SrI₂:Eu²⁺ and CaI₂:Eu²⁺ are two of the brightest known scintillators, but they both suffer from self-absorption. Their respective undoped isostructural compounds EuI₂ and YbI₂ are not suitable for scintillation due to the additional problem of concentration quenching. These compounds can however be doped with Sm²⁺ to turn them into near-infrared emitting scintillators, with the additional benefit that the self-absorption probability of the Sm²⁺ emission is low. Here, the scintillation properties of SrI₂:1%Sm²⁺, EuI₂:4%Sm²⁺, and YbI₂:1%Sm²⁺ single crystals are assessed which were grown by the vertical Bridgman technique. SrI₂:1%Sm²⁺ and EuI₂:4%Sm²⁺ fall within the ideal wavelength range for detection with silicon based photodetectors and are spectroscopically very similar to each other. However, the key difference is that the scintillation decay time of EuI₂:4%Sm²⁺ is 1.1μs, much shorter than the 1.8μs of SrI₂:1%Sm²⁺. Both SrI₂:Sm²⁺ and EuI₂:Sm²⁺ are identified as interesting candidates for further optimisation in the development of near-infrared emitting scintillators.

X-ray photon-counting detectors (PCDs) are a rapidly developing technology. Current PCDs used in medical imaging are based on CdTe, CZT, or Si semiconductor detectors, which directly convert X-ray photons into electrical pulses. An alternative approach is to combine ultrafast scintillators with silicon photomultipliers (SiPMs). Here, an overview is presented of different classes of scintillators, with the aim of assessing their potential application in scintillator-SiPM based indirect X-ray PCDs. To this end, three figures of merit (FOMs) are defined: the pulse intensity, the pulse duration, and the pulse quality. These FOMs quantify how characteristics such as light yield, pulse shape, and energy resolution affect the suitability of scintillators for application in indirect PCDs. These FOMs are based on emissive characteristics; a fourth FOM (ρZ_eff^3.5) is used to also take stopping power into account. Other important properties for the selection process include low self-absorption, low after-glow, possibility to produce sub-mm pitch pixel arrays, and cost-effectiveness. It is shown that material classes with promising emission properties are Ce³⁺- or Pr³⁺-doped materials, near band gap exciton emitters, plastics, and core-valence materials. Possible shortcomings of each of these groups, e.g., suboptimal emission wavelength, nonproportionality, and density, are discussed. Additionally, the engineering approach of quenching the scintillator emission, resulting in a targeted shortening of the decay time, and the possibility of codoping are explored. When selecting and/or engineering a material, it is important to consider not only the characteristics of the scintillator but also relevant SiPM properties, such as recharge time and photodetection efficiency.

While Eu²⁺ → Eu³⁺ energy transfer is well known, in this study the energy transfer from Eu³⁺ to Eu²⁺ is reported for the first time. The predominant condition for Eu³⁺ → Eu²⁺ energy transfer is a Eu²⁺ 4f⁵5d band at lower energy than the position of the Eu³⁺ 4f⁶[⁵D₀] level, which is fulfilled in Eu-doped CaO. X-ray powder diffraction, Eu Mössbauer spectroscopy and optical absorption measurements are employed to determine the Eu³⁺ and Eu²⁺ concentrations in the prepared CaO:1at.%Eu samples. Synthesis in an H₂/N₂ atmosphere and addition of graphite powder as a reducing agent to the starting mixture are found to result in respective Eu³⁺ and Eu²⁺ concentrations of 0.6–0.7% and 0.3–0.4%. For this sample, the Eu³⁺ → Eu²⁺ energy transfer efficiency is estimated to be high (> 90%). This is explained by the high oscillator strength of the 4f⁷ → 4f⁶5d excitation transition of the Eu²⁺ ion to which energy is transferred. As the Eu²⁺ 4f⁵5d band lies below the Eu³⁺ 4f⁶[⁵D₀] level, Eu³⁺ does not act as a killer center for the near-infrared (NIR) Eu²⁺ emission at about 720 nm. Therefore, a full reduction of Eu³⁺ is not required to attain a high quantum efficiency. Implications of the demonstrated Eu³⁺ → Eu²⁺ energy transfer for application of long wavelength Eu²⁺ phosphors are discussed.

Recent research activity on Sm²⁺-doped compounds has significantly increased the amount of available data on 4f⁵5d → 4f⁶ decay times. This enabled the systematic comparison of spectroscopic and time resolved luminescence data to theoretical models describing the interplay between the 4f⁵5d and 4f⁶[⁵D₀] excited states on the observed decay time. A Boltzmann distribution between the population of the excited states is assumed, introducing a dependence of the observed 4f⁵5d → 4f⁶ decay time on the energy gap between the 4f⁵5d and 4f⁶[⁵D₀] levels and temperature. The model is used to interpret the origin of the large variation in reported 4f⁵5d → 4f⁶ decay times through literature, and links their temperature dependence to applications such as luminescence thermometry and near-infrared scintillation. The model is further applied to the analogous situation of close lying 4f^n-15d and 4fⁿ states in Eu²⁺ (⁶P_7/2) and Pr³⁺ (¹S₀).

LaBr₃:Ce³⁺ is a compound with excellent scintillation properties, but its ultraviolet emission does not match well with the detection efficiency curves of silicon based photodetectors. In this work, Sm²⁺ is studied as an activator for LaBr₃ as its near-infrared emission can be detected with close to 100% efficiency by such photodetectors. LaBr₃:Sm²⁺ single crystals were grown with and without co-doping of Ce³⁺ or Pr³⁺. The samples were studied by means of X-ray excited and photoluminescence spectroscopy at temperatures between 10 K and 300 K. Their spectroscopic properties are compared to LaBr₃:Ce³⁺ and LaBr₃:Eu²⁺. The effect of using Ce³⁺ or Pr³⁺ as scintillation sensitiser for Sm²⁺ is assessed. It is found that energy transfer from host to Sm²⁺ greatly improves upon Ce³⁺ co-doping, but the quenching temperature of the Sm²⁺ emission decreases. The quenching mechanism of both the Ce³⁺ and Sm²⁺ emission in LaBr₃ is elaborated on. Furthermore, the effect of charge compensating defects on the light yield and spectroscopic properties is discussed.

The benefits of doping Cs₄EuBr₆ and Cs₄EuI₆ with Sm²⁺ are studied for near-infrared scintillator applications. It is shown that undoped Cs₄EuI₆ suffers from a high probability of self-absorption, which is almost completely absent in Cs₄EuI₆:2% Sm. Sm²⁺ doping is also used to gain insight in the migration rate of Eu²⁺ excitations in Cs₄EuBr₆ and Cs₄EuI₆, which shows that concentration quenching is weak, but still significant in the undoped compounds. Both self-absorption and concentration quenching are linked to the spectral overlap of the Eu²⁺ excitation and emission spectra which were studied between 10 K and 300 K. The scintillation characteristics of Cs₄EuI₆:2% Sm is compared to that of the undoped samples. An improvement of energy resolution from 11% to 7.5% is found upon doping Cs₄EuI₆ with 2% Sm and the scintillation decay time shortens from 4.8 s to 3.5 s in samples of around 3 mm in size.

Fast energy transfer from Yb²⁺ to Sm²⁺ is a requirement when using Yb²⁺ as a sensitiser for Sm²⁺ emission for near-infrared scintillator applications. This cannot be achieved through dipole-dipole interactions due to the spin-forbidden nature of the involved Yb²⁺ transition, making the rate of energy transfer too slow for application. This work explores whether exploiting the exchange interaction by increasing the Yb²⁺ concentration to 99% is an effective way to increase the rate at which energy is transferred from Yb²⁺ to Sm²⁺. The scintillation characteristics of CsYbBr₃:1%Sm, CsYbI₃:1%Sm and YbCl₂:1%Sm single crystals were studied through ¹³⁷Cs excited pulse height spectra, X-ray excited decay and X-ray excited luminescence spectra. An energy resolution of 7% and a light yield of 30,000 ph/MeV was achieved with CsYbI₃:1%Sm. Photoluminescence spectroscopy and decay studies were performed to study the band structure and relaxation dynamics.

The feasiblity of using Yb²⁺ as a scintillation sensitiser for CsBa₂I₅:Sm²⁺ near-infrared scintillators has been assessed. CsBa₂I₅ samples with concentrations ranging from 0.3% to 2% Yb²⁺ and 0–1% Sm²⁺ have been studied. The scintillation properties have been determined and the dynamics of the scintillation mechanism have been studied through photoluminescence measurements. Radiationless energy transfer between Yb²⁺ ions plays a key role in increasing the ratio between the spin-forbidden and spin-allowed emission with increasing Yb²⁺ concentration in samples where Yb²⁺ is the only dopant. In samples co-doped with Sm²⁺, the Yb²⁺ 4f¹³[F2_7/2]5d₁[LS] and 4f¹³[F2_7/2]5d₁[HS] states both serve as donor states for radiationless energy transfer to Sm²⁺ with a rate of energy transfer that is inversely proportional to the luminescence lifetime the respective donor states. At a Sm²⁺ concentration of 1%, 85% of the Yb²⁺ excitations are transferred to Sm²⁺ through radiationless energy transfer. Almost all of the remaining Yb²⁺ emission is reabsorbed by Sm²⁺, resulting in nearly complete energy transfer.

We have designed and modeled a novel optical system composed of a Laue lens coupled to an x-ray tube that produces a focused beam in an energy range near 100 keV (? = 12.4 picometer). One application of this system is radiation therapy where it could enable treatment units that are considerably simpler and lower in cost than present technologies relying on linear accelerators. The Laue lens is made of Silicon Laue components which exploit the silicon pore optics technology. The lens concentrates photons to a small region thus allowing high dose rates at the focal area with very much lower dose rates at the skin and superficial regions. Monte Carlo simulations with Geant4 indicate a dose deposition rate of 0.2 Gy min-1 in a cylindrical volume of 0.7 mm diameter and 10 mm length, and a dose ratio of 72 at the surface (skin) compared to the focus placed 10 cm within a water phantom. Work is ongoing to newer generation crystal technologies to increase dose rate.