Scintillation Properties of Metal-Organic Framework-Based Composites for Time-of-Flight Positron Emission Tomography Imaging

Scintillation is a phenomenon wherein the absorption of high-energy radiation in a material leads to the emission of light from matter due to radiative recombination of excited luminescent centers. Scintillating materials are currently widely used in many detection systems addressing different fields, such as high-energy physics (HEP), homeland security, and medical imaging. In the latter, the positron emission tomography (PET) technique is of significant importance for clinical oncology; the image reconstruction exploits the simultaneous detection of two back-to-back $\gamma$ photons emitted following the annihilation process of a positron, originating from the injected radiotracer, with an electron of the surrounding tissue. A large number of clinical studies have unequivocally demonstrated that a significant improvement concerning image quality and better signal-to-noise ratio (SNR) can be achieved by the use of the difference of the arrival times of the two annihilation photons, i.e. the time-of-flight (TOF). However, the effectiveness of this improvement is strongly correlated with the coincidence time resolution (CTR) of a detector, defined as the full-width-at-half maximum (FWHM) of the time delay distribution of arrival photons on detectors from a point source. The CTR was recently reduced down to ~215 ps by Siemens Healthineers, corresponding to a spatial uncertainty of ~3cm. Continuous research is done every day to further improve and push it up to the intrinsic limit of ~1.5 mm. This Master thesis project concerns the investigation of the scintillation properties of nanocomposite scintillators based on fluorescent metal-organic-frameworks (MOF) nanocrystals designed for the TOF-PET imaging. Their framework integrates a heavy-metal (Zr) node sensitizer and an antracene-based emitter, both embedded in polymer matrices: poly(methyl methacrylate) (PMMA) or poly(dimethylsiloxane) (PDMS). These materials have been prepared with different concentrations of MOFs (0.05%, 0.1%, 0.5%, and 2.5%) and characterized at the Materials Science Department (UNIMIB) to determine their photoluminescence (PL) emission spectra and PL lifetimes. The accurate investigation of their scintillation properties related to the TOF-PET imaging has been carried out in the CERN Crystal Clear laboratory in Geneva, Switzerland. Scintillation light yield (LY) upon gamma irradiation has been measured using a $^{137}$Cs source, coupling nanocomposite samples to a silicon photomultiplier (SiPM) and comparing the signal with that of a known inorganic scintillator. The LY for the PMMA:2.5\% MOF was ~20% with respect to Bi$_{4}$Ge$_{3}$O$_{12}$ (BGO) standard reference with small differences for other samples. Rise and decay times ($\tau_{r}$ and $\tau_{d}$, respectively) have been studied using pulsed X-rays with 40 keV maximum energy. The scintillation emission rate has been detected by a Hybrid PMT operating in time-correlated single-photon counting (TCSPC) mode, measuring the photon arrival time delay with respect to the laser excitation pulse. The measured rise time was evaluated to be in the sub-100 ps range, challenging the performance limits of the used setup, while the effective decay time is around 1 ns for all the tested samples. The measured quantities (LY, $\tau_{r}$ and $\tau_{d}$) allow estimating the theoretical CTR value, or in other words its intrinsic lower limit, of the material through the below equation deriving from an analytic model studied by S. Vinogradov, which results to be as low as ~118 ps. $CTR_{intr.~l.~l.} = 3.33\sqrt{\tau_{d}\frac{1.57 \cdot \tau_r + 1.13 \cdot \sigma_{SPTR+PTS}}{LY_{@Energy}}}$ In this expression, the term $\sigma_{SPTR+PTS}$ denotes the convolution of the single photon time resolution (SPTR) of the SiPM used in the experimental analysis of the CTR with the photon transfer time spread (PTS) of the sample. Instead, the time resolution of MOF nanocomposite measured experimentally has been investigated by coupling it with a SiPM against another known reference crystal (Lu$_{2}$SiO$_{5}$:Ce or LSO) and by using a $^{22}$Na source, which emits two 511 keV photons back-to-back in coincidence. After correcting for the reference detector, an effective CTR of ~230 ps was evaluated for PMMA:2.5% MOF. The results prove that MOFs-based composites could be a viable alternative as radiation detectors for TOF-PET imaging due to their fast scintillation since a prototype scintillator already shows the state-of-the-art CTR values for commercial devices. Further improvements must be addressed: in particular, considering the fast response of the composite scintillators, efforts must be devoted to enhancing the LY, by increasing the efficiency of ionizing radiation absorption and reducing light losses due to the internal scattering and self-absorption, such as using others or-ganic ligands or dopants to increase the Stokes shift. Moreover, next developments could regard the implementation of MOFs-based composites in heterostructure scintillators, composed of al-ternating plates of MOFs-based composites and high density crystals able to efficiently absorb the ionizing radiation. Therefore, the number of events detected in the overall system would in-crease leading to a prospective increase of LY and consequently even to a better CTR.