On the Luminescence of (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
upon X-ray Exposure
Max-Fabian Volhard and Thomas Jüstel
Department of Chemical Engineering, Münster University of Applied Sciences,
Stegerwaldstrasse 39, D-48565 Steinfurt, Germany
Keywords: Solid State Actinometer, (Ba
1-X/2
Sr
1-X/2
)
2
SiO
4
:Eu
x
, Ortho-Silicates, X-ray Excitation, X-ray Luminescence.
Abstract: Eu
2+
doped ortho-silicates are widely applied as luminescent materials in phosphor converted light emitting
diodes (pcLEDs) (Park et al. 2003; Jüstel et al. 2012), due to their high quantum yield, strong absorption,
short decay time, and long-term stability upon blue light excitation.
The optical properties of (Ba
1-x/2
Sr
1-x/2
)
2
SiO
4
:Eu
x
upon x-ray radiation are, however, much less known and
were thus analysed in this work. It turned out that the activator ions in Eu
3+
activated ortho-silicates are
reduced to Eu
2+
upon excitation by high energy radiation (> 5.0 eV). This reduction process can be
monitored by the fading of the red line emission of Eu
3+
in the range between 590 and 710 nm, originating
from the [Xe]4f
6
-[Xe]4f
6
transitions of Eu
3+
. At the same time, a novel green broad emission band caused by
the interconfigurational transition [Xe]4f
7
-[Xe]4f
6
5d
1
shows up (fig. 1). This spectral change is a function of
the irradiation period and can be quantified by the colour point shift of the emitted spectrum as well (fig. 2).
1 INTRODUCTION
Luminescent materials for the detection of high
energy radiation are widely applied as storage
materials. These storage phosphors consist of a host
material comprising defects. Such defects are e.g.
hole traps located above the valence band or electron
traps below the conduction band. The storage
material is charged by high energy radiation, which
promote charge carriers from the valence band to the
conduction band. There are two mechanisms to
release the charge carriers. In the first case the traps
are not deep enough and the charge carriers have the
chance to leave the traps at room temperature. This
process is known as afterglow or persistent
luminescence. If the traps are deep enough, much
deeper than the energy related to ambient
temperature, there is no way for the charge carriers
to leave the traps without stimulation via lattice
vibration or optical photons (Kulesza et al. 2015;
Blasse & Grabmaier 1994).
Storage phosphors such as BaFBr:Eu
2+
or
BaFCl:Eu
2+
(Li et al. 2002; Secu et al. 2000) are
common for this application area. In order to analyse
the radiation dose of a storage phosphor, the
functional principle via stimulation of the traps is
complicated with a rather high relative uncertainty
(Kulesza et al. 2015; Blasse & Grabmaier 1994).
This work describes the detection of high energy
radiation via the change of photoluminescence
properties. It turned out that the activator ions in
Eu
3+
activated ortho-silicates are reduced to Eu
2+
upon excitation by high energy radiation (> 5.0 eV).
This reduction process can be monitored by the
fading of the red line emission of Eu
3+
in the range
between 590 and 710 nm, originating from the
[Xe]4f
6
-[Xe]4f
6
transitions of Eu
3+
. Other materials
where the change of luminescence upon high energy
radiation has already been described are Ga
2
O
3
(from red to blue) and Sr
2
SiO
4
(from orange-red to
yellow-white) (Nag & Kutty 2004; Layek et al.
2015).
Therefore, these Eu
3+
doped ortho-silicates are
promising candidates for the application in a solid
state actinometer, which can be used to monitor a
perceived x-ray radiation dose. Another application
field could be the 2D x-ray mapping in flat
detectors, wherein each phosphor particle can be
individually read out after the x-ray exposure.
2 EXPERIMENTAL
All samples were synthesized by a high temperature
solid state reaction. BaCO
3
(Alfa Aesar, 99,8%),
SrCO
3
(Aldrich, 99,9%), Eu
2
O
3
(Treibacher,
122
Volhard M. and JÃijstel T.
On the Luminescence of (Ba0.5Sr0.5)2SiO4:Eu3+ upon X-ray Exposure.
DOI: 10.5220/0006269101220125
In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 122-125
ISBN: 978-989-758-223-3
Copyright
c
 2017 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

99,99%) and SiO
2
(Merck) were used as starting
materials without further purification. The educts
were weighted in stoichiometric amounts,
thoroughly blended as acetone slurry in an agate
mortar. For the substitution of Eu
3+
for Sr
2+
/Ba
2+
no
charge compensation was performed. The
suspension was dried at ambient temperatures and
was then transferred into an alumina boat. The
powder was calcined at 1350 °C for 4 h in an oxygen
atmosphere (Westfalen AG). After firing the
phosphor was cooled down and ground to a fine
powder again.
The phase purity of the samples was validated by
using x-ray powder diffraction (XRD). The XRD
measurements were performed on a Rigaku Miniflex
II (Cu Kα; 30 kV; 15 mA), operating between 10°
and 80° (2θ) with a step width of 0,02° and an
integration time of 5°/min.
Optical properties like PLE and PL were measured
on an Edinburgh Instruments FLS 920, which is
equipped with an Xe arc lamp (450 W) and a cooled
single photon PMT detection unit (Hamamatsu
R2658P). Diffuse reflectance spectra were measured
using an Edinburgh Instruments FS 920
spectrometer. This device is equipped with Xe arc
lamp, a cooled single photon PMT detection
Hamamatsu R928 and a spectralon sphere coated
with Teflon. It was measured against a white
standard (BaSO
4
, 99,998%).
X-ray exposure tests were measured using a Rigaku
Miniflex II (Cu Kα; 30 kV; 15 mA; 450 W). The
samples were transferred into sample carriers and
irradiated for different time periods between 40° and
41° (2θ).
3 RESULTS AND DISCUSSION
The phase purity of (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
was
proven by XRD. The herein investigated ortho-
silicates crystallise in an orthorhombic crystal
structure with a space group Pnma (#62) wherein
two different Ba
2+
/Sr
2+
sites exist (Wang et al.
2013). Eu
3+
(0,112 nm CN = 9) is a somewhat
smaller than Ba
2+
/Sr
2+
(Ba
2+
: 0,147 nm; Sr
2+
: 0,131
nm; CN = 9) and occurs on the Ba
2+
-sites (Ahrens
1952). Figure 1 shows the XRD pattern of
(Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
and the respective ICDD
reference card of the orthorhombic Ba
2
SiO
4
. The
comparison confirms that there is no phase impurity
present.
10 20 30 40 50 60 70 80
Normalized Intensity (a.u.)
(Sr
0.495
Ba
0.495
Eu
0.01
)
2
SiO
4
4 h, 1350 °C, O
2
2  (°)
Ba
2
SiO
4
ICDD: 04-011-2153
Figure 1: XRD pattern of (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
and ICDD
reference card 04-011-2153 of Ba
2
SiO
4
.
Optical properties of chemically reduced
(Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
2+
were investigated by
measuring the PLE, PL and reflectance spectra
(Figure 2). The excitation spectrum was monitored
at λ
em
= 612 nm and it is shown a broad excitation
band between approx. 250 nm to 500 nm with a
maximum at about 427 nm caused by [Xe]4f
7
-
[Xe]4f
6
5d
1
transition. A green broad emission band
between 470 nm to 650 nm with a maximum at
about 524 nm was observed monitoring at 427 nm.
(Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
shows a high reflectance
among 500 – 800 nm and a strong absorption band
below 500 nm resulted by [Xe]4f
7
-[Xe]4f
6
5d
1
transition.
200 250 300 350 400 450 500 550 600 650 700 750 800
0,0
0,2
0,4
0,6
0,8
1,0
Normalized Intensity (a.u.)
Wavelength [nm]
(Ba,Sr)
2
SiO
4
:Eu
2+
Excitation ( 
Em
= 524 nm)
Emission ( 
Ex
= 427 nm)
Reflectance
0
20
40
60
80
100
Reflectance [%]
5 4,5 4 3,5 3 2,5 26,2 1,55
Energy (eV)
Figure 2: Room temperature PLE, PL and Reflectance
spectra of (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
2+
.
Figure 3 depicts the photoluminescence and the
reflectance of a typical (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
sample. The excitation spectrum was monitored at
λ
em
= 612 nm. Many parity forbidden [Xe]4f
6
-
[Xe]4f
6
transitions are observed, as it is typical for
trivalent Europium. The
7
F
0

5
L
7
(394 nm)
transition shows the highest absorption strength. PL
On the Luminescence of (Ba0.5Sr0.5)2SiO4:Eu3+ upon X-ray Exposure
123

of the Eu
3+
phosphor was recorded for excitation at
394 nm. There are a set of emission lines
corresponding to the intraconfigurational
5
D
0

7
F
J
(J = 0-5) transitions. In the visible region the
reflectance is nearly 100% and decrease below 350
nm due to
7
F
0

5
H
6
transition (Baur & Jüstel 2015;
QIAO et al. 2009).
200 250 300 350 400 450 500 550 600 650 700 750 800
0,0
0,2
0,4
0,6
0,8
1,0
Normalized Intensity [a.u.]
Wavelength [nm]
(Sr
0.4875
Ba
0.4875
Eu
0.025
)
2
SiO
4
Excitation ( 
Em
= 612 nm)
Emission ( 
Ex
= 394 nm)
Reflectance
0
20
40
60
80
100
Reflectance [%]
5 4,5 4 3,5 3 2,5 26,2 1,55
Energy (eV)
Figure 3: Room temperature emission, excitation, and
reflection spectra of (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
.
It was observed that the described ortho-silicate
reacts with high energy radiation (> 5.0 eV) and the
activator cation was reduced from Eu
3+
to Eu
2+
.
Figure 4 illustrate the emission spectra monitored at
λ
ex
= 394 nm in respect of x-ray exposure time.
Without x-ray radiation there is only the Eu
3+
lines
emission observed. The exposure time was increased
in 30 s steps and measured the PL properties. It was
shown that the line emission of Eu
3+
decreases with
exposure time and the green broad band emission
(Eu
2+
) was increased. In temperature phosphor
thermometry it is well proved that the temperature is
measured via the ratio of two emission lines/bands.
This method should also be possible for the
measurement of the radiation dose (Hertle et al.
2017; Rabasović et al. 2016). Figure 5 shows
(Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
after x-ray exposure. It was
used an ortho-silicate without any stabilisation (e.g.
Li
+
or Na
+
), because a stabilised Eu
3+
avoid
reduction. There is a possibility for a back reaction
to Eu
3+
. At 500 °C (or higher) for 4 h in air, it was
measured that all Eu
2+
particles were oxidized.
450 500 550 600 650 700 750
0
2000
4000
6000
8000
10000
12000
14000
16000
Intensity [cts]
Wavelength [nm]
180 sec. X-Rays
150 sec. X-Rays
120 sec. X-Rays
90 sec. X-Rays
60 sec. X-Rays
30 sec. X-Rays
0 sec. X-Rays
(Sr
0.495
Ba
0.495
Eu
0.01
)
2
SiO
4

Ex
= 394 nm
Figure 4: Emission spectra of (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
upon
394 nm excitation after exposure to x-ray radiation as
function of the exposure period.
Figure 5: Luminescence of (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
(red
emission) and (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
2+
(green emission) after
x-ray exposure.
The perceived emission colour is a result of additive
colour mixing of the line emission of remaining Eu
3+
particles and of the broad band emission of Eu
2+
-
particles. The CIE 1931 colour diagram depicts the
colour points of a (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
sample
treated for different exposure times. It nicely shows
the colour change (Figure 6 and Table 1).
Table 1: Calculated colour points of (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
upon 394 nm excitation after exposure to x-ray radiation
as function of the exposure period.
Time [s] x y
0 0.4699 0.3923
30 0.3984 0.4676
60 0.3702 0.4974
90 0.3611 0.5088
120 0.3496 0.5203
150 0.3422 0.5285
180 0.335 0.5357
PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology
124

420
470
480
490
500
510
520
530
540
550
560
570
580
590
600
610
680
0
30
60
90
120
150
180
x
y
Figure 6: CIE1931 colour triangle comprising the colour
points x, y of a (Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
sample upon 394 nm
excitation after exposure to x-ray radiation as function of
the exposure period.
4 CONCLUSIONS
A solid state reaction was used to prepare
(Ba
0.5
Sr
0.5
)
2
SiO
4
:Eu
3+
without applying co-dopants
for valence state stabilisation. This phosphor
exhibits strong 4f-4f line emission at around 590,
613, and 705 nm. Upon x-ray exposure, the
photoluminescence changes to a green broad band
emission, which can be assigned to the formation of
divalent Europium. First results show that a strong
dependence between exposure time and the intensity
of Eu
2+
emission exists.
Therefore, these Eu
3+
doped ortho-silicates are
promising candidates for the application in a solid
state actinometer, which can be used to monitor a
perceived x-ray radiation dose. Another application
field could be the 2D x-ray mapping in flat
detectors, wherein each phosphor particle can be
individually read out after the x-ray exposure. The
detectors can be easily reactivated, since it was
shown that the process was completely reversible at
a temperature of about 500 °C.
ACKNOWLEDGEMENTS
The authors are grateful to Merck KGaA Darmstadt,
Germany for their generous financial support.
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On the Luminescence of (Ba0.5Sr0.5)2SiO4:Eu3+ upon X-ray Exposure
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