Objective.Lutetium oxide (Lu2O3), with its high density (9.4 g cm-3), presents a compelling scintillation host for detecting 511 keV annihilation photons in positron emission tomography (PET). Despite its favorable density, the practical deployment of Lu2O3-based scintillators for PET has faced limitations due to difficulties in crystal growth and inappropriate decay time. Recent progress in ceramic processing has facilitated the development of transparent Lu2O3ceramics, while targeted doping strategies have significantly improved their luminescence performance. This study evaluates the performance of Lu2O3:Yb and a newly developed ceramic scintillator of (Lu,Y)2O3:La, a modified Lu2O3-based compound incorporating yttrium (Y) and doped with lanthanum (La).Approach.Various ceramic disks were fabricated and cut into 3 × 3 × 5 mm3samples. The performance of both Lu2O3:Yb and (Lu,Y)2O3:La ceramic samples in terms of decay time, energy resolution, and coincidence timing resolution (CTR) was assessed. Decay time measurements were conducted using waveform data collected from samples mounted on an H10580 photomultiplier tube (PMT) and irradiated with 511 keV photons from a22Na source. Energy and CTRs were evaluated using both PMT and silicon photomultiplier setups, arranged in coincidence with a reference lutetium-yttrium oxyorthosilicate (LYSO) detector of the same size.Main results.All three (Lu,Y)2O3:La ceramic scintillator samples exhibited a triple exponential decay profile and were dominated by a slow component ranging from 1379.3 to 1515.6 ns. The best energy resolution of 15.4% at 511 keV and the best CTR of 237.9 ps full width at half maximum (FWHM) were observed for the same sample. In contrast, a fast decay time of 1.6 ns was observed for the Lu2O3:Yb samples, which exhibited CTR values ranging from 237.9 ps to 261.4 ps FWHM, while the photopeak at 511 keV was difficult to distinguish. These CTR values were estimated between two identical ceramic samples, derived from coincidence measurements of each ceramic sample against the LYSO reference detector. The (Lu,Y)2O3:La samples achieved CTR values comparable to those of the Lu2O3:Yb samples, as their much higher light yield offsets the disadvantage associated with their slower decay time.Significance.These results highlight the promising potential of the (Lu,Y)2O3:La ceramic scintillators for PET applications, especially for time-of-flight PET.

Ceramic scintillators provide many advantages over single-crystal scintillators, including cost-effective production due to shorter processing times and lower fabrication temperatures. Polycrystalline scintillators with garnet structure are of particular interest due to their cubic crystal structure, high density and high effective atomic number Zeff. To this end, Gd3(Al,Ga)5O12:Ce3+ ceramics doped with La3+ were produced and their microstructure and scintillation properties were analyzed. The produced ceramics were translucent and showed the characteristic d-f emission of Ce3+ in the green spectral range upon excitation with X-rays. The light yield and energy resolution were found to be 23,900 photons x MeV-1 and 14 % at 662 keV, respectively.

Thallium chloride (TlCl) is a promising material for radiation detection due to its high gamma-ray stopping power and cost-effective manufacturing. In its undoped form, TlCl emits prompt Cherenkov photons, while double doping with Be and TlI induces scintillation, making it a candidate for Positron Emission Tomography (PET). However, the Be,TlI doping scheme presents challenges, including limited luminosity. To address this, we investigated alternative dopants to enhance material’s scintillation properties. We evaluated various iodide and other compounds, including AgI, CuCl, HgI2, SmI2, and EuI2. Although some dopants exhibited X-ray-induced emission, their light yield remained lower than the reference Be,TlI composition. The most promising alternatives were TlI-only and SmI2-doped samples, both achieving a light yield of at least 350 ph/MeV, approximately half that of the reference composition and slightly faster overall decay. These findings provide new insights into optimizing TlCl-based scintillators for radiation detection applications.

High-energy X-ray radiography is a key non-intrusive inspection technique for cargo screening and nuclear security. However, the performance of current detector systems is limited by the properties of the scintillator materials used. Multi-component garnet scintillators have the potential to address these limitations. In this work, we investigate the physical and scintillation properties of(Y0.08Gd0.08Tb0.75Lu0.08)3Al5O12:Ce 1% fabricated from commercial powder via uniaxial hot pressing (UHP) and hot isostatic pressing (HIP). Both fabrication routes produced dense ceramics with cubic garnet crystal structure (space group Ia-3d), with a lattice parameter of 12.06 & Aring;. The UHP and HIP ceramics display nearly identical optical and scintillation properties. Photoluminescence measurements demonstrate efficient Tb3+-Ce3+ energy transfer, and under X-ray irradiation emission from only the Ce3+ 4f05d1-4f1 (2F5/2, 2F7/2) transitions are observed. An absolute light yield of 46,000 ph/MeV was measured, placing this composition among the brightest aluminum garnet scintillators reported to date. Moreover, the light yield remains unchanged after exposure doses up to 100 Mrad under 9 MeV X-ray irradiation, demonstrating excellent radiation hardness. These results highlight Tb-rich multi-component garnet ceramics as strong candidates for radiation-hard scintillators for high-energy X-ray radiography applications.

Crystals of undoped and Eu2+-doped TlSr2Br5 of up to 16 mm in diameter and 50 mm in length were grown by the vertical Bridgman technique. TlSr2Br5 has the monoclinic crystal structure with space group P21/c. Its density and Zeff are 5.03 g/cm3 and 58.6, respectively. Radioluminescence spectra of undoped and Eu2+-doped TlSr2Br5 feature a broad emission band peaking at about 440 nm for undoped and 520 nm for Eu2+-doped crystals. The light yield of undoped TlSr2Br5 is 42 000 ph/MeV, which increases to 55 000 ph/MeV for Eu2+-doped crystals. The energy resolution at 662 keV (Cs-137) is about 5%-6% full-width at half-maximum (FWHM). The scintillation decay of undoped and Eu2+-doped TlSr2Br5 is on the order of hundreds of nanoseconds

Lu2O3 has emerged as a highly attractive host material for gamma-ray detection, primarily due to its high density (9.4 g/cm3) and effective atomic number (68). Various dopants have been explored to enhance its scintillation properties, with notable examples including Eu3+ and Yb3+. While Eu3+ doping results in high luminosity, it suffers from a prolonged decay time in the millisecond range. On the other hand, Yb3+ produces a fast scintillation response in the nanosecond range, but at the cost of a significantly lower light yield. Both dopants present limitations for gamma-ray spectroscopy.Recently, we developed a novel variant of Lu2O3 doped with La3+. Given the high melting point of the host material (2490 °C), ceramic consolidation techniques were employed during synthesis, leveraging the optically isotropic cubic structure of the material. This composition offers a balanced set of properties, exhibiting a light yield as high as 20,000 photons/MeV and two dominant decay time components at 530 ns and 1230 ns. Additionally, the energy resolution at 662 keV was measured to be 5.3 %, which can be attributed to the material's highly proportional response.Optimization of La3+ concentration revealed that the best results were achieved at a doping level of around 5 %. The material demonstrated excellent timing properties, with a fast rise time, <600 ps, and a single-channel timing resolution measured at 511 keV is 307ps. This composition is highly suited for radiation detection applications where intrinsic background noise is not a concern, such as in radiography. Furthermore, its superior timing characteristics and high stopping power make it attractive for Positron Emission Tomography (PET).

Radiation detectors are crucial in a wide variety of research and commercial applications, such as oil and gas exploration, medical imaging, nuclear nonproliferation, and homeland security. Neutron and gamma-ray detectors are fundamental components in portal monitors at ports and border crossings, bolstering national security against radiological threats. This study presents a dual-mode scintillator, undoped and Tl-doped 6Li-Cs3Cu2I5, and demonstrates its potential as a promising material for simultaneous thermal neutron and gamma-ray detection. We explore the Bridgman growth of both undoped and thallium-doped Li -> Cu and Li -> Cs substitutional systems with various Li doping levels and assess their impact on scintillation properties. Under 662 keV gamma-ray excitation, the undoped crystals had light yields up to 35,900 ph/MeV, with energy resolutions down to 4.5%. The Tl-doped crystals performed better than the undoped crystals, with light yields peaking at 65,900 ph/MeV and energy resolutions as low as 3.5%. When exposed to a moderated 252Cf excitation source, our crystals had light yields between 102,900 and 167,200 photons per thermal neutron capture, with a full energy thermal neutron peak reaching 3 MeV in gamma equivalent energy. Pulse shape discrimination studies reveal well-separated gamma and neutron events, resulting in a figure-of-merit (FOM) as high as 3.7. These findings highlight the potential of Li-doped Cs3Cu2I5 as a viable candidate for next-generation dual-mode scintillators.

The Cherenkov emission in inorganic crystal scintillators has been shown to dramatically improve time resolution for time-of-flight positron emission tomography (TOF-PET) for in slow scintillators with a high refractive index such as bismuth germanium oxide (BGO). This is due to the faster nature of the Cherenkov emission (tens of picoseconds) with respect to scintillation (nanosecond). However, the presence of slower scintillation light and the inability of existing detectors to distinguish between Cherenkov and scintillation make it difficult for BGO to achieve a good CTR for all the detected coincidence events. In this paper, we exploit the difference between the Cherenkov and scintillation emission spectra and use dichroic filters to enhance the Cherenkov over scintillation ratio. Dichroic filters transmit or reflect photons based on their wavelength, with a photon attenuation lower than 10%. We investigate several shortpass and longpass dichroic filters in a single-photon configuration with BGO and find the optimal filter that maximizes the Cherenkov over scintillation ratio. We demonstrate that we can enhance the ratio of Cherenkov to scintillation photons by a factor of 2.17 ± 0.38 by employing a shortpass dichroic filter with a cut-off wavelength of 450 nm for BGO, and by a factor of 2.87 ± 0.40 using a longpass dichroic filter with a cut-on wavelength of 550 nm for iodine/beryllium-doped thallium chloride.

Gamma-ray spectroscopy is an essential tool in nuclear science, nuclear security, and environmental monitoring. However, challenges arise in interpreting spectral data due to the presence of low counts, multiple sources, and dynamic backgrounds. To address these issues, a novel feature-driven analytical approach for gamma-ray spectral analysis using machine-learning techniques is developed. The method utilizes a series of random forest models for in-distribution (ID) multi-label classification, and the model-derived feature importance values to guide the out-of-distribution (OOD) detection task. The performance of this approach is quantitatively evaluated across various spectral parameters, including acquisition time, number of sources, energy of an OOD source, and background composition. Increasing the acquisition time from 1 s to 100 s leads to improved performance for multi-label classification, with 22 sources achieving F1-scores >= 0.9 after 50 s acquisitions for a CLLBC handheld detector and a standoff distance of 30 cm. The feature-driven analytical approach also demonstrates robustness when handling complex source mixtures. Furthermore, it provides contextual energetic information for OOD detection. The results presented here highlight the interpretability of the approach, establishing clear links between the spectral features and underlying physics. Moreover, the approach effectively distinguishes overlapping spectral signatures of different ID gamma-ray sources, enhancing human reliability in machine learning-based gamma-ray spectral analysis. The feature-driven analytical approach offers a promising solution to automate gamma-ray spectral analysis by addressing existing limitations and providing insights into performance across diverse spectral parameters.

Development of new scintillator materials is a continuous effort, which recently has been focused on materials with higher stopping power. Higher stopping power can be achieved if the compositions include elements such as Tl (Z=81) or Lu (Z=71), as the compounds gain higher densities and effective atomic numbers. In context of medical imaging this translates into high detection efficiency (count rates), therefore, better image quality (statistics, thinner films) or lower irradiation doses to patients in addition to lowering of cost. Many known scintillator hosts, commercial or in research stages, are alkali metal halides (Cs, K, Rb). Often these monovalent ions can be replaced with monovalent Tl. Since Tl has a higher atomic number than for example Cs (55), this increases the stopping power of modified compounds. A good example of an enhanced host is Ce doped Tl2LaCl5 (5.2 g/cm(3)), that mirrors less dense Ce doped K2LaCl5 (2.89 g/cm(3)). Tl substation also increased the luminosity to >60,000 ph/MeV, as it often leads to a reduction in the bandgap. Another example is the dual mode (gamma/neutron) Ce doped Cs2LiYCl6 scintillator (density 3.31 g/cm(3)). Substitution creates Ce doped Tl2LiYCl6 with density of 4.5 g/cm(3), with much better stopping power and 20% higher light yield. Binary Tl-compounds are also of interest, although mostly they are semiconductors. Notable example of a scintillator is double doped TlCl with Be, I. This scintillator offers fast Cherenkov emission topped off with scintillation signal for achieving better energy resolution. Another family of interesting and dense compositions is based on Lu2O3 ceramics. Lu2O3 is one of the densest hosts (9.2 g/cm(3)) available offering high stopping power. Lu2O3 doped with Eu3+ is known to be a high luminosity scintillator, however, this emission is very slow (1-3 ms), which limits its utility. On the other hand, ultra-fast, 1 ns, scintillation can be achieved with the Yb3+ doping that can be used for timing or high count-rate applications. However, while fast, Yb3+ doped Lu2O3 has very low luminosity. Recently, we have shown a middle ground performance, with Lu2O3 doped with La3+. This composition generates scintillation with 1,000 ns decay time and up to 20,000 ph/MeV luminosity. Moreover, the material demonstrates very good energy resolution.

High-temperature-capable detectors are essential for nuclear material accounting in advanced fuel cycles like Molten Salt Reactors (MSRs). Dual-mode scintillators, capable of detecting gamma-rays and neutrons simultaneously, are highly advantageous for such environments. They enhance measurement accuracy and efficiency. $\mathrm{Cs}_{2} \mathrm{LiLa}(\mathrm{Br}, \mathrm{Cl})_{6}$ (CLLBC) is a notable dual-mode scintillator, renowned for its excellent gamma detection and ability to discriminate gamma-rays from neutrons. It’s readily available commercially from RMD. At room temperature, CLLBC shows a $3.0 \%$ energy resolution at 662 keV and a light yield of approximately $45,000 \mathrm{ph} / \mathrm{MeV}$. Its gamma-neutron pulse shape discrimination figure of merit stands at about 3.5 at room temperature. A high temperature (HT) detector was designed and manufactured using the CLLBC scintillator and a high temperature PMT (R1288AH-27) from Hamamatsu. The detector was characterized from $25^{\circ} \mathrm{C}$ to $175^{\circ} \mathrm{C}$ with various gamma sources and digital pulse processing MCA. The 662 keV energy resolution was measured to be $5 \%$ at RT and $7.8 \%$ at $175^{\circ} \mathrm{C}$. We were able to resolve gamma energies from 60 keV to 1.5 MeV when a combination of multiple sources was used. In this presentation we will discuss the data and the significance of radioisotope identification and quantification for NMA applications at high temperatures and count rates.

Key characteristics of brain PET scanners include spatial resolution and sensitivity, requiring detectors to have both high 3D resolution and high detection efficiency. We are developing a high- density ceramic scintillator for brain PET applications based on lutetium oxide, Lu2O3:La (LO). Compared with state-of-the-art scintillators such as lutetium (yttrium) oxyorthosilicate (L(Y)SO), LO exhibits a higher density (9.4 g/cm3 for LO vs. 7.2 g/cm3 for L(Y)SO) and effective atomic number (Z) (68 for LO vs. 65 for L(Y)SO), providing higher stopping power and photoelectric ratio. Our results show that a ~ 11% energy resolution can be obtained from a 1 x 1 x 5 mm3 LO element. Currently, we are fabricating a 4 x 4 array of 0.9 x 0.9 x 15 mm3 LO crystals with a 1.0 mm pitch. The performance of a dual-ended readout detector based on this LO array and position-sensitive silicon photomultipliers (PS-SiPMs) will be reported and compared to a detector based on an L(Y)SO array with the same dimensions at the conference.

A Monte Carlo model is used to investigate electron-hole (e-h) generation created by incident gamma-ray radiation in LaB3, CeBr3, and Cs2LiLaBr6 (CLLB) scintillators. Our approach follows the detailed energy loss mechanisms and describes the microscopic structure of ionization tracks in order to address differences in the scintillation properties of these three materials. The mean energy required to create an e-h pair, W, theoretical light yield, and the spatial distribution of e-h pairs are determined. We found that W approaches constant and similar values at high incident energies for LaB3 and CLLB, suggesting that these materials should have similar light yields. However, the experimental light yield of CLLB is almost half that of LaBr3. Unlike for LaBr3, W of CeBr3 increases with increasing energy excitation and shows a nonlinear behavior in e-h creation, potentially explaining slightly lower light yield and worse energy resolution of CeBr3. Furthermore, we observed that the spatial distributions of electron-hole pairs in LaBr3 and CeBr3 are very similar, while the number of high-density e-h domains in CLLB is greater in comparison. This discrepancy could explain the lower light yield of CLLB. The thermalization model of e-h pairs showed that the longitudinal optical phonon energy has a profound effect on the thermalization time and distance of e-h pairs, leading to a much higher density of excitation in LaBr3 but a more diffuse one in CLLB. This effect leads to steeper gradients in LaBr3, resulting in varying density effects and worse proportionality, while CLLB suffers from more uniform but more pronounced quenching. The fraction of nonradiatively recombined electrons in LaBr3 and CLLB was estimated to be 30% and 45%, respectively. These results correlate well with experimental observations of the scintillation properties of these materials. The approach can be used to predict the expected properties of new materials and support further development of existing materials.

TlCl emerges as a promising scintillation material for Positron-Emission-Tomography (PET), boasting a high density of $7 \mathrm{~g} / \mathrm{cm}^{3}$ and an effective atomic number of 77, coupled with rapid Cherenkov emission. Its appealing attributes include a low melting point of $430^{\circ} \mathrm{C}$ and a straightforward cubic structure, suggesting potential for cost-effective production. Currently, the material is dually doped with Be and I, yielding a scintillation light yield ranging from 500 to 1000 $\mathrm{ph} / \mathrm{MeV}$, accompanied by a multicomponent decay profile. While this Be-I doping combination stands as the current benchmark, it presents certain production challenges and could benefit from enhanced prompt luminosity. Consequently, our research delved into alternative dopants for TlCl, aiming to optimize its scintillation properties. Our investigation encompassed various dopants (and doping levels), including $\mathrm{Ag}, \mathrm{Cu}, \mathrm{Hg}, \mathrm{Sm}, \mathrm{Eu}$, among others. Although some of these dopants exhibit emission under x-ray excitation, their light yield thus still falls short of the reference composition (Be, I). In this presentation, we present findings from our exploration, showcasing images of grown crystals, emission spectra, pulse height spectra, and, in select cases, decay times.

Scintillators are central to the performance of positron emission tomography (PET) scanners. Lutetium-yttrium oxyorthosilicate (LYSO) has emerged as a leading scintillator due to its appropriate attributes for PET, yet it presents challenges such as high cost. In this study, we explore the potential of a novel ultra-dense transparent ceramic scintillator: lutetium oxide doped with lanthanum as an alternative for PET imaging. Ceramic scintillator samples with a size of 3 × 3 × 5 mm 3 were fabricated and compared with LYSO in terms of timing and energy resolution characteristics. The experimental setup involved two Near Ultraviolet High-Density Silicon Photomultipliers (NUV-HD SiPMs) connected to readout boards for extracting timing and energy signals. Energy resolutions and coincidence timing resolutions (CTRs) at different thresholds were calculated using digitized waveforms. Results indicated comparable light yield and energy resolution between LYSO and the ceramic scintillators. In terms of timing properties, the new ceramic scintillators showed appropriate timing performance for the use in Time-of-Flight (TOF) PET applications.

In this paper we are reporting on the progress and scale up of the Cerium-doped Tl2LiYCl6 (TLYC) scintillator. The boules up to 2-inch in diameter were grown by the Bridgman method at Fisk University. The lapped and polished samples were characterized for their scintillation properties including gamma-neutron dual mode detection. The 1.5-inch diameter samples demonstrated the energy resolution as good as 4.5% (FWHM) at 662 keV. The neutron peak due to 6Li(n, α) reaction was observed at 1.8 MeV gamma equivalent energy. Gamma-neutron pulse shape discrimination (PSD) was demonstrated with the Figure-of-Merit (FOM) of 1.8. The hermetically encapsulated samples were also evaluated for their radiation hardness with irradiation up to 1 Mrad.

The material requirements for gamma-ray detectors for medical imaging applications are multi-fold and sensitivity is often overlooked. High effective atomic number (Z eff ) Cherenkov radiators have raised the attention in the community due to their potential for harvesting prompt photons.A material with one of the highest Z eff and thus low gamma-ray attenuation length is thallium chloride (TlCl). By doping TlCl with beryllium (Be) or iodine (I), scintillation photons are produced upon gamma-ray interaction on top of the prompt Cherenkov photon signal. We report on the performance of TlCl: Be, I for fast timing applications, in particular time-of-flight positron emission tomography and prompt gamma imaging.Scintillation with a measured effective decay time of about 60 ns can be used for energy discrimination. On top, about 3% of the photons are produced promptly, forming a precise time-tagger. The timing capability of different geometries and readout strategies was investigated utilizing both populations of photons. For instance with silicon photomultiplier readout and absence of energy discrimination, a CTR of about 360 ps FWHM was measured for small (~3x3x3 mm³) TlCl crystals. With energy discrimination and time walk correction this figure improves to 230 ps, while with micro-channel plate photomultiplier tube readout even sub-100ps timing performance is possible.

Activated by charge transfer luminescence ytterbium doped lutetium oxide (Lu 2 O 3 :Yb) is an ultrafast scintillator. Combined with its high density (9.4 g/cm 3 ), it is a promising ultrafast inorganic scintillator for breaking the picosecond (ps) timing barrier for future high energy physics (HEP) time of flight (TOF) and ultrafast calorimetry applications. We report gamma rayinduced radiation damage in Lu 2 O 3 :Yb and Lu 2x Y 2(1-x) O 3 :Yb ceramic samples fabricated in Radiation Monitoring Devices Inc. All samples show X-ray excited luminescence peaked at 370 nm with an ultrafast decay time of one ns observed by using a microchannel plate-photomultiplier tube-based test bench. Pulse height spectra and light output as a function of integration time were measured before and after irradiation up to 20 Mrad. This is a part of an on-going investigation for developing ultrafast inorganic scintillators for future HEP experiments.

Multi-mode radiation detectors capable of gamma-ray spectroscopy are needed as the next generation of radiation portal monitors (RPMs) at ports of entry along the nation’s borders. Inorganic scintillators are difficult to procure in large volumes needed for portal monitoring and their cost can be prohibitive. Plastic scintillators that are currently used in RPMs can be produced in very large volumes at low cost, but due to their low density and low effective atomic number (Zeff), are limited to gross gamma-ray counting. To meet the performance and size requirements needed for RPMs, RMD and SNL are developing metal-loaded Polymer Organic Glass Scintillators (POGS). In this work we show progress in the formulation chemistry of metal-loaded POGS. We have fabricated high quality samples of tin-loaded POGS up to 2" diameter sizes resulting in light yields >10,000 photons/MeV and energy resolutions <10 % @ 662 keV. We have performed successful fusing of individual tin-POGS elements, performed accelerated aging studies, and GEANT4 simulations to establish a working model of tin-POGS, and compared gamma-ray and neutron efficiencies of OGS, tin-loaded POGS and EJ-200.