arXiv subjects

Luka Pavelić

Publications and source records attributed to Luka Pavelić.

5 recordsLinked to original sources

arXiv2026

Omnidirectional Radiation Detector with Perpendicular Dual Silicon Photomultiplier Readout - Directional Sensitivity and Machine Learning Source Positioning

Directional information of incident gamma rays in radiation detection is essential to many applications, from medical imaging to mapping radioactive contamination after nuclear accidents, characterization of nuclear waste during decommissioning, and nuclear security. In this work, we present a radiation detector capable of detecting gamma-ray photons in 4$π$, utilizing Gadolinium Aluminum Gallium Garnet (GAGG) scintillating crystals and silicon photomultipliers (SiPMs). The GAGG crystals are assembled in a cubical 4$\times$4$\times$4 matrix, and their light output is read out by SiPMs from two neighboring sides. The size of each crystal is 3$\times$3$\times$3 mm$^3$, while the matrix pitch is 3.2 mm, matching the size and the pitch of the 4$\times$4 SiPMs for one-to-one coupling. The layers perpendicular to the SiPMs are separated by optical reflectors, providing efficient light collection while lowering the probability of inter-crystal leakage. The design of the radiation detector offers high detection efficiency and full-view imaging of gamma-ray sources through Compton scattering. Monte Carlo simulations were performed in Geant4 to evaluate the detection efficiency of the proposed design due to its geometrical non-uniformity. Machine learning models developed with the XGBoost algorithm were trained and tested on the simulated data to assess the capability of the detector to localize point sources of 511, 662, and 1275 keV energies. We find that the proposed design does not influence the collection efficiency of the high-energy gamma-ray photons and offers sensitivity to the direction of the incoming gamma rays. The trained models are capable of determining the source-to-detector distance of the Na-22 and Cs-137 point sources from measurements, thus providing initial conditions for faster image reconstruction.

physics.ins-det↗

arXiv2026

Positron Emission Tomography with quantum-entangled Compton events: first imaging results at clinically relevant activities

In Positron Emission Tomography, a potential, yet unutilized enhancement, may come from exploiting the quantum entanglement of the annihilation quanta, inscribed in the correlation of their polarizations. To investigate this, we built a PET demonstrator capable of measuring polarization correlations of annihilation quanta by their Compton scattering, based on single-layer scintillator polarimeters. We present a detailed study of the imaging of two $^{68}$Ge line sources, 45 MBq each, to extract the spatial resolution and assess image quality. The results show that a spatial resolution of 2.5$\pm$0.1 mm is obtained using single-pixel events, while resolutions obtained with polarization-correlated Compton events range from 3.6$\pm$0.3 mm to 4.9$\pm$0.3 mm, depending on data selection criteria. We also found that the polarization-correlated Compton events exhibit up to 20% higher average signal to random background ratio compared to the single-pixel events. We also present the first imaging of the NEMA NU-4 phantom filled with a $^{68}$Ga solution of 378 MBq initial activity, successfully combining polarization-correlated events with conventional single-pixel event selection. Based on the extracted spatial resolution, signal-to-background, signal-to-noise, contrast, and contrast-to-noise ratio, we estimate that up to 10% sensitivity increase may be attained by exploiting the polarization-correlated events, while preserving a high image quality.

physics.ins-det↗

arXiv2025

Towards polarization-enhanced PET: Study of random background in polarization-correlated Compton events

Positron Emission Tomography (PET) is a medical imaging modality that utilizes positron-emitting isotopes, such as Ga-68 and F-18, for many diagnostic purposes. The positron annihilates with an electron from the surrounding area, creating two photons of 511 keV energy and opposite momenta, entangled in their orthogonal polarizations. When each photon undergoes a Compton scattering process, the difference of their azimuthal scattering angles reflects the initial orthogonality of their polarizations, peaking at $\pm$90$^{\circ}$. This type of correlation is not yet utilized in conventional PET scanners, but could potentially offer an energy-independent method for background reduction. Measurements of these kinds of correlations can be achieved using Compton polarimeters, built from a single layer of segmented scintillating crystals such as Gadolinium Aluminium Gallium Garnet doped with Cerium (GAGG:Ce), read out by silicon photomultipliers (SiPMs). In this paper, we study the signal-to-random background ratios in measurements of these correlated annihilation photons from coincidence time spectra across clinically relevant source activities, from $\sim$200 MBq to $\sim$378 MBq. These are then compared to the standard single-pixel (photoelectric) measurements. We find that the signal-to-random background ratios (SBRs) obtained from the polarization-correlated events for Compton scattering angles $θ_{1,2}\in[72^{\circ}, 90^{\circ}]$ and azimuthal angle difference $Δϕ=90^{\circ}\pm20^{\circ}$ are consistently higher than those from single-pixel events, with the ratio of their SBR values of 1.23. The SBR of the selected events also increases with the polarimetric modulation factor $μ$, gaining $\sim$50\% in value during the experiment.

physics.ins-det↗

arXiv2024

Investigation of the spatial resolution of PET imaging system measuring polarization-correlated Compton events

Recent studies of positron emission tomography (PET) devices have shown that the detection of polarization-correlated annihilation quanta can potentially reduce the background and creation of false lines of response (LORs) leading to improved image quality. We developed a novel PET demonstrator system, capable of measuring correlated gamma photons with single-layer Compton polarimeters to explore the potential of the method. We tested the system using sources with clinically relevant activities at the University Hospital Centre Zagreb. Here we present, for the first time, the images of two Ge-68 line sources, reconstructed solely from the correlated annihilation events. The spatial resolution at two different diameters is determined and compared to the one obtained from events with photoelectric interaction.

physics.ins-det↗

arXiv2020

Single-layer Compton detectors for measurement of polarization correlations of annihilation quanta

Measurement of gamma-ray polarization can provide valuable insight in different areas of physics. A possible application is in Positron Emission Tomography, where annihilation quanta with orthogonal polarizations are emitted. Since polarization can be measured via Compton scattering, the initial orthogonality of polarizations can be translated to correlation of azimuthal scattering angles, and this correlation may be exploited as an additional handle to identify the true coincidence events. To examine the concept of using the polarization correlations in PET, we have used a system of two compact, position and energy-sensitive Compton detectors in coincidence mode. Each consists of a single matrix of scintillation pixels, read-out by an array of Silicon photomultipliers. The Compton events in each module are clearly identified and the scattering angles are reconstructed from the energy deposition and event topology. We have extracted the polarimetric modulation factors from the distributions of the difference of the azimuthal angles of the two Compton-scattered gammas and studied their dependence on Compton scattering angles $θ$ and on azimuthal resolution $Δϕ$. For scattering angles around $θ_{1,2}=82^\circ$, where the maximum modulation is expected, the modulation factors from $μ=0.15\pm 0.01$ to $μ=0.27\pm 0.02$ have been measured, depending on the azimuthal resolution, which is governed by event topology in the detectors. Analogously, for scattering around $θ_{1,2}=70^\circ$, modulation factors from $μ=0.12\pm 0.01$ to $μ=0.21\pm 0.02$ have been obtained. The results show that the measurement of the polarization correlations of annihilation quanta are feasible with compact single-layer, single-side read-out detectors, which may be used to build cost-efficient systems for various applications where gamma-ray polarization information is of interest.

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