Synthesis and characterization of scintillating lead-iodide-based nanocrystals

ABSTRACT Lead-iodide-based nanocrystals were synthesized by dissolution of commercial lead iodide powder in a coordinating solvent, tetrahydrofuran, and subsequent re-crystallization af ter an optimum addition of methanol. The nanocrystals were characterized by transmission electron microscopy, scanning electron microscopy, energy dispersive spectroscopy (EDS), steady state UV-visible optical absorption and photoluminescence spectroscopy, and by photoluminescence lifetime and quantum efficiency measurem ents. The radiation hardness of the synthesized material was tested using a 137 Cs gamma source. Scintillation was observed from the lead-iodide based material exposed to low-level gamma irradiation. Keywords: Positron emission tomography; Radiation detectors; Nanoscintillators; Lead iodide; Iodolaurionite 1. INTRODUCTION Semiconductor nanocrystals (NCs) or quantum dots (QDs) have been extensively investigated over the last decade for a variety of biomedical, biochemical sensing, and optoelectronic applications. An area that has received relatively little attention so far is the use of NCs as gamma or X-ray detectors in applications such as positron emission tomography (PET), digital radiography, dosimetry, and nuclear medicine. In a typical radiation detection system, conversion of the incident energy of ionizing radiation is accomplished by using scintillating materi als that emit photons in the visible/UV spectral range, subsequently collected by a photosensitive element. Compared to currently used scintillating particles of the micrometer size, NCs offer the prospect of significantly improved performance. Due to their small size, they are expected to have better solubility in organic polymer or inorganic sol-gel host materials and to cause much less scattering, which should result in higher efficiency of the scintillator. Due to three-dimensional confinement and much better overlap of electron and hole wavefunctions, the optical transitions are expected to be much faster than in bulk scintillators, which should eliminate the major problem of relatively slow response of scintillator detectors. Lead-based compound NCs are of interest as potential novel sc intillation materials due to thei r high density and the high atomic weight of Pb. While the bulk materials may have poor efficiency of light emission at room temperature, the effects of quantum confinement are expected to greatly enhan ce the probability of radiative transitions, as well as reduce the radiative recombination lifetime. In this paper, after a short discussion of the principles of PET and of the advantages of using lead-iodide-based scintillators for PET, the synthesis procedure of the crystals will be described. Next, we present results of characterization of the lead-iodide-based material using transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), steady state UV-VIS optical absorption and photoluminescence (PL) spectroscopy, and PL lifetime and quantum efficiency measurements. Subsequently, we describe the results of radiation hardness testing of the synthesized material, and show evidence of room-temperature scintillation under gamma irradiation. Finally, the material parameters relevant to PET are compared to those of lutetium-yttrium oxyorthosilicate (LYSO), a scintillator used in some time-of-flight PET systems [Pidol 2004].