rus
Issue #5/2025
S. E. Sarkisov, V. A. Yusim, D. N. Chausov
Crystal Scintillator With a Sensitized Circuit for Converting Ionizing Radiation Into the Visible Spectral Band
Crystal Scintillator With a Sensitized Circuit for Converting Ionizing Radiation Into the Visible Spectral Band
DOI: 10.22184/1993-7296.FRos.2025.19.5.408.418
The inorganic compounds based on crystalline solid solutions are proposed as the materials for visually observable transformation of ionizing radiation. The energy spectra of Ca1‑xLuxF2+x: Eu2+ crystals have showed that the light output is consistently increased upon an increase in the parameter Zeffect related to the advanced LuF3 content. The efficient excitation energy transfer between the impurity rare-earth ions Eu2+ and Pr3+ in the mixed crystals of Ca1‑xLuxF2+x synthesized by the horizontal directional crystallization has been studied. The results of luminescence studies during registration of the γ-, X-ray and short-wave UVC radiation confirm the process of excitation energy transfer from the Eu2+ ions to Pr3+ in Ca1‑xLuxF2+x and allow to consider the Eu2+ ions as an efficient sensitizer of Pr3+ ions. The sensitization-enhanced radiation of Pr3+ ions converts the UV radiation of Eu2+ excited by the γ-quanta into the visible spectral band.
The inorganic compounds based on crystalline solid solutions are proposed as the materials for visually observable transformation of ionizing radiation. The energy spectra of Ca1‑xLuxF2+x: Eu2+ crystals have showed that the light output is consistently increased upon an increase in the parameter Zeffect related to the advanced LuF3 content. The efficient excitation energy transfer between the impurity rare-earth ions Eu2+ and Pr3+ in the mixed crystals of Ca1‑xLuxF2+x synthesized by the horizontal directional crystallization has been studied. The results of luminescence studies during registration of the γ-, X-ray and short-wave UVC radiation confirm the process of excitation energy transfer from the Eu2+ ions to Pr3+ in Ca1‑xLuxF2+x and allow to consider the Eu2+ ions as an efficient sensitizer of Pr3+ ions. The sensitization-enhanced radiation of Pr3+ ions converts the UV radiation of Eu2+ excited by the γ-quanta into the visible spectral band.
Теги: mixed fluorides physical characteristics rare-earth ions scintillation characteristics spectroscopic properties редкоземельные ионы смешанные фториды спектроскопические свойства сцинтилляционные характеристики физические характеристики
Crystal Scintillator With a Sensitized Circuit for Converting Ionizing Radiation
Into the Visible Spectral Band
S. E. Sarkisov 1, V. A. Yusim 1–3, D. N. Chausov 2
National Research Center “Kurchatov Institute”, Moscow, Russia
Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia
Moscow Institute of Physics and Technology (National Research University), Dolgoprudny, Moscow region, Russia
The inorganic compounds based on crystalline solid solutions are proposed as the materials for visually observable transformation of ionizing radiation. The energy spectra of Ca1‑xLuxF2+x: Eu2+ crystals have showed that the light output is consistently increased upon an increase in the parameter Zeffect related to the advanced LuF3 content. The efficient excitation energy transfer between the impurity rare-earth ions Eu2+ and Pr3+ in the mixed crystals of Ca1‑xLuxF2+x synthesized by the horizontal directional crystallization has been studied. The results of luminescence studies during registration of the γ-, X-ray and short-wave UVC radiation confirm the process of excitation energy transfer from the Eu2+ ions to Pr3+ in Ca1‑xLuxF2+x and allow to consider the Eu2+ ions as an efficient sensitizer of Pr3+ ions. The sensitization-enhanced radiation of Pr3+ ions converts the UV radiation of Eu2+ excited by the γ-quanta into the visible spectral band.
Key words: mixed fluorides, rare-earth ions, physical characteristics, scintillation characteristics, spectroscopic properties.
Article received: June 29, 2025
Article accepted: July 21, 2025
INTRODUCTION
Among the large number of crystals activated by the trivalent rare-earth ions (R3+) used as the scintillators of ionizing radiation, a special place is held by the mixed fluoride compounds with a disordered structure [1, 2]. In comparison with the simple crystals (CaF2, SrF2, etc.), the loose crystal lattice of mixed compounds increases the isomorphic content, while facilitating introduction of the necessary impurities into their composition, including the simultaneous matrix doping with several R3+ ions [3, 4]. The well-known disordered fluoride compounds are the two-component crystalline systems based on the solid solutions MeF2 – RF3 (Me=Ca, Sr, Ba; R=Y, La, Gd, Lu) with a cubic fluorite structure (space group Oh5–Fm3m) and RF3–MeF2 with the trigonal tysonite structure (space group D3d4–P3c1). The present paper continues our exploratory studies [5, 6] of scintillation materials based on the crystals with the large values of effective atomic numbers (Zeff) that determine the high capture cross sections of X- and γ-radiation quanta, i. e. the larger Z, the greater the photoemissive effect glow that allows increasing the light output. In these studies, for the crystals activated by the Ce3+ and Pr3+ ionsin order to develop the heavy scintillators, gadolinium trifluoride was used as an additional matrix component that simultaneously served as a sensitizer for the radiation of impurity rare-earth ions.
When the Eu2+ ion was used as an activator, due to the identity of its energy state diagram with the Gd3+ ion [7], the compositions of mixed crystalline matrices should contain the second RF3 -matrix components that are spectrally neutral in relation to the Eu2+ ions. Such fluorite matrices can be Ca1‑xRxF2+x (R=Y, La, Lu). In this paper, the excitation energy transfer mechanisms between the Eu2+ and Pr3+ ions in the mixed crystal Ca1‑xLuxF2+x are studied to assess the prospective use of this scintillator for visualization of the ionizing radiation in a wide range from the soft γ-radiation (from 100 keV) to the ultraviolet spectroscopy (above 4 eV) in the visible light region. Due to the long de-excitation time (~110 μs) of the Pr3+ ionsin crystals with a fluorite structure [8], the material is classified as a slow scintillator, suitable for use in the hand-held measuring devices for radioactive contamination detection and measurement, as well as control over the nuclear materials. The multifunctionality of this scintillation material prepared on the basis of a “heavy” matrix Ca1‑xLuxF2+x with an efficient luminescence sensitization system Eu2+–Pr3+ is characterized by the ability to detect γ–, X– and far UV radiation with conversion to the visible spectral region in the visualizer mode. In the applied field, the studies of the crystals Ca1‑xLuxF2+x : Eu2+, Pr3+ are aimed at the possible development of a single-crystal ionizing radiation detector that is an alternative to the combined multifunctional detectors with comprehensive scintillation compositions.
RESEARCH EQUIPMENT AND METHODS
For these studies, a series of mixed double fluoride mixed crystals CaF2-хLuF3, with the compositions of Ca1‑xLuxEuyPrzF2+x+y+z (x = 0.05; 0.1; y = 0; 0.002; z = 0; 0.003; 0.01; 0.02) were grown by the horizontally directed crystallization (HDC) method. A spectrophotometer SF‑56 was used to measure the absorption spectra. The luminescence studies were performed using a diffraction monochromator MDR‑206 (operational range: 190–5 000 nm) and a multiplier phototube Hamamatsu R6355 (sensitivity range: 185–850 nm with a maximum value at λ = 530 nm) and 100 W Xe lamps, model LAX–C100 (radiation band: 240–1 000 nm) with the narrow-band filters for various UV regions, allowing the monochromatic radiation generation. When measuring the X-ray luminescence in the X-ray radiation device, a BSV‑29(Co) tube was used. The energy (amplitude) spectra of the light pulses of crystals were obtained and registered from a radioactive gamma source 137Cs (Eγ = 662 keV) according to the working procedure [5].
EXPERIMENTAL PROCEDURES AND RESEARCH RESULTS
The peculiarity of fluoride growing with europium is that a high reduction potential is generated in the graphite heating unit of the growth setup and the Eu3+ ions that have a low chemical potential and a high electron affinity, are already reduced to the divalent state Eu2+ during the growth process at T = 420–600 °C with the formation of EuF2:
EuF3 + C0 → EuF2 + CF
Since the energy state diagram of the Eu2+ levels is similar to that ofthe Gd3+ ion, then being in a single impurity pair, the latter quenches the luminescence of Eu2+. Therefore, the studies [9] have shown a zero light output of the bright CaF2: Eu2+ scintillator when GdF3 is introduced into the matrix. In this case, LuF3 was selected as an additional matrix component to CaF2 where due to the electron sheath completely filled to 4f14, the Lu3+ ions demonstrate spectroscopic neutrality in the crystals to the remaining R3+ ions with unfilled 4fN sheaths from Ce3+ to Yb3+. In addition, lutetium is the heaviest element among the lanthanides both in terms of the atomic weight and density (9.8 g/cm3) and has the smallest atomic and ionic radii. For comparison, the ionic radius of Ca2+ = 1.26 angstroms,and the radius of Lu3+ that generates a common lattice with it, is = 1.12 angstroms. Table 1 shows the values of parameter Zeff for two crystals with the lowest and highest LuF3 content.
The energy levels of RE elements determine the state of electron and hole traps in the crystal. The rare-earth ions which 4f levels in the divalent state (R2+) are located below the bottom of the conduction band with ΔE > 13 eV, can capture free electrons being formed when the matrix is exposed to γ- or X-ray radiation. In calcium fluoride, the ground state of 4f Eu2+ ions has the highest ΔE ≈ 3.7 eV among the R2+ ions that classifies them as the deep electron traps.
In the fluorite crystals the state 5d1 of Eu2+ is almost at the same level with the excited level 3P2 of the mixed multiplet 1I6 + 3PJ(2,1,0) of the Pr3+ ion that leads to the efficient Eu2+ → Pr3+ energy transfer and an increased light output of these materials. This fact allows us to consider Eu2+ as a sensitizer of the Pr3+ ion. For this ionic pair, the efficient energy transfer occurs as a result of a noticeable overlap of the 5d → 4f (8S7/2) emission band of the Eu2+ ions with the excitation band 3H4 → 3PJ(0,1,2) of the Pr3+ ions as a part of the pair emissive luminescence relaxation mechanism for the donor-acceptor pairs. In the mixed crystals of solid solutions CaF2–LuF3 : Eu2+, Pr3+ with an increase in the second rare-earth component content in the matrix, the clusters are generated [10]. The clustering process should contribute to the improved energy transfer between two ions Eu2+ and Pr3+ by increasing the probable location of praseodymium ions in close proximity to the neighboring europium ions [11].
Fig. 1 shows a photograph of scintillation for three crystal samples in the visible spectral region, namely, the violet glow at 435 nm in the 5d → 8S7/2 channel of the crystal Ca0.95Lu0.05F2: 0.2 mol.% Eu2+, medium blue glow in the cascade luminescence channels 1S0 → 3P2 (455 nm – blue [12]) and 3Р0 → 3H4 (480 nm – light blue), 0.3 mol.% Pr3+ ions in the crystal Ca0.95Lu0.05F2, bright (white) glow of the multiband (integrated) blue-green-red luminescence sensitized by the Eu2+ ions withthe maximum emission wavelengths of 480, 520, 610 nm, corresponding to the transitions3P0 → 3HJ(4,5,6) 1 mol. % Pr3+ ions in the crystal Ca0.95Lu0.05F2: 0.2 mol.% Eu2+ when irradiated with the X-ray device March‑200 (the end radiation output in a solid angle of ~140°, 360‑degree irradiation, radiation dose at a distance of 500 mm from the focus of the tube: 4R/min).
Fig. 2 shows the amplitude spectra of the Ca1‑xLuxF2+x : Eu2+ crystals. It is evident that even a small increase in the atomic number of the material Zeff (Table 1) leads to the photopeak shift from the concentration of LuF3 (0.1 mol.%, Fig. 2 (2)) to LuF3 (10 mol.%, Fig. 2 (1)) in a higher-energy spectral region.
Table 2 provides the calculation results for light outputs based on the obtained amplitude spectra of the studied crystals by comparing the positions of the total absorption peak maxima with the position of the total absorption peak maximum of the standard sample [13] using the following formula:
LYX = LYNaI,
where LYX is the light output of the measured scintillator, LYNaI is the light output of the standard sample NaI(Tl) = 39 000 F / MeV, AX and ANaI are the positions of photopeaks in the amplitude distribution (channels) of the energy spectrum for the relevant samples.
Figure 3 shows the spectra of the Eu2+ and Pr3+ ions in Ca0.95Lu0.05F2.05 that are related to the process of excitation energy transfer Eu2+ → Pr3+. Luminescence of the Eu2+ ions from the 5d–level on the interconfiguration 4f65d1 → 4f7(8S7/2) transition allowed by the selection rules [15] with λ = 435 nm (the band is shown by a solid line) was excited by UV into a broad absorption band of the Eu2+ ions with a maximum at 300 nm (indicated by a dotted line) associated with the transitions 8S7/2 → 6P7/2, 6IJ(7/2,9/2,11/2),6D9/2. The dots in Fig. 3 indicate the band of intermultiplet absorption of the Pr3+ ions from the ground state 3H4 to the excited states 3PJ(2,1,0).
When the crystal is exposed to the γ-quanta or X-radiation after generation of a free electron matrix in the conduction band and their subsequent multiphonon nonradiative relaxation through the 4fn states to the 5d level, the 5d→4f luminescence of the Eu2+ ions occurs as shown in Fig.
4. In all the indicated cases of exposure of the crystal to the γ-, X- (X-ray) and UV excitation sources, the energy transfer from the sensitizer (Eu2+) to the luminescent Pr3+ ions is performed according to the cross-relaxation scheme: Eu2+(5d,3H4) X Pr3+(8S7/2,3P2).
Figure 5 shows the sensitized luminescence spectra of the Pr3+ ions caused by the excitation energy transfer from the Eu2+ ions according to the cross-relaxation scheme as a result of overlap of the emission band 4f65d1 → 4f7(8S7/2) of the Eu2+ ions with the absorption bands 3H4 → 3PJ(2,1,0) of the Pr3+ ions (see Fig. 3). With the increased concentration of the Pr3+ ions from 1 mol. %, the luminescence on the resonance channel 3P0–3H4 is quenched from the ground state of 3H4 as a result of reabsorption and the brightest luminescence is developed in the red region with λ = 610 nm, belonging to the non-resonant transition 3P0 → 3H6, the final state energy of which 3H6 is located ~4 000 cm−1 higher than the ground state of 3H4 [16, 17]. The spectral lines belonging to these luminescence channels are highlighted separately in Fig. 5.
A significant drop in the luminescence intensity of the Eu2+ ions at the transition 4f65d1 → 4f7(8S7/2) shown in Fig. 6 demonstrates a noticeable effect of decreasing the distance (R) between ions with the increasing concentration of Pr3+ ions on the energy transfer process intensity Eu2+ → Pr3+ by the Forster mechanism (E ≈ 1/R6) [18].
The pair Eu2+–Pr3+ can develop a cooperative process called “quantum cutting” that increases the luminescence efficiency of the active medium as a result of conversion of a high-energy photon into two photons with lower energy [19]. A high-energy UV photon developed into the excited state 6D9/2 (E ≈ 40,000 cm‑1) of the Eu2+ ions can transfer its energy to two Pr3+ ions that will then emit two photons in the visible region from the metastable level 3P0 (E ≈ 20,000 cm‑1). It can increase the luminescence intensity of the Pr3+ ions in the Ca1‑xLuxF2+x: Eu2+, Pr3+ crystal. Numerous studies show that the “quantum cutting” mechanism in the activated inorganic media allows increasing the quantum luminescence yield by more than 100% [20–22]. The influence of the sensitization mechanism by the Eu2+ ions of the luminescence of Pr3+ ions is demonstrated by the spectra shown in Fig. 7, obtained upon excitation by the UV radiation. The luminescence spectrum of the Ca0.937Lu0.05F2 crystal with 0.3 mol.% Eu2+ + 1.0 mol.% Pr3+ was obtained at λexcit. ≈ 300 nm in the UV absorption band of europium (Fig. 2), and without europium upon excitation with λexcit. ≈ 470 nm in the bands of 3P1 + 1I6,3P0 of the Pr3+ ions (1.0 mol.%).
The studies have shown that with an increase in the concentration of Pr3+ ions above 1 mol.% in the crystal, a significant overall concentration quenching of the luminescence of praseodymium ions is observed, including in the non-resonance channels (Fig. 7b).
The observed quenching may be related to the formation of paired Pr(I)–Pr(II) centers and cross-relaxation caused by interactions between the neighboring Pr3+ ions along (1) the path Pr(I)[3P0,3H4] X Pr(II)[3F2,1D2] or along the path (2) Pr(I)[3P0,3H4] X Pr(II)[1G4,1G4], as shown in Fig. 4b. Thus, the optimal concentration of the Pr3+ ions corresponds to 0.2–0.3 mol.% for luminescence excitation at the resonance transition 3P0 → 3H4 in the blue region and 1 mol.% at the transition 3P0 → 3H6 in the red spectral region.
CONCLUSION
The non-stoichiometric Ca1‑xLuxF2+x crystals belonging to the fluorite structural type based on a single-component CaF2 matrix have been studied. The energy spectra and calculated light output of these compounds with an admixture of the Eu2+ and Pr3 ions allow them to be classified as the bright scintillators emitting in the green–blue (480–500 nm) spectral range of the Pr3 ionic wavelengths. An increase in the light output with increasing LuF3 content in the mixed crystalline matrix has been shown. The comprehensive spectral and luminescent studies have demonstrated the existence of efficient excitation energy transfer from the 5d state of the Eu2+ ions to the 3PJ levels of the 4f state of the Pr3+ ions due to the cross–relaxation mechanism. The Eu2+ ions are an efficient sensitizer of the Pr3+ ions, allowing to increase the excitation cross section of the Pr3+ ions. As a result, this can lead to a cooperative process (“quantum cutting”) in the Eu2+–Pr3+ pair, while increasing the luminescence efficiency of the Pr3+ ionsof the Ca1‑x–y–zLuxEuyPrzF2 crystal.
ACKNOWLEDGEMENT
The works were performed within the framework of the public assignment of the National Research Center “Kurchatov Institute” (order No.8 dated January 09, 2025).
ABOUT AUTHORS
Sarkisov Stepan Ervandovich, Cand. of Sc. (Phys.&Math.), Deputy Head of Department, National Research Center “Kurchatov Institute”, e-mail: dr.stevesarkisov@gmail.com; Moscow, Russia.
Yusim Valentin Alexandrovich, Cand. of Sc. (Eng.), Leading Researcher, National Research Center “Kurchatov Institute”, Leading Researcher, Laboratory of Photonics and Organic Electronics, Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow; Associate Professor, Moscow Institute of Physics and Technology (National Research University), e-mail: ValentinYusim@mail.ru; Dolgoprudny, Moscow reg., Russia.
ORCID: 0000-0003-4536-058X
Chausov Denis Nikolaevich, Dr.of Sc. (Phys.&Math.), Associate Professor, Head of the Laboratory of Photonics and Organic Electronics, Prokhorov General Physics Institute of the Russian Academy of Sciences, e-mail: d.chausov@yandex.ru; Moscow, Russia.
ORCID ID: 0000-0002-1287-6427
Contribution of the authors
The authors contributed equally to this research and the preparation of the article.
Conflict of interest
The authors declare that there is no conflict of interest.
Into the Visible Spectral Band
S. E. Sarkisov 1, V. A. Yusim 1–3, D. N. Chausov 2
National Research Center “Kurchatov Institute”, Moscow, Russia
Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow, Russia
Moscow Institute of Physics and Technology (National Research University), Dolgoprudny, Moscow region, Russia
The inorganic compounds based on crystalline solid solutions are proposed as the materials for visually observable transformation of ionizing radiation. The energy spectra of Ca1‑xLuxF2+x: Eu2+ crystals have showed that the light output is consistently increased upon an increase in the parameter Zeffect related to the advanced LuF3 content. The efficient excitation energy transfer between the impurity rare-earth ions Eu2+ and Pr3+ in the mixed crystals of Ca1‑xLuxF2+x synthesized by the horizontal directional crystallization has been studied. The results of luminescence studies during registration of the γ-, X-ray and short-wave UVC radiation confirm the process of excitation energy transfer from the Eu2+ ions to Pr3+ in Ca1‑xLuxF2+x and allow to consider the Eu2+ ions as an efficient sensitizer of Pr3+ ions. The sensitization-enhanced radiation of Pr3+ ions converts the UV radiation of Eu2+ excited by the γ-quanta into the visible spectral band.
Key words: mixed fluorides, rare-earth ions, physical characteristics, scintillation characteristics, spectroscopic properties.
Article received: June 29, 2025
Article accepted: July 21, 2025
INTRODUCTION
Among the large number of crystals activated by the trivalent rare-earth ions (R3+) used as the scintillators of ionizing radiation, a special place is held by the mixed fluoride compounds with a disordered structure [1, 2]. In comparison with the simple crystals (CaF2, SrF2, etc.), the loose crystal lattice of mixed compounds increases the isomorphic content, while facilitating introduction of the necessary impurities into their composition, including the simultaneous matrix doping with several R3+ ions [3, 4]. The well-known disordered fluoride compounds are the two-component crystalline systems based on the solid solutions MeF2 – RF3 (Me=Ca, Sr, Ba; R=Y, La, Gd, Lu) with a cubic fluorite structure (space group Oh5–Fm3m) and RF3–MeF2 with the trigonal tysonite structure (space group D3d4–P3c1). The present paper continues our exploratory studies [5, 6] of scintillation materials based on the crystals with the large values of effective atomic numbers (Zeff) that determine the high capture cross sections of X- and γ-radiation quanta, i. e. the larger Z, the greater the photoemissive effect glow that allows increasing the light output. In these studies, for the crystals activated by the Ce3+ and Pr3+ ionsin order to develop the heavy scintillators, gadolinium trifluoride was used as an additional matrix component that simultaneously served as a sensitizer for the radiation of impurity rare-earth ions.
When the Eu2+ ion was used as an activator, due to the identity of its energy state diagram with the Gd3+ ion [7], the compositions of mixed crystalline matrices should contain the second RF3 -matrix components that are spectrally neutral in relation to the Eu2+ ions. Such fluorite matrices can be Ca1‑xRxF2+x (R=Y, La, Lu). In this paper, the excitation energy transfer mechanisms between the Eu2+ and Pr3+ ions in the mixed crystal Ca1‑xLuxF2+x are studied to assess the prospective use of this scintillator for visualization of the ionizing radiation in a wide range from the soft γ-radiation (from 100 keV) to the ultraviolet spectroscopy (above 4 eV) in the visible light region. Due to the long de-excitation time (~110 μs) of the Pr3+ ionsin crystals with a fluorite structure [8], the material is classified as a slow scintillator, suitable for use in the hand-held measuring devices for radioactive contamination detection and measurement, as well as control over the nuclear materials. The multifunctionality of this scintillation material prepared on the basis of a “heavy” matrix Ca1‑xLuxF2+x with an efficient luminescence sensitization system Eu2+–Pr3+ is characterized by the ability to detect γ–, X– and far UV radiation with conversion to the visible spectral region in the visualizer mode. In the applied field, the studies of the crystals Ca1‑xLuxF2+x : Eu2+, Pr3+ are aimed at the possible development of a single-crystal ionizing radiation detector that is an alternative to the combined multifunctional detectors with comprehensive scintillation compositions.
RESEARCH EQUIPMENT AND METHODS
For these studies, a series of mixed double fluoride mixed crystals CaF2-хLuF3, with the compositions of Ca1‑xLuxEuyPrzF2+x+y+z (x = 0.05; 0.1; y = 0; 0.002; z = 0; 0.003; 0.01; 0.02) were grown by the horizontally directed crystallization (HDC) method. A spectrophotometer SF‑56 was used to measure the absorption spectra. The luminescence studies were performed using a diffraction monochromator MDR‑206 (operational range: 190–5 000 nm) and a multiplier phototube Hamamatsu R6355 (sensitivity range: 185–850 nm with a maximum value at λ = 530 nm) and 100 W Xe lamps, model LAX–C100 (radiation band: 240–1 000 nm) with the narrow-band filters for various UV regions, allowing the monochromatic radiation generation. When measuring the X-ray luminescence in the X-ray radiation device, a BSV‑29(Co) tube was used. The energy (amplitude) spectra of the light pulses of crystals were obtained and registered from a radioactive gamma source 137Cs (Eγ = 662 keV) according to the working procedure [5].
EXPERIMENTAL PROCEDURES AND RESEARCH RESULTS
The peculiarity of fluoride growing with europium is that a high reduction potential is generated in the graphite heating unit of the growth setup and the Eu3+ ions that have a low chemical potential and a high electron affinity, are already reduced to the divalent state Eu2+ during the growth process at T = 420–600 °C with the formation of EuF2:
EuF3 + C0 → EuF2 + CF
Since the energy state diagram of the Eu2+ levels is similar to that ofthe Gd3+ ion, then being in a single impurity pair, the latter quenches the luminescence of Eu2+. Therefore, the studies [9] have shown a zero light output of the bright CaF2: Eu2+ scintillator when GdF3 is introduced into the matrix. In this case, LuF3 was selected as an additional matrix component to CaF2 where due to the electron sheath completely filled to 4f14, the Lu3+ ions demonstrate spectroscopic neutrality in the crystals to the remaining R3+ ions with unfilled 4fN sheaths from Ce3+ to Yb3+. In addition, lutetium is the heaviest element among the lanthanides both in terms of the atomic weight and density (9.8 g/cm3) and has the smallest atomic and ionic radii. For comparison, the ionic radius of Ca2+ = 1.26 angstroms,and the radius of Lu3+ that generates a common lattice with it, is = 1.12 angstroms. Table 1 shows the values of parameter Zeff for two crystals with the lowest and highest LuF3 content.
The energy levels of RE elements determine the state of electron and hole traps in the crystal. The rare-earth ions which 4f levels in the divalent state (R2+) are located below the bottom of the conduction band with ΔE > 13 eV, can capture free electrons being formed when the matrix is exposed to γ- or X-ray radiation. In calcium fluoride, the ground state of 4f Eu2+ ions has the highest ΔE ≈ 3.7 eV among the R2+ ions that classifies them as the deep electron traps.
In the fluorite crystals the state 5d1 of Eu2+ is almost at the same level with the excited level 3P2 of the mixed multiplet 1I6 + 3PJ(2,1,0) of the Pr3+ ion that leads to the efficient Eu2+ → Pr3+ energy transfer and an increased light output of these materials. This fact allows us to consider Eu2+ as a sensitizer of the Pr3+ ion. For this ionic pair, the efficient energy transfer occurs as a result of a noticeable overlap of the 5d → 4f (8S7/2) emission band of the Eu2+ ions with the excitation band 3H4 → 3PJ(0,1,2) of the Pr3+ ions as a part of the pair emissive luminescence relaxation mechanism for the donor-acceptor pairs. In the mixed crystals of solid solutions CaF2–LuF3 : Eu2+, Pr3+ with an increase in the second rare-earth component content in the matrix, the clusters are generated [10]. The clustering process should contribute to the improved energy transfer between two ions Eu2+ and Pr3+ by increasing the probable location of praseodymium ions in close proximity to the neighboring europium ions [11].
Fig. 1 shows a photograph of scintillation for three crystal samples in the visible spectral region, namely, the violet glow at 435 nm in the 5d → 8S7/2 channel of the crystal Ca0.95Lu0.05F2: 0.2 mol.% Eu2+, medium blue glow in the cascade luminescence channels 1S0 → 3P2 (455 nm – blue [12]) and 3Р0 → 3H4 (480 nm – light blue), 0.3 mol.% Pr3+ ions in the crystal Ca0.95Lu0.05F2, bright (white) glow of the multiband (integrated) blue-green-red luminescence sensitized by the Eu2+ ions withthe maximum emission wavelengths of 480, 520, 610 nm, corresponding to the transitions3P0 → 3HJ(4,5,6) 1 mol. % Pr3+ ions in the crystal Ca0.95Lu0.05F2: 0.2 mol.% Eu2+ when irradiated with the X-ray device March‑200 (the end radiation output in a solid angle of ~140°, 360‑degree irradiation, radiation dose at a distance of 500 mm from the focus of the tube: 4R/min).
Fig. 2 shows the amplitude spectra of the Ca1‑xLuxF2+x : Eu2+ crystals. It is evident that even a small increase in the atomic number of the material Zeff (Table 1) leads to the photopeak shift from the concentration of LuF3 (0.1 mol.%, Fig. 2 (2)) to LuF3 (10 mol.%, Fig. 2 (1)) in a higher-energy spectral region.
Table 2 provides the calculation results for light outputs based on the obtained amplitude spectra of the studied crystals by comparing the positions of the total absorption peak maxima with the position of the total absorption peak maximum of the standard sample [13] using the following formula:
LYX = LYNaI,
where LYX is the light output of the measured scintillator, LYNaI is the light output of the standard sample NaI(Tl) = 39 000 F / MeV, AX and ANaI are the positions of photopeaks in the amplitude distribution (channels) of the energy spectrum for the relevant samples.
Figure 3 shows the spectra of the Eu2+ and Pr3+ ions in Ca0.95Lu0.05F2.05 that are related to the process of excitation energy transfer Eu2+ → Pr3+. Luminescence of the Eu2+ ions from the 5d–level on the interconfiguration 4f65d1 → 4f7(8S7/2) transition allowed by the selection rules [15] with λ = 435 nm (the band is shown by a solid line) was excited by UV into a broad absorption band of the Eu2+ ions with a maximum at 300 nm (indicated by a dotted line) associated with the transitions 8S7/2 → 6P7/2, 6IJ(7/2,9/2,11/2),6D9/2. The dots in Fig. 3 indicate the band of intermultiplet absorption of the Pr3+ ions from the ground state 3H4 to the excited states 3PJ(2,1,0).
When the crystal is exposed to the γ-quanta or X-radiation after generation of a free electron matrix in the conduction band and their subsequent multiphonon nonradiative relaxation through the 4fn states to the 5d level, the 5d→4f luminescence of the Eu2+ ions occurs as shown in Fig.
4. In all the indicated cases of exposure of the crystal to the γ-, X- (X-ray) and UV excitation sources, the energy transfer from the sensitizer (Eu2+) to the luminescent Pr3+ ions is performed according to the cross-relaxation scheme: Eu2+(5d,3H4) X Pr3+(8S7/2,3P2).
Figure 5 shows the sensitized luminescence spectra of the Pr3+ ions caused by the excitation energy transfer from the Eu2+ ions according to the cross-relaxation scheme as a result of overlap of the emission band 4f65d1 → 4f7(8S7/2) of the Eu2+ ions with the absorption bands 3H4 → 3PJ(2,1,0) of the Pr3+ ions (see Fig. 3). With the increased concentration of the Pr3+ ions from 1 mol. %, the luminescence on the resonance channel 3P0–3H4 is quenched from the ground state of 3H4 as a result of reabsorption and the brightest luminescence is developed in the red region with λ = 610 nm, belonging to the non-resonant transition 3P0 → 3H6, the final state energy of which 3H6 is located ~4 000 cm−1 higher than the ground state of 3H4 [16, 17]. The spectral lines belonging to these luminescence channels are highlighted separately in Fig. 5.
A significant drop in the luminescence intensity of the Eu2+ ions at the transition 4f65d1 → 4f7(8S7/2) shown in Fig. 6 demonstrates a noticeable effect of decreasing the distance (R) between ions with the increasing concentration of Pr3+ ions on the energy transfer process intensity Eu2+ → Pr3+ by the Forster mechanism (E ≈ 1/R6) [18].
The pair Eu2+–Pr3+ can develop a cooperative process called “quantum cutting” that increases the luminescence efficiency of the active medium as a result of conversion of a high-energy photon into two photons with lower energy [19]. A high-energy UV photon developed into the excited state 6D9/2 (E ≈ 40,000 cm‑1) of the Eu2+ ions can transfer its energy to two Pr3+ ions that will then emit two photons in the visible region from the metastable level 3P0 (E ≈ 20,000 cm‑1). It can increase the luminescence intensity of the Pr3+ ions in the Ca1‑xLuxF2+x: Eu2+, Pr3+ crystal. Numerous studies show that the “quantum cutting” mechanism in the activated inorganic media allows increasing the quantum luminescence yield by more than 100% [20–22]. The influence of the sensitization mechanism by the Eu2+ ions of the luminescence of Pr3+ ions is demonstrated by the spectra shown in Fig. 7, obtained upon excitation by the UV radiation. The luminescence spectrum of the Ca0.937Lu0.05F2 crystal with 0.3 mol.% Eu2+ + 1.0 mol.% Pr3+ was obtained at λexcit. ≈ 300 nm in the UV absorption band of europium (Fig. 2), and without europium upon excitation with λexcit. ≈ 470 nm in the bands of 3P1 + 1I6,3P0 of the Pr3+ ions (1.0 mol.%).
The studies have shown that with an increase in the concentration of Pr3+ ions above 1 mol.% in the crystal, a significant overall concentration quenching of the luminescence of praseodymium ions is observed, including in the non-resonance channels (Fig. 7b).
The observed quenching may be related to the formation of paired Pr(I)–Pr(II) centers and cross-relaxation caused by interactions between the neighboring Pr3+ ions along (1) the path Pr(I)[3P0,3H4] X Pr(II)[3F2,1D2] or along the path (2) Pr(I)[3P0,3H4] X Pr(II)[1G4,1G4], as shown in Fig. 4b. Thus, the optimal concentration of the Pr3+ ions corresponds to 0.2–0.3 mol.% for luminescence excitation at the resonance transition 3P0 → 3H4 in the blue region and 1 mol.% at the transition 3P0 → 3H6 in the red spectral region.
CONCLUSION
The non-stoichiometric Ca1‑xLuxF2+x crystals belonging to the fluorite structural type based on a single-component CaF2 matrix have been studied. The energy spectra and calculated light output of these compounds with an admixture of the Eu2+ and Pr3 ions allow them to be classified as the bright scintillators emitting in the green–blue (480–500 nm) spectral range of the Pr3 ionic wavelengths. An increase in the light output with increasing LuF3 content in the mixed crystalline matrix has been shown. The comprehensive spectral and luminescent studies have demonstrated the existence of efficient excitation energy transfer from the 5d state of the Eu2+ ions to the 3PJ levels of the 4f state of the Pr3+ ions due to the cross–relaxation mechanism. The Eu2+ ions are an efficient sensitizer of the Pr3+ ions, allowing to increase the excitation cross section of the Pr3+ ions. As a result, this can lead to a cooperative process (“quantum cutting”) in the Eu2+–Pr3+ pair, while increasing the luminescence efficiency of the Pr3+ ionsof the Ca1‑x–y–zLuxEuyPrzF2 crystal.
ACKNOWLEDGEMENT
The works were performed within the framework of the public assignment of the National Research Center “Kurchatov Institute” (order No.8 dated January 09, 2025).
ABOUT AUTHORS
Sarkisov Stepan Ervandovich, Cand. of Sc. (Phys.&Math.), Deputy Head of Department, National Research Center “Kurchatov Institute”, e-mail: dr.stevesarkisov@gmail.com; Moscow, Russia.
Yusim Valentin Alexandrovich, Cand. of Sc. (Eng.), Leading Researcher, National Research Center “Kurchatov Institute”, Leading Researcher, Laboratory of Photonics and Organic Electronics, Prokhorov General Physics Institute of the Russian Academy of Sciences, Moscow; Associate Professor, Moscow Institute of Physics and Technology (National Research University), e-mail: ValentinYusim@mail.ru; Dolgoprudny, Moscow reg., Russia.
ORCID: 0000-0003-4536-058X
Chausov Denis Nikolaevich, Dr.of Sc. (Phys.&Math.), Associate Professor, Head of the Laboratory of Photonics and Organic Electronics, Prokhorov General Physics Institute of the Russian Academy of Sciences, e-mail: d.chausov@yandex.ru; Moscow, Russia.
ORCID ID: 0000-0002-1287-6427
Contribution of the authors
The authors contributed equally to this research and the preparation of the article.
Conflict of interest
The authors declare that there is no conflict of interest.
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