Novel materials for radiation detectors

Resume : The development of ionizing radiation detection system over large areas is a crucial task in different fields such as nuclear waste management, radiotherapy or personal protection devices. Despite the excellent detecting performance exhibited by the inorganic materials (e.g. a-Se, CZT…), the increasing quest for flexible, portable, low cost and low power consumption sensors pushed the scientific community to look for alternative materials and technologies able to fulfill these new kinds of requirement. The recent impressive progress in the field of organic semiconductors and perovskites (both in terms of material technologies and device progress) assessed their large potential for the development of direct detection of ionizing radiation, as they proven the ability to detect X-ray, coupled to unique properties such as solution-processability, cost-effective fabrication, scalability to large area systems. and the possibility to be deposited on non-conventional substrates. Despite the implementation of top performing detectors in form of single crystals and thick films, they do not fully exploit the solution processability potential, as the scalability over large area on flexible plastic substrates. Indeed, flexible X-ray detectors, employing active films with a thickness of tens of microns or less, are lightweight devices that can be operated at low-voltages and strongly limit the use of toxic materials and precursors. Moreover, flexible detectors are extremely appealing because they surface conformability open to innovative and unprecedent functionalities in markets such as wearable electronics, smart walls, nuclear waste pipe testing for industrial and citizens’ security, cultural heritage, and space missions. Recent results on thin film direct detectors fabricated from solution-grown organic semiconductors and perovskites will be reported with a focus on the role of crystalline grains and defects in the modelling of the observed X-ray detection processes. Moreover, the possibilities and the limits of flexibility and human-tissue equivalence will be critically discussed for their applications in medical dosimetry. Finally, the latest technological achievements and the new challenges opened by the possibility to detect new radiation sources (e.g. protons beams, high energy gammas) will be described.

Resume : Radiation therapy is effective for cancer treatment, but the risks of malignancies induced by peripheral radiation in healthy tissues surrounding the target volumes represents a serious concern. Inhomogeneities in the target (muscles/bones/adipose tissue), positioning of the patient, and even minor movements, can alter the received dose and targeted volume and thus affect the outcome of the procedure. Therefore, being able to accurately measure radiation doses is a critical aspect of diagnostics and treatment. In this presentation I will introduce a new type of radiation dosimeter, the RAD-OFET (RAdiation Detector based on Organic Field-Effect Transistor), which can measure with high precision the dose being delivered and ensure that for nearby regions an acceptable level of low dose is being received. The operation of RAD-OFETs relies on monitoring the trap generation/annihilation in organic semiconductors using transistor characterization. Placement of the sensor directly onto the human body, coupled with the similarity in the Z-number between the electronically active layers and the human tissue, allows for direct measurement of the radiation dose, eliminating the need for extensive data processing faced by current technologies. The direct consequence is a greater precision and lower complexity in the medical equipment. We found that our RAD-OFETs are sensitive to doses relevant to many radiation treatment procedures and are robust when incorporated into conformal large-area electronic applications. Their adoption in clinical settings will facilitate the application of therapeutic radiation with high precision, a process that will increase the effectiveness on treating cancerous tissue and minimize the impact on the surrounding healthy cells. These results uncover new opportunities for organic circuits that will improve the quality of healthcare through better, lower cost in vivo dose monitoring during radiation therapy. Organic Field‐Effect Transistors as Flexible, Tissue‐Equivalent Radiation Dosimeters in Medical Applications, Andrew M. Zeidell, Tong Ren, David S. Filston, Hamna F. Iqbal, Emma Holland, J. Daniel Bourland, John E. Anthony, and Oana D. Jurchescu, Adv. Sci. 7 (18), 2001522 (2020)

Resume : CsPbCl3/Br3 are inorganic perovskite materials with a potential in high energy radiation detection, coming from a combination of a lower mean ionization energy and a lower room temperature dark current than for conventional semiconductor materials as Si. The ability of easily depositing them on almost any kind of substrates, even flexible, adds further appeal to this emerging class of materials. In this work, we have grown magnetron sputtered inorganic perovskite CsPbCl3 thin films (500nm-1.5um) directly on plastic rigid and flexible substrates equipped with interdigitated electrodes to manufacture novel perovskite radiation detectors. The high quality of the deposited material, in terms of morphology, structure and stoichiometry has been verified by means of a thorough optical and electrical characterization. The devices have been tested for the first time at the Trento Proton Therapy Center experimental beam line under proton beams with energy in the range 83-220MeV and beam current in the range 1-10nA. Their current response at constant bias has been monitored in real-time configuration. The devices showed stable signals, high signal-to-noise ratios even at moderate applied voltages and a linear response of the current as a function of the proton flux. Performances in transmission have been also tested by placing the two devices along the same beam-line, one downstream the other. Experimental results will be presented and discussed in detail.

Resume : 1) Purpose/Objective. There is growing interest in the development of novel materials and devices capable of ionizing radiation detection for medical applications. To meet the demands of modern wearable detectors, such devices should be mechanically flexible, low-cost, and able to produce a response to radiation that is equivalent to human tissue. They require also a response with no dose rate dependence, linearity over a wide dynamic range of doses and stability with time and accumulated dose. At the Centre for Medical Radiation Physics, we assessed the potential use of organic photodetectors in the area of radiotherapy using direct and indirect detection technologies. 2) Material/Methods. We investigated devices fabricated with polymer donor P3HT and non-fullerene acceptor o-IDTBR to those fabricated with benchmark materials P3HT and PCBM. We report the optoelectronic and X-ray dosimetric response stability of a tissue equivalent organic photodetector fabricated with solution-based non-inks onto a flexible Kapton substrate. Indirect detection of X-rays was achieved via coupling of organic photodiodes with a plastic scintillator. The OPDs were tested prior to and following irradiation up to a total dose of 40kGy by a cobalt-60 gamma source. The OPDs response was tested by exposure to ionising radiation by indirect interaction by a plastic scintillator (BC430, Saint-Gobain Crystals)). Energy dependence of the sensor was tested by photon irradiation from kV to MV energy and MeV electrons using medical linear accelerators. In order to characterise the detector’s response under different irradiation dose rates, response was recorded by varying the source to surface distance from 90cm up to 300cm. Percentage depth dose and output factor measurements were obtained in reference conditions and compared to an IC (CC13). 3) Results. Although direct detection of MV x-ray photons is possible in thick BHJ devices (>400nm), the strong dose rate dependence (up to 250%) compromises their use for dosimetry in radiotherapy due to the distortion introduced in profiling and depth dose response. Sensors coupled to a scintillator instead displayed excellent response linearity with dose, with sensitivities to 6 MV photons of 263.4 ± 0.6 pC/cGy and 114.2 ± 0.7 pC/cGy recorded for 400 nm P3HT:PCBM and P3HT:o-IDTBR detectors, respectively. The samples after pre-irradiation, show that the response to irradiation by 6MV photons is consistent and reproducible when used with a scintillator. We observed no dose rate response variation despite the low mobility of organic semiconductors. Energy dependence measurements highlighted that the photodetectors were almost tissue equivalent, though an under-response in devices compared to water by up to a factor of 2.3 for photon energies of 30 to 200 keV was found due to the response of the plastic scintillator. The P3HT:o-IDTBR device exhibited a far higher stability to radiation, showing just an 18.4% reduction in performance when exposed to large radiation doses of up to 10 kGy. 4) Conclusion. We have successfully proven the linearity, dose rate and energy independence of organic based photodiodes combined to a plastic scintillator in reference conditions to 6MV photons. The radiation hardness of these devices up to 40kGy has proven their longevity for applications in radiation environments and when coupled with a plastic scintillator have the potential to be used as real-time dosimeters for radiotherapy treatment.

Resume : In recent decades organic electronics have entered mainstream use in consumer electronics found in households around the world. I will discuss radiation sensors based on organic semiconductor technology, and in particular applications related to detection of hadronic radiation. This includes α particles as well as protons, neutrons and elastic and inelastic scattering resulting from neutron interactions. Recent developments include the use of scalable solution processed organic semiconductor sensors. The potential for low cost fabrication and low power consumption make this type of radiation sensor attractive compared with the more costly inorganic variants that are commonplace in both industrial and scientific applications.

Resume : The ideal photodetector is the one able to detect every single incoming photon. In particular, in X-ray medical imaging, the radiation dose for patients can then approach its fundamentally lowest limit set by the Poisson photon statistics. Such near-to-ideal X-ray detection characteristics have been demonstrated with only a few semiconductor materials such as Si and CdTe; however, their industrial deployment in medical diagnostics is still impeded by elaborate and costly fabrication processes. Hybrid metal halide perovskites – newcomer semiconductors – make for a viable alternative owing to their scalable, inexpensive, robust, and versatile solution growth and recent demonstrations of single gamma-photon counting under high applied bias voltages. The major hurdle with perovskites as mixed electronic-ionic conductors, however, arises from the rapid material's degradation under high electric field, thus far used in perovskite X-ray detectors. Here we show that both near-to-ideal and long-term stable performance of perovskite X-ray detectors can be attained in the photovoltaic mode of operation at zero-voltage bias, employing thick and uniform methylammonium lead iodide (MAPbI3) single crystal (SC) films (up to 300 µm), solution-grown directly on hole-transporting electrodes. The operational device stability is equivalent to the intrinsic chemical shelf lifetime of MAPbI3, being at least one year in the studied case. Detection efficiency of 88% and noise equivalent dose of 90 pGyair (lower than the dose of a single incident photon) are obtained with 18 keV X-rays, allowing for single-photon counting, as well as low-dose and energy-resolved X-ray imaging. These findings benchmark hybrid perovskites as practically suited materials for developing low-cost commercial detector arrays for X-ray imaging technologies.

Resume : Metal halide perovskite single crystals show their potential as next-generation gamma-ray detectors due to high stopping power, defect-tolerance nature and large mobility-lifetime product. Among them inorganic CsPbBr3 is particularly attractive due to its high stability under large electric field and has been demonstrated to make the best resolution detectors so far. However, it is extremely hard to grow high performance CsPbBr3 crystals with high yield by solution method. Here, we report solution-grown Formamidinium Cesium Lead Bromide (FACsPbBr3) single crystals for room-temperature gamma-ray spectroscopy detector. High yield centimeter sized FACsPbBr3 crystals are grown from solution by employing low purity lead bromide (98%) precursors. The introduction of FA into CsPbBr3 enhance crystal quality by eliminating the immiscible phase gap between the growth temperature and room temperature. As-grown FACsPbBr3 exhibits a high resistivity of ~2 × 1010 Ω cm, and hole and electron mobility-lifetime products of ~3.2 × 10-3 cm2 V-1 and ~2.2 × 10-3 cm2 V-1, respectively. The FACsPbBr3 spectrum detectors with asymmetrical metal electrode configuration achieved a resolution of 3.7% or better for 662 keV 137Cs γ-rays. This method reduces manufacturing costs of the high-quality crystal production at least 10 times compared with melting methods due to the high yield, low cost precursor materials and energy efficient growth process.

Resume : Highly sensitive hard radiation detectors operating at room temperature are greatly desired for a broad variety of applications. The perovskite semiconductor CsPbBr3 exhibits a high spectral resolution of ?-rays at room temperature. CsPbBr3 is air stable, non-hygroscopic, and possesses high effective atomic number Z-eff of 65.9. We present an overview of the research progress on the bulk CsPbBr3 crystals studied so far and detector fabrication and characterization of single crystals. We discuss effective procedures to produce highly pure CsPbBr3 and the growth of single crystals with enhanced quality and larger size using the Bridgman method. These crystals show significantly reduced levels of impurity (9 ppm of the combined amount of 69 different impurity elements as measured by Glow Discharge Mass Spectrometry. CsPbBr3 perovskite?s defect tolerance tends to screen electrically activated defects, which enables the remarkable transport properties of both carriers (> 5x10-3 cm2/V for h; > 10-3 cm2/V for e). Our CsPbBr3 detector has shown remarkable energy resolving capability under both X and ? rays, particularly in achieving 3.9% (4.8 keV, FWHM) energy resolution for 122 keV 57Co ?-ray with good temporal stability. The hole lifetime in CsPbBr3 detector-grade single crystal was observed to be well over 25 ?s. 2x2 pixelated devices resolve 137Cs 662-keV ?-rays with 1.4% energy resolution, as well as other X- and ?-rays with energies ranging from tens of keV to over 1 MeV in unipolar hole-only sensing modes.

Resume : Perovskite - a wonder material for X-ray detection Ion migration is a well-known problem in perovskite materials. It causes baseline drift, lowers imaging resolution, accelerated decomposition and device performance degradation. In particular in X-ray detectors, the effect of ion migration is more obvious under working bias. The first principals study reveals that the 0D structure perovskite would show effectively reduced ion migration between neighbouring unit cells compared with the popular 2D and 3D perovskites. A nucleation-controlled strategy is developed to grow superior inch-sized high-quality 0D-structured lead-free (CH3NH3)3Bi2I9 perovskite single crystals (MA3Bi2I9 PSCs) with significantly lower ion migration, much reduced dark current and better environmental stability compared to other perovskite materials, enabling us to design and fabricate a new type of 0D-structured lead-free perovskite X-ray detector. It is found that the X-ray detectors show surprisingly high sensitivity, 15 times more than that of the state-of-the-art commercial ?-Se detectors, with very low detection limit that is desired for medical diagnostics, material inspection, etc. Furthermore, their response time is as short as 0.98 ms, the shortest among all X-ray detectors reported in literature, which may allow us to develop an X-ray screening system with reduced X-ray dose and improved resolution.

Resume : Radiation detection is of outmost importance in fundamental scientific research and applications including medical diagnostics, homeland security, environmental monitoring, and non-destructive inspection in industrial manufacturing. Lead halide perovskites (LHP) are rapidly emerging as high-Z materials for next generation of solution processable scintillators and photoconductors for ionizing radiation detection. To unlock their full potential as reliable and cost-effective alternatives to conventional scintillators, LHP urge to conjugate high scintillation yields with emission stability over prolonged exposure to high doses of ionizing radiation. To date, however, no definitive solution has been devised to suppress parasitic processes affecting the scintillation efficiency and kinetics in LHP and nothing is known of their radiation hardness for doses above a few kGy. Here, we demonstrate, for the first time, that CsPbBr3 nanocrystals (NCs) exhibit exceptional radiation hardness for 60Co gamma radiation doses as high as 1 MGy. Side-by-side spectroscopic and radiometric experiments further highlight that, despite their defect tolerance, scintillators based on standard CsPbBr3 NCs suffer from electron trapping in highly dense surface traps. This limitation is effectively overcome through a post synthesis surface fluorination treatment resulting in over 500% enhancement of the scintillation efficiency which becomes comparable to commercial scintillator crystals, while still retaining exceptional levels of radiation harness. These results have profound implications for the widespread of LHPs in radiation detection schemes for high-energy physics, nuclear monitoring, nuclear batteries and space-grade solar cells where high radiation hardness is critical for successful and long-running operation, as well as for ultra-stable scintillators in medicine, environmental/industrial monitoring and border control.

Resume : X-ray detection, which plays a significant role in medical and industrial fields, usually relies on inorganic scintillators to convert X-ray to visible photons. Though efficient scintillators are generally inorganics, many high quantum yield fluorescent organic molecules have been tested as scintillators; however, high-energy radiation can ionize molecules and create secondary electrons and ions. As a result, a high fraction of triplet states is generated, acting as scintillation loss channels. Here, we firstly developed a quantitative model to measure and calculate the generated singlet/triplet ratio for organic molecules under X-ray excitation, and found the triplets were dominating. Inspired by the development of organic light emitting diode, we realize that the X-ray induced triplet excitons can be exploited for emission through thermally activated up-conversion in a very rapid manner. We reported scintillators based on three thermally activated delayed fluorescence (TADF) molecules with different emission bands, which showed significantly higher efficiency than conventional anthracene-based scintillators. X-ray imaging with 16.6 lp mm-1 resolution was also demonstrated. These results highlight the importance of efficient and prompt harvesting of triplet excitons for efficient X-ray scintillation and radiation detection. Lastly, we will report our latest discovery that those knowledge learned from classic excitonic molecules can also be applied to perovskite scintillators with strong exciton binding, especially the perovskite scintillators based on self-trapped exciton emissions.

Resume : Organic/inorganic metal halide perovskites are promising semiconductor materials for the development of next generation radiation detectors due to their large absorption cross-section for high-energy ionizing photons, superior optoelectronic properties and low fabrication cost. However, growth of high quality and large-size single crystals are still challenging. In this study, we performed the growth and characterization of methylammonium lead halide single crystals for X-ray detector application. The inverse temperature crystallization (ITC) technique was employed to grow large-size bulk single crystals by repeated seeding. By optimizing the molar ratio, temperature and other conditions, up to 10mm x10mm x 5mm sized crystals were obtained. Both methylammonium lead iodide (MAPbI3) and methylammonium lead bromide (MAPbBr3) crystals were grown. These crystals were characterized using X-ray diffraction, which confirmed their (100) orientation. Transmittance measurements were performed to estimate their optical bandgaps, which were 1.51eV and 2.2eV for iodide and bromide crystals, respectively. Finally detectors were fabricated by evaporating metal electrodes on both sides of the crystal. Current-voltage measurements performed at room temperature showed low dark current. The resistivity of the iodide crystals was around 2x10^8 ohm cm, whereas that of bromide crystals was around 7x10^6 ohm cm. We further confirmed the X-ray sensitivity by irradiating the detectors with white X-rays. Details on crystal growth and the detector characterization as well as detectors’ performance stabilities will be presented at the meeting.

Resume : Advanced synthetic methods to generate nano- and microparticles hold great promise for controlling their properties and enable improved fabrication processes of functional materials and composites bridging several size ranges and properties from the nanoscale to the macroscale. Luminescent particles can be applied in a wide field of applications such as lighting, display, or scintillator technologies, provided their efficiency and density is sufficiently high. Here, we present our efforts to expand the synthetic toolbox towards high density caesium hafnium halide double perovskites and lutetium hydroxy chlorides. For generating Cs2HfF6 (CHF), Cs2HfCl6 (CHC), and Lu(OH)2Cl (LHC) particles, three different synthetic approaches were chosen: a heating-up synthesis in high boiling organic solvents, an emulsion synthesis, and a microwave assisted solvothermal synthesis, respectively. A heating-up approach was suitable to generate colloidal CHF nanocrystals at a relatively mild temperature of 160 ºC. The obtained nanopowder shows no intrinsic band to band recombination due to the large bandgap of Cs2HfF6. However, a likely defect-related emission is observed in pure CHF particles, which is enhanced by incorporating dopants such as Mn(II) and Eu(III) ions. Secondly, Cs2HfCl6 is an intrinsically fluorescent and radioluminescent material. A water in oil microemulsion synthesis was chosen to generate CHC with sizes scaled down to the microscale. The low solubility of the precursor salts in organic solvents, prevents the use of classical heating-up methods to synthesise CHC. The microparticles produced by this method showed the same absorption and emission profiles as reported bulk samples.[1] Lastly, Lu(OH)2Cl microparticles were obtained via a microwave assisted benzyl alcohol route with sizes ranging from 200 nm to 10 µm depending on the synthesis parameters reaction temperature and duration. Eu-doped samples showed the typical europium emission profile with a prominent red fluorescence.[2] All the obtained particles’ structure and morphology were analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) while their optical properties were characterized by photoluminescence spectroscopy (PL). We discuss how various high-density materials can be obtained as nano- and/or micropowders using a range of synthetic techniques. Based on these results, the appropriate synthetic route can be selected to ensure suitable properties like luminescence, dispersibility, morphology, or size, depending on specific applications. [1] R. Král, V. Babin, E. Mihóková, M. Buryi, V. V. Laguta, K. Nitsch and M. Nikl, J. Phys. Chem. C, 2017, 121, 12375–12382. [2] M. Fellner, A. Soppelsa and A. Lauria, Crystals, 2021, 11, 992.

Resume : Halide based perovskite materials have generated strong and growing attention in the last years for being hopeful candidates for optoelectronic device applications. With their defect tolerance nature, sufficient mobility–lifetime product, and simple crystal growth from solution, organic-inorganic (hybrid) halide perovskite materials bring an unprecedented opportunity for radiation detection in direct mode. They show good X-ray absorption, thanks to the presence of heavy elements. Additionally, single-crystalline halide perovskites exhibit no grain boundaries and possess low trap densities. We optimized the growth of methylammonium lead tribromide (MAPbBr3) single crystals (SC) in dimethylformamide (DMF) [1] for X-ray radiation detection [2]. SCs were grown via Modified Inverse Temperature Crystallization (MITC). Their optoelectronic properties were evaluated under X-ray illumination showing good sensitivity but large dark current. To overcome this problem, we looked at the impact of small modifications of the perovskite chemistry. In this work, two different approaches have been explored: a deliberate modification within the halide elements (anion engineering), and a possible unintentional alteration by the environment by growing SCs under different atmospheric conditions. Firstly, bromine was partially substituted by chlorine in MAPbBr3 leading to doped SCs of general formula MAPb(Br1−xClx)3. Crystals were grown from solutions with different Cl contents (%Clsol). Their respective optical band gaps Eg were determined via UV-visible spectroscopy as a function of Cl content. Their study by energy dispersive X-Ray analysis (EDX) via field emission scanning electron microscopy (FESEM), and X-ray powder diffraction (XRD) allowed determining the Cl content inside the crystal lattice (%Clcryst, x in the general formula above), and cell parameters a via LeBail refinement, indicating a typical solid solution behaviour. After crystal polishing and Cr electrodes deposition, the optoelectronic performance of 3 to 4 SC devices per composition were tested via J-V sweeps and response under X-ray radiation, hinting at the existence of a potential optimal composition for MAPb(Br0.85Cl0.15)3 at 50 V/mm. A decrease in hole mobility and potential rise in charge carriers lifetime were also observed, potentially linked to an improvement of μτ product for both charge carriers and trap passivation in the material [3, 4]. Secondly, two extreme atmospheric conditions were set while growing SCs: non-controlled laboratory conditions and dry Ar-atmosphere conditions in glovebox. H2O and O2 surface and bulk contents were evaluated by Fourier transform IR (FTIR), Raman and X-ray photoelectron (XPS) spectroscopies The difference in H2O and O2 content in crystal growth of both atmospheres do not show any significant impact on optoelectronic performance for X-ray detection, thus showing MAPbBr3 resilience toward the incorporation of humidity, O2 or CO2. REFERENCES [1] S. Amari, J.-M. Verilhac, E. Gros D’Aillon, A. Ibanez, and J. Zaccaro, ‘Optimization of the Growth Conditions for High Quality CH 3 NH 3 PbBr 3 Hybrid Perovskite Single Crystals’, Cryst. Growth Des., vol. 20, no. 3, pp. 1665–1672, Mar. 2020, doi: 10.1021/acs.cgd.9b01429. [2] O. Baussens et al., ‘An insight into the charge carriers transport properties and electric field distribution of CH 3 NH 3 PbBr 3 thick single crystals’, Appl. Phys. Lett., vol. 117, no. 4, p. 041904, Jul. 2020, doi: 10.1063/5.0011713. [3] X. Wang et al., « Ion Migrations in Lead Halide Perovskite Single Crystals with Different Halide Components », Phys. Status Solidi B, p. 1900784, févr. 2020, doi: 10.1002/pssb.201900784. [4] Z. Li et al., « Optoelectronic Dichotomy of Mixed Halide CH 3 NH 3 Pb(Br 1– x Cl x ) 3 Single Crystals: Surface versus Bulk Photoluminescence », J. Am. Chem. Soc., vol. 140, no 37, p. 11811‑11819, sept. 2018, doi: 10.1021/jacs.8b07560.

Resume : In this work, we present a novel sustainable and flexible composite detector with the capability to detect thermal neutrons. Its characteristics and physical properties make it suitable to be employed in scenarios where non-standard geometries are needed, for example, to optimize the detector performance and/or maximize the neutron detection efficiency. Currently, there is an important need to find alternatives to the 3He proportional counter, which has been the most used thermal neutron detector, because there is a severe shortage of 3He worldwide. In this work, the proposed neutron detector can represent one alternative. The scintillator is based on a fully enriched Lithium Tetraborate preparation (LiBO). The presence of 6Li and 10B in the material makes the scintillator highly sensible to thermal neutrons. The LiBO preparation is mixed with the inorganic scintillation powder ZnS:Ag (EJ-600, from Eljen Technology), all this mix is dispersed in a PDMS (Polydimethylsiloxane) matrix. Three samples were produced varying the percentage of the ZnS:Ag/LiBO mix in the PDMS matrix. The scintillator samples consisted of three flexible sheets (disks of 50 mm diameter by 0.4 mm thick.). The samples were coupled to a 2” Hamamatsu PMT (H1949-51). The neutron detection efficiency of the different scintillators was measured using a neutron flux generated at the CN facility of Legnaro National Laboratories (LNL), Italy, and with a 252Cf source. Optimum results were obtained compared to the performance of the commercial EJ-420 scintillator (from Eljen Technology), reaching up to 38% of relative neutron detection efficiency. Also, the intrinsic response of the proposed neutron detector to the presence of a gamma-ray field was evaluated using a set of calibration gamma sources. It was found an excellent gamma rejection ratio parameter of GRR<1e-11. Furthermore, the response of the detector was modeled using the Monte Carlo method (Geant4 v10.7), the comparison between the experimental and the simulation results shows that only 11 % of the neutron captures that occurred in the scintillator material are able to produce a measurable event. On the other hand, searching for a triple particle discrimination with a single hybrid unit, another experimental configuration of the LiBO/PDMS detector was studied by coupling the proposed scintillator to a commercial plastic scintillator (EJ-299 from Eljen Technology) with fast neutron/gamma discrimination capabilities. As a consequence, a compact detector capable of detecting and discriminating between gamma-rays, fast and thermal neutrons was obtained. Excellent discrimination was found between the three types of particles, with a FoM of 1.32±0.01 between gamma-rays and fast neutrons, and a FoM of 3.74±0.02 between thermal and fast neutrons. All the characteristics studied and presented in this work verify that the LiBO/PDMS detector represents a good candidate to replace standard thermal neutron detectors.

Resume : An emerging class of novel materials, that of the organic-inorganic hybrid semiconductors which is usually named perovskites, has been used along with polymer matrices for futuristic applications. Perovskites have the general formula AxMyXz (A is an amine, M is a metal and X a halide anion) and in many cases are semiconductors 3D, 2D, 1D or 0D. These exhibit excellent optoelectronic properties such as increased light absorption coefficient in the UV–vis range, high and balanced carrier mobility as well as long carrier diffusion length, non-linear optical effects and efficient luminescence. Within a decade, halide perovskite solar cells have shown power conversion efficiency higher than 22.7%. Moreover, these excellent optical and electronic properties place halide perovskites as outstanding candidates for other optoelectronic and electronic devices such as light emitting diodes (LEDs), photodetectors, detectors and lasing materials. Perovskites and related devices are prone to humidity degradation and an interesting approach to alleviate this is the incorporation of additives. In the present work, we present novel organic additives and polymers. Specifically, we present results regarding the addition of sugar – like molecules as well as addition of PMMA along with secondary polymers. These additives have been investigated along with 3D, 2D and quasi-2D hybrid organic-inorganic perovskite materials. Some preliminary results for related sensing devices are included.

Resume : Metal-organic Frameworks (MOFs) are family of materials which have harnessed tremendous research interests in gas adsorption, photocatalysis and drug delivery owing to their vast versatility and tunability, with seemingly endless metal and organic linker combinations. Initial research into MOFs and MOF composites as X-ray scintillators has shown promise with low detection limits and good image resolutions achieved.[1] One of the main advantages of MOFs is their versatility allowing for efficient design using heavy metal atoms within the framework to obtain excellent attenuation efficiencies. Importantly, a unique feature due to their high porosity, MOFs can be further used to encapsulate known scintillating materials and the ability to tune the radioluminescence (RL) wavelength, lifetimes and improve radioluminescent efficiencies. However, MOFs typical powder morphologies limits their attenuation efficiencies and thus RL due to their low packing densities. Recently, we have been able to develop high density MOF structures through advanced synthesis, engineering and densification of the building block constituents, providing an exciting opportunity for highly attenuating scintillators with unique multifunctionality from encapsulated species.[2] Herein, we propose the development of a novel highly dense MOF composite scintillator with excellent attenuation efficiencies, radiation hardness and tunable RL, alongside excellent stability under environmental stresses. To determine suitable candidate materials, we used database screening with defined parameters such a Z number, density and porosity. We then use a simple and scalable sol-gel approach to prepare highly dense, mechanically stable MOF scintillators with controllable shapes, size and thickness capable of eventually being directly integrated on top of pixelated detectors. We will further present a technique we have designed to characterise our scintillators in conjunction with industry to perform time resolved X-ray radioluminescence experiments in an accessible and user-friendly way. These TCSPC radioluminescence capabilities allows us to extract spectral and time resolved response of our scintillators and understand the fast kinetics with nanosecond resolution. With this work we can quickly screen materials to design next generation scintillators with fast response and spectral outputs on demand, opening new avenues for MOFs and other potential materials, impacting strategic fields such as healthcare and security. [1] a) Wang, J. et al. Matter. 2022, 5, 253; b) Wang, C. et al. J. Am. Chem. Soc. 2014, 136, 6171; c) Lu, J. et l. Dalton Trans., 2019, 48, 1722. [2] a) Tian, T. et al. Nat. Mater. 2018, 17, 174; b) Connolly, B. M. et al. Nat. Commun. 2019, 10, 2345.

Resume : Image sensors and artificial vision rely on spectral-selective photodetectors that respond to light of a certain wavelength range. Existing spectral-selective photodetectors require optical filters because most light-absorbing materials on the market are responsive to a broad spectrum. The employment of filters also prevents further vertical integration of the spectral-selective photodetectors, which is key to high-resolution imaging. In this study, we report an integrated spectral-selective perovskite/organic hybrid photodetector. The device responds to two distinct spectral ranges, depending on the bias across the device. The FAPbBr1.5Cl1.5 perovskite layer in the hybrid photodetector acts as an active layer for the short-wavelength light and a long-wavelength bandpass. The long-wavelength portion of the incoming light penetrates the perovskite layer and is absorbed by the poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]:[6,6]-phenyl-C71-butyric acid methyl ester organic bulk heterojunction. Both layers have a high light absorption coefficient and balanced electron/hole mobility, allowing efficient light absorption, carrier generation, and charge transport. A pristine poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b?]dithiophene))-alt-(5,5-(1?,3?-di-2-thienyl-5?,7?-bis(2-ethylhexyl)benzo[1?,2?-c:4?,5?-c?]dithiophene-4,8-dione)] interlayer is inserted between the two active layers to optically separate the detection windows of two active layers by absorping incoming light up to 680 nm after the absorption of perovskite layer. The interlayer also electronically creates an energy barrier to control the electron transport between the perovskite and to organic bulk heterojunction layers by the applied bias. By applying opposite biases, charge carriers from either of the active layers are extracted. In one example, we demonstrate the spectral-selective detection of blue (up to 480 nm) and near-infrared (680 to 780 nm) light with high responsivities of around 200 mA/W and 320 mA/W, respectively. The response range of the perovskite layer can be further adjusted by varying the halide composition of the material. With low spectral crosstalk, the perovskite/organic hybrid photodetector opens an exciting opportunity for integrated band-selective photodetection.

Resume : The direct detection of 5-MeV protons by flexible organic thin film devices is here reported for the first time [1]. Mechanical flexibility, portability, low cost of fabrication and human tissue equivalence are important properties which make this technology an excellent candidate for the development of wearable proton dosimeters to be employed in several areas such as in the medical field (i.e. during proton therapy treatments). In this application, ion beams are used for the controlled treatment of cancer by the delivery of the prescribed amount of dose to the tumor [2]. To assure the safe, effective and consistent radiation delivery, accurate monitoring of the beams and measurements of the absorbed dose are essential [3]. The here reported detectors have a 2-terminals co-planar architecture in which the organic semiconducting layer (i.e. bis(triisopropylgermylethynyl)-pentacene [4], [5]) is deposited from solution on the top of two interdigitated gold electrodes. The devices are fabricated onto polymeric foils assuring the full mechanical flexibility of the detecting system. This study illustrates how, once the proton beam impinges onto the sensor, because of the release of energy into both the organic semiconductors and polymeric foil, the detecting signal is formed by two different contributions. On one side, the energy absorbed from the proton beam by the organic semiconductor is registered by an instantaneous increase of current ruled by the photoconductive gain effect. This mechanism has been already reported and discussed in literature for X-Ray detection by organic thin film based devices [6], [7]. On the other side, the energy released in the plastic substrate generates an accumulation of trapped charges which induces an increase of the device conductivity proportionally to the integral dose absorbed by the system. Thus, exploiting the coupling between the organic semiconductor and the plastic substrate, it is possible to measure simultaneously and independently the real-time and the cumulative dose absorbed by the device recording two different information regarding the irradiation condition of the sensing system. References [1] I. Fratelli et al., Sci. Adv., vol. 7, no. 16, p. eabf4462, Apr. 2021, doi: 0.1126/sciadv.abf4462. [2] S. Giordanengo, et al., Phys. Medica, vol. 43, no. October, pp. 79–99, 2017, doi: 10.1016/j.ejmp.2017.10.013. [3] H. Paganetti, Proton Therapy Physics. CRC Press, 2012. [4] J. C. Sorli et al., Chem. Mater., vol. 31, no. 17, pp. 6615–6623, 2019, doi: 10.1021/acs.chemmater.9b00546. [5] A. Ciavatti et al., Adv. Funct. Mater., vol. 29, no. 21, pp. 1–8, 2019, doi: 10.1002/adfm.201806119. [6] L. Basiricò et al., Nat. Commun., vol. 7, no. 1, p. 13063, Dec. 2016, doi: 10.1038/ncomms13063. [7] I. Temiño et al., Nat. Commun., vol. 11, no. 1, pp. 1–10, 2020, doi: 10.1038/s41467-020-15974-7.

Resume : Early work of halide perovskites in solar cell research, where they were proven to be an effective family of materials for increasing power conversion efficiency, attracted the attention of researchers seeking new alternative radiation detectors because of the outstanding optoelectronic properties demonstrated by perovskites. Metal halide perovskites are ideal candidates for direct (semiconductor) and indirect (scintillator) radiation detection due to their high charge carrier mobility, long diffusion length, tunable bandgap, high quantum yield, high absorption coefficient, and flexible chemistry. Through their processing versatility, a broad range of hybrid and inorganic perovskites have been pursued to advance the field of radiation detector technologies over the past few years. These perovskites come in the form of nanoparticles/wires/sheets, polycrystalline thin films, and bulk single crystals; the latter two being most popular in ionizing radiation detection (e.g., alpha particle, X-ray, and gamma). Perovskite single crystals have been of particular interest due to better optoelectronic properties than their polycrystalline film counterparts (lower trap density and absence of grain boundaries). Fortunately, perovskite single crystals can be grown by low-cost and fast solution methods besides melt growth methods. This motivated our research group to explore an underutilized gel-growth technique to grow high quality perovskite single crystals. The gel-growth method is highly advantageous as it enables precise control of solute-diffusion, allows crystals to be grown at various temperatures and enables an experimental environment with lower concentration of material defects. In this regard, our research group successfully used the gel-growth technique to grow large, nonlinear optical crystals of bismuth chloride hydrate thiourea (BCHT). This progress initiated our effort to grow perovskite single crystals based on the diffusion of reagents in gel medium. One of the focuses of this work is on the bulk crystal growth of CsPb2Br5, which has been studied much less than the extensively studied CsPbBr3 perovskites. In this presentation, we report the first synthesis of CsPb2Br5 bulk single crystals by the gel-growth method. A series of materials characterization and detector tests will be presented and discussed. These efforts offer a new opportunity to explore the potential of CsPb2Br5 for ionizing radiation detection.

Resume : Cs2Ag0.6Na0.4In0.85Bi0.15Cl6 is an exciting new material in the field of lead-free inorganic perovskite scintillators. The material benefits from a large Stokes-shift, ensuring low levels of self-absorption. By tuning the ratios of the central cations for the double perovskite, the X-ray sensitivity and light yield has been optimised, as shown by Zhu et al. [1]. Bulk crystals of the material have been synthesised, and 5 mm diameter pellets pressed from powder obtained from ball milling the crystals have been made. The synthesis and pellet making procedures will be outlined, and a systematic investigation into the optimisation of this process will be discussed, including pressing pressure and milling speed. Results from both the initial crystals and processed material will be presented. Initial findings in the synthesis of Cs2Ag0.6Na0.4In0.85Bi0.15Cl6 nanoparticles of will be presented, and applications for these quantum dots explored, including the fabrication of nanocomposite materials. [1] Zhu, W., Ma, W., Su, Y., Chen, Z., Chen, X., Ma, Y., Bai, L., Xiao, W., Liu, T., Zhu, H. and Liu, X., 2020. Low-dose real-time X-ray imaging with nontoxic double perovskite scintillators. Light: Science & Applications, 9(1), pp.1-10.

Resume : Among the next-generation technologies based on organic semiconductors, photodetection is emerging as one of the most promising directions, with tremendous opportunities for applications in medical imaging, X-ray detection and wearable sensors.[1,2] With the performance of organic photodetectors (OPDs) currently rivalling silicon-based technology, on-going research aims to capitalize on the unique advantages of molecular semiconductors related to their tunable spectral response, flexible form factors and large-area solution-based fabrication. Polarization sensitivity represents yet another highly desirable functionality for advanced optical systems, but to-date remains largely unexplored due to the complexity of enabling polarized response without compromising the overall device performance.[3] Here we present OPDs capable of direct detection of linearly polarized light without any additional optical filtering. Polarization sensitivity is enabled by directional alignment of bulk-heterojunction active layers, which elegantly exploits the intrinsically anisotropic optoelectronic properties of molecular semiconductors. In particular, we present a scalable solution-based processing approach based on co-deposition with ‘crystallizable solvents’ by gas-assisted blade-coating.[4] Using benchmark materials such as P3HT, N2200 and IDTBR, we show that this versatile method is capable of orienting both polymer:polymer and polymer:NFA blends. To further improve device performance, we advance a novel post-processing scheme based on backfilling and plasma etching to enable dark-current suppression by over three orders of magnitude. Due to enhanced long-range order via directional orientation, the resulting polarized P3HT:N2200 OPDs exhibit an overall enhancement of specific detectivity compared to reference isotropic devices, as well as a 3-fold increase of 3 dB bandwidth up to 0.75 MHz – the highest reported for all-polymer OPDs to-date. The maximum polarization ratios of 3.5 are governed by material selection, with both broad- and narrow-band polarization response attainable. The application potential is demonstrated via polarimetry-inspired characterization of the photoelastic response of elastomeric films under tensile deformation, with relevance to advanced optical sensing in industrial and ‘soft electronics’ contexts. In summary, we present a promising ‘roadmap’ for the advance of next-generation OPDs with unique application-driven functionalities and microstructure-derived performance enhancements. The remaining challenges related to the hitherto incomplete understanding of photoconversion processes, extension to X-ray detection, and printing-based fabrication of complex devices arrays, will also be discussed. [1] Strobel et al., Flex. Print. Electron. 2019, 4, 043001; [2] Posar et al., Adv. Mater. Technol. 2021, 6, 2001298; [3] Altaqui et al., Sci. Adv. 2021, 7, eabe3196; [4] Perevedentsev et al., Adv. Optical Mater. 2022, accepted.

Resume : X-ray detection is widely used for scientific study, medical diagnosis, safety surveillance, and so on. Indirect detection means employing scintillators to first convert X-rays into visible light and then detected by a Si photodetector, while direct detection means directly converting incident X-rays into electricity by a semiconductor. In this presentation, I will talk about recent progress on exploring new metal halides for indirect and direct X-rays detection. Specifically, I will first discuss the synthesis, characterization, and imaging application of some copper halide based scintillators (CsCuI3, RbAgBr2, et. al.) with the emphasis on self-trapped exciton emission and its advantages for X-rays imaging. I will then discuss the use of lead halide perovskite for direct X-ray detection, mainly including single crystal growth, film fabrication, and TFT integration. Last, I will also briefly present the use of hybrid perovskites for mixed field radiation.

Resume : Here I will give insight into the uniqueness and challenges of charge transport in halide perovskites. Despite the tremendous efforts in the study of their electric properties, the free carriers - defects interactions and some critical defect properties are still unclear in methylammonium lead halide perovskites (MHPs). Here we use a multi-method approach to quantify and characterize defects in single crystal MAPbI3 giving a cross-checked overview of their properties. Time of Flight current waveform spectroscopy (ToF CWF) reveals the interaction of carriers with five shallow and deep defects. Photo Hall (PHES) and Thermoelectric effect spectroscopy (TEES) assess the defect density, cross-section, and relative (to the valence band) energy. The detailed reconstruction of free carrier relaxation through Monte Carlo (MC) simulation allow quantifying the lifetime, mobility, and diffusion length of hole and electron separately. We demonstrate that the dominant part of defects releases free carriers after trapping; this happens without non-radiative recombination with consequent positive effects on the photoconversion and charge transport properties. On the other hand, shallow traps decrease the drift mobility sensibly. Our results provide a trustworthy picture for future consideration on the defect properties in MAPbI3 thanks to the verification of our statements with multiple methods. Our results will be key for the optimization of the charge transport properties and defects in MHP and will contribute to the research aiming to improve their stability.

Resume : Methylammonium lead tribromide (MAPbBr3), thanks to its excellent optical and transport properties and its easy and low-cost fabrication process, is one of the most promising materials for next generation X- and gamma-ray detectors. Indeed, devices based on MAPbBr3 single crystals demonstrated record-breaking sensitivities, overcoming those of commercially available detectors based on cadmium zinc telluride and selenium. [1] Despite this, thorough studies on the effects of ionizing radiation on the photo-physical properties, trap states and long-term stability of the material are still lacking. Such studies are of fundamental importance to identify the factors that currently limit the device performance and indicate how the fabrication process should be controlled to overcome them. In this work, [2] we studied the effect of controlled X-ray dose on the opto-electronic properties of MAPbBr3 single crystals in the near-surface region by means of Surface-Photovoltage Spectroscopy (SPS) and Photoluminescence spectroscopy. We found that exposure to irradiation in the relevant dose range for diagnostic applications (from tens to hundreds of Gy) induces changes in the nature of the excitonic specie in the material. By means of X-ray Photoelectron Spectroscopy, we correlated this change to the formation of bromine vacancies upon irradiation, suggesting this effect is due to a modification in the dielectric screening of charge carriers. We also found this effect to be reversible upon storage in ambient conditions, indicating that reaction with environmental oxygen and water can be beneficial to the recovery of the material’s properties. [1] L. Basiricò, A. Ciavatti, B. Fraboni, Advanced Materials Technologies 2021, 6, 2000475. [2] G. Armaroli, L. Ferlauto, F. Lédée, M. Lini, A. Ciavatti, A. Kovtun, F. Borgatti, G. Calabrese, S. Milita, B. Fraboni, D. Cavalcoli, ACS Appl. Mater. Interfaces 2021, acsami.1c16072.

Resume : Current room temperature semiconductor detectors suffer from a high cost to grow and/or complicated pulse processing schemes to achieve energy resolutions around 1% at 662 keV. Metal halide perovskites (MHPs) are a unique material class that holds promise to advance solid-state radiation sensing. MHPs exhibit a suitable band gap for room temperature operation and they can be grown in solution, which suggests that their cost of production may be an order of magnitude or more cheaper than current commercial options. However, MHPs are still relatively new as applied to radiation sensing, and many questions remain. These include defect formation during growth, potential defect passivation through dopants, and ionic conductivity and long-term stability. Furthermore, additional advances in post-growth processing techniques is required, including appropriate chemomechanical processing, surface passivation, metallization scheme, and packaging. In this presentation, the progress and achievements made at the University of Tennessee in developing MHPs for radiation sensing will be presented, and a discussion of remaining challenges will be discussed.

Resume : Lead halide perovskites have made remarkable progress in optoelectronics due to the excellent and unique optoelectronic properties. However, the instability and the toxicity of lead-based perovskite have greatly hindered the commercial applications. Alternatively, lead-free perovskites with good stability and nontoxicity thus have fascinated strong attention for direct radiation detection. In this review, we summarize the current research status of X-ray detector based on lead-free halide perovskites. First, the synthesis methods of lead-free perovskites including film and single crystals are discussed. In addition, we also presented the properties of these materials and the detectors, which could give a better understanding and designing satisfactory devices. Finally, conclusions and perspectives for developing high-performance lead-free perovskite radiation detectors are also provided.

Resume : The demand for large area, low cost and flexible high-energy radiation detection systems for medical imaging and public security, has pushed the research to develop novel detectors combining high sensitivity and low-cost fabrication processes. Recently, lead-halide perovskites emerged as a very promising novel class of materials for X- and gamma-ray detection. Their success can be attributed to the excellent perovskite optoelectronic properties. The presence of heavy elements like Pb, Br or I inside the lead-halide perovskite structure ensures a high effective atomic number, resulting in a high absorption coefficient in the ionizing radiation energy range. For efficient detection, high absorption is not enough, the charge transport properties of the material also play a crucial role. Diffusion length of over 1μm and long lifetime of carriers have been attributed to perovskite even in polycrystalline form. Moreover, these materials can be fabricated at low temperature from solution opening the possibility to low-cost, large-area and flexible detectors. Among the lead-halide perovskite, 2D layered hybrid perovskites have recently attracted increasing attention as active layers in LEDs and UV-Vis photodetector. 2D perovskites crystallize in a natural self-assembled quantum well-like structure and possess several interesting features. Even if the transport properties of 2D perovskite are inferior to the 3D perovskite, they are an interesting alternative because of their superior environmental stability. In this work, we fabricated a direct X-ray detector based on PEA2PbBr4 directly deposited on PET substrate with pre-patterned interdigitated metal contacts. The perovskite deposited by single step spin-coating forms a polycrystalline film with an average grain size of 33.5 ± 8.3 μm and thickness of 1.9 ± 0.8 μm. The transport properties and sensing performances of the devices were tested first under UV light and then under X-ray radiation in the energy range 40-150keV. Under pulsed laser, the material showed a fast response with a rise time of 147 ± 10 ns. The excellent X-ray detection performances are demonstrated by the two figures of merits: sensitivity and limit of detection. The devices showed higher sensitivity under 150kVp X-ray radiation with a value of 806 μC Gy−1 cm−2. The higher bandgap and lower defective states of the material, in comparison to 3D perovskite, lead to dark current values 2 x 10-13 A. The extremely low and stable dark current allowed to detect very low incident dose rates. We demonstrated a very low limit of detection with values down to 42 nGy/s. Additionally, the tested devices exhibit exceptionally stable current response under continuous operation at constant irradiation and bias with negligible dark current drift, assessing the material robustness. The performances of the X-ray detectors fabricated on flexible substrates have been tested under different bending conditions to assess the effect of the mechanical stress on the perovskite layer. [1] Lédée, F., Ciavatti, A., Verdi, M., Basiricò, L., & Fraboni, B. (2021). Ultra‐Stable and Robust Response to X‐Rays in 2D Layered Perovskite Micro‐Crystalline Films Directly Deposited on Flexible Substrate. Advanced Optical Materials, 2101145.

Resume : Two dimensional (2D) organic-inorganic hybrid halide perovskites have emerged as promising semiconducting materials due to their unique range of optoelectronic properties, facile fabrication strategies, structural diversity and superb ambient stability. Recently, 2D perovskite single crystal-based X-ray detectors attracted significant attention due to their high charge carrier mobility-lifetime (μτ) product, low defect density, suppressed ion migration, adjustable bandgap and low dosage X-ray imaging properties. Specifically, the bulky organic spacer used to template the inorganic lead-halide lattice in 2D perovskite can efficiently suppress ion migration even for large applied bias in X-ray detection. Here, in the present work, we have fabricated the Ruddlesden Popper (RP) phase n=1 layered 2D perovskite single crystals by incorporating linear chain butyl amine (BA) organic spacer into the inorganic PbI6- lattice through slow cooling method. The as fabricated centimetre size (~1 cm) (BA)2PbI4 2D perovskite crystals has shown large μτ product of 3.1×10-3 cm2V-1s-1. Furthermore, the 2D (BA)2PbI4 perovskite single crystal exhibits an intrinsic property with high bulk resistivity of 5.42 × 10^10 Ω cm, which is beneficial for low device noise for hard X-ray detection. Ultimately, the 2D perovskite single crystal has shown promising X-Ray response with high sensitivity of 17.8 μC/Gycm2. Our study indicates that (BA)2PbI4 2D perovskite single crystals have a great potential towards efficient X-Ray detection.