample thickness increased. Monoliths loaded with large band gap high-Z nanoparticles suffered from light yield deterioration due to inefficient non-radiative energy transfer to the polymer matrix. Cadmium zinc sulfide quantum dots improved F�rster resonance energy transfer (FRET) from generated excitons to organic species due to their luminescence properties. However the low-Z nature of these quantum dots barely provided noticeable photopeak signals on the pulse height spectrum.
This dissertation strives to overcome the obstacles researchers encountered in the field of spectroscopic scintillation study. One focus is to find a new luminescent quantum dot with high Z to improve the photopeak signal of the detector. Inorganic lead halide perovskite nanocrystals have been shown to have high photoluminescence quantum yield (PLQY), fast emission decay, facile size control and most importantly, high Z (ZPb = 82 and ZCs = 55). Cesium lead bromide perovskite nanocrystals were synthesized at a mild temperature, with a square shape capped with oleic acid as the ligand. Ligand exchange of nanocrystals was performed to alleviate phase separation during the in-situ copolymerization with the monomers. Thermal curing was first conducted, yet only produced an opaque monolith. UV curing of the nanocomposites, on the other hand, led to a much more transparent monolith, suggesting instability of the nanocrystals at the elevated temperature. Different organic primary dyes were mixed with the nanocrystals and the monomers, and the resulting solutions were cured to form nanocomposite scintillators. However, all dyes got bleached in the presence of the nanocrystals under UV irradiation. Pulse height spectrum of nanocomposites loaded with perovskite nanocrystals was recorded. The light yield was rather low, and no full gamma photopeak was observed.
Liquid matrix was then chosen to substitute polymer matrix in the nanocomposites and scintillation solution can be prepared without polymerization. Cesium lead bromide perovskite nanocrystals were synthesized and purified following the previous protocol, without treatment by polymerizable ligand. The primary dye concentration was adjusted in a broad range, from 0.01 to 1.5 wt%, to study non-radiative energy transfer from perovskite nanocrystals to the dye molecules. Highest light yield of around 10000 photons/MeV was achieved on the liquid scintillator loaded with 20 wt% of nanocrystals at optimized organic dye concentration. Nanocrystal loading was then increased to 60 wt%. In our best demonstration, light yield around 9000 photons/MeV and deconvoluted photopeak energy resolution of 10.6% were achieved with a much more prominent gamma photopeak signal, showing potential of the high-Z luminescent perovskite nanocrystals for spectroscopic scintillators.
The obtained photopeak energy resolution was lower than that obtained from the CdZnS-quantum-dot-based nanocomposite scintillators. Many causes for the lower energy resolution were examined. One of the important reasons is that the scintillation emission spectrum from the perovskite liquid scintillators peaks in the green wavelength range where the photodetection efficiency of the photomultiplier tube (PMT) is low. The light yield produced from the PMT is low, and thus the gamma photoelectron energy resolution is low. Scintillation solution loaded with hafnium oxide nanoparticles emitting blue light was then prepared. At 50 wt% loading of the nanoparticles, the transmittance at the emission peak wavelength is still higher than 80%. At 40 wt% loading, the scintillation solution showed light yield of around 6300 photons/MeV with deconvoluted photopeak resolution of 5.3%. Solvent additives were added to the system for more efficient exciton energy transfer from the primary solvent to the primary dye and 25 wt% of naphthalene improved light yield by about 40%. Finally, a large liquid scintillator (20 mm diameter and 20 mm thickness) was prepared containing 40 wt% of nanocrystals as well as solvent additives. Light yield of 8500 photons/MeV and photopeak resolution of 11.6% were achieved. Although there is still room for energy resolution improvement, for the first time we observed a prominent gamma photopeak with an organic liquid scintillator loaded with high-Z nanoparticles.