Development of Composite Scintillators Based on the LuAG: Pr Single Crystalline Films and LuAG:Sc Single Crystals

The scintillation properties of novel type of composite scintillator based on Lu3Al5O12:Pr (LuAG:Pr) single crystalline film (SCF) and LuAG:Sc substrate grown by the liquid-phase epitaxy method are considered in this work. The registration of α-particles and γ-quanta in such types of composites occurs by means of separation of the scintillation decay kinetics of SCF and crystal parts, respectively. Namely, under excitation by α-particles of 241Am (5.5 MeV) source and γ-quanta of 137Cs (662 keV) source, the large differences in the respective scintillation decay kinetics and decay time values tα and tγ are observed for the LuAG:Pr SCF/LuAG:Sc SC composite scintillator with various film thicknesses. Furthermore, the best tγ/tα ratio above 4.5 is achieved for such types of epitaxial structure with SCF and substrate thicknesses of 17 μm and about 0.5 mm, respectively. The development types of composite scintillators can be successfully applied for simultaneous registration of α-particles and γ-quanta in the mixed radiation fluxes.

The LPE method permits us to choose the film scintillator thickness close to the pathway of registering particles in scintillation materials. Namely, the thickness of film scintillators, which are necessary for the complete stopping of α-particles from 239 Pu and 241 Am sources with an energy of 5.15 and 5.5 MeV, respectively, typically lies in the 12-15 µm range [21]. Furthermore, the LPE technology offers also wide possibility of the development of composite scintillator materials of a "phoswich type" (phosphor sandwich). Such types of scintillators enable separate registration of the various components in the mixed ionization fluxes. Namely, the LPE-grown epitaxial structures of the garnets, containing SCF and substrate scintillators, can be used for registration of α-particles (absorbed only in the SCF part) and γ-rays (stopped mainly in SC substrate). For this reason, the composite scintillators based on the mentioned epitaxial structures can be used in nuclear research, radiation monitoring, microtomography and many other devices for detection of ionization radiation.
The first example of LPE grown composite scintillators was produced in 1990 [21]. These composite scintillators were based on the epitaxial structures of Y 3 Al 5 O 12 (YAG) garnet. Namely, the two types of YAG:Ce SCF/YAG:Nd SC and YAG:Ce SCF/YAG:Sc SC composite scintillators were grown by using the LPE method for simultaneous registration of αand β-particles, as well as α-particles and X-rays or low-energy γ-quanta [21]. Meanwhile, due to the low density, = 4.52 g/cm 3 , and small effective atomic number, Z eff = 29, the YAG SC substrates can be used only for detection of low-energy types of ionizing radiations.
The LuAG garnet host possesses a significantly higher value, = 6.7 g/cm 3 and Z eff = 61 in comparison with YAG [22]. For this reason, crystals of LuAG:Ce, LuAG:Pr and LuAG:Sc garnets are the well-known scintillators for radiation monitoring and computer tomography [23,24]. Furthermore, the LuAG host is a very prospective material for the creation of composite scintillators as well. Recently, several types of epitaxial structures based on the Ce 3+ and Pr 3+ doped SCFs and Ce 3+ , Pr 3+ and Sc 3+ doped SC of LuAG garnets were successfully grown by using the LPE method, and the scintillation properties of respective composite scintillators were investigated [25][26][27]. Namely, we confirm in these works that the LuAG:Pr SCF/LuAG:Ce SC, LuAG:Sc SCF/LuAG:Ce SC and (Lu,Tb)AG:Ce SCF/LuAG:Pr SC epitaxial structures can be used for the detection of α-particles and γ-rays by means of the differences in the pulse height spectra and decay kinetics the SCF and SC parts of composite scintillators.
The present work involved searching for the new types of effective composite scintillators for simultaneous registration of the different components of mixed ionizing fluxes, including α-particles and γ-rays with improved functional properties. In this work, we present the results on crystallization and investigation of the optical and scintillation properties of composite scintillators based on the LuAG:Pr SCF/LuAG:Sc SC epitaxial structures grown by the LPE method.

Growth of Composite Detectors
The LuAG:Sc substrates with the 10 × 10 mm 2 size and the 0.5 mm thickness were used for creation of the composite scintillators ( Figure 1). The substrates were prepared from LuAG:Sc crystals with a Sc concentration of 0.25 at.%, grown by using the Czochralski method in Crutur Ltd., Czech Republic. the composite scintillators based on the mentioned epitaxial structures can be used clear research, radiation monitoring, microtomography and many other devices fo tion of ionization radiation.
The first example of LPE grown composite scintillators was produced in 19 These composite scintillators were based on the epitaxial structures of Y3Al5O12 garnet. Namely, the two types of YAG:Ce SCF/YAG:Nd SC and YAG:Ce SCF/YAG composite scintillators were grown by using the LPE method for simultaneous r tion of α-and β-particles, as well as α-particles and X-rays or low-energy γ-quan Meanwhile, due to the low density, ρ = 4.52 g/cm 3 , and small effective atomic num = 29, the YAG SC substrates can be used only for detection of low-energy types of io radiations.
The LuAG garnet host possesses a significantly higher value, ρ = 6.7 g/cm 3 an 61 in comparison with YAG [22]. For this reason, crystals of LuAG:Ce, LuAG: LuAG:Sc garnets are the well-known scintillators for radiation monitoring and co tomography [23,24]. Furthermore, the LuAG host is a very prospective material creation of composite scintillators as well. Recently, several types of epitaxial str based on the Ce 3+ and Pr 3+ doped SCFs and Ce 3+ , Pr 3+ and Sc 3+ doped SC of LuAG were successfully grown by using the LPE method, and the scintillation propertie spective composite scintillators were investigated [25][26][27]. Namely, we confirm i works that the LuAG:Pr SCF/LuAG:Ce SC, LuAG:Sc SCF/LuAG:Ce SC and (Lu,Tb) SCF/LuAG:Pr SC epitaxial structures can be used for the detection of α-particles rays by means of the differences in the pulse height spectra and decay kinetics t and SC parts of composite scintillators.
The present work involved searching for the new types of effective composi tillators for simultaneous registration of the different components of mixed io fluxes, including α-particles and γ-rays with improved functional properties. In thi we present the results on crystallization and investigation of the optical and scint properties of composite scintillators based on the LuAG:Pr SCF/LuAG:Sc SC ep structures grown by the LPE method.

Growth of Composite Detectors
The LuAG:Sc substrates with the 10*10 mm 2 size and the 0.5 mm thickness we for creation of the composite scintillators ( Figure 1). The substrates were prepare LuAG:Sc crystals with a Sc concentration of 0.25 at.%, grown by using the Czoc method in Crutur Ltd., Czech Republic.  The set of LuAG:Pr SCF/LuAG SC composite scintillators was grown by using the LPE method from the super-cooled melt solutions based on the PbO-B 2 O 3 flux (see Reference [16] for details). Figure 1 shows an example of the mentioned composite scintillators. Later, two epitaxial structures with different LuAG:Pr SCF thicknesses (21 and 17 µm), grown onto the LuAG:Sc SC substrates with a thickness of 0.5 mm, were selected for investigation of the scintillation properties of this type of composite scintillators (Table 1). The sample LuAG:Pr SCF was crystalized also onto undoped YAG substrate at relatively the same growth conditions for comparison with the properties of composite scintillators. The growth conditions of the LuAG:Pr SCF and LuAG:Pr SCF/LuAG:Sc SC epitaxial structures, selected for investigation of the content and structural properties, as well for studying their absorption, cathodoluminescent and scintillation properties, were summarized in Table 1.
The structural quality of the composite scintillators was characterized by the X-ray diffraction ( Figure 2). From the respective XRD patterns of these SCFs, we can also calculate the lattice constants of the SCF and SC parts of epitaxial structures and estimate the misfit between their lattice constants m = (a SCF − a sub )/a sub *100% (Figure 2a). Namely, the LuAG:Sc substrate and a1 LuAG:Pr SCF possess the lattice constants of 11.9103 Å and 11.9157 Å, respectively, and the SCF/substrate misfit value m = 0.045% (Figure 2a). The set of LuAG:Pr SCF/LuAG SC composite scintillators was grown by using the LPE method from the super-cooled melt solutions based on the PbO-B2O3 flux (see Reference [16] for details). Figure 1 shows an example of the mentioned composite scintillators. Later, two epitaxial structures with different LuAG:Pr SCF thicknesses (21 and 17 μm), grown onto the LuAG:Sc SC substrates with a thickness of 0.5 mm, were selected for investigation of the scintillation properties of this type of composite scintillators (Table 1). The sample LuAG:Pr SCF was crystalized also onto undoped YAG substrate at relatively the same growth conditions for comparison with the properties of composite scintillators. The growth conditions of the LuAG:Pr SCF and LuAG:Pr SCF/LuAG:Sc SC epitaxial structures, selected for investigation of the content and structural properties, as well for studying their absorption, cathodoluminescent and scintillation properties, were summarized in Table 1.
The structural quality of the composite scintillators was characterized by the X-ray diffraction ( Figure 2). From the respective XRD patterns of these SCFs, we can also calculate the lattice constants of the SCF and SC parts of epitaxial structures and estimate the misfit between their lattice constants m = (aSCF − asub)/asub*100% (Figure 2a). Namely, the LuAG:Sc substrate and a1 LuAG:Pr SCF possess the lattice constants of 11.9103 Å and 11.9157 Å, respectively, and the SCF/substrate misfit value m = 0.045% (Figure 2a). For study of the uniformity of LuAG:Pr SCFs LPE grown onto LuAG:Sc SC substrates at so small SCF/substrate misfit the rocking curves (RCs) of these samples in the 2θ scan mode were measured and compared (Figure 2b). RCs of the mentioned samples were recorded by using CuKα radiation in a double crystal spectrometer with a silicon monochromator.
As can be seen from Figure 2b, the RCs of LuAG:Sc and LuAG:Pr substrate show very good symmetry and uniformity of peaks in the ω scan mode. Meanwhile, the FWHM values of RCs for LuAG:Pr SCF are equal to 0.032 degrees, which is 1.6 times larger than For study of the uniformity of LuAG:Pr SCFs LPE grown onto LuAG:Sc SC substrates at so small SCF/substrate misfit the rocking curves (RCs) of these samples in the 2θ scan mode were measured and compared (Figure 2b). RCs of the mentioned samples were recorded by using Cu Kα radiation in a double crystal spectrometer with a silicon monochromator.
As can be seen from Figure 2b, the RCs of LuAG:Sc and LuAG:Pr substrate show very good symmetry and uniformity of peaks in the ω scan mode. Meanwhile, the FWHM values of RCs for LuAG:Pr SCF are equal to 0.032 degrees, which is 1.6 times larger than FWHM values of 0.02 degrees for the corresponding peaks in LuAG:Sc substrate (Figure 2b). At the same time, the structural quality of the LuAG:Pr SCF, which is proportional to the FWHM of rocking curves, is very high due to lower SCF-substrate misfit values m.

Experimental Results
The absorption spectra, cathodoluminescence (CL) spectra, scintillation light yield (LY), energy resolution and scintillation decay kinetics measurements under excitation by α-particles and γ-quanta were used for characterization of the properties of LuAG:Sc substrate and two samples LuAG:Pr SCF/LuAG:Sc SC composite scintillators.
The absorption spectra in the 200-1100 nm range were measured using by a UV-Vis Jasco 760 spectrometer. The CL spectra in the 200-925 nm range were measured by using a Stellar Net spectrometer with TE-cooled CCD detector under excitation by electron gun from SEM JEOL JSM-820 electron microscope working at U = 30 kV, I = 0.1 µA. The pulse height spectra (PHS) of all SCFs and SC substrate samples were measured with a shaping time of 12 µs, using the setup based on a Hamamatsu H6521 photomultiplier (PMP) and multi-channel analyzer (MCA) under excitation by α-particles of 239 Pu (5.15 MeV) source, and the results of these measurements were used for determination of their scintillation LY (Table 1). Namely, these PHS were compared with the spectra of standard YAG:Ce SCF sample with a photoelectron yield of 360 phels/MeV and a LY of 2650 photons/MeV [17] and also with the reference LuAG:Sc substrate. The scintillation decay kinetics under the mentioned α-particles excitation were measured in the detector based on the Hamamatsu H6521 PMT and digital Tektronix TDS3052 oscilloscope. All the measurements were performed at room temperature (RT).
The scintillation LY and decay kinetics of the selected samples of composite scintillators (see Table 1) were also tested by using the setup based on a HPMT DEP PP0475B hybrid PMT, Ortec 672 spectroscopy preamplifier and 927 ASPEC MCA and PC control. The PHSs were measured under excitation by α-particles of 241 Am (5.5 MeV) and γ-rays of 137 Cs (662 keV) sources, respectively.
It is worth noting here that the α-particles of 239 Pu and 241 Am sources absorb only in SCF scintillators because the pathway of α-particles in the studied materials lies in the 12-15 µm range. Figure 3 presents the absorption spectra of the LuAG:Pr SCF/LuAG:Sc SC composite scintillators measured in the comparison with the absorption spectra of the LuAG:Pr SCF, grown onto undoped YAG substrate. The E 2 and E 1 absorption bands, peaked at 239 and 284 nm, are related to the allowed 4f-5d 1,2 transitions of Pr 3+ ions in LuAG:Pr SCFs. The other absorption bands, which peaked in 260-262 nm and below 200 nm, correspond to the 1 S 0 → 3 P 1 and 1 S 0 → 1 P 1 transitions of Pb 2+ flux impurity in these SCFs, respectively. The absorption spectra of LuAG:Sc substrate show only the wide low-intensive band peaked around 257 nm ( Figure 3). Most probably, this absorption band is connected with very common O 2− →Fe 3+ charge transfer transitions of Fe 3+ trace impurity [28,29] in the raw oxides used for LuAG:Pr SC growth.

Cathodoluminescence Spectra (CL)
The CL spectra of two LuAG:Pr SCF/LuAG:Sc SC composite scintillators with different SCF thicknesses in comparison with CL spectrum of LuAG:Sc substrate are shown in Figure 4. The peaks at 275 nm in CL spectrum LuAG:Sc SC substrate are related to the luminescence of excitons localized and bound with Sc 3+ isoelectronic impurities in Al 3+ octahedral positions of the garnet host [21,25]. The CL spectra of LuAG:Sc SC substrate also possess the low-intensive emission band peaked at 595 nm. Most probably, this band corresponds to the luminescence of dimer or more complex charged oxygen vacancies with one or two trapped electrons [30,31]. The luminescence of such centers has been recently observed in several oxide hosts, namely in (Y,Lu)AlO 3 perovskites [31]. Taking into account this assumption, the luminescence of such defect centers in LuAG:Sc SC can be excited via the UV luminescence of Sc 3+ centers. It is also worth noting that, due to the overlap with the corresponding Pr 3 + absorption bands (Figure 3), UV light from the LuAG: Sc substrate may be partially reabsorbed by Pr3 + ions in SCF scintillators.

Cathodoluminescence Spectra (CL)
The CL spectra of two LuAG:Pr SCF/LuAG:Sc SC composite scintillators with different SCF thicknesses in comparison with CL spectrum of LuAG:Sc substrate are shown in Figure 4. The peaks at 275 nm in CL spectrum LuAG:Sc SC substrate are related to the luminescence of excitons localized and bound with Sc 3+ isoelectronic impurities in Al 3+ octahedral positions of the garnet host [21,25]. The CL spectra of LuAG:Sc SC substrate also possess the low-intensive emission band peaked at 595 nm. Most probably, this band corresponds to the luminescence of dimer or more complex charged oxygen vacancies with one or two trapped electrons [30,31]. The luminescence of such centers has been recently observed in several oxide hosts, namely in (Y,Lu)AlO3 perovskites [31]. Taking into account this assumption, the luminescence of such defect centers in LuAG:Sc SC can be excited via the UV luminescence of Sc 3+ centers. It is also worth noting that, due to the overlap with the corresponding Pr 3 + absorption bands (Figure 3), UV light from the LuAG: Sc substrate may be partially reabsorbed by Pr3 + ions in SCF scintillators.    (3,4) in comparison with the absorption spectra of LuAG:Pr SCF grown onto YAG substrate (1) and LuAG:Sc substrate (2).

Cathodoluminescence Spectra (CL)
The CL spectra of two LuAG:Pr SCF/LuAG:Sc SC composite scintillators with different SCF thicknesses in comparison with CL spectrum of LuAG:Sc substrate are shown in Figure 4. The peaks at 275 nm in CL spectrum LuAG:Sc SC substrate are related to the luminescence of excitons localized and bound with Sc 3+ isoelectronic impurities in Al 3+ octahedral positions of the garnet host [21,25]. The CL spectra of LuAG:Sc SC substrate also possess the low-intensive emission band peaked at 595 nm. Most probably, this band corresponds to the luminescence of dimer or more complex charged oxygen vacancies with one or two trapped electrons [30,31]. The luminescence of such centers has been recently observed in several oxide hosts, namely in (Y,Lu)AlO3 perovskites [31]. Taking into account this assumption, the luminescence of such defect centers in LuAG:Sc SC can be excited via the UV luminescence of Sc 3+ centers. It is also worth noting that, due to the overlap with the corresponding Pr 3 + absorption bands (Figure 3), UV light from the LuAG: Sc substrate may be partially reabsorbed by Pr3 + ions in SCF scintillators.   The CL spectra of LuAG:Pr SCF and LuAG:Pr SCF/LuAG:Sc SC epitaxial structures ( Figure 4, curves 2 and 3) shows two intensive UV emission bands, peaked at 309 and 379 nm, related to the 5d 1 -4f( 3 H 4-6 , 3 H 5 , 4 F 2 ; 1 G 4 ) transitions of Pr 3+ ions. The narrow emission bands in the visible range of LuAG:Pr SCF are related to the 4f-4f transitions of Pr 3+ ions from 3 P 0 -and 1 D 2 levels to 3 H j states. In LuAG:Pr SCF/LuAG:Sc SC epitaxial structures, these sharp Pr 3+ emission bands overlaps with the wide band of defect centers in LuAG:Sc substrate peaked at 595 nm. Most probably, the luminescence of these defect centers in LuAG:Sc SC is excited by the UV emission of Pr 3+ ions in the respective LuAG:Pr SCFs.

Pulse Height Spectra
The PHS of two LuAG:Pr SCF/LuAG:Sc SC composite scintillator samples under excitation by α-particles and γ-ray of 241 Am and 137 Cs sources are shown in Figure 5a (Table 1). Meanwhile, the variations in the content of Pb 2+ flux related impurity in LuAG:Pr SCFs grown from PbO based flux at different temperatures (Table 1) result also in the notable deviation of their scintillation LY (Figure 5a) (see also References [32,33]).
spectively. The main peaks in Figure 5a correspond to the total absorption of α-particles with an energy of 5.5 MeV. The peaks in the left part of the spectrum are related to the absorption of the 59.6 keV low-energy line of 241 Am source. The positions and shape of the main photopeaks are slightly different for various SCF and substrate scintillators. This means that α-particles excite only LuAG:Pr SCF parts of composite scintillators. These results on the LY measurements of composite scintillators under α-particle excitation by 241 Am source (Figure 5a) are coherent with the LY of these samples under excitation by 239 Pu source (Table 1). Meanwhile, the variations in the content of Pb 2+ flux related impurity in LuAG:Pr SCFs grown from PbO based flux at different temperatures (Table 1) result also in the notable deviation of their scintillation LY (Figure 5a) (see also References [32,33]). Under excitation of LuAG:Pr SCF/LuAG:Ce SC composite scintillators by γ-rays from 137 Cs source, the significant Compton scattering tools are present in PHS (Figure 5b; see also Reference [27]). Meanwhile, the last peaks in PHS corresponds to the total absorption of γ quanta with an energy of 662 keV (Figure 5b). The low-energy line of 137 Cs source corresponds to additional peak at energy of 32 keV.
It is important to note here the different positions of main PHS photopeaks for LuAG:Pr SCF/LuAG:Sc SC epitaxial structures and LuAG:Sc substrate (Figure 5b). This means that γ-rays, apart from excitation of the substrate, excite SCF scintillators as well. For this reason, the total scintillation LY of LuAG:Pr SCF/LuAG:Sc SC composite scintillators under γ-ray excitation will be also affected by LY of SCF scintillators. As a consequence, the position of the main PHS photopeaks corresponding to the absorption of 662 keV quanta of 137 Cs source depends notably on the thickness and LY of the SCF scintillators.
It should be noted here that, in the case of excitation of with an energy of 662 keV of the 137 Cs radioisotope, the total attenuation coefficient of the LuAG host is ~0.095 cm 2 /g [27]. Taking into account the same attenuation factors for the film and the substrate, the LuAG:Pr SCFs with a total thickness of 34-42 μm on both parts of the substrate can absorb gamma quanta in the amount of 6.8-8.5%, compared to 100% for the LuAG substrate with a thickness of 500 μm. Therefore, the effect of influence of SCFs on total scintillation LY and decay kinetics of the tested composite scintillators is predicted a priori. Under excitation of LuAG:Pr SCF/LuAG:Ce SC composite scintillators by γ-rays from 137 Cs source, the significant Compton scattering tools are present in PHS (Figure 5b; see also Reference [27]). Meanwhile, the last peaks in PHS corresponds to the total absorption of γ quanta with an energy of 662 keV (Figure 5b). The low-energy line of 137 Cs source corresponds to additional peak at energy of 32 keV.
It is important to note here the different positions of main PHS photopeaks for LuAG:Pr SCF/LuAG:Sc SC epitaxial structures and LuAG:Sc substrate (Figure 5b). This means that γ-rays, apart from excitation of the substrate, excite SCF scintillators as well. For this reason, the total scintillation LY of LuAG:Pr SCF/LuAG:Sc SC composite scintillators under γ-ray excitation will be also affected by LY of SCF scintillators. As a consequence, the position of the main PHS photopeaks corresponding to the absorption of 662 keV quanta of 137 Cs source depends notably on the thickness and LY of the SCF scintillators.
It should be noted here that, in the case of excitation of with an energy of 662 keV of the 137 Cs radioisotope, the total attenuation coefficient of the LuAG host is~0.095 cm 2 /g [27]. Taking into account the same attenuation factors for the film and the substrate, the LuAG:Pr SCFs with a total thickness of 34-42 µm on both parts of the substrate can absorb gamma quanta in the amount of 6.8-8.5%, compared to 100% for the LuAG substrate with a thickness of 500 µm. Therefore, the effect of influence of SCFs on total scintillation LY and decay kinetics of the tested composite scintillators is predicted a priori.

LY
The variations of LY (in ph/MeV) of LuAG:Pr/LuAG:Sc epitaxial structures and LuAG:Sc substrate measured within the 0.5-10 μs shaping time interval under α-particle excitation by 241 Am (5.5 MeV) and γ-ray excitation by 137 Cs (662 keV) are shown in Figure  6a,b, respectively. Furthermore, the values of LYα and LYγ and their LYα/LYγ ratios for the mentioned scintillators, detected with "fast" (0.5 μs) and "slow" (10 μs) shaping times under α-particles and γ-rays excitation, respectively, are presented in Table 2.  In the case of α-particles excitation, the maximal LY values of 1735 and 1457 ph/MeV for LuAG:Sc substrate and LuAG:Pr SCF/LuAG:Sc SC a1 epitaxial structure, respectively, are reached at a shaping time of 6 μs ( Figure 6a). Meanwhile, in the case of γ-ray excitation, the LY maxima of these scintillators, being equal to 7819 and 5012 ph/MeV, respectively, are reached already at a shaping time of 10 μs (Figure 6b). We have also found that the LYα/LYγ value for LuAG:Sc SC scintillator varies in the 0.19-0.23 range within the 0.5-10 μs shaping time interval (Table 2), and such a value is coherent with the value of 0.2 for YAG:Ce SC scintillator [21,27].

Energy Resolution
The variation of the energy resolution of LuAG:Sc SC substrate and LuAG:Pr SCF/LuAG:Sc SC epitaxial structures, detected with the 0.5-10 μs shaping time under excitation by α-particles and γ-rays, is shown in Figure 7a,b, respectively. As can be seen from Figure 7a, the energy resolution of substrate and a1 composite sample under registration of α-particles shows opposite trend on the LY/shaping time dependences ( Figure  6) and lies in the 13.2-30.4% and 14.4-32.3% ranges, respectively. On the contrary, the  In the case of α-particles excitation, the maximal LY values of 1735 and 1457 ph/MeV for LuAG:Sc substrate and LuAG:Pr SCF/LuAG:Sc SC a1 epitaxial structure, respectively, are reached at a shaping time of 6 µs (Figure 6a). Meanwhile, in the case of γ-ray excitation, the LY maxima of these scintillators, being equal to 7819 and 5012 ph/MeV, respectively, are reached already at a shaping time of 10 µs (Figure 6b). We have also found that the LY α /LY γ value for LuAG:Sc SC scintillator varies in the 0.19-0.23 range within the 0.5-10 µs shaping time interval (Table 2), and such a value is coherent with the value of 0.2 for YAG:Ce SC scintillator [21,27].

Energy Resolution
The variation of the energy resolution of LuAG:Sc SC substrate and LuAG:Pr SCF/ LuAG:Sc SC epitaxial structures, detected with the 0.5-10 µs shaping time under excitation by α-particles and γ-rays, is shown in Figure 7a,b, respectively. As can be seen from Figure 7a, the energy resolution of substrate and a1 composite sample under registration of α-particles shows opposite trend on the LY/shaping time dependences ( Figure 6) and lies in the 13.2-30.4% and 14.4-32.3% ranges, respectively. On the contrary, the energy resolution of a2 composite sample at the detection of α-particles shows significantly less variation on the shaping time and lies between 16.1% and 18.4% in whole 0.5-10 ms range. However, under detection of γ-rays, the energy resolution of a1 and a2 composite samples is notably changed within the shaping time and lies in the 13.5-18.7% and 13.8-17.8% ranges, respectively. energy resolution of a2 composite sample at the detection of α-particles shows significantly less variation on the shaping time and lies between 16.1% and 18.4% in whole 0.5-10 ms range. However, under detection of γ-rays, the energy resolution of a1 and a2 composite samples is notably changed within the shaping time and lies in the 13.5-18.7% and 13.8-17.8% ranges, respectively. Meanwhile, the energy resolution of the LuAG:Sc Sc substrate under detection of γrays is notably better and lies in the 7.7-14.3% range (Figure 7b).

Scintillation Decay Kinetics
In the case of creation of composite scintillators, based on the LPE grown epitaxial structures of garnet compounds, it is very important to analyze firstly the scintillation decay profiles of SC substrates under α-particle and γ-ray excitation in a wide range of decay time and emission intensity. Namely, the scintillation decay kinetics of LuAG:Sc SC substrate under α-particle and γ -ray excitation is presented in Figure 8. The tα/tγ or tγ/tα ratios of scintillation intensity decay to 1/e, 0.1 and 0.05 levels were used also for description of the difference between the respective scintillation decay profiles under α-particles and γ-quanta excitation (Table 3).  Meanwhile, the energy resolution of the LuAG:Sc Sc substrate under detection of γ-rays is notably better and lies in the 7.7-14.3% range (Figure 7b).

Scintillation Decay Kinetics
In the case of creation of composite scintillators, based on the LPE grown epitaxial structures of garnet compounds, it is very important to analyze firstly the scintillation decay profiles of SC substrates under α-particle and γ-ray excitation in a wide range of decay time and emission intensity. Namely, the scintillation decay kinetics of LuAG:Sc SC substrate under α-particle and γ -ray excitation is presented in Figure 8. The t α /t γ or t γ /t α ratios of scintillation intensity decay to 1/e, 0.1 and 0.05 levels were used also for description of the difference between the respective scintillation decay profiles under α-particles and γ-quanta excitation (Table 3). energy resolution of a2 composite sample at the detection of α-particles shows significantly less variation on the shaping time and lies between 16.1% and 18.4% in whole 0.5-10 ms range. However, under detection of γ-rays, the energy resolution of a1 and a2 composite samples is notably changed within the shaping time and lies in the 13.5-18.7% and 13.8-17.8% ranges, respectively. Meanwhile, the energy resolution of the LuAG:Sc Sc substrate under detection of γrays is notably better and lies in the 7.7-14.3% range (Figure 7b).

Scintillation Decay Kinetics
In the case of creation of composite scintillators, based on the LPE grown epitaxial structures of garnet compounds, it is very important to analyze firstly the scintillation decay profiles of SC substrates under α-particle and γ-ray excitation in a wide range of decay time and emission intensity. Namely, the scintillation decay kinetics of LuAG:Sc SC substrate under α-particle and γ -ray excitation is presented in Figure 8. The tα/tγ or tγ/tα ratios of scintillation intensity decay to 1/e, 0.1 and 0.05 levels were used also for description of the difference between the respective scintillation decay profiles under α-particles and γ-quanta excitation (Table 3).  The rate of separation of the scintillation decay profiles under detection of α-particles and γ-quanta can be significantly improved in the composite scintillators based on the SCFs and crystals of different garnet compounds in comparison with respective crystalsubstrates ( Figure 9). The following experiments of LuAG:Pr SCF/LuAG:Sc SC LPE grown epitaxial structures confirm such a suggestion. Namely, the separation of the scintillating decay profiles of SCF and SC substrate parts of such composite scintillator can be obtained in the wide 100-3500 ns time interval, where the scintillation response is significantly faster under γ ray excitation than that under α-particle excitation (Figure 9). Table 3. Time dependence of intensity of scintillation decay of LuAG:Sc SC substrate from the initial value at t = 0 to 1/e, 0.1 and 0.05 levels.

Intensity
LuAG:Sc Substrate t α , ns t γ , ns t α /t γ or t γ /t α Ratio  Table 3. Time dependence of intensity of scintillation decay of LuAG:Sc SC substrate from the initial value at t = 0 to 1/e, 0.1 and 0.05 levels. The rate of separation of the scintillation decay profiles under detection of α-particles and γ-quanta can be significantly improved in the composite scintillators based on the SCFs and crystals of different garnet compounds in comparison with respective crystalsubstrates (Figure 9). The following experiments of LuAG:Pr SCF/LuAG:Sc SC LPE grown epitaxial structures confirm such a suggestion. Namely, the separation of the scintillating decay profiles of SCF and SC substrate parts of such composite scintillator can be obtained in the wide 100-3500 ns time interval, where the scintillation response is significantly faster under γ ray excitation than that under α-particle excitation (Figure 9). The abovementioned conclusion is illustrated also by comparison of the differences in the decay times of the intensity decay to 1/e, 0.1, 0.05 and 0.01 levels under α-particle and γ-rays excitation (so called tγ/tα ratio) in the LuAG:Pr SCF/LuAG:Sc SC composite scintillators and the reference LuAG:Sc substrate ( Figure 10 and Table 4). Specifically, for LuAG:Pr SCF/LuAG:Sc SC a2 sample, the tγ/tα ratio is significantly large between 1/e to 0.01 levels of intensity decay in comparison with a1 sample (Figure 9). Furthermore, the best separation of the scintillation decay from the SCF and SC parts of such composite scintillator, being equal to tγ/tα = 1.55-3.3, can be obtained in the relatively narrow range from 0.065 to 0.01 levels in the time intervals 75-3500 ns, which are shown by the dashed line in Figure 9b. The abovementioned conclusion is illustrated also by comparison of the differences in the decay times of the intensity decay to 1/e, 0.1, 0.05 and 0.01 levels under α-particle and γ-rays excitation (so called t γ/ t α ratio) in the LuAG:Pr SCF/LuAG:Sc SC composite scintillators and the reference LuAG:Sc substrate ( Figure 10 and Table 4). Specifically, for LuAG:Pr SCF/LuAG:Sc SC a2 sample, the t γ/ t α ratio is significantly large between 1/e to 0.01 levels of intensity decay in comparison with a1 sample (Figure 9). Furthermore, the best separation of the scintillation decay from the SCF and SC parts of such composite scintillator, being equal to t γ/ t α = 1.55-3.3, can be obtained in the relatively narrow range from 0.065 to 0.01 levels in the time intervals 75-3500 ns, which are shown by the dashed line in Figure 9b. Table 4. Time dependence of scintillation intensity decay from the initial value at t = 0 to 1/e, 0.1 and 0.05 level for LuAG:Pr SCF/LuAG:Sc SC a1 and a2 composite scintillator samples under α-particle excitation by 241 Am source and γ-ray excitation by 137 Cs source.

Discussion
For analysis of the differences between the decay profiles of composite scintillator under α-particle and γ-ray excitation, the most important value is tα/tγ or tγ/tα ratio, which needs to be "as large as it is possible" in the broad time interval for the selected combination of scintillation materials used for SCF and substrate parts [21]. It is acceptable that the scintillation response from SCF and substrate scintillators can be readily separated if tα/tγ or tγ/tα ratio exceeds 1.5 [34]. In this case, the so-called Δ parameter (introduced as difference tγ/tα-1 for more clear interpretation of the rate of γ/α discrimination) overcomes the 0.5 value (Figure 10). Such a demand is fully filled for the a1 and a2 samples of LuAG:Ce SCF/LuAG:Sc SC epitaxial structures ( Figure 10, curves 2 and 3, respectively).
The reasons for the mentioned differences in the separation of the scintillation profiles under α-particle and γ-ray excitations can be related to (i) the different interaction processes of the particles and quanta with the same garnet host and/or to (ii) the different thicknesses of SCF and substrate parts of the composite scintillator. Generally, the shape of scintillation decay profile of epitaxial structure under γ-ray excitation will depend also on the ratio between SCF and substrate thickness and their scintillation LY.
Therefore, the selection of the suitable ratio of thickness of SCF and substrate parts is very important for the optimization of the figure of merit of composite scintillator.
In overall case, it is optimal that the thickness of SCF scintillator is slightly exceeds the penetration depth of detected particles. For SCF of LuAG garnet, such thickness is equal to 12-15 μm for detection of particles with an energy of 5.15-5.5 MeV [11]. In this case, the unwanted absorption of γ-rays by SCF scintillator will be minimal ( Figure 11). On the other hand, due to the demand for the absorption of γ-quanta with the energy in the tens-hundreds KeV range, the thickness of LuAG:Sc substrate needs to be "as thick as it is possible". Meanwhile, taking into account the need of stable mounting of thick and

Discussion
For analysis of the differences between the decay profiles of composite scintillator under α-particle and γ-ray excitation, the most important value is t α/ t γ or t γ/ t α ratio, which needs to be "as large as it is possible" in the broad time interval for the selected combination of scintillation materials used for SCF and substrate parts [21]. It is acceptable that the scintillation response from SCF and substrate scintillators can be readily separated if t α/ t γ or t γ/ t α ratio exceeds 1.5 [34]. In this case, the so-called ∆ parameter (introduced as difference t γ/ t α − 1 for more clear interpretation of the rate of γ/α discrimination) overcomes the 0.5 value (Figure 10). Such a demand is fully filled for the a1 and a2 samples of LuAG:Ce SCF/LuAG:Sc SC epitaxial structures ( Figure 10, curves 2 and 3, respectively).
The reasons for the mentioned differences in the separation of the scintillation profiles under α-particle and γ-ray excitations can be related to (i) the different interaction processes of the particles and quanta with the same garnet host and/or to (ii) the different thicknesses of SCF and substrate parts of the composite scintillator. Generally, the shape of scintillation decay profile of epitaxial structure under γ-ray excitation will depend also on the ratio between SCF and substrate thickness and their scintillation LY.
Therefore, the selection of the suitable ratio of thickness of SCF and substrate parts is very important for the optimization of the figure of merit of composite scintillator.
In overall case, it is optimal that the thickness of SCF scintillator is slightly exceeds the penetration depth of detected particles. For SCF of LuAG garnet, such thickness is equal to 12-15 µm for detection of particles with an energy of 5.15-5.5 MeV [11]. In this case, the unwanted absorption of γ-rays by SCF scintillator will be minimal ( Figure 11). On the other hand, due to the demand for the absorption of γ-quanta with the energy in the tens-hundreds KeV range, the thickness of LuAG:Sc substrate needs to be "as thick as it is possible". Meanwhile, taking into account the need of stable mounting of thick and heavy substrate in Pt holder for LPE growth, the most optimal thicknesses of LuAG:Sc substrates lie in the 0.5-1 mm range. Taking into account the abovementioned, the optimal values of t γ/ t α ratio at registration of α-particles and γ-quanta, using LuAG:Pr SCF/LuAG:Sc SC composite scintillator, being equal to 1.55-3.3 at intensity decay from 0.065 to 0.01 levels in the 75-3500 ns time interval, are obtained for the a2 sample with a SCF thickness of 17 µm and a substrate thickness of 0.5 mm (Figure 9b). heavy substrate in Pt holder for LPE growth, the most optimal thicknesses of LuAG:Sc substrates lie in the 0.5-1 mm range. Taking into account the abovementioned, the optimal values of tγ/tα ratio at registration of α-particles and γ-quanta, using LuAG:Pr SCF/LuAG:Sc SC composite scintillator, being equal to 1.55-3.3 at intensity decay from 0.065 to 0.01 levels in the 75-3500 ns time interval, are obtained for the a2 sample with a SCF thickness of 17 μm and a substrate thickness of 0.5 mm (Figure 9b).

Conclusions
The new type of composite scintillator, based on LuAG:Pr SCFs with the thickness in the 17-21 μm range, and LuAG:Sc substrate with a thickness of 0.5 mm, prepared from the respective single crystal (SC) with a Sc content of 0.25 at.%, was grown by using the LPE method from melt solutions, using PbO-B2O3 flux.
The notable differences in the scintillation decay kinetics of LuAG:Pr SCF/LuAG:Sc SC epitaxial structures are observed within the two decades of intensity decay from 1.0 to 0.01 levels in the wide time interval up to 3500 ns under excitation by α-particles by 241 Am (5.5 MeV) and γ-quanta by 137 Cs (0.662 MeV) sources. Such differences can be characterized by the decay time ratio tγ/tα, which, for this type of composite scintillator (at SCF and a substrate thickness of 17 μm and 0.5 mm, respectively), reaches the largest values within tγ/tα = 1.55-3.3 range at the intensity decay from the 0.065 level down to 0.01 level in the time interval from 75 to 3500 ns. Such value of tγ/tα ratio allows one to easily perform the time discrimination of the signals detected by the SCF and SC parts of composite scintillators. For this reason, the LPE grown LuAG:Pr SCF/and LuAG:Sc SC epitaxial structures can be successfully used for the simultaneous detection of α-particles and γ-rays in the mixed fluxes of ionization radiation.

Conclusions
The new type of composite scintillator, based on LuAG:Pr SCFs with the thickness in the 17-21 µm range, and LuAG:Sc substrate with a thickness of 0.5 mm, prepared from the respective single crystal (SC) with a Sc content of 0.25 at.%, was grown by using the LPE method from melt solutions, using PbO-B 2 O 3 flux.
The notable differences in the scintillation decay kinetics of LuAG:Pr SCF/LuAG:Sc SC epitaxial structures are observed within the two decades of intensity decay from 1.0 to 0.01 levels in the wide time interval up to 3500 ns under excitation by α-particles by 241 Am (5.5 MeV) and γ-quanta by 137 Cs (0.662 MeV) sources. Such differences can be characterized by the decay time ratio t γ /t α , which, for this type of composite scintillator (at SCF and a substrate thickness of 17 µm and 0.5 mm, respectively), reaches the largest values within t γ /t α = 1.55-3.3 range at the intensity decay from the 0.065 level down to 0.01 level in the time interval from 75 to 3500 ns. Such value of t γ /t α ratio allows one to easily perform the time discrimination of the signals detected by the SCF and SC parts of composite scintillators. For this reason, the LPE grown LuAG:Pr SCF/and LuAG:Sc SC epitaxial structures can be successfully used for the simultaneous detection of α-particles and γ-rays in the mixed fluxes of ionization radiation.