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Abstract In this review, modern insights into the synthesis and fundamental optical properties as well as applications of divalent lanthanides as spectral energy converters are discussed, which have especially evolved within the last two decades. An emphasis is put on the state-of-the-art knowledge about the luminescence of other divalent lanthanides than the most commonly applied representative Eu2+ and what special features in their luminescence arises due to the different number of 4f n electrons. In particular, the luminescence of Sm2+, Tm2+, Yb2+ and the less stable ions Nd2+ and Dy2+ will be regarded. A broad overview over the synthetic approaches to inorganic compounds based on divalent lanthanide ions will be conveyed as well and shall demonstrate the fascinating developments in the field of inorganic solid-state chemistry of the lanthanides. Besides their fundamental optical properties, also applications of the presented divalent lanthanide activators are discussed. Sm2+ is a well-known temperature and pressure sensor due to its interconvertible 4f‐4f and 5d-4f transitions. Moreover, nanocrystalline BaFCl:Sm2+ has an appreciable potential as novel X-ray storage phosphor. Tm2+ is interesting for upconversion in photovoltaic and lasing applications, whereas Yb2+ in halides may be a promising alternative to Eu2+ for scintillation materials and phosphors for light-emitting diodes (LEDs). Nd2+ and Dy2+ are currently emerging as novel near infrared (NIR)-absorbing and emitting activators that might become interesting as lasing ions. The spectroscopy of these unusual lanthanide ions provides many non-trivial features due to the excited two-open shell 4fn-15d1 configuration overlapping with the excited 4f n levels. Finally, a perspective for future developments will be given. [ABSTRACT FROM AUTHOR]

This work describes the relation between crystallite size and scintillation properties of Eu 3+ -doped yttrium oxide. Nanocrystalline Y 2 O 3 :Eu 3+ 1.0 mol% powders were prepared by Pechini method and thermal treated at 700–1100 °C. The thermal treatment temperature has influence on the crystallite sizes of Y 2 O 3 :Eu 3+ nanocrystals and, consequently, on the structural and electronic defects concentration, which contributes to nonradiative recombination routes in scintillation materials. This relationship between the crystallite sizes and scintillation properties was investigated by X-ray diffraction and X-ray excited optical luminescence measurements. [ABSTRACT FROM AUTHOR]

Sodium decatungstoeuropate, Na 9 [Eu(W 5 O 18 ) 2 ]·14H 2 O, is a promising material to convert X-ray into visible light. In this work, temperature and exposure time-dependent scintillation of the Na 9 [Eu(W 5 O 18 ) 2 ]·14H 2 O sample under X-ray excitation were evaluated. Light emission intensity decreases along with X-ray exposure time, probably due to electronic defects at (W 5 O 18 ) 6- groups, leading to a lower efficiency of the LMCT (O→W) process, which is determinant in the scintillation mechanism of this material. After X-ray exposition, there is a partial recovery of the initial emission intensity, indicating that defects created are reversible. With temperature increasing, changes in the emission spectra evidence the decreasing of the local symmetry around the Eu 3+ ions, caused by reticular vibration and dehydration process. The luminescence intensity dependence with temperature suggests the possibility to use this material as molecular thermometer in two different temperatures ranges. [ABSTRACT FROM AUTHOR]

Scintillation properties of the new Tl 2 LaCl 5 : xCe 3+ where x=0, 0.5, 1 and 10 mol % (TLC: Ce 3+ ) single crystals are presented. Two zones vertical Bridgman technique is used for the growth of this scintillation material. High Z-number (79) of this material offer excellent detection efficiency for X- and γ-rays. Grown samples are characterized under X- and γ-rays excitation in order to find the emission wavelength, energy resolution, light yield, and decay time. Emission spectra exhibit Ce 3+ emission bands between 350–550 nm and peaking at 389 nm. The obtained energy resolution and light yield increases with high Ce-concentration. Energy resolution and light yield of 10%Ce-doped sample are found to be 6.9% (FWHM) and 51,000±5000 ph/MeV, respectively at room temperature. For the grown samples, two exponential decay time constants are found. The measured decay time constants showed variation in their values with respect to Ce-concentrations in the host crystal. Fast decay time constant of 31 ns with 87% light emission is found for 10%Ce sample. Scintillation results suggested that TLC will be one of the promising next generation scintillator for the medical imaging techniques such as Positron Emission Tomography (PET). [ABSTRACT FROM AUTHOR]

The Tm-doped MgAl 2 O 4 single crystals were synthesized by using Xe arc lamps-equipped floating zone furnace. The synthesized crystals exhibited luminescence due to the 4f-4f transitions of Tm3+ in photoluminescence (PL), scintillation, and thermally stimulated luminescence (TSL). In all of PL, scintillation, and TSL, the 0.5% Tm-doped crystal showed the highest luminescence intensity. In TSL dose response functions, the 0.5% Tm-doped crystal showed sensitivity from X-ray irradiation with 0.1 mGy. In addition, we measured linear energy transfer (LET) dependence on TSL glow curve of the crystal and X-ray imaging test taken by TSL. We revealed the spatial resolution of the crystal was 100 μm. • We have characterized the photoluminescence, scintillation, and dosimetric properties of Tm-doped MgAl 2 O 4 single crystals. • All the crystals showed luminescence properties due to Tm3+ ion. • We revealed possibility of Tm-doped MgAl 2 O 4 crystal for personal dosimetry, 2D dosimetry, and heavy ion beam measurement. [ABSTRACT FROM AUTHOR]

Optically stimulated luminescence (OSL) is a known property in many scintillators, and investigating the branching ratio of thermalized carriers to this photon-emission mechanism could aid both scintillator development and OSL-based dosimetry. We present a novel method for determining the branching ratio of thermalized carriers to OSL in scintillators while monitoring the energy deposition from 511 keV gamma photons on an event-by-event basis. We demonstrate the method using a cerium-doped lutetium oxyorthosilicate (LYSO:Ce) crystal, and validate that the observed signal indeed originates from OSL photons by estimating the lifetime of the OSL traps, which is consistent with values found in the literature. We report a branching ratio of thermalized carriers to the OSL traps in LYSO:Ce of (1.0±0.2)% upon excitation with 511 keV gamma photons, using a one-trap-one-recombination-center model. • New approach for assessing a dosimetric property in scintillators. • Branching ratio to OSL traps in LYSO:Ce reported for the first time. • OSL yield of LYSO:Ce is found to be 400 ± 80 photons per deposited MeV. [ABSTRACT FROM AUTHOR]

Y 3 Al 5 O 12 :Ce (YAG:Ce) polycrystalline ceramics with 0.1 and 0.3 mol% Ce substituting for Y were fabricated by laser sintering and characterized in their microstructure, luminescence, thermoluminescence and scintillation in comparison to a single crystal. The polycrystalline ceramics had cubic single crystallographic phase and relative densities of 97 ± 1 and 99 ± 1% for the 0.1 and 0.3 mol% Ce concentrations, respectively. The combined results of attenuated total reflection Fourier transform infrared (ATR FTIR) spectroscopy and photoluminescence measurements showed the polycrystalline ceramics fabricated by laser sintering to contain a higher level of structural disorder than the single crystal. The consequent broadening of the luminescence bands caused enhanced self-absorption in the polycrystalline ceramics. Radioluminescence (RL) under X-ray excitation, optical transmission and photoluminescence measurements revealed the presence of numerous types of point defects, including F centers, F+ centers/perturbed F+ centers and Y Al anti-sites. A detailed investigation of the thermoluminescence of YAG:Ce was executed using different methods and a complex behavior deviating from the first order kinetics mechanism was observed. The scintillation response of the polycrystalline ceramics was evaluated under low energy γ-ray irradiation. • Detailed comparison between laser-sintered ceramic and single crystal YAG:Ce. • Comprehensive thermoluminescence analysis. • Ceramics have higher level of structural disorder and shallower traps. [ABSTRACT FROM AUTHOR]

Hafnium-based materials are attractive for a scintillator owing to their high density and effective atomic number. In this study, single crystals of Ce-doped calcium hafnate perovskite (Ce:CaHfO 3) were synthesized from a Ca-rich composition, and photoluminescence (PL) and scintillation properties were investigated. The X-ray diffraction patterns revealed that the synthesized single crystals were a single phase of CaHfO 3. The Ce:CaHfO 3 exhibited blue luminescence at 430 nm and fast decay due to the 5d–4f transition of Ce3+ in both PL and scintillation. From the measurement of pulse height spectra under 137Cs irradiation, the highest light yield of 7800 photons/MeV was observed in the 3% Ce:CaHfO 3. • Single crystals of Ce-doped calcium hafnate perovskite (Ce:CaHfO 3) were successfully synthesized from a Ca-rich composition. • Ce:CaHfO 3 exhibited blue luminescence and fast decay due to the 5d–4f transition of Ce3+ in both PL and scintillation. • The light yield of 7800 photons/MeV was observed in the 3% Ce:CaHfO 3 under 137Cs gamma-ray irradiation. [ABSTRACT FROM AUTHOR]

The optical and scintillation properties of Ce 3+ -doped YGd 2 Al 5− x Ga x O 12 ( x =2,3,4) single crystals were investigated. With increasing Ga content in the garnet host, the 5d 1 absorption and emission bands shift toward higher energy due to the decrease in the crystal field splitting of the 5d levels. Temperature dependences of absorption and emission spectra and of photoluminescence decays of both prompt and delayed components were measured. Light yield (LY) and its dependence on an amplifier shaping time were measured under excitation with γ-rays. High LY value of 38,000 photons/MeV was obtained for a YGd 2 Al 2 Ga 3 O 12 :Ce sample. Scintillation decay was measured with an extended dynamical and temporal scale under the nanosecond-pulsed soft X-ray excitation. The relative content of the fastest component in the scintillation response increases with increasing Ga content. [ABSTRACT FROM AUTHOR]

The scintillation and luminescence properties of SrI 2 :1% Sm 2+ and SrI 2 :1% Er 3+ are reported. Single crystals were grown by the vertical Bridgman technique. Broad band Sm 2+ 5d–4f emission was observed centered at 750 nm with a radiative lifetime of 1.25 µs. The photoluminescence decay time increases with temperature due to self-absorption. With Er 3+ a clear charge transfer band was identified at 295 nm. The spectroscopic properties of Sm 2+ and Er 3+ are compared with those of Eu 2+ , Yb 2+ , Ce 3+ , and Nd 3+ doped in SrI 2 . The results are combined to construct a diagram with the vacuum referred electron binding energies for all divalent and trivalent lanthanide ions in SrI 2 which is fully consistent with the spectroscopic observations. [ABSTRACT FROM AUTHOR]

Energy transfer from donor Gd 3+ to acceptor Ce 3+ ions was studied in low doped lutetium aluminum garnet, LuAG:Ce,Gd. High purity single crystalline films were prepared by liquid phase epitaxy. The mechanism of nonradiative energy transfer from 6 P J (Gd 3+ ) multiplet to crystal field split 5d 2 ( 2 D) states of Ce 3+ was established as long-range dipole–dipole interaction and the average critical transfer distance between Gd 3+ and Ce 3+ ions was found ~14 Å at room temperature. It is shown that the single step energy transfer between donor–acceptor pairs is dominant while migration of excitation energy within the donor Gd 3+ subsystem is only a small perturbation in the energy transfer mechanism in the studied low doped garnets. [ABSTRACT FROM AUTHOR]

The effects of post-fabrication thermal processing in O 2 flux on the luminescence and scintillation of a Y 2 O 3 :Tm transparent ceramic were investigated. The results showed that the strategy of post-fabrication processing can be beneficial to the performance of the ceramics, depending on the cumulative processing time. After the first hour of processing, about 40% enhancement in the luminescence output together with about 20% enhancement in the scintillation light yield were obtained. The enhancements were tentatively assigned to the incorporation of oxygen into vacancy sites. Longer cumulative processing times lead to the incorporation of oxygen as interstitials that is detrimental to scintillation light yield but not to luminescence output. This work also revealed that thermoluminescence measurements are a useful tool to predict scintillation light yield of Y 2 O 3 :Tm. [ABSTRACT FROM AUTHOR]

Growth, luminescence and scintillation characterizations of the new Tl 2 LiGdCl 6 :Ce 3+ (TLGC:Ce 3+ ) single crystals are reported. This crystal was grown by the Bridgman technique and the composition belongs to the elpasolites family. X-ray induced emission spectra of the Ce-doped samples show the emission between 360 and 440 nm and peaking at 387 nm. The emission was attributed to the Ce 3+ ions in the host lattice. The highest light yield of 58,000±6000 ph/MeV and 4.6±0.2% (FWHM) energy resolution was obtained for 10% Ce doped sample. Three decay time components were observed for the 10% Ce-concentration under γ-ray excitation at room temperature. Among the decay time constants, the fast decay time constant of 34 ns (81%) due to Ce 3+ luminescence was obtained. Further investigation to improve scintillation material is underway. [ABSTRACT FROM AUTHOR]

Gd 3+ -doped borosilicate glasses are prepared in different melting atmosphere. Absorption spectra, decay time, luminescence spectra under UV and X-ray excitation are investigated. With melting atmosphere changing from air to CO, the luminescence intensities of Gd 3+ at 313 nm under the excitation of UV and X-ray are both enhanced. This mainly results from the reduction of Gd 3+ , which is validated by electron paramagnetic resonance (EPR). The optimal Gd 2 O 3 content for the glasses prepared under CO atmosphere is 7.5 mol%, whose integral scintillation efficiency is 20% compared with Bi 4 Ge 3 O 12 . [ABSTRACT FROM AUTHOR]

Single crystals of Ce-doped Cs 3 LaCl 6 and Cs 3 LaBr 6 show promising scintillation properties in gamma-ray detection. Their thermal response in the range from 40 K to 500 K is discussed in this work. The steady state photoluminescence spectra, photoluminescence decay time, and X-ray excited radioluminescence of Ce-doped and undoped Cs 3 LaCl 6 and Cs 3 LaBr 6 were studied. The photoluminescence and radioluminescence emission intensity of the Ce-doped crystals decreases less than 25% from 40 K to 500 K. The photoluminescence decay time of Ce 3+ increases with increasing temperature. [ABSTRACT FROM AUTHOR]

Abstract: We report on the luminescence and scintillation characterizations of cerium doped Cs2LiGdBr6 (CLGB) single crystal. This is a new scintillation material and belongs to elpasolite family. Single crystals of CLGB: Ce3+ with dimensions up to ∅10×5mm3 are successfully grown by the Bridgman technique. Under X-ray excitation, broad emission band between 370 to 478nm wavelengths is obtained. Such emission is attributed to the cerium ion. At 662keV γ-rays CLGB: Ce3+ shows energy resolutions of 7.1%, 9.4%, and 8.6% (FWHM) at 1%, 5%, and 10% Ce-concentrations, respectively. Light yield of CLGB depends on cerium concentration and maximum light yield of ~30,500ph/MeV is measured for 10% Ce sample crystal. Under γ-ray excitation, CLGB: Ce3+ crystals showed three exponential decay time components. [Copyright &y& Elsevier]

This study describes a simple route to the synthesis of the Ba 1-x MgAl 10 O 17 : Eu x (x = 0, 0.02, 0.035, 0.055, and 0.07) nanoparticles based on the solution combustion synthesis method. The morphology, luminescence properties, and structural analysis of the synthesized compounds were performed by field emission scanning electron microscopy (FE-SEM), photoluminescence (PL) spectra, energy dispersive analysis of the X-ray (EDS), and X-ray diffraction (XRD) techniques. The photoluminescence study showed an intense blue band centered at~447 nm caused by Eu2+. It was observed in all the samples. The radiation response of the synthesized BAM nanoparticles (NPs), were measured using a241Am alpha source. The results indicated that the BaMgAl 10 O 17 : Eu2+ (BAM: Eu) NPs have a good absolute efficiency and can be used as a suitable candidate to fabricate efficient scintillators for radiation detection. • A simple route to the synthesis of the BaMgAl 10 O 17 :Eu+2 nanoparticles is described. • The photoluminescence study showed an intense blue band centered at 447 nm. • The radiation response of BaMgAl 10 O 17 :Eu+2 is studied with 241Am alpha source. • The BaMgAl 10 O 17 :Eu+2 could act as scintillator for radiation detection. [ABSTRACT FROM AUTHOR]

Scheelite related rare-earth double molybdate with the formula of NaLa(MoO 4) 2 is a promising scintillator for investigating the neutrinoless double beta decay process. In this study, NaLa(MoO 4) 2 crystal was grown by the vertical Bridgman method using spontaneous crystallization. The as-grown NaLa(MoO 4) 2 crystal exhibited yellow and dark blue in color (from top to the bottom). The powder x-ray diffraction and optical transmission spectra of the crystal were measured at room temperature. The temperature dependence of the luminescence and scintillation properties were measured under the excitations of 4.42 eV UV light from a LED source and β-particles from a90Sr source in 10–300 K range, respectively. The transmittance of the annealed NaLa(MoO 4) 2 crystal sample with the thickness of 10 mm was about 65–72% above the fundamental absorption edge. The light yield under UV and β-particles excitations were found to be enhanced upon cooling and exhibited maximum intensity at 10 K. The luminescence light yield of NaLa(MoO 4) 2 crystal was estimated to be about half of that of CaMoO 4 crystal and 18 times as much as that of Li 2 MoO 4 crystal at 10 K. The scintillation light yield of NaLa(MoO 4) 2 crystal reached the maximum at 10 K and was 70% of Li 2 MoO 4 crystal. The shortest average decay time of 6.12 μs was obtained at 250 K, which was further slowed down upon cooling of the crystal and the longest average decay time of 442.77 μs was found to be at 10 K. Considering the possibility of crystal growth with large volume and high quality, the luminescence and scintillation properties of NaLa(MoO 4) 2 crystal revealed that this scintillation material may be a promising candidate as a cryogenic detector for investigating the neutrinoless double beta decay (0νββ) search. • NaLa(MoO 4) 2 crystal has been grown using vertical Bridgman method. • The transmittance of annealed NaLa(MoO 4) 2 crystal with the thickness of 10 mm was about 65–72%. • The luminescence light yield increased with the decreasing temperature. • The light yield of NaLa(MoO 4) 2 crystal was 18 times as much as that of Li 2 MoO 4 crystal at 10 K. • The scintillation light yield of NaLa(MoO 4) 2 crystal was 70% of Li 2 MoO 4 crystal at 10 K. [ABSTRACT FROM AUTHOR]

Single crystal of Ce doped Gd 3 Ga 3 Al 2 O 12 is one of the most studied advanced scintillators currently. The scintillation kinetics has been investigated with different excitations. The higher excitation density of alpha particles quenches the scintillation light yield but on the contrary the decay curve gets accelerated with gamma rays unlike that have been typically observed for conventionally used halide scintillators. In view of resolving this unusual behavior, a thorough investigation has been first time performed by carrying out systematic experiments on temperature dependent scintillation properties with gamma and alpha irradiation. The dependence of light yield for both radiations behaves differently with temperature with a notable early onset of quenching for charged particles. All three decay components also behave differently with temperature to indicate the various mechanisms governing the relaxation of excited charged carriers. At some temperatures the decay time components indeed behave in the similar manner as measured for halide crystals. The maximum difference in decay components carrying major amplitude occurs close to room temperatures leading to better discrimination ability. A strong correlation with thermo-luminescence properties has been observed to investigate the role of defect structure. A comprehensive model based on the prominent role of charged carriers released from trap centers along with the energy transfer and migration via Gd sub-lattice in governing the scintillation properties has been convincingly presented. It has demonstrated precisely the unusual behavior of this material but can be extended more considerably to describe various properties of other scintillators as well. • Single crystals of Ce doped Gd 3 Ga 3 Al 2 O 12 were grown by Czochralski method. • The light yield quenches under massive particle radiations but the decay time slows down. • A model is proposed to explain this strike difference from halide scintillator. • Temperature dependent kinetics studied under different radiations. • The role of defect structure and host energy transfer with excitation kinetics for alpha and gamma. [ABSTRACT FROM AUTHOR]