1
Radiation Effects on the Photoluminescence
of Rare-earth Doped Pyrochlore Powders
S. L. Weeden-Wright, S. L. Gollub, R. Harl, A. B. Hmelo, D. M. Fleetwood, B. R. Rogers,
R. D. Schrimpf, and D. G. Walker
Abstract—This work examined the sensitivity of the
luminescence spectra of europium-doped lanthanum zirconate to
different types and amounts of radiation exposure. For the
samples and radiation sources used in this work (X-rays and
protons), changes in the photoluminescence of europium-doped
lanthanum zirconate were minimal. However, when the phosphor
was paired with an alternate luminescent material, which had a
differing radiation response, relative changes in the
photoluminescence of the two materials were correlated to the
radiation exposure.
Index
Terms—photoluminescence,
proton
irradiation,
pyrochlore phosphors, radiation effects, X-ray irradiation
I. INTRODUCTION
V
arious phosphors have been used as radiation detectors
and dosimeters for a range of applications. Specifically,
phosphors have been utilized as scintillation detectors and
storage phosphors. Scintillation detectors use phosphors to
convert incident X-ray or gamma-ray radiation into UV or
visible light. The light emitted is immediately captured by a
photodector, typically a photomultiplier tube, and is analyzed
for dosimetry [1-2]. Unlike scintillation technology, which
detects incident radiation at the time of exposure, storage
phosphors are able to maintain radiation information several
days after the initial exposure. Storage phosphors, whose
matrix materials are generally salts, have been used for several
decades as dosimeters in space and medical applications [3-5].
When exposed to ionizing radiation, storage phosphors trap
electrons and holes at defect sites. Trapped electrons and holes
recombine to emit light when stimulated optically or
thermally, also refered to as Optically or Thermally Stimulated
Luminescence (OSL or TSL). Emission from the
recombination of electron-hole pairs can be correlated to the
dose of the incident radiation. Current storage phosphor
technology includes detection for X-rays, gamma-rays,
Manuscript received September 28, 2012. This work was supported in part
by the Defense Threat Reduction Agency (DTRA) under the Grant HDTRA110-1-0112.
S. L. Weeden-Wright, D. M. Fleetwood, and R. D. Schrimpf are with
Department of Electrical Engineering and Computer Science, Vanderbilt
University, Nashville, TN 37215 USA (phone: 206-225-1010; e-mail:
[email protected]).
S. L. Gollub and D. G. Walker are with the Department of Mechanical
Engineering, Vanderbilt University, Nashville, TN 37215 USA.
R. Harl, and B. R. Rogers are with the Department of Chemical
Engineering, Vanderbilt University, Nashville, TN 37215 USA.
neutrons and thermal neutrons. There have been various other
studies investigating the radiation effects on ceramic
phosphors. For example, Hollerman et al. reported degradation
of photoluminescence during proton irradiation of rare earth
oxides, including yttrium aluminum garnet (YAG) phosphor
powders [6]. Kaczmarek et al. studied the effects of proton
and gamma exposure on the absorption coefficients of various
oxide compounds, including YAG phosphors [7]. Despite the
history of phosphor powders and their use as detectors, little is
known about how radiation affects the photoluminescence
spectra of phosphor materials.
The effects of radiation on the luminescence of europiumdoped lanthanum zirconate (LZO) are reported in this work.
Rare-earth doped ceramic oxides are effectively inert under
various ambient conditions, including high temperatures,
humidity and oxidizing environments. Consequently, any
radiation-induced defects are less likely to anneal in ceramic
oxides. Europium-doped LZO has a photoluminescence
spectrum that is highly affected by the state of the crystalline
structure surrounding the activator. Damage to the crystalline
structure from radiation-induced defects has the potential to
change the photoluminescence of europium-doped LZO.
Europium-doped LZO powders were fabricated using a
combustion synthesis method and irradiated to quantify the
sensitivity of the photoluminescence to different types and
amounts of radiation. Two different sample sets were created
in order to understand the radiation response of LZO by itself
and the radiation reponse of LZO when another luminescent
material is present. Lanthanum zirconate samples were
exposed to ionizing and non-ionizing radiation. Changes in the
photoluminescence spectra, including shifts in peaks or
changes in intensity, were compared before and after radiation
exposure to quantify the radiation responses of these two
material systems.
Exposure to either ionizing or non-ionizing radiation did not
produce shifts in the wavelengths of the photoluminescence
peaks. Changes in intensity were found for LZO-only samples
exposed to ionizing and non-ionizing radiation. However, the
changes in intensity were relatively small and inconsistent
from run to run. Unexpected and promising results were found
after exposure to energetic protons of samples with a
secondary luminescent material. Changes in the differing
responses of the two photoluminescence spectra were
compared and correlated with increased proton exposure.
A. B. Hmelo is with the Department of Physics and Astronomy, Vanderbilt
University, Nashville, TN 37215 USA.
2
5
D ï 7F
0
1
D ï7F
5
0
2
Intensity (a.u.)
5
560
D0 ï7F4
5
7
D0 ï F0
5
D0 ï7F3
580
600
620
640
660
680
700
720
Wavelength (nm)
(a)
(b)
Fig. 1. (a) 1/8 of the pyrochlore unit cell, after [11]. (b) Luminescence spectra of Eu-doped lanthanum zirconate, for an excitation wavelength of 395 nm. Russel
Saunder’s Term Symbol is used to indicated various magnetic and electric dipole transitions within th 4f orbital of the europium ion.
II. EXPERIMENTAL DETAILS
A. Eu-doped Lanthanum Zirconate
Lanthanum zirconate (LZO) is a pyrochloric compound.
When doped with rare-earth ions, LZO luminesces.
Pyrochlores have a chemical form of A2B2O7, where La3+ or
trivalent europium occupy the A site and Zr4+ occupies the B
site. Pyrochlores, specifically rare earth zirconates (A2Zr2O7),
have complex crystal structures that lend themselves to a
variety of physical and chemical properties, including stability
in extreme environments. Pyrochlores have been used as hightemperature thermographic phosphors for their intense light
emission and high-temperature stability [8]. Pyrochlores have
been used to insulate vulnerable mechanical components from
prolonged exposure to extreme temperatures because of their
high thermal expansion coefficient, high melting point and
low thermal conductivity [9].
The trivalent rare-earth ion Eu3+ is used as an activator in
lanthanum zirconate powders. Trivalent europium substitutes
readily for the La3+ ion site in the pyrochlore crystalline
structure (Fig. 1a), where a concentration of 5% (percent
replacement of La) was used in order to maximize emission
intensity. The Eu3+ activator produces emission in the red
spectral region with various bands corresponding to optical
transitions within the 4f orbital. Fig. 2b provides a
representative example of phosphor photoluminescence under
UV excitation. The sharp Eu3+ emission lines provide
information regarding the state of the crystalline structure
surrounding the activator. For example, relative peak
intensities between the magnetic and electric dipole transitions
indicate the probabilities of these transitions and are highly
affected by the geometrical symmetry surrounding the
europium ion [10]. Splitting within the various electric and
magnetic dipole transitions is highly affected by the state of
the host lattice material.
Emission lines are denoted by the Russel Saunder’s Term
Symbol, 2S+1LJ, which defines transitions by their quantum
numbers and characterizes the various electronic transitions
within the Eu3+ ion. Optical transitions occur from the 5D0-7FJ
(J = 0, 1, 2 and 3) states and are present between 560 and 680
nm. The emission spectrum for a single representative sample
of europium-doped lanthanum zirconate is shown in Fig. 1b.
Emission bands between 610 and 635 nm are caused by
electric dipole transitions from the 5D0-7F2 electronic states.
The sharp emission bands between 580-610 nm and 635-660
nm are the magnetic dipole transitions from the 5D0-7F1 and
the 5D0-7F3 states, respectively.
B. Sample Preparation
In order to understand radiation effects on lanthanum
zirconate independent from other materials, a sample set was
created by mounting the phosphor on an aluminum substrate
without the use of a binder. The non-binder-assisted technique
is advantageous as there are no adverse darkening affects and
radiation-induced damage observed will not be obscured by
any possible binder interactions. Powders are inherently
difficult to securely mount onto a substrate using a non-binder
technique. Non-binder-assisted samples were prepared by
pressing the powder into a gap fabricated in an aluminum
substrate (Fig. 2a). The aluminum substrate is approximately
0.5 - 1.0 mm thick and 1 cm2 in area with a hole 2 mm in
diameter, drilled approximately 25 µm into the face of the
substrate.
Cyanoacrylate, a common glue, was used in this work
initially as a binder. It is common for phosphor powders to be
mounted using a binder or glue for ease of application onto a
substrate. Cyanoacrylate luminesces within the spectral range
of Eu3+, when excited under UV at 395 nm. Comparing results
from LZO-only and binder-assisted LZO samples yields an
understanding of how a secondary luminescent material may
affect the radiation response of europium-doped LZO’s
(a)
(b)
Fig. 2. (a) Example of a generic phosphor powder directly mounted into an
aluminum substrate, without the use of any binder. (b) Example of the
photoluminescence of a generic phosphor.
3
Intensity (a.u.)
Cyanoacrylate on Al Substrate
LZO:Eu 5%ïCyanoacrylate Sample
LZO:Eu 5% only Sample
580
600
620
640
660
680
700
Wavelength (nm)
Fig. 3. Emission spectrum, around the 5D0-7F3 magnetic dipole transition, of
cyanoacrylate on aluminum substrate, cyanoacrylate with LZO on aluminum
substrate and a lanthanum zirconate only sample.
photoluminescence. The preparation of binder-assisted
samples is considerably simplified compared to the nonbinder-assisted samples. Binder-assisted samples were
fabricated by applying a small amount of LZO powders on a
thin aluminum substrate using cyanoacrylate. The spectral
emission of binder-assisted contain an additional peak near
647 nm associated near the 5D0-7F3 magnetic dipole transition
of europium. The secondary luminescent peak is not
associated with magnetic dipole transition, but with the
cyanoacrylate emission. Emission from cyanoacrylate was
confirmed by comparing photoluminescence spectra for
cyanoacrylate-only, LZO-cyanoacrylate and LZO-only
mounted on an aluminum substrates. The 647 nm peak was
present for all cases which cyanoacrylate was used in the
sample. Whereas, the photoluminescence of the LZO-only
sample contained no peaks at 647 nm, which would be
indicative of magnetic dipole transitions within the europium
ion (Fig. 3).
The use of a binder is advantageous for adhering powders
onto a substrate such that there is no material loss; however,
binders can have undesirable consequences as mentioned
earlier. Hollerman et al. showed that a yttrium aluminum
garnet (YAG) powder mounted on an aluminum substrate
using a polysiloxane binder darkened due to release of carbon
after exposure to energetic protons [6]. Darkening from a
binder has the disadvantage of possible luminescence intensity
degradation. Such consequences are considered in this work
and implications of darkening are discussed in further detail in
the experimental results section.
0.25
Intensity (a.u.)
0.2
0.15
0.1
C. Photoluminescence Measurements
Photoluminescence spectra measurements were taken preand post-irradiation. After a baseline measurement, each
sample was exposed to increasing amounts of radiation, and
photoluminescence spectra were taken between each step
stress. Emission spectra measurements were compared to the
baseline in order to understand and quantify the dose
dependencies of all radiation-induced changes.
Radiation-induced changes in the photoluminescence were
quantified by monitoring the photoluminescence intensity and
the characteristic shape of the emission curve. Changes in
absolute intensity measurements for exposure experiments are
quantified by monitoring maximum intensities. The maximum
intensity for the emission spectrum at a given exposure is
normalized by the pre-irradiation maximum intensity for the
data set. Error bars, which are reported along with changes in
maximum intensities, are estimates of uncertainty for intensity
measurements of a single sample and were obtained by
characterizing the same sample multiple times.
Variability testing for intensity measurements was
performed by taking five separate measurements on three
different samples. To obtain a single variabiliy measurement a
fresh sample was placed into an optically black (dark) box, the
emission spectrum was measured and the sample was
removed. This process was repeated five times for each nonbinder-assisted sample. Error bars are defined as the relative
standard deviation at the maximum intensity which is
calculated using the set of spectra from the worst-case sample
(sample with the highest intensity variability). Fig. 4 depicts
un-normalized emission spectra for the worst-case non-binderassisted sample. The maximum intensity is indicated by the
vertical bar and, for the europium-doped LZO fabricated for
this work, always occurs near 630 nm.
Changes within the characteristic shape of the emission
spectra include any shifts of the peak wavelengths in the
spectra, any missing peaks, and additional peaks or changes in
relative peak intensities after exposure. Intensity
measurements are inherently variable; consequently, it is
advantageous for radiation-induced effects to manifest
themselves through changes in the characteristic shape of the
photoluminescence. Changes in the shape of the emission
spectra are monitored by comparing pre- and post-irradiation
normalized photoluminescence spectra.
Samples were placed in an optically black (dark) box where
they were excited using a Xenon lamp and monochromator.
Photoluminescence measurements were taken using a
spectrometer. Samples were exposed to 10-keV X-rays using
an ARACOR Model 4100 irradiator. Additionally, samples
were exposed to 1 MeV protons in one of the following
facilities: the Pelletron accelerator or the High Voltage
Engineering AN-2000 Van de Graaff accelerator, both of
which are located at Vanderbilt University.
m/!
0.05
III. EXPERIMENTAL RESULTS
0
540
560
580
600
620
640
660
680
700
720
Wavelength (nm)
Fig. 4. Un-normalized emission spectrum for worst-case sample in nonbinder-assisted variability measurements. The vertical line depicts maximum
intensity, and horizontal lines represent variability.
Neither proton or X-ray irradiation (10-keV X-ray or 1 MeV
proton) of non-binder-assisted samples yielded any changes in
the characteristic shape of the emission spectra. However,
intensity changes were found after X-ray irradiation. The
Change in Maximum Intensity
Change in Maximum Intensity
4
2
1.5
Baseline
1
0.5
0 1
10
2
3
10
4
10
10
2
1.5
Baseline
1
0.5
0 13
10
14
10
Total Dose krad(SiO2)
15
16
10
17
10
Total Fluence (cmï2)
10
(a)
(b)
Fig. 5. Maximum intensities, normalized by pre-irradiation maximum intensity, versus (a) total 10 keV X-ray dose, and (b) total 1 MeV proton fluence for the
LZO only sample set.
absolute intensity of LZO:Eu 5% samples exposed to X-ray
doses greater than 500 krad(SiO2) showed a slight decrease
from the baseline, seen in Fig. 5a. However, decreases in
intensity were relatively small, approximately 10% from the
pre-irradiation value. Inconclusive results were found for
intensity measurements of non-binder-assisted samples
exposed up to 1x1016 1-MeV proton fluence at the Pelletron
facility, as shown in Fig. 5b.
Binder-assisted samples were exposed to 1-MeV protons in
the Van de Graaff facility. Samples exposed to 1 MeV protons
were found to have a correlation between changes in peak
intensities of the europium electric dipole and the
cyanoacrylate peaks. The integral normalized emission spectra
at various exposures, ranging from 1014 to 1016 protons-cm-2,
are shown in Fig. 6a. The two 5D0-7F2 electric dipole peaks
between 610 and 630 nm will be referred to as peaks “1” and
“2”, respectively. The cyanoacrylate emission, located at
approximately 647 nm, will be refered to as peak “3”. Relative
peak intensities were computed by fitting the integral
normalized peaks with gaussians using a nonlinear least
squares method. The relative peak intensity between the two
electric dipole peaks remains constant with increasing
fluence, as expected from the non-binder-assisted sample
exposure results. However, the relative peak intensity for
peaks 1 to 3 decreases with increasing fluence. Relative peak
intensities versus total fluence for the ratios of peaks 1 to 3
(black data set) and peaks 1 to 2 (blue data set) are shown in
Fig. 6b. The ratio of peaks 1 to 3 decreases by about 60% for
Peak 1
0.8
Peak 2
Peak 3
0.56
0.2
Eu3+ Emission
0
600
IV. CONCLUSIONS
Photoluminescence measurements, specifically absolute
intensity and characteristic shape of the spectra, were
monitored before and after exposures for two sample
preparations and exposure types. Photoluminescence intensity
measurements are highly variable from run to run. Therefore,
general changes in post-exposure intensity measurements were
reported along with estimates of the absolute intensity’s
uncertainty for all non-binder-assisted sample sets.
1.3
605
610
615
620
625
Cyanoacrylate
630
635
Wavelength (nm)
640
645
650
655
Relative Peak Intensity
Normalized Intensity
1.0
samples exposed to 1016 protons-cm-2 compared to the
baseline.
In addition to the unexpected response to energetic protons,
binder-assisted samples also demonstrated darkening of the
phosphors after exposure. As cited by Hollerman et al.,
darkening is caused by the release of carbon from the binder
after proton irradiation [6]. The darkening is unlikely to be the
source of changes in the relative peak intensities between the
europium and cyanoacrylate peaks. If darkening were the
source of the changes seen, it would be expected to yield an
equivalent decrease over the entire spectra. Changes in relative
peak intensities could be due to various complicated
interactions between the energetic protons, cyanoacrylate and
lanthanum zirconate. However, the cyanoacrylate emission
may also provide an integrated reference such that changes in
the intensity of europium-doped lanthanum zirconate can be
measured.
1.2
Baseline
1.1
1
0.9
Peaks 1:2
0.8
0.7
0.6
0.5
0.4
0.3 13
10
Peaks 1:3
14
10
15
10
ï2
Total Fluence (cm )
16
10
17
10
(a)
(b)
Fig. 6. (a) Integral normalized emission spectra (between 600 and 655 nm) for binder-assisted LZO:Eu 5%. Arrows indicate increasing proton exposure, from
pre-irradiation to 1016 cm-2. (b) Relative peak intensities versus fluence for two samples. Relative peak intensities are between Eu3+ electric dipole transition near
613 nm and cyanoacrylate emission (black data set), and between the Eu3+ electric dipole transitions (blue data set).
5
Data from non-binder-assisted samples showed no changes
within the characteristic shape of the emission spectra post
exposure. Changes were found within the maximum intensity
measurements after X-ray exposures of non-binder-assisted
samples. However, decreases found in maximum intensity
measurements, amounting to approximately 10% from the
baseline, did not occur until a high total dose of 500
krad(SiO2). Changes in maximum intensities for non-binderassisted samples exposed to 1 MeV protons were highly
variable from run to run and yielded no discernable trend.
Binder-assisted samples exposed to 1 MeV protons exhibited a
systematic decrease of relative peak intensity between the
phosphor and cyanoacrylate peaks with increasing fluence.
Quantifying
radiation-induced
changes
in
the
photoluminescence for a single material is an exceptionally
difficult task. The results from the LZO-only samples indicate
that the luminescence of europium-doped lanthanum zirconate
alone is not particularly sensitive to radiation, at least for the
radiation sources considered here. When cyanoacrylate was
present, changes in the relative intensities of the luminescence
of the LZO and the binder were measured.
ACKNOWLEDGMENT
Portions of this work were performed at the Vanderbilt
Institute of Nanoscale Science and Engineering (VINSE),
using facilities renovated under NSF ARI-R2 DMR-0963361.
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