Persistent luminescence in powdered and ceramic polycrystalline Gd 3 Al 2 Ga 3 O 12 :Ce

Abstract

This paper studies powders of Gd3Ga3Al2O12:Ce, a promising scintillator composition, as a possible object for express pre-characterization of scintillation kinetics and level of persistent luminescence. Garnet phase powders with uniform microstructure, consisting of 1-2 μm grains, were obtained by co-precipitation approach. It was shown, that both scintillation decay time and presence of persistent luminescence are influenced by both powder thermal treatment temperature and strong Ga deficit.

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Persistent luminescence in powdered and ceramic polycrystalline Gd3Al2Ga3O12:Ce
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Persistent luminescence in powdered and ceramic
polycrystalline Gd3Al2Ga3O12:Ce
G Dosovitskiy1, A Fedorov2, V Mechinsky3, A Borisevich3, A Dosovitskiy4,
E Tret’jak5, M Korjik2,3
1 Institute of Chemical Reagents and High Purity Chemical Substances IREA, 107076
Moscow, Russia
2 Radiation Instruments and New Components, 220113 Minsk, Belarus
3 Research Institute for Nuclear Problems, 220030 Minsk, Belarus
4 NeoChem, 117647 Moscow, Russia
5 Research Institute for Physical Chemical Problems, 220030 Minsk, Belarus
E-mail: george.dos@gmail.com
Abstract. This paper studies powders of Gd3Ga3Al2O12:Ce, a promising scintillator
composition, as a possible object for express pre-characterization of scintillation kinetics and
level of persistent luminescence. Garnet phase powders with uniform microstructure,
consisting of 1-2 μm grains, were obtained by co-precipitation approach. It was shown, that
both scintillation decay time and presence of persistent luminescence are influenced by both
powder thermal treatment temperature and strong Ga deficit.
1. Introduction
Scintillator crystals and phosphors based on Ce-activated mixed garnets Gd3(Ga,Al)5O12 (GGAG) get
increasing interest recent years [1], as they have promising applications on large markets, such as
medical imaging. Light yield of 55000 Ph/MeV was obtained for Gd3Ga3Al2O12:Ce transparent
ceramic [2] and single crystal scintillators [3]. At the same time, strong persistent luminescence was
observed for Gd3(Ga,Al)5O12:Ce ceramics and single crystals grown from raw materials of 99,995% or
better purity [2,4]. Moreover, garnets with Al/Ga solid solution turned out to be promising materials
for long glowing phosphors – strong persistent luminescence was observed in Y3(Ga,Al)5O12:Ce,Cr
ceramic samples [5]. High level of afterglow is a major drawback in application of this material in
scintillation detectors, so studying this effect and its minimizing is a mandatory and an important issue
for its future application.
A number of works was dedicated to optimizing host compound composition. Ogiegło et al [6]
found the most intensive photoluminescence in powder samples of Gd3GaxAl5-xO12:Ce row at x = 2.
For single crystalline materials the most used composition of choice is Gd3Ga3Al2O12:Ce [3,7-10].
As a complex oxide solid solution, GGAG system allows wide composition variations. Scintillation
kinetics depending on composition was studied in a number of works [3,7-10], results are summarized
in Table 1. One can see that kinetics data differ for identical compositions. Inhomogeneous element
distribution could have effect on scintillation properties, such inhomogeneity was reported e.g. for
complex (Lu,Gd)3(Ga,Al)5O12:Ce garnet crystals grown by micropulling-down technique [7]. One of
possible reasons for that is incontrollable Ga content in a crystal. According to thermodynamic data,
maximum evaporation rate of Ga oxide could reach 5*10-5 – 5*10-3 mol*cm-2*s-1 in a temperature
1
2016 International Conference on Defects in Insulating Materials (ICDIM 2016) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 169 (2017) 012014 doi:10.1088/1757-899X/169/1/012014
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range 1800-2000 °C in oxidizing conditions [11]. It is known, that Gd2O3-Ga2O3 system has a
substantial homogeneity range around a garnet phase – 4% according to [12] and 2,5% according to
[13]. Wide homogeneity region accommodating Ga deficit was found for Y2O3-Gd2O3-Ga2O3-Al2O3
system in [2]. Certain homogeneity region was found for Gd2O3-Ga2O3-Al2O3 system as well [14],
which means that scintillator could accommodate a lot of defects, caused by Ga deficit, and still
comprise a single-phase garnet.
Table 1. Scintillation kinetics of GGAG, depending on composition, basing on data from [3,7-10].
Reference:
Composition: Gd3GaxAl(5-x)O12:Ce
x=3
x=2.7
x=2.4
x=2
[3]
LY, Ph/MeV
55 000
58 000
46 000
τsc, ns
(P, %)
97 (80)
353 (20)
172 (88)
1932 (12)
138 (71)
649 (29)
[7]
LY, Ph/MeV
42 217
45 931
τsc, ns
(P, %)
52.8 (73)
282 (27)
221 (100)
[8]
LY, Ph/MeV
56 500
55 600
44 600
τsc, ns
(P, %)
150(61)
490 (39)
150(61)
490 (39)
130(42)
620(31)
6700(27)
[9]
LY, Ph/MeV
45 260
40 740
τsc, ns
(P, %)
89(73)
286(27)
136(69)
647(31)
[10]
(10x10x10
mm)
LY, Ph/MeV
35 600
τsc, ns
(P, %)
156(76)
565(24)
[10]
(2x2x10 mm)
LY, Ph/MeV
34 700
τsc, ns
(P, %)
101(65)
319(35)
Influence of Ga deficit on persistent luminescence is controversial yet. As could be concluded from
[2], Ga deficit could be a possible reason for persistent luminescence in Gd1.49Y1.49Ce0.02Ga2.2Al2.8O12
ceramics. In [1], on the contrary, composition shift towards excess of Gd leads to increase of light
yield and decrease of afterglow in powders with compositions close to (Gd0.988Ce0.012)3(Al0.56Ga0.44)5O12.
Existence of correlation between luminescent properties of GGAG polycrystalline ceramic samples
and single crystals of corresponding composition was shown in [15], which justifies using
polycrystalline samples for preliminary studies of single crystal scintillators.
In order to define an origin of persistent luminescence in Gd3Al2Ga3O12:Ce we have examined
microstructure properties and scintillation kinetics of powder samples and ceramics, which are
synthesized at temperatures up to 1600 °C. Ga volatility is much lower at these conditions, so powder
composition could be better determined then for crystals.
2. Experimental
2.1. Samples preparation
Powders were obtained by co-precipitation technique. Nitrate solutions with 1 mole/l concentrations
were prepared from Gd oxide (5N), Ga nitrate (4N), Al nitrate (5N), NH4-Ce nitrate (4N), then mixed
2
2016 International Conference on Defects in Insulating Materials (ICDIM 2016) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 169 (2017) 012014 doi:10.1088/1757-899X/169/1/012014

in proportion to match desired composition of Gd2.97Ce0.03Ga3-xAl2O12. This joint solution was added to
ammonia solution with concentration 2.5 mole/l at a rate 30 ml/min with constant stirring. Then the
precipitate slurry was kept boiling for 30 minutes, then filtered, washed with water, dried at 100 °C,
crushed, sieved through 100 μm sieve and calcined at 600 °C and then sintered at 1000-1600 °C in air
for 2 hours in molten silica or corundum crucibles.
2.2. Samples characterization
Powders microstructure was observed using Jeol JSM 7100 F scanning electron microscope with EDX
detector Oxford Instruments X-maxN for local element analysis. Powder X-ray diffractometer GBC
Scientific eMMA with Cu Kα radiation was used for phase analysis. Thermogravimetry and
differential thermal analysis were performed using Thermo Scientific SDT Q600 analyzer.
Photoluminescence spectra were measured using Lumex Fluorat-02-Panorama spectrometer with a
fiber lightguide attachment.
2.3. Scintillation measurements
22Na source with gamma emission lines of 511 keV and 1,27 MeV and activity of 1000 Bq was used
for scintillation measurements. Decay curves were registered in 1 μs range using start-stop method
with ratio of starts to stops 0.02.
3. Results and discussion
3.1. Composition and microstructure
When precipitation was conducted according to the procedure, which works for YAG precipitation
[16], using excess of precipitant 20-50%, then 3-7% of Ga remained in mother solution. This
corresponds to Ga hydroxide solubility in ammonia water solutions [17]. Boiling precipitate slurry for
30 minutes allows to decrease Ga losses to no more than 0.5%. Filtered and dried precipitate forms
dense xerogel translucent pieces, consisting of 10-20 nm particles (Figure 1) with homogeneous
elements (Gd, Ga, Al) distribution according to EDX mapping (Figure 2).
Heating the precipitate up to 1200 °C leads to a mass loss of 28%, which corresponds to processes
of release of physically and chemically linked water and precipitate decomposition with
transformation of Gd, Ga, Al, Ce hydroxides to oxide form. Exothermal peak is found at 880 °C on
DTA curve, which, most probably, corresponds to garnet phase formation (Figure 3). In
correspondence with this, powders sintered at 1000-1600 °C contain garnet phase (most close to PDF
73-1371) with peaks seemingly narrowing with temperature increase up to sintering temperature of
1400 °C, which indicates increase of their crystallinity with thermal treatment temperature (Figure 4).
Figure 1. SEM images
of a GGAG xerogel.
3
2016 International Conference on Defects in Insulating Materials (ICDIM 2016) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 169 (2017) 012014 doi:10.1088/1757-899X/169/1/012014

Figure 2. EDX element
mapping of a GGAG
xerogel particle.
Figure 3. TG-DTA curve for GGAG precipitate,
heating rate 10 °C/min.
Figure 4. XRD patterns for GGAG powders,
sintered at different temperatures
Samples microstructure observed by SEM gradually changes with powder sintering temperature
increase (Figure 5). At lower sintering temperatures two types of grains could be seen on images:
larger, 0.5-2 μm size and smaller, ~50 nm for 1000 °C and ~100 nm for 1200 °C. Powder particles
contain noticeable amount of 50-100 nm pores. Raising sintering temperature leads to a more uniform
microstructure: at 1400 °C 0.3-1 μm grains and 0.2-0.3 μm pores, at 1600 °C 1-2 μm grains and up to
1 μm pores (not shown on the image). Faceting of separate grains can be noticed after 1600 °C.
4
2016 International Conference on Defects in Insulating Materials (ICDIM 2016) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 169 (2017) 012014 doi:10.1088/1757-899X/169/1/012014

Figure 5. SEM images of
a GGAG powder, sintered
at different temperatures.
3.2. Luminescent and scintillation properties
Photoluminescence of obtained GGAG powders is typical for Ce-activated garnets with two broad
excitation bands (4f-5d transitions 2F5/2 → 2D5/2 with maximum at 340 nm, 2F5/2 → 2D3/2 with maximum
at 445 nm) and one emission band (5d-4f transition 2D3/2 → 2F7/2, 2F5/2 with maximum at 535 nm)
(Figure 6). Ga content variation ±1% does not change bands positions.
Figure 6. Photoexcitation and
photoluminescence spectra
(normalized) for GGAG powder
with varying Ga content.
Scintillation decay curves were measured to determine samples scintillation kinetics. Background
of random coincidences in scintillation kinetics measurements by start-stop method can be used for
qualitative estimation of samples persistent luminescence. Light yields were not specially measured, as
at the time of the research there was no appropriate technique to adequately estimate light yields on
powder samples, to their values are not reported. They could be qualitatively estimated from necessary
measurement time to collect enough statistics in kinetic measurements. According to that light yields
were the same order of magnitude.
For all samples kinetics contained 2 or 3 exponential components, faster one (τ1) around 40 ns?
slower ones more than 100 ns and averaged scintillation time in the range of 54-81 ns (Table 2).
5
2016 International Conference on Defects in Insulating Materials (ICDIM 2016) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 169 (2017) 012014 doi:10.1088/1757-899X/169/1/012014

Comparing kinetics measurements for samples with different Ga content, one can state, that average
scintillation time is higher for samples with 5% Ga deficit (70-80 ns) compared to samples with Ga
variation of 1% (55-60 ns). Background level in kinetics measurements is close for them, tenting to be
higher for Ga-deficit sample (Figure 7).
Effect of increase of sample thermal treatment temperature can be anticipated from comparison of
measurements results for GGAG powder sintered at 1000 °C and GGAG ceramics of the same
composition sintered at 1600 °C (Figure 8). It leads to insignificant change of scintillation decay curve
and averaged scintillation decay time – 67 ns for 1000 °C powder and 72 ns for 1600 °C ceramics. At
the same time, there is also a noticeable difference in background level, indicating that low
temperature sample has noticeably lower level of persistent luminescence.
As a proposal for future work, described synthesis approach could be used to prepare homogeneous
pre-synthesized raw materials for single crystal growth and test its influence on single crystalline
material kinetics and level of persistent luminescence.
Figure 7. Scintillation
decay curves for GGAG
powder sintered at
1600 °C with
composition
Gd2.97Ce0.03Ga2.85Al2O12
(a) and
Gd2.97Ce0.03Ga3Al2O12 (b).
1 channel equals 1 ns.
Dashed line indicates
background level
estimation.
Figure 8. Scintillation
decay curves for GGAG
powder sintered at
1000 °C (a) and GGAG
ceramics of the same
composition sintered at
1600 °C (b). 1 channel
equals 1 ns. Dashed line
indicates background
level estimation.
6
2016 International Conference on Defects in Insulating Materials (ICDIM 2016) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 169 (2017) 012014 doi:10.1088/1757-899X/169/1/012014

Table 2. Scintillation decay components for GGAG samples of different compositions: powder
sintered at 1000 °C, ceramics sintered at 1600 °C, all other – powders sintered at 1600 °C.
Composition:
τ1, ns
P1, %
τ2, ns
P2, %
τ3, ns
P3, %
τaverage, ns
Gd2.97Ce0.03Ga3.03Al2O12
48
81%
108
19%
'
'
60'
Gd2.97Ce0.03Ga3Al2O12
37
79%
117
21%
'
'
54'
Gd2.97Ce0.03Ga2.97Al2O12
41
59%
82
41%
'
'
58'
Gd2.97Ce0.03Ga2.85Al2O12
42
67%
132
33%
'
'
70'
Gd2.97Ce0.03Ga2.85Al2O12
47
72%
177
28%
'
'
81'
Gd2.97Ce0.03Ga2.85Al2O12 1000
37'
56%'
80'
32%'
172'
12%'
67'
Gd2.97Ce0.03Ga2.85Al2O12 ceram.
45'
67%'
128'
32%'
'
'
72'
4. Conclusions
Powder polycrystalline materials could be obtained with homogeneous composition, well-defined μm-
size grains and well-formed garnet phase. Deficit of Ga in powder samples was shown to slow down
scintillation kinetics, increasing averaged scintillation time from 55-60 ns to 70-80 ns. Slightly higher
level of background in kinetic measurements was observed in these samples, indicating higher level of
persistent luminescence. GGAG ceramics, heat treated at 1600 °C has shown higher level of
afterglow, compared to powder heat treated at 1000 °C. So, to use polycrystalline samples as objects
for express pre-characterization of single crystal materials, such temperature-dependent parameters as
microstructure and crystallinity should be taken into account.
Acknowledgements
Work was supported by Russian Ministry of Science and Education, Subsidy agreement
№ 14.625.21.0033 dated 27.10.2015, project identifier RFMEFI62515X0033.
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2016 International Conference on Defects in Insulating Materials (ICDIM 2016) IOP Publishing
IOP Conf. Series: Materials Science and Engineering 169 (2017) 012014 doi:10.1088/1757-899X/169/1/012014

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IOP Conf. Series: Materials Science and Engineering 169 (2017) 012014 doi:10.1088/1757-899X/169/1/012014