Needs, trends, and advances in scintillators for radiographic imaging and tomography

Scintillators are important materials for radiographic imaging and tomography (RadIT), when ionizing radiations are used to reveal internal structures of materials. Since its invention by R\"ontgen, RadIT now come in many modalities such as absorption-based X-ray radiography, phase contrast X-ray imaging, coherent X-ray diffractive imaging, high-energy X- and $\gamma-$ray radiography at above 1 MeV, X-ray computed tomography (CT), proton imaging and tomography (IT), neutron IT, positron emission tomography (PET), high-energy electron radiography, muon tomography, etc. Spatial, temporal resolution, sensitivity, and radiation hardness, among others, are common metrics for RadIT performance, which are enabled by, in addition to scintillators, advances in high-luminosity accelerators and high-power lasers, photodetectors especially CMOS pixelated sensor arrays, and lately data science. Medical imaging, nondestructive testing, nuclear safety and safeguards are traditional RadIT applications. Examples of growing or emerging applications include space, additive manufacturing, machine vision, and virtual reality or `metaverse'. Scintillator metrics such as light yield and decay time are correlated to RadIT metrics. More than 160 kinds of scintillators and applications are presented during the SCINT22 conference. New trends include inorganic and organic scintillator heterostructures, liquid phase synthesis of perovskites and $\mu$m-thick films, use of multiphysics models and data science to guide scintillator development, structural innovations such as photonic crystals, nanoscintillators enhanced by the Purcell effect, novel scintillator fibers, and multilayer configurations. Opportunities exist through optimization of RadIT with reduced radiation dose, data-driven measurements, photon/particle counting and tracking methods supplementing time-integrated measurements, and multimodal RadIT.Comment: 45 pages, 43 Figures, SCINT22 conference overvie

Neutron Imaging with Timepix Coupled Lithium Indium Diselenide

The material lithium indium diselenide, a single crystal neutron sensitive semiconductor, has demonstrated its capabilities as a high resolution imaging device. The sensor was prepared with a 55 μ m pitch array of gold contacts, designed to couple with the Timepix imaging ASIC. The resulting device was tested at the High Flux Isotope Reactor, demonstrating a response to cold neutrons when enriched in 95% 6 Li. The imaging system performed a series of experiments resulting in a <200 μ m resolution limit with the Paul Scherrer Institute (PSI) Siemens star mask and a feature resolution of 34 μ m with a knife-edge test. Furthermore, the system was able to resolve the University of Tennessee logo inscribed into a 3D printed 1 cm 3 plastic block. This technology marks the application of high resolution neutron imaging using a direct readout semiconductor

Bridgman Growth of Laser-Cooling-Grade LiLuF 4 :Yb 3+ Single Crystals

The first demonstration of solid-state laser cooling in fluoride grown by the Bridgman method is reported. We present advances in the Bridgman crystal growth of Yb3+-doped LiLuF4 (LLF:Yb) single crystals in a radio-frequencyheated furnace. COMSOL Multiphysics numerical simulations are used to investigate the thermal gradients within the crucible during the crystal growth. Optical spectroscopy and laser-cooling efficiency measurements of three LLF:Yb crystals as well as laser cooling of a LLF:5%Yb crystal in a double-pass geometry from room temperature to 195 K are reported. Solid-state laser cooling is only possible in materials having extremely high chemical purity and crystal quality. The vertical Bridgman method is well suited for the growth of high-quality crystals on the few gram scale, a quantity that is compatible with purification techniques that aim to exceed the 99.999−99.9999% purity that is typical of commercial precursor materials. The results demonstrate that the small-scale Bridgman growth of LLF:Yb in glassy-carbon crucibles is able to produce laser-coolinggrade crystals, opening a new route to produce high-performance materials for solid-state optical refrigerators and radiation-balanced lasers