On-line induced absorption measurement on PbWO4, YAlO3:Ce and CsI scintillating crystals

On-line measured radiation-induced absorption spectra and their kinetics are presented for the doped PbWO₄, YAlO₃:Ce and CsI crystals. In the doubly doped PbWO₄:Mo,Y crystals the saturation level of the induced absorption increased with Mo concentration up to 2750 ppm. With a higher concentration it again gradually decreased. Yttrium co-doping around 100 ppm improved considerably the radiation hardness of Mo-doped PbWO₄. Characteristic recovery time of these crystals was about 30 min. Positive influence of Zr⁺⁴ co-doping on characteristics of a set of YAlO₃:Ce samples resulted in decrease in the induced absorption intensity. The presence of very slow recovery processes was in good agreement with thermoluminescence characteristics above room temperature reported earlier. An absorption band round 420 nm appeared in as-grown CsI crystals doped with Tl, and was related to the oxygen contamination.     

Influence of PbF2 and MoO3 on properties of PbWO4 crystals

The influence of doping (with F, Gd and Mo ions) and co-doping (with F, Gd or F, Gd, Mo ions) of PbWO₄ crystals was investigated. It is shown that even a small amount of specific impurities (especially F ions) causes an important redistribution of the point defects and changes the physical properties of the crystals. The cell parameters were refined from X-ray powder diffraction data. It was found that the cell parameters of F-doped PbWO₄ samples are larger than those of undoped samples. Photoluminescence and optical transmission spectra of pure and doped PbWO₄ were investigated.     

闪烁晶体的发光研究进展

概述了近年来闪烁体发光研究的进展,主要介绍用于未来高能物理实验的新型闪烁体发光机理研究,选取我们在研BaF₂,BaF₂:RE,CeF₃以及PbWO₄中的一些新进展。重点谈及三点:(1)在BaF₂的“价带^→芯带”跃迁发光研究基础上进行稀土(Gd³⁺-Eu³⁺)掺杂时观察到的量子剪裁以及对多光子发光的新思考;(2)CeF₃晶体发光的级联能量传递中,Ce³⁺(290nm发射带)与缺陷发光中心(340nm发射带)间能量传递及其传递效率的温度依赖;(3)PbWO₄晶体的发光中心研究中,提出以“WO₄^-2+O_i”绿光中心替代“WO³+F”中心观点的依据。同时也简介了医用闪烁体的最新进展。     

Scintillation properties of PbWO4 crystals doped with the rare-earth ions

In PbWO₄(PWO) crystals grown by Czochralski method the influence of atmosphere of the growth (O₂, air) and doping with the rare-earth ions of different types (A³⁺=Lu³⁺, Gd³⁺,Tb³⁺,Eu³⁺ as well as doubly doped A³⁺–Li⁺) on light yield and luminescence decay were analyzed. PWO scintillator with the ultra-fast (τ=0.5 ns) main component of luminescence decay (87% of total light yield) was obtained using the O₂-growth atmosphere and doping by Eu₂O₃ at a concentration of 5000 ppm. It is concluded that the decrease of decay constant of the main scintillation component is the result of the resonant energy transfer between the centers of “blue” PWO luminescence (λ_max=420 nm) and the 4f–4f-transitions of Eu³⁺ ions in this spectral region.     

PbWO4闪烁晶体的发光动力学模型

在对PbWO₄闪烁晶体的光谱特性、发光衰减及其温度依赖以及热释光的研究基础上,并结合理论计算,提出了PbWO₄晶体发光的动力学模型,给出了PbWO₄晶体的基本能带结构及激子发光中心能态、陷阱能级在能隙中的位置。用此模型可以完整说明PbWO₄的发光过程,特别是导致室温下发光效率低的原因。最后还对其主发射成分蓝、绿发光中心的起源作了简要讨论。     

Decay kinetics of the green emission in tungstates and molybdates

Luminescence characteristics of a number of undoped and variously doped PbWO₄ crystals were studied at 0.4–400 K by the time-resolved spectroscopy and compared with those of ZnWO₄,CdWO₄ and PbMoO₄ crystals. Two types of green emission centres are detected in PbWO₄ crystals. The centres of the first type are responsible for the low-temperature 2.3–2.4 eV emission observed under excitation around 3.90–3.95 eV. The structure and parameters of their relaxed excited states were determined. It was concluded that the origin of defects responsible for the green emission of the first type could vary for different crystals. The centres of the second type with the emission around 2.5 eV appear in crystals containing oxygen vacancies after the thermal destruction of Pb⁺-WO₃ centres at T>180 K. Decomposition of the exciton and various defect-related states was also studied, and activation energies of this process were calculated.     

Red emission of PbWO4 crystals

Luminescence characteristics of a large number of undoped and doped PbWO₄ crystals, grown by the Czochralski or Bridgman method, as-grown or annealed in the nitrogen atmosphere or in air, were studied in the 4.2–300 K temperature range. Two types of red emission centres were found. The centres with the emission band, peaking at 4.2 K at 1.57 eV, were observed in most of the crystals studied. The centres with the emission band, peaking at 4.2 K at 1.48 eV, were observed only in the PbWO₄ : Mo⁶⁺, Y³⁺ crystal. It is suggested that incompletely compensated lead vacancies are responsible for the appearance of the red emission.     

Fast heavy scintillators on the basis of tungstate single crystals

New very promising heavy fast PbWO₄ crystals as radiators for dense high resolution photon calorimeters were studied in this paper. Radiation detector parameters obtained with the real-size calirometer cells are presented. It was shown that owing to high density and transparency, quick response (4 ns), high radiation stability (about 10 mrad) with energy resolution 1% at an energy above 1 GeV, the PbWO₄-based scintillators are a most advantageous candidate for the creation of total absorption compact detectors of EM caorimeters.     

Electron paramagnetic resonance study of copper impurity charge-states in PbWO4 scintillator

The study of two types of Cu²⁺ centres observed in nominally pure PbWO₄ crystals grown by Bridgman and Czochralski methods was carried out by electron paramagnetic resonance (EPR). One of centres, called Cu²⁺(I), arises after oxygen compensation, while the second one, called Cu²⁺(II) requires, in addition, infrared illumination at low temperatures, being thermally stable only up to 22–23 K. The EPR spectra of both Cu²⁺ centres are described by rhombic symmetry g-tensors with the Z-principal axis lying close to, or in the (ab) plane of the crystal. It is proposed that in both centres the Cu²⁺ ions substitute for Pb²⁺ ions. The additional reduction of the local crystal-field symmetry is connected with a CuWO₄ wolframite-type lattice distortion for the Cu(I) and a Jahn–Teller distortion of the regular PbWO₄ lattice for the Cu(II). It was also found that either IR irradiation or thermal heating activate the transfer of an electron between two closely spaced Cu²⁺ centres.     

Time-resolved luminescence of complex wide-gap oxide crystals under inner-shell excitation

In order to investigate the soft X-ray energy transformation in oxide detectors the optical spectra of several wide-gap oxide crystals were analyzed. The time-resolved luminescence (2.5–10.5 eV) and luminescence excitation spectra (50–200 and 500–630 eV) as well as decay kinetics of luminescence at 10 and 295 K were recorded using the synchrotron radiation from BW3 channel (HASYLAB, DESY). Several analogous features were discovered in the excitation spectra of both intrinsic self-trapped exciton luminescence and recombination luminescence for BeO, BeAl₂O₄, Be₂SiO₄ and AlPO₄ crystals under inner-shell excitation. Simultaneously, the excitation of Ce³⁺-luminescence in scintillating Be₂La₂O₅-Ce crystals significantly differs.     

Scintillation properties of the Yb-doped YAlO3 crystals

Yb³⁺-doped YAlO₃ single crystals were grown by the Czochralski method with a radio frequency heating system. The starting melt compositions of Y_1−xYb_xAlO₃ were varied as x=0.02, 0.1, 0.2, 0.3 and 0.45. Photo- and thermo-luminescence characteristics including emission decays were measured within 4–300 K. Very fast charge transfer luminescence of Yb³⁺ was found, with the typical double peak spectrum at 345 and 525 nm. At room temperature the photoluminescence and scintillation decay curves show a leading subnanosecond decay component without any additional slower processes. The high density of Yb-rich YAlO₃ makes this material a promising candidate for fast scintillators.     

Controllable preparation and disorder-dependent photoluminescence of morphologically different C60 microcrystals

Different C₆₀ crystals were synthesized by precipitation from a mixture of the good solvent m-xylene and the poor solvent isopropyl alcohol. The samples were characterized by scanning electron microscopy (SEM), Raman spectroscopy, thermogravimetric analysis, and high resolution transmission electron microscope (HRTEM). We found that the morphologies and sizes of the samples could be controlled by adjusting the volume ratio between the good and poor solvents. Especially, an unexpected short flower column-like crystal was synthesized at low ratios (from 1:6 to 1:12). Room temperature photoluminescence (PL) and HRTEM studies of the C₆₀ crystal samples reveal that the PL efficiency of the crystals decreases with increasing crystalline order and that the disordered C₆₀ crystals synthesized at the ratio of 1:2 show 10 times higher PL efficiency than that of pristine C₆₀. The mechanism of the growth process of these C₆₀ crystals was also studied by replacing the good solvents m-xylene with toluene and mesitylene.     

Radiation-induced processes and defects in alkali and alkaline-earth borate crystals

The paper presents the results of a study of the radiation-induced processes and defects in nonlinear optical crystals Li₂B₄O₇ (LTB), LiB₃O₅ (LBO), CsLiB₆O₁₀, KB₅O₈·4H₂O, β-BaB₂O₄. It was revealed that a pulsed electron beam irradiation at 290 K forms the radiation-induced pairs of the ‘vacancy—interstitial atom’ defects in the cation sublattice of these crystals. This gives rise to a creation of metastable electronic (interstitial atom) and hole (small-radius polaron near the cation vacancy) centers in high concentrations. Optical hole-transitions from the local level of the trapped hole centers to the valence band states are responsible for the transient optical absorptions (TOA) of borates in the visible and UV spectral ranges. A sublattice of the weakly bound mobile lithium cations in LTB and LBO favors a spatial separation of the radiation-induced pair defects ‘hole polaron near Li-vacancy—mobile interstitial Li⁰ atom’. Their decay rated by the electron–hole nonradiative tunnel recombination determines a peculiar feature of the TOA decay kinetics in LTB and LBO.     

Scintillation properties of Yb-doped garnet crystals

Luminescence properties of heavily Yb-doped Lu₃Al₅O₁₂ (Yb:LuAG) and Lu₃Ga₅O₁₂ (Yb:LGG) single crystals grown by micro-pulling-down method were characterized. Charge transfer luminescence of Yb³⁺ was observed in both crystals. Photoluminescence spectra, temperature dependence of emission intensity and decay kinetics of these crystals were studied. Mean decay time of about 24 ns at 90 K (Yb5%:LuAG) and 21 ns at 110 K (Yb5%:LuGG) were observed. Strong thermal quenching at room temperature was measured for both Yb:LuAG and Yb:LGG.     

二氧化钛微晶结构相变与光致发光

以钛酸四丁脂为前驱体制备了二氧化钛(TiO₂)胶体,将其粉末在不同温度下作热处理,采用差热分析、X射线衍射、荧光光谱等手段对样品进行测试。结果表明,随着热处理温度升高,TiO₂由板钛矿相经锐钛矿相向金红石相转变,在808℃左右出现一级相变。原始粉末样品以及在600℃以下热处理的样品,在400nm处可观察到TiO₂纳米晶的带边发光,在470nm处可观察到表面态发光;当热处理温度达到850℃时,主晶相转变为金红石相,400nm处的带边发光峰消失,位于470nm处发光峰成为最强峰,但强度减弱,并且样品发光的波长范围明显变窄。其原因是,随着热处理温度升高,晶粒不断长大,量子限域效应减弱乃至消失,晶粒的表面状况发生变化,导致样品的荧光发射行为发生变化。     

Excited-state dynamics of Yb in LiCaAlF6 single crystal

Single crystals of Yb²⁺-doped LiCaAlF₆ were grown by the Czochralski technique under CF₄ atmosphere. Photoluminescence, thermally stimulated luminescence and decay kinetics of Yb²⁺ centre in the LiCaAlF₆ host were measured in the 4–300 K temperature interval. Phenomenological two excited-state-level model is introduced to obtain quantitative characteristics of the excited state dynamics of Yb²⁺. The role of Yb²⁺ centre in trapping processes is discussed. Moreover, defect centres related to LiCaAlF₆ host were found.     

EPR characterization of Mn impurity ions in PbWO4 single crystals

The electron paramagnetic resonance (EPR) properties of the Mn²⁺ ions in PbWO₄ single crystals grown by the Czochralski method have been investigated in the X-band microwave frequency, at T=20 K. The angular dependence of the EPR line positions obtained by rotating the magnetic field in the main crystallographic planes shows that the local symmetry at the Mn²⁺ impurity ions is tetragonal, strongly suggesting that the Mn²⁺ ions substitute for the Pb²⁺ lattice cations, without charge compensation. The resulting spin Hamiltonian parameters compare well with the corresponding values for the Mn²⁺ ions in other isomorphous tungstates. The observed strong angular variation of the EPR linewidth has been quantitatively described considering a random distribution of lattice strains.     

热分解含硫金属有机配合物制备近红外PbS量子点

以Pb(NO₃)₂, Na(S₂CNEt₂)·3H₂O为反应物, 在去离子水中合成含硫金属有机配合物Pb(S₂CNEt₂)₂. 氩气保护下, 在油酸和十八烯混合溶液中热分解前躯体Pb(S₂CNEt₂)₂, 反应时间分别为30, 60, 90, 120 min, 获得PbS量子点样品a, b, c, d. 通过红外光谱分析和热重-差热等手段对前躯体进行表征, 证明配体Na(S₂CNEt₂)·3H₂O中的两个硫原子与Pb²⁺配位成功. PbS量子点样品X射线衍射和透射电子显微镜分析表明, 合成的PbS为类球形纯立方晶系PbS纳米晶; 对PbS量子点样品紫外-可见吸收光谱和光致发光谱进行研究发现, 吸收光谱和光致发光谱随着反应时间的增加顺序红移, 表明优化热分解反应时间可以调控PbS量子点的吸收光谱和光致发光谱. PbS量子点样品a发射峰在1080 nm, 与硅基太阳能电池相匹配, 可作为硅基荧光太阳能聚集器的荧光材料. 关键词： 热分解法 含硫金属有机配合物 PbS量子点 反应时间     

近紫外光激发黄色荧光粉Ba3Y4O9：Dy3+的制备及发光特性

通过高温固相法合成了一系列Ba₃Y_4-xO₉：xDy³⁺荧光粉材料。利用X射线粉末衍射、荧光光谱和荧光寿命对样品进行了表征。实验表明,样品的激发光谱由一系列线状峰组成,峰值分别位于328,355,368,386,427,456,471 nm。在355 nm激发下,荧光粉在490 nm（⁴F_9/2→⁶H_15/2）和580 nm（⁴F_9/2→⁶H_13/2）处有很强的发射,发射光谱的色坐标位于黄光区域。研究了不同Dy³⁺掺杂浓度对样品发光性质的影响,发现样品的发光随着Dy³⁺浓度的增大而增强,但光谱形状基本保持不变,表明Dy³⁺占据了基质中低对称性的Y³⁺格位。当Dy³⁺摩尔分数x=0.08时出现发光强度猝灭现象,浓度猝灭机理为电偶极-电偶极相互作用。样品的发光寿命随着Dy³⁺浓度的增大逐渐减小,进一步证明了Dy³⁺离子之间存在着能量传递现象。Ba₃Y₄O₉：Dy³⁺荧光粉的发光位于黄光区域,有较好的热稳定性,是潜在的白光LED用荧光粉材料。     

Influence of the light illumination on thermoluminescence properties of PbWO4

The thermally stimulated properties of PbWO₄ in the temperature range 90–400 K have been investigated on undoped and terbium-doped crystals after X-ray irradiation at 90 K. Doping with terbium changes the concentration of shallow traps, which are responsible for retrapping free electrons and holes. Light illumination can change the distribution of the traps. The optically stimulated luminescence is observed. The influence of light illumination on the TSL curves and emission properties is studied. The possible mechanism of TSL phototransformation is discussed.