Recent research in the field of phosphor and scintillator materials and related detectors is reviewed. After a historical introduction the fundamental issues are explained regarding the interaction of x-ray radiation with a solid state. Crucial parameters and characteristics important for the performance of these materials in applications, including the employed measurement methods, are described. Extended description of the materials currently in use or under intense study is given. Scintillation detector configurations are further briefly overviewed and selected applications are mentioned in more detail to provide an illustration.

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Scintillation detectors for x-rays

A review of medical diagnostic imaging methods utilizing x-rays or gamma rays and the application and development of inorganic scintillators is presented.

https://iopscience.iop.org/article/10.1088/0031-9155/47/8/201/pdf

Inorganic scintillators in medical imaging.

Recent progress in quantum cutting phosphors

A review is presented of recent inorganic scintillator R&D. Attention is focused on Ce doped gamma-ray and thermal-neutron scintillators for counting applications.

Inorganic-scintillator development

Using epitaxy and the misfit strain imposed by an underlying substrate, it is possible to elastically strain oxide thin films to percent levels—far beyond where they would crack in bulk. Under such strains, the properties of oxides can be dramatically altered. In this article, we review the use of elastic strain to enhance ferroics, materials containing domains that can be moved through the application of an electric field (ferroelectric), a magnetic field (ferromagnetic), or stress (ferroelastic). We describe examples of transmuting oxides that are neither ferroelectric nor ferromagnetic in their unstrained state into ferroelectrics, ferromagnets, or materials that are both at the same time (multiferroics). Elastic strain can also be used to enhance the properties of known ferroic oxides or to create new tunable microwave dielectrics with performance that rivals that of existing materials. Results show that for thin films of ferroic oxides, elastic strain is a viable alternative to the traditional method of chemical substitution to lower the energy of a desired ground state relative to that of competing ground states to create materials with superior properties.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769414000013

Elastic strain engineering of ferroic oxides

A new scintillator, LuAlO/sub 3/:Ce, has been found to have properties that put it in the forefront when stopping power and timing is of importance. This material, an extension of other well known cerium-doped scintillators, the yttrium-based orthoaluminate and garnet, can be readily pulled from the melt, and displays particularly promising performance. We summarize the results achieved when Y is replaced by Lu in this class of oxide crystals. >

LuAlO/sub 3/:Ce and other aluminate scintillators

Scintillation properties of selected oxide monocrystals activated with Ce and Pr

In this paper we report measurements of x-ray- and vacuum UV-excited luminescence, luminescence excitation spectra and time profiles, low temperature thermoluminescence and isothermal decays as well as x-ray- and gamma-excited scintillation time profiles and scintillation light yields at various temperatures on BaF2:Ce and, for reference, on undoped BaF2, two well known scintillator materials. For the first time we find that all these results can be consistently interpreted in the frame of a model that includes Ce-related recombination centres and several charge `traps'. The charge trapping at most of these `traps' has its origin in self-trapping and trapping of holes at regular (Vk) and interstitial (H) fluorine sites. We have identified and characterized two different modes of thermally activated Vk release that precede radiative recombination at Ce sites. These two modes are responsible for a 7 ns rise time and a slower 114 ns components in the scintillation time profile at room temperature (297 K) that produce about 67% of the scintillation light detected within a 0.5 µs time window. The remaining 33% is due to a prompt component decaying with the Ce3+ radiative lifetime of about 30 ns that originates from the direct recombination of charge carriers at Ce3+ ions. We also estimate that scintillation light loss due to even slower components, at 23.1 µs (H centres), 1.1 ms and 7 ms (VkA and VkA' centres), exceeds at least threefold the amount of light emitted in the 0.5 µs time window. Therefore in addition to their well known role as defect centres actively participating in the formation of stable radiation damage centres these species are also involved in the radiative recombination process itself. The perspectives of improvements in performance of the BaF2:Ce scintillator are also briefly discussed.

Radioluminescence and recombination processes in BaF2:Ce

In this paper we report measurements of thermoluminescence in the temperature range of 20–370 K, isothermal decays, pulsed vacuum ultraviolet and γ-excited luminescence time profiles at various temperatures on cerium-activated orthoaluminate (LuAlO3:Ce, LuAP), a new and promising scintillator material. We demonstrate that results of all these experiments can be consistently explained by assuming a recombination mechanism of scintillation light production in the LuAP scintillator. Using a simple first-order kinetic model that includes Ce3+ ions as recombination centres and a number of electron traps, we extract from experimental data the basic trap parameters (energy depths and frequency factors). Consequently we identify nine traps that are responsible for undesired features of the LuAP scintillator, such as a reduced scintillation light output, a relatively long scintillation rise time and slow scintillation components (afterglow) at room temperature. We demonstrate that some of these traps are responsible for large variations of the scintillation light yield with temperature as reported earlier. Although the deepest traps do not alter scintillation time profiles, they are responsible for a significant scintillation light loss and are, therefore, detrimental to scintillation performance of the material. We observe that there is an apparent correlation between trap depths and frequency factors for at least five of the traps that may fit some more general pattern involving various groupings of all the traps. This, in turn, would indicate that traps in LuAP are not unrelated and are due, most likely, to a series of native defects in the LuAP crystal structure. Although the specific identity of traps remains unknown, the performance of the LuAP scintillator is now, in practical terms, fully understood and can be described numerically at any temperature using a model and a set of parameters given in this paper. It is clear that any major improvement of the material would require that traps are eliminated or that their influence on the scintillation process is minimized.

https://iopscience.iop.org/article/10.1088/0953-8984/13/42/318/pdf

Electron traps and scintillation mechanism in LuAlO3:Ce

In recent years the scintillation properties of several cerium-doped rare earth oxyorthosilicate scintillators, Ln/sub 2/SiO/sub 5/:Ce where Ln = Y, La - Lu, have been reported and, in some cases, extensively studied. In addition, binary and ternary compounds such as (Lu,Y)/sub 2/SiO/sub 5/:Ce, (Lu,Gd)/sub 2/SiO/sub 5/ and (Lu,Y,Gd)/sub 2/SiO/sub 5/:Ce have been reported. All of these crystals have either monoclinic P or C structures with characteristic SiO/sub 4/ tetrahedra and trivalent cations occupying two unique crystallographic positions. The trivalent cerium activator ions are assumed to occupy the cation lattice sites and possibly interstitial positions as well. The excited 5d state of Ce/sup 3+/ is split into 3 observable levels with luminescence emission occurring only from the lowest 5d level to the 4f ground state (/spl sim/3 eV) with a Stokes shift of /spl sim/ 0.5 eV. The band gap is about 6 eV, and the index of refraction is close to 1.8, with some variation according to crystallographic axes. Despite these similarities, important differences remain among the crystals including scintillation efficiency, decay time, rise time, and afterglow. In this paper, we report thermoluminescence measurements between 10K and 350K that allow the determination of trapping levels that may influence scintillation properties. The thermoluminescence data shows that the various scintillators compositions have surprisingly dissimilar sets of traps that may at least partially explain some of the differences in their scintillation properties.

D. Wisniewski

Papers

LuAlO/sub 3/:Ce and other aluminate scintillators

Scintillation properties of selected oxide monocrystals activated with Ce and Pr

Radioluminescence and recombination processes in BaF2:Ce

Electron traps and scintillation mechanism in LuAlO3:Ce

Thermoluminescence and scintillation properties of rare earth oxyorthosilicate scintillators