Scintillation properties of Nd, Tm, and Er doped LuF3 scintillators in the vacuum ultra violet region

In order to develop novel vacuum ultra violet (VUV) emitting scintillators, we grew Nd 0.5%, Tm 0.5%, and Er 0.5% doped LuF₃ scintillators by the μ-pulling down method, because LuF₃ has a very wide band gap and Nd³⁺, Tm³⁺, and Er³⁺ luminescence centers show fast and intense 5d-4f emission in VUV region. Transmittance and X-ray induced radioluminescence were studied in these three samples using our original spectrometer made by Bunkou-Keiki company. In the VUV region, transmittance of 20-60% was achieved for all the samples. The emission peaks appeared at approximately 180, 165, and 164 nm for Nd³⁺, Tm³⁺, and Er³⁺ doped LuF₃, respectively. Using PMT R8778 (Hamamatsu), we measured their light yields under ²⁴¹Am α-ray excitation. Compared with Nd:LaF₃ scintillator, which has 33 photoelectrons/5.5 MeV α, Nd:LuF₃ and Tm:LuF₃ showed 900±90 and 170±20 ph/5.5 MeV-α, respectively. Only for the Nd doped one, we can detect ¹³⁷Cs 662 keV γ-ray photoabsorption peak and the light yield of 1200±120 ph/MeV was measured. We also investigated their decay time profiles by picosecond pulse X-ray equipped streak camera, and the main decay component of Nd:LuF₃ turned out to be 7.63 ns.     

Crystal growth and scintillation properties of Nd-doped Lu3Al5O12 single crystals with different Nd concentrations

Nd 0.1%, 0.5%, 1% and 3% doped Lu₃Al₅O₁₂ (Nd:LuAG) single crystals were grown in the nitrogen atmosphere by the micro-pulling down (μ-PD) method. The grown crystals had a single-phase confirmed by powder XRD analysis. In absorption spectra, some weak absorption lines due to Nd³⁺ 4f-4f transitions were observed and their intensity increased with the increase of Nd concentration. When excited by ²⁴¹Am α-ray, a broad emission peak due to defects in the host lattice at 320 nm and some sharp lines due to Nd³⁺ 4f-4f transitions at wavelength longer than 400 nm were observed. The decay time profiles of Nd:LuAG under γ-ray excitation were well approximated by two exponential function of 340-760 ns and 3-5 μs for each sample. By pulse height measurement using ¹³⁷Cs, Nd 0.5%:LuAG showed the highest light yield of 7600 ± 760 photons/MeV.     

Crystal growth and scintillation characteristics of the Nd doped LiLuF4 single crystals

Sixty millimeter diameter single crystal of Nd³⁺ doped LiLuF₄ was successfully grown by the Czochralski technique. No remarkable absorption due to unfavorable impurities was observed from optical absorption measurements in the vacuum ultra-violet spectral region. The high crystallinity and homogeneous luminescence characteristics were found from X-ray rocking curve and cathode-ray luminescence respectively. X-ray excited luminescence spectrum was measured and the significant 4f²5d-4f³ luminescence at 182 nm was observed in the grown crystal. The pulse height spectrum was taken upon γ-ray irradiation. As a result, the grown crystals demonstrated sufficient response to the γ-ray showing the light yield of 420 ± 30 photons/MeV. The decay curve under α-ray irradiation was also investigated and described by two component decay kinetics which consists of the decay constants of 34 and 450 ns.     

Crystal growth and scintillation properties of (NaxCa1−2xLux−yNdy)F2 single crystals

Na_xCa_1−2xLu_x−yNd_yF₂ single crystals were grown from the melt using the precise atmosphere control type Micro-Pulling-Down (μ-PD) method to investigate their potential as a vacuum-ultraviolet (VUV) scintillators. The grown crystals were single-phase materials with fluorite-type structure (Fm-3m, Z = 4) as confirmed by XRD. The crystals demonstrated 80-90% transmittance above 200 nm wavelength and Nd³⁺ 5d-4f luminescence (when exited by X-ray) observed around 185 nm. The radioluminescence measurements under 5.5 MeV α-ray excitation (²⁴¹Am) demonstrated the light yield of 48 [Ph/5.5 MeV-α] and the decay time of 6.4-7.7 ns.     

Fast d-f emission in Ce, Pr and Nd activated RbCl

In the search for new scintillator materials, Ce³⁺ doped chlorides are a promising class of materials, combining a high efficiency and fast response time. Even shorter response times may be achieved by replacing Ce³⁺ by Pr³⁺ or Nd³⁺ as the lifetime of the d-f emission is substantially shorter for these ions. Here we report on the luminescence properties of Ce³⁺, Pr³⁺ and Nd³⁺ in RbCl and investigate the potential as a scintillator material. Under UV excitation Ce³⁺ shows d-f emission between 325 and 425 nm. The emission originates from multiple (differently charge compensated) Ce³⁺ sites. The luminescence lifetime varies with wavelength and is ∼40 ns for the longer wavelength emission. For RbCl:Pr³⁺ three d-f emission band are observed between 250 and 350 nm which can be assigned to transitions from the lowest energy fd state to different ³H_J (J = 4-6) states within the 4f² configuration of Pr³⁺. The decay time is ∼17 ns. For the Nd³⁺ activated sample a weak emission band around 220 nm is observed only at 8 K which may be due to d-f emission. The very short lifetime (4 ns) is faster than the radiative lifetime, indicating that the d-f emission is quenched by relaxation to lower lying 4f³ states or by the process of photoionization. Under VUV excitation at wavelengths below 175 nm (the bandgap of RbCl) the d-f emission is very weak for Ce³⁺, Pr³⁺ and Nd³⁺ doped RbCl and the emission spectra are dominated by defect related emission. This indicates that energy transfer from the host lattice to the fd states is inefficient which prevents application as a scintillator material.     

Crystal growth and optical properties of the Nd doped LuF3 single crystals

Thirty millimeter diameter single crystal of Nd³⁺ doped LuF₃ was grown using LiF as solvent. The single phase crystallization was confirmed by the powder X-ray diffraction, and high structural perfection was demonstrated by X-ray rocking curve (XRC) measurements. FWHM of XRC for 220 reflection was 32 arcsec. No remarkable absorption due to unfavorable impurities was observed from optical absorption measurements in the VUV spectral region. The crystal showed the VUV luminescence peaking around 178 nm that is consistent with the 4f²5d-4f³ transition of Nd³⁺ ion. The luminescence intensity of Nd:LuF₃ under X-ray irradiation was significantly higher than that of reported VUV scintillators such as Nd:LaF₃ or Nd:LiLuF₄.     

Crystal growth and scintillation properties of Ce and Eu doped LiSrAlF6

Ce and Eu doped LiSrAlF₆ (LiSAF) single crystals for the neutron detection with different dopant concentrations were grown by the micro-pulling-down method (μ-PD). In Ce:LiSAF, intense emission peaks due to Ce³⁺ 5d-4f transitions were observed at approximately 315 and 335 nm in photo- and α-ray induced radio-luminescence spectra. In case of Eu:LiSAFs, an intense emission peak at 375 nm due to Eu²⁺ 5d-4f transition was observed in the radio-luminescence spectra. The pulse height spectra and decay time profiles were measured under ²⁵²Cf neutron irradiation to examine the neutron response. The Ce 3% and Eu 2% doped LiSAF showed the highest light yield of 2860 ph/n with 19 ns main decay time component and 24,000 ph/n with 1610 ns.     

Structural and optical properties of neodymium-doped lutetium fluoride thin films grown by pulsed laser deposition

Neodymium-doped lutetium fluoride (Nd³⁺:LuF₃) thin films were successfully grown on MgF₂ (0 0 1) substrates by pulsed laser deposition (PLD). It is void of cracks that are otherwise prevalent due to structural phase transitions in Nd³⁺:LuF₃ during thin film deposition and bulk crystal growth. Cathodoluminescence (CL) spectra revealed multiple emission peaks, with a dominant peak in the vacuum ultraviolet (VUV) region at 179 nm. This peak has a decay time of 6.7 ns. The ability to grow high quality Nd³⁺-doped fluoride thin films would enable fabrication of VUV light-emitting devices that will enhance applications requiring efficient VUV light sources.     

Optical properties and gamma-ray response of Czochralski grown Pr:Lu3Al5O12 scintillating garnet crystals with different Pr content

Pr:LuAG single crystalline scintillators with different Pr³⁺ concentration, 0.1, 0.18, and 0.22 mol% were grown by the Czochralski method. The crystals were cut to dimensions of 2.2 × 2.2 × 15 mm³ and polished, simulating sensors for Positron Emission Tomography (PET). Their absorption coefficients were examined, and the absorption strength was found to be proportional to the Pr concentration. The α-ray induced emission spectra of the samples demonstrated two emission lines peaking at 310 and 370 nm. The emission intensities in the radio luminescence spectra were also proportional to the Pr content. The absolute light yields and intrinsic energy resolution under γ-ray irradiation were evaluated at +20, 0, and −20 °C using avalanche photodiode as a photodetector. Pr 0.22% doped crystal had strongest light output of 16 400 ph/MeV, and its intrinsic energy resolution was around few % at several hundred keV. When coupled with PMT, the decay time was around 25 ns, and it was almost independent on concentration.     

Scintillation and optical properties of Pb-doped YCa4O(BO3)3 crystals

This communication reports optical properties and radiation responses of Pb²⁺ 0.5 and 1.0 mol%-doped YCa₄O(BO₃)₃ (YCOB) single crystals grown by the micro-pulling-down (μ-PD) method for neutron scintillator applications. The crystals had no impurity phases according to the results of X-ray powder diffraction. These Pb²⁺-doped crystals demonstrated blue-light luminescence at 330 nm because of Pb²⁺¹S₀-³P_0,1 transition in the photoluminescence spectra. The main emission decay component was determined to be about 250-260 ns under 260 nm excitation wavelength. When irradiated by a ²⁵²Cf source, the relative light yield of 0.5% Pb²⁺-doped crystal was about 300 ph/n that was determined using the light yield of a reference Li-glass scintillator.     

Crystal growth and scintillation properties of Er-doped Lu3Al5O12 single crystals

Er-doped Lu₃Al₅O₁₂ (Er:LuAG) single crystalline scintillators with different Er concentrations of 0.1, 0.5, 1, and 3% were grown by the micro-pulling-down (μ-PD) method. The grown crystals were composed of single-phase material, as demonstrated by powder X-ray diffraction (XRD). The radioluminescence spectra measured under ²⁴¹Am α-ray excitation indicated host emission at approximately 350 nm and Er³⁺ 4f-4f emissions. According to the pulse height spectra recorded under γ-ray irradiation, the 0.5% Er:LuAG exhibited the highest peak channel among the samples. The γ-ray excited decay time profiles were well fitted by the two-component exponential approximation (0.8 μs and 6-10 μs).     

Eu-doped LiF-SrF2 eutectic scintillators for neutron detection

Eu²⁺ 0.05%, 0.1%, and 0.2% activated LiF-SrF₂ eutectic scintillators were prepared by the Bridgman method using ⁶Li enriched (95%) raw material. The α-ray-induced radio luminescence spectra showed intense emission peak at 430 nm due to an emission from Eu²⁺ 5d-4f transition in the Eu:SrF₂ layers. When excited by ²⁵²Cf neutrons, all the samples exhibited almost the same light yields of 5000-7000 ph/n with a typical decay times of several hundreds ns.     

Luminescence properties and gamma-ray response of the Ce and Ca co-doped (Gd,Y)F3 single crystals

The Ca0.5% and Ce1%, 3%, 7%, 10% co-doped Gd_0.5Y_0.5F₃ single crystals were grown by the μ-PD method. In the Ca0.5% and Ce3% co-doped sample, Ce³⁺-perturbed luminescence at 380 nm was observed with 32.4 ns photoluminescence decay time. The energy transfer in the sequence of the regular Ce³⁺→ (Gd³⁺)_n→ the perturbed Ce³⁺ sites was evidenced through observation of decay time shortening of the regular Ce³⁺ and Gd³⁺ centers and the change between the Gd³⁺ and Ce³⁺-perturbed emission intensity. The gamma-ray excited scintillation response of the Ca0.5%, Ce7% co-doped Gd_0.5Y_0.5F₃ sample was investigated with the help of the pulse height spectra and the light yield, energy resolution and non-proportionality was evaluated in the interval of energies of 59.4-1274 keV.     

Basic study of Europium doped LiCaAlF6 scintillator and its capability for thermal neutron imaging application

Eu²⁺ 0.1, 0.5, 1, and 2 mol% doped LiCaAlF₆ single crystalline scintillators were grown by the micro-pulling down (μ-PD) method. Eu²⁺ 2 mol% doped LiCaAlF₆ was also prepared using the Czochralski method. In the transmittance spectra, 4f-5d absorption lines appeared around 200-220 and 290-350 nm. An intense emission at 375 nm due to Eu²⁺ 5d-4f transition was observed under ²⁴¹Am α-ray excitation. When ²⁵²Cf excited pulse height spectra were measured, Eu 2% doped one showed the highest light yield of 29,000 ph/n with 1.15 μs decay time. Using the 2 inchφ Czochralski grown one coupled with the position sensitive photomultiplier tube covered by Cd mask with various size (1, 2, 3, and 5 mm) pin holes, thermal neutron imaging was examined. As a result, the spatial resolution turned out to be better than 1 mm.     

Crystal growth and thermal neutron scintillation response of Ce doped KLiYF5

K⁶Li(Y_1−xCe_x)F₅ (x = 0.003, 0.02) single crystals were grown from the melt using the precise atmosphere control type Micro-Pulling-Down (μ-PD) method to examine their potential as a new thermal neutron scintillators. The grown crystals were single-phase materials as confirmed by XRD. The crystals demonstrated 40-60% transmittance above 320 nm and Ce³⁺ 5d-4f luminescence observed around 340 nm when exited by α-ray. The radio luminescence measurements under thermal neutron excitation (²⁵²Cf) demonstrated the light yield of 890 (Ph/neutron) and the decay time excited by α-ray exhibited 20 and 259 ns.     

Mid-infrared emission characteristics and energy transfer processes in doubly doped Tm, Tb: KPb2Br5 and Tm, Nd: KPb2Br5

We present spectroscopic studies on the ∼5 μm mid-infrared emission and energy transfer properties of Tb³⁺ doped KPb₂Br₅ and Nd³⁺ doped KPb₂Br₅ sensitized by Tm³⁺ ions. A series of co-doped Tm, Tb: KPb₂Br₅ and Tm, Nd: KPb₂Br₅ samples were prepared from purified starting materials of PbBr₂, KBr, and rare-earth bromides. Resonant excitation into the ³H₆ → ³F₄ absorption transition of Tm³⁺ at ∼1.76 μm resulted in an enhanced 5 μm emission from Tb³⁺ and Nd³⁺ ions in Tm, Tb: KPb₂Br₅ and Tm, Nd: KPb₂Br₅, respectively. The existence of energy transfer between Tm → Tb and Tm → Nd in KPB was further evidenced by the quenching of the emission decay times of the ³F₄ → ³H₆ transition of Tm³⁺ in doubly doped Tm, Tb: KPb₂Br₅ and Tm, Nd: KPb₂Br₅ compared to singly doped Tm: KPb₂Br₅.     

Transparent glass ceramic containing Er:CaF2 nano-crystals prepared by sol-gel method

Er³⁺ doped SiO₂-CaF₂ transparent glass ceramic was prepared by sol-gel method. The microstructural evolution of the samples was studied with X-ray diffraction (XRD), transmission electron microscopy (TEM), and infrared spectra (IR). After heat-treatment at 900 °C, the Si-OH bonds and other organic groups were basically eliminated. The CaF₂ crystallites in the sample heat-treated at 900 °C are 10-20 nm in size, distributed homogeneously among the amorphous silica matrix. The efficient upconversion emission for Er^3+.4F_9/2 → ⁴I_15/2 transition was recorded under 980 nm excitation, which could be ascribed to the incorporation of Er³⁺ ions into the CaF₂ nano-crystals with low phonon energy.     

Yb:CaF2 grown by liquid phase epitaxy

Ytterbium doped CaF₂ crystalline layers have been grown for the first time from high temperature solutions at controlled atmosphere by using the liquid phase epitaxy technique. Doped layers having thicknesses between a few microns to a hundred of microns have been grown onto non-oriented and (1 1 1) oriented CaF₂ substrates. The Yb³⁺:CaF₂ layers show structural properties very close to the undoped substrate without any further thermal treatment. Registration of room temperature emission spectra and fluorescence lifetime measurements performed with epitaxial layers corresponding to different ytterbium concentrations show similar spectroscopic properties as in the bulk crystals.     

Structure and scintillation of Eu-activated solid solutions in the BaBr2-BaI2 system

We report the structure and scintillation of Eu²⁺-activated solid solutions in the BaBr₂-BaI₂ system. Samples were synthesized in the form of ∼1 mm size crystals by melting the reactants in a sealed quartz tube followed by slow cooling. The solid solutions form an orthorhombic PbCl₂-type crystal structure with an ordered arrangement of the anions. Upon optical and X-ray excitation, the Eu²⁺-activated samples show an intense emission centered between 410 and 423 nm. The samples exhibit a fast decay characteristic of Eu²⁺, with the primary decay time between 315 and 600 ns for ∼75% of the total emitted light. Light yields for the compositions are compared to a newly discovered scintillator, BaBrI:Eu²⁺, measured under identical conditions.     

Luminescence and scintillation properties of rare-earth-doped LuF3 scintillation crystals

The Nd-doped and Er-doped LuF₃ single crystals were grown by the micro-pulling-down method to study their scintillation properties in the vacuum-ultraviolet (VUV) region. The doubly Nd–Er codoped single crystal was grown to study possibility of scintillation performance improvement by energy transfer from Er³⁺ to Nd³⁺ ions. The LiF flux was to avoid phase transition below melting temperature. The 1%Nd-doped sample showed the highest overall scintillation efficiency under X-ray excitation which was 7 times as high as that of the LaF₃:Nd 8% standard. The leading Nd³⁺ 5d–4f emission was situated at 176 nm, while the Er³⁺ 5d–4f emission for Er-doped samples was observed at 163 nm, which better matches the sensitivity of some VUV-sensitive photodetectors. The optimum Er concentration was determined to be around 1–3 mol%. No Er³⁺ 5d–4f emission was observed for the doubly Er,Nd-codoped sample due to energy transfer from the Er³⁺ to Nd³⁺ ions. Slight improvement of the light yield was observed in the doubly-doped sample with respect to the Nd-only doped one.