Neutron detection with LiCaAlF6 scintillator doped with 3d-transition metal ions

Capability of thermal neutron detection was examined for LiCaAlF₆ (LiCAF) scintillators doped with 3d-transition metal ions. Their radioluminescence spectra were measured with an 241-Am source to simulate ⁶Li(n, α)³H reaction. The sufficiently intense radioluminescence was observed for the Mn, Co and Cu dopants, while only a weak one was observed for Ti, V, Fe and Ni. A Mn doped LiCAF crystal, which showed the highest radioluminescence intensity, was coupled with a Si avalanche photodiode for the examination of its neutron response. It was confirmed that the average current of the photodiode clearly increased under excitation with 13.5 meV neutron flux.     

Luminescence of Ce3+ ion in aluminate based phosphors for scintillator

Ce³⁺ activated aluminate based phosphors prepared by combustion synthesis are reported here. The emission of Ce³⁺ ion is observed around 321–354 nm in the ultraviolet region of the spectrum. The intensity of strong photoluminescence emission peak of Ce³⁺ ion increases when Mg is added to CaAl₁₂O₁₉ phosphor. The Ce³⁺ emission in CaAl₁₂O₁₉:Ce,Mg phosphor may be useful for scintillation application.     

Calibrated U-TH glasses for neutron dosimetry and determination of uranium and thorium concentration by the fission track method

We have prepared a series of standard uranium glasses. A primary standard containing 41 ppm of added U had its concentration and isotope ratio verified by mass spectrometry and by fission track techniques. Secondary standards were compared to the primary standard using the fission track technique. These glasses are useful for measuring both Th and U in solids. The U concentration in an unknown is determined by comparing the fission track density induced by thermal neutron irradiation with the corresponding density in the standard. Th is measured by a cyclotron irradiation using the same U standards. The glasses are also useful for neutron dosimetry. Investigators may obtain small samples of most of these standards from R. H. Brill of the Corning Museum of Glass. Although most of the glasses were fabricated from depleted uranium, one standard containing natural uranium is available. A standard Th glass with concentration of 41 ppm was also made.     

Room-temperature scintillation properties of cerium-doped REOX (RE=Y, La, Gd, and Lu; X=F, Cl, Br, and I)

The scintillation properties of cerium-doped oxyhalides following the general formula REOX (RE=Y, La, Gd, and Lu; X=F, Cl, Br, and I) are reported. These materials were synthesized under dry conditions as microcrystalline powders from conventional solid state reactions. The room temperature X-ray excited emission and scintillation decay curves were measured and analyzed for each material. Additionally, the hygroscopic nature of the oxychlorides and oxybromides was compared to that of their corresponding rare earth halides. The yttrium, lanthanum, and gadolinium oxychlorides, and all of the oxybromides and oxyiodides are found to be activated by Ce³⁺. GdOBr doped with 0.5% Ce³⁺ has the highest light output with a relative luminosity of about one-half that of LaBr₃: Ce³⁺. It displays a single exponential decay of 30 ns.     

Fast scintillation processes in CsCl crystals comprising semiconductor nanocrystals

We analyzed the scintillation mechanism in CsCl single crystals comprising CsPbCl₃-like semiconductor nanocrystals. A decay component with a subnanosecond lifetime was observed and ascribed to the exciton luminescence in the nanocrystals. This is the first observation of scintillation from semiconductor nanocrystals. In addition, slower components were observed and ascribed to the intraionic transition at isolated Pb²⁺ ions and the Auger-free luminescence in the CsCl matrix. Furthermore, it was shown that the nanocrystals absorbed and re-emitted the scintillation photons from the isolated Pb²⁺ ions. The results in this report clearly indicates that the semiconductor nanocrystals function as ultrafast “luminescent centers,” and complicated scintillation dynamics are observed due to the luminescence from different components in the crystal and the radiative energy transfer between them.     

Excitation mechanisms and effects of dopant concentration in Gd2O2S:Tb phosphor

Gadolinium oxysulfide powders doped with different Tb³⁺ concentrations were prepared from sulfur vaporization on rare earths’ basic carbonate precursors. Single-phase Gd₂O₂S samples were obtained, with Tb³⁺ doping up to 9 at%. The study of the excitation mechanisms revealed that the Tb³⁺ emission might occur after the direct Tb³⁺ excitation either by energy transfer from Gd³⁺ or from the phosphor host. The characteristic terbium emission lines were observed, resulting from the radiative decay from ⁵D₃ or ⁵D₄ to ⁷F_j levels. The cross-relaxation phenomenon was observed and its effects on the materials emission color were discussed based on the CIE diagram. By using time-resolved spectroscopy, ⁵D₃→⁷F_J and ⁵D₄→⁷F_J transitions were separated.     

Characterizations of Pr-doped Yb3Al5O12 single crystals for scintillator applications

Yb₃Al₅O₁₂ (YbAG) single crystals doped with different concentrations of Pr were synthesized by the Floating Zone (FZ) method. Then, we evaluated their basic optical and scintillation properties. All the samples showed photoluminescence (PL) with two emission bands appeared approximately 300–500 nm and 550–600 nm due to the charge transfer luminescence of Yb³⁺ and intrinsic luminescence of the garnet structure, respectively. A PL decay profile of each sample was approximated by a sum of two exponential decay functions, and the obtained decay times were 1 ns and 3–4 ns. In the scintillation spectra, we observed emission peaks in the ranges from 300 to 400 nm and from 450 to 550 nm for all the samples. The origins of these emissions were attributed to charge transfer luminescence of Yb³⁺ and intrinsic luminescence of the garnet structure, respectively. The scintillation decay times became longer with increasing the Pr concentrations. Among the present samples, the 0.1% Pr-doped sample showed the lowest scintillation afterglow level. In addition, pulse height spectrum of 5.5 MeV α-rays was demonstrated using the Pr-doped YbAG, and we confirmed that all the samples showed a full energy deposited peak. Above all, the 0.1% Pr-doped sample showed the highest light yield with a value of 14 ph/MeV under α-rays excitation.     

Radiation induced colour centers and damage in YAlO 3 :Ce and YAlO 3 :Ce,Zr scintillators

Radiation induced absorption spectra (irradiation up to 500 v Gy by 60 Co) and TSL of a set of YAP:Ce crystals co-doped by Zr 4+ were investigated. Positive effect of Zr 4+ co-doping was found, namely reduction of both the defect creation and presence of TSL active traps. Up to three absorption bands were observed in the radiation induced absorption spectra. They probably have different origin - from colour centers in the green part of the spectra to change of the Ce 3+ valency in the near UV.     

Eu and Rb co-doped LiCaAlF6 scintillators for neutron detection

Eu and Rb co-doped LiCaAlF₆ (LiCAF) single crystals with different dopant concentrations were grown by the micro-pulling-down method for neutron detection. Their transmittance spectra showed strong absorption bands at 200–220 and 290–350 nm, and under ²⁴¹Am alpha-ray excitation, their radioluminescence spectra exhibited an intense emission peak at 373 nm that was attributed to the Eu²⁺ 5d–4f transition. These results were consistent with those for the Rb-free Eu:LiCAF. The highest light yield among the grown crystals was 36,000 ph/n, which was 20% greater than that of the Rb-free crystal. In addition, the neutron-excited scintillation decay times were 650–750 ns slower than that of the Rb-free Eu:LiCAF.     

Effects of Na and K co-doping on growth and scintillation properties of Eu:SrI2 crystals

We grew Na and K co-doped Eu:SrI₂ [Na,Eu:SrI₂ and K,Eu:SrI₂] crystals by a modified micro-pulling-down method to reveal the co-doping effects on the crystal growth and scintillation properties. The non-codoped, Na0.5%, Na1.0%, K0.5% and K1.0%,Eu:SrI₂ crystals indicated high transparency while the milky parts were generated in the Na5.0% and K5.0%,Eu:SrI₂ crystals. The light yields of Na,Eu:SrI₂ and K,Eu:SrI₂ crystals under γ-ray irradiation were decreased by the Na and K co-doping. On the other hand, there was a small change within 940–1020 ns in the decay times by the Na and K co-doping. In the light yield proportionality under γ-ray irradiation, the non-proportionality in the low energy region was improved by Na and K co-doping.