Scintillation materials convert high-energy radiation into many visible photons and, in combination with a photodetector, are used as ionizing radiation detectors. Since the discovery of ionizing radiation, there have been intensive research efforts in finding new, better performing scintillators, resulting in the development of a large variety of scintillation materials. Each scintillation material is tailored to a specific application to have the best performance. With ever-increasing material demands set by the various applications of scintillators, the search for even better performing scintillation materials remains an active field. This thesis explores newavenues of scintillation materials research in order to find the next-generation of scintillation materials.@en

This paper provides an overview and interpretation of the spectroscopic data of the Bi3+ activator ion in 117 different inorganic compounds. The energies of the metal-to-metal charge transfer and the interconfigurational transitions of Bi3+ were collected from the archival literature. Using these energies, in combination with the electron binding energies in the host conduction and valence band, the binding energies in the 6s ground state and 6p excited state were determined relative to the vacuum level. The locations of the Bi3+ energy levels within the forbidden gap of the host compound provides valuable insight in the physical properties of the Bi3+ activator ion in different compounds.@en

An overview of the spectroscopic data on Tl+ and Pb2+ in 37 and 126 different inorganic compounds, respectively, is presented. Using the metal-to-metal charge transfer and A-band transition energies, the electron binding energies in the S01 ground state and P13 excited state of Tl+ and Pb2+ were determined relative to the vacuum level. By constructing vacuum referred binding energy (VRBE) diagrams that display the energy levels of the Tl+ and Pb2+ activator ion together with that of the host compound, insight in the luminescent properties of these activated compounds are gained. The obtained VRBEs of the electron in the S01 and P13 states of Tl+ and Pb2+ are compared to those of Bi3+.@en

An overview of the spectroscopic data on Tl+ and Pb2+ in 37 and 126 different inorganic compounds, respectively, is presented. Using the metal-to-metal charge transfer and A-band transition energies, the electron binding energies in the S01 ground state and P13 excited state of Tl+ and Pb2+ were determined relative to the vacuum level. By constructing vacuum referred binding energy (VRBE) diagrams that display the energy levels of the Tl+ and Pb2+ activator ion together with that of the host compound, insight in the luminescent properties of these activated compounds are gained. The obtained VRBEs of the electron in the S01 and P13 states of Tl+ and Pb2+ are compared to those of Bi3+.@en

The vacuum referred binding energy of the electron in the Bi2+ ground state has been determined in 15 different compounds. This shows that the electron binding energy in the ground state of Bi2+ is at lower (more negative) energy compared to the electron binding energy in the excited state of Bi3+. This means that electron transfer from the excited state of one Bi3+ to a neighboring Bi3+, forming a Bi2+−Bi4+ pair, acts as a quenching route for the Bi3+ emission. Electron back transfer in the Bi2+−Bi4+ pair is then suggested to be the origin for the frequently observed pair emission. This paper shows that vacuum referred electron binding energy diagrams can provide a unique physical insight in the properties of inorganic compounds.@en

YPO4 doped with Bi3+ and/or Tb3+ samples were prepared in air. X-ray excited luminescence measurements showed emission from isolated Bi3+ and Bi-pairs, and also emission from Bi2+ was observed. Based on the obtained spectroscopic data, the electron binding energies in the ground and excited states of Bi3+ and Bi2+ were placed inside the vacuum referred binding energy (VRBE) scheme, and this was used to explain the luminescence of bismuth doped YPO4. The VRBE scheme and additional thermoluminescence glow curves show that bismuth can act both as electron and as hole trap in YPO4.@en

YPO4 doped with Bi3+ and/or Tb3+ samples were prepared in air. X-ray excited luminescence measurements showed emission from isolated Bi3+ and Bi-pairs, and also emission from Bi2+ was observed. Based on the obtained spectroscopic data, the electron binding energies in the ground and excited states of Bi3+ and Bi2+ were placed inside the vacuum referred binding energy (VRBE) scheme, and this was used to explain the luminescence of bismuth doped YPO4. The VRBE scheme and additional thermoluminescence glow curves show that bismuth can act both as electron and as hole trap in YPO4.@en

YPO4 doped with Bi3+ and/or Tb3+ samples were prepared in air. X-ray excited luminescence measurements showed emission from isolated Bi3+ and Bi-pairs, and also emission from Bi2+ was observed. Based on the obtained spectroscopic data, the electron binding energies in the ground and excited states of Bi3+ and Bi2+ were placed inside the vacuum referred binding energy (VRBE) scheme, and this was used to explain the luminescence of bismuth doped YPO4. The VRBE scheme and additional thermoluminescence glow curves show that bismuth can act both as electron and as hole trap in YPO4.@en

The luminescence and scintillation properties of SrI 2 single crystals doped with 5% Eu 2+ and 0.05%, 0.2% and 0.5% Sm 2+ are evaluated. X-ray excited and photoluminescence measurements show energy transfer from excited Eu 2+ ions to Sm 2+ ions. At a concentration of 0.5% Sm 2+ , the luminescence consists almost entirely of 740 nm emission from Sm 2+ 5d-4f transitions. Co-doping SrI 2 :5% Eu 2+ with Sm 2+ provides a novel method to bypass the self-absorption problem encountered in large SrI 2 :Eu 2+ crystals and, at the same time, provides a unique near-infrared emitting scintillator with a light yield of approximately 40,000 photons/MeV. @en

Bismuth-doped Li2BaP2O7 was prepared in air, showing no Bi3+ or Bi2+ related photoluminescence even at 10 K. Absorption measurements showed that only Bi3+ was present in the as-prepared samples of which the emission is completely quenched. During X-ray excitation the characteristic deep-red radioluminescence of Bi2+ was observed. After X-ray irradiation, this red luminescence of Bi2+ could be excited optically, indicating that upon X-ray irradiation the Bi3+ is reduced to Bi2+. Based on the spectroscopic results, the Bi3+ and Bi2+ energy levels were estimated in a vacuum referred binding energy (VRBE) scheme and were used to explain the observed luminescence behavior. The VRBE scheme provided an interpretation for the commonly observed Bi3+ pair emission in bismuth-doped compounds. In the case of Li2BaP2O7:Bi it was used to explain the self-quenching behavior of Bi3+. These findings show that is possible to initially dope compounds with Bi3+ ions while only radioluminescence is observed from Bi2+ when the sample is exposed to high-energy excitation. This phenomenon can be used to fabricate new types of luminescent materials.@en