A deeper insight into (Lu,Y)AG : Pr scintillator crystals

Interior of Czochralski-grown (Lu,Y)AG:Pr crystals has been examined by means of several techniques, such as X-Ray Photoelectron Spectroscopy, X-Ray Diffraction, Time-of-Flight Secondary Ion Mass Spectrometry, and magnetic susceptibility measurements. Additionally, their luminescence has been monitored at various combinations of a double-beam (X-ray/IR) excitation

Negatively Charged Excitons and Photoluminescence in Asymmetric Quantum Well

We study photoluminescence (PL) of charged excitons ($X^-$) in narrow asymmetric quantum wells in high magnetic fields B. The binding of all $X^-$ states strongly depends on the separation $\delta$ of electron and hole layers. The most sensitive is the ``bright'' singlet, whose binding energy decreases quickly with increasing $\delta$ even at relatively small B. As a result, the value of B at which the singlet--triplet crossing occurs in the $X^-$ spectrum also depends on $\delta$ and decreases from 35 T in a symmetric 10 nm GaAs well to 16 T for $\delta=0.5$ nm. Since the critical values of $\delta$ at which different $X^-$ states unbind are surprisingly small compared to the well width, the observation of strongly bound $X^-$ states in an experimental PL spectrum implies virtually no layer displacement in the sample. This casts doubt on the interpretation of PL spectra of heterojunctions in terms of $X^-$ recombination

Scintillation mechanisms in rare earth orthophosphates

In this Communication we present results of our studies on scintillation properties of rare earth orthophosphates. Despite of the PO{sub 4} group high frequency vibrations, which limit the conversion efficiency in orthophosphate lattices, some of them are very promising, as evidenced by the exceptionally good scintillation properties of LuPO{sub 4}:Ce. We have studied the following orthophosphate crystals: YbPO{sub 4}:Ce, YbPO{sub 4}, and Lu{sub x}Yb{sup 1-x}PO{sub 4}:Ce. Both Ce and Yb ions scintillate (through d-f transition and charge transfer state decay, respectively) but, unfortunately, they also show strong tendency toward mutual quenching. We propose that nonradiative Yb-Ce energy transfer and the nonradiative decay of the (Yb{sup 2+} + Ce{sup 4+}) charge transfer state are responsible for the observed effect. Therefore ytterbium does not provide cheaper substitute for the optically inactive RE ion (lutetium) in the orthophosphate lattice, on the contrary, it has to be carefully avoided whenever a Ce-activation is used

Scintillation and radioluminescence mechanism in β-Ga2O3 semiconducting single crystals

In this paper, we present the results of experiments on samples of β-Ga2O3 single crystals under a project aimed at assessing and improving the scintillation performance of this material by studying scintillation and radioluminescence mechanism and its limitations. In addition to standard experiments, such as scintillation light yields and time profiles, radio-, and thermoluminescence, we developed and tested a new and promising two-beam experiment, in which a sample is excited by an X-ray beam and additionally stimulated by an IR laser diode.Fe and Mg doping compensate for the inherent n-type conductivity of β-Ga2O3 to obtain semi-insulating single crystals for large-area substrates and wafers. At the same time, residual Fe and Ir are ubiquitous uncontrolled impurities leached from the Ir crucibles used to grow large bulk crystals by the Czochralski method.For these experiments, we selected four samples cut from the Czochralski grown 2-cm diameter β-Ga2O3 single crystal boules; one with a reduced Fe content, two unintentionally Fe- and Ir-doped (UID) with lower and higher Fe content, and one doped with Mg.We find that steady-state radioluminescence spectra measured at temperatures between 10 and 350 K are dominated by the UV emission peaking at about 350–370 nm. Unfortunately, even for the best sample with a reduced Fe-content, the intensity of this emission drops precipitously with the temperature down to about 10 % at 300 K.From the two-beam experiments, we conclude that recombination via inadvertent Fe impurity involving three charge states (2+, 3+, and 4+) may reduce a steady-state UV emission of β-Ga2O3 under X-ray excitation by as much as 60–70 %, one-third to one-half of which is due to the recombination (specific for Fe-doped β-Ga2O3) involving the 4+ and 3+ charge states of Fe and the remaining 50–70 % being due to a more familiar route typical of other oxides, involving the 2+ and 3+ charge states of Fe. These losses are at higher temperatures enhanced by a thermally activated redistribution of self-trapped holes (STHs). In addition, the trapping of electrons by Fe and holes by Mg, Fe, and Ir may be responsible for scintillation light loss and reduction of the zero-time amplitude essential for the fast timing scintillation applications.Despite indirect evidence of competitive recombination in β-Ga2O3 involving a deep Ir3+/4+ donor level, we could not quantitatively assess losses of the UV steady state radioluminescence light due to the inadvertent Ir impurity

Dynamical equilibrium between excitons and trions in CdTe quantum wells in high magnetic fields

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33000 photons per MeV from mixed (Lu0.75Y0.25)3Al5O12:Pr scintillator crystals

(LuxY1-x)3Al5O12:Pr (x = 0.25, 0.50, 0.75) crystals have been grown by the Czochralski method and their scintillation properties have been examined. Compared to the well-respected LuAG:Pr scintillator, which has so extensively been studied in the recent years, the new mixed LuYAG:Pr crystals display markedly higher light yields, regardless of the value of x. In particular, (Lu0.75Y0.25)3Al5O12:0.2%Pr characterized by a yield of 33000 ph/MeV, an energy resolution of 4.4% (at 662 keV), and a density of 6.2 g/cm3, seems to be an ideal candidate to supercede Lu3Al5O12:0.2%Pr (19000 ph/MeV, 4.6%, 6.7 g/cm3) in various applications. The observed enhancement of light output following the partial substitution of lutetium by yttrium is most probably related to some specific differences in distributions of shallow traps in particular materials.RST/Radiation, Science and TechnologyApplied Science