APPLICATION
www.rsc.org/materials | Journal of Materials Chemistry
Development and characterization of highly efficient new cerium doped rare
earth halide scintillator materials
K. W. Krämer,*a P. Dorenbos,b H. U. Güdela and C. W. E. van Eijkb
Received 23rd February 2006, Accepted 26th April 2006
First published as an Advance Article on the web 18th May 2006
DOI: 10.1039/b602762h
Inorganic scintillators for c-ray detection are used in many fields from medical-diagnostic imaging
to industrial measuring systems. Accordingly they face various demands with respect to e.g.
response time, light yield, and energy resolution. The development of the new family of rare-earth
halide scintillators LaCl3 : Ce3+, LaBr3 : Ce3+, and LuI3 : Ce3+ is reviewed and their properties are
compared to those of the established materials NaI : Tl+ and Lu2SiO5 : Ce3+. Extraordinary
properties such as the short lifetime of 15 ns and the narrow energy resolution of 2.6% in
LaBr3 : 5% Ce3+, or the high light yield of 95000 photons per MeV in LuI3 : 5% Ce3+ improve the
performance of scintillators in existing applications and open the way to novel ones. The
development of these new materials was empowered by the close interdisciplinary collaboration
between scientists in the fields of inorganic chemistry, optical spectroscopy, and applied physics.
1. Introduction
Inorganic scintillators have a special place in radiation detection in many sectors of fundamental and applied research, in
most of the medical-diagnostic imaging modalities, and in
many industrial measuring systems, see e.g. refs. 1 and 2. The
variety of applications and the ever-growing wish for better
data imply a diversity of detector requirements that continue
to change. In this paper we focus on the counting of c-rays. In
this case optimal scintillator requirements are:
1) fast response time (10–100 ns) and fast signal rise time for
good time resolution and handling of high counting rates,
a
Department of Chemistry and Biochemistry, University of Bern,
Freiestrasse 3, CH-3000 Bern 9, Switzerland.
E-mail: [email protected]; [email protected]
b
Faculty of Applied Sciences, Delft University of Technology, Mekelweg
15, 2629 JB Delft, The Netherlands. E-mail: [email protected];
[email protected]
2) high light yield (>50000 photons per MeV of absorbed
gamma ray energy) for good energy, time and position
resolution,
3) proportional response for good energy resolution, and
4) high density, r, and high atomic number, Z, for high
gamma ray detection efficiency.
There are many other requirements, of which matching of
the scintillation-wavelength spectrum with the sensitivity curve
of the light sensor (silicon diode or photomultiplier tube), the
possibility to grow large crystals (#1 dm3) and a modest price
per cm3 are very important.
Often the specifications of available inorganic scintillators
do not meet the requirements. Therefore, research and
development to introduce novel scintillator materials is
imperative. Scintillator development as described in this paper
is based on the application of the Ce3+ ion as a luminescence
centre. The allowed 5d–4f transition makes response times in
the 10–100 ns range possible. The host material has to be
Karl Krämer received his
Chemistry diploma (1988)
and Dr. rer. nat. (1991) from
the Justus-Liebig University,
Giessen, Germany. He is group
leader of solid state analytics
at
the
Department
of
Chemistry of the University
of Bern, Switzerland, with
research interests in the
synthesis of anhydrous metal
halides, crystal growth, magnetic neutron scattering, and
optical spectroscopy.
Karl W. Krämer
This journal is ß The Royal Society of Chemistry 2006
Pieter Dorenbos
Pieter Dorenbos graduated in
solid state physics in 1983 and
completed his Ph.D. in 1988 on
ionic conductivity in fluorite
crystals both at the University
of Groningen, The Netherlands.
In 1988 he started at Delft
University of Technology on a
research project to develop new
inorganic scintillators. Later he
specialized on the 5d level
energies of lanthanide ions,
particularly Ce3+, in inorganic
compounds. As associate professor he is now leading the
scintillator and luminescence
materials research at Delft
University of Technology.
J. Mater. Chem., 2006, 16, 2773–2780 | 2773
selected for its high r, Z, and it should have a relatively small
gap between the valence and conduction band (Egap) for
realising >50000 photons per MeV, the ultimate number of
photons per MeV being proportional to 1/Egap. The host
material must be able to incorporate the Ce3+ dopant ions in a
controlled way.
We present results of research concerning new cerium doped
rare-earth halide scintillators that show an unprecedented
combination of high light yield, fast response and excellent
energy resolution. These scintillators have a large impact on
the field of inorganic scintillator application. They have
the potential to replace existing scintillators for numerous
applications. The first devices based on the new materials are
commercially available.
2. New scintillator materials
2.1. The potential of Ce3+ doped materials in light emitting
devices
When we set out, more than a decade ago, to develop new
materials and explore their capability for scintillation detection, this was a rather fundamental research project. We did
not have in mind to improve a particular property of existing
scintillator materials which was thought to be poor. Rather,
our broad objective was to explore the light emission properties and the potential for scintillator applications of Ce3+ in a
range of suitable crystalline environments. The choice of Ce3+
was mainly based on the known light emission properties in
the near ultraviolet (UV) and visible (VIS) spectral region for
Ce3+ in various lattices. The choice of host lattices was based
on our experience in the single-crystal growth of heavy halide
materials and the general knowledge that such materials have a
large attenuation coefficient for c-ray photons.
Ce3+ is the first member of the series of trivalent lanthanide
ions, many of which play an important role in light emitting
devices. To name just a few: Tb3+ and Eu3+ are the active ions
in many green and red phosphors, respectively, and they are
used both in lighting and display devices.3,4 Lasers based on
Nd3+ doped crystals and glasses are among the most important
high-power lasers for a multitude of applications. Er3+ doped
lasers and laser amplifiers are at the heart of fiber based
telecommunications. The main reason that light emission
processes are unusually competitive against non-radiative
relaxation processes in lanthanide materials lies in the strong
shielding of the 4f electrons from the environment. This
reduces electron–phonon coupling and thus the probability of
transforming electronic excitation energy to vibrational energy
and eventually heat.
Ce3+ plays a very special role among the lanthanides. Having
only one 4f electron, there are only two terms arising from this
electron configuration: the 2F5/2 ground term and the 2F7/2
excited term at roughly 2200 cm21. Beyond this energy,
throughout the near-infrared (NIR) and part of the VIS at
least up to the blue and often into the near UV, Ce3+ doped
materials are transparent, which is important for some of their
applications in phosphors (vide infra). The onset of intense
broad absorptions strongly depends on the environment. It
can be as high as 460 nm in Ce3+ doped Y3Al5O12 (YAG) and
as low as 280 nm in Ce3+ doped LaCl3. These are 4f–5d
excitations, and the lowest-energy state deriving from the 5d1
electron configuration is usually emissive, with the broad
emission maximum ranging from 560 nm in YAG : Ce3+ to
350 nm in LaCl3 : Ce3+.5
Fig. 1 shows the spectroscopically relevant states and
transitions in a configurational coordinate diagram for
octahedral coordination. With the 4f electron promoted to
the 5d orbital, the radius of the lanthanide ion increases,
and the excited states have different Ce–ligand equilibrium
distances. The energy difference between the excited states 2Eg
and 2T2g for Ce3+ at sites with Oh point symmetry corresponds
to the crystal-field parameter 10 Dq of the 5d electron.
Its value typically decreases from roughly 27000 cm21 to
22000 cm21 to 18000 cm21 to 16000 cm21 in fluorides, oxides,
chlorides, and bromides, respectively.6 4f–5d transitions are
parity allowed, thus the intense absorptions (e values typically
in the order of 1000 l mol21 cm21) and the relatively short
radiative lifetimes of less than 60 ns.
The chemical structure and tunability of the position of the
strong 4f–5d absorptions and the corresponding emissions
make Ce3+ an important ingredient in light emitting materials.
Classical green lighting phosphors such as CeMgAl11O19 :
Tb3+ are excited by a mercury emission at 254 nm, and Ce3+
acts as a sensitizer.7 The 4f–5d absorption at 254 nm leads to
efficient excitation, which is then almost quantitatively
transferred to 4f–4f excited states of Tb3+ around 330 nm,
where the spectral overlap with the Ce3+ emission is biggest.
Hans Güdel received his Ph.D.
in Natural Sciences in 1969
from the University of Bern,
Switzerland. His research
interests range from molecular
nanomagnets to light-emitting
materials. He is professor of
chemistry at the University of
Bern.
Hans U. Güdel
2774 | J. Mater. Chem., 2006, 16, 2773–2780
Carel W. E. van Eijk
Carel van Eijk received his
Engineering diploma and Dr.
in Applied Sciences degree
from the Delft University of
Technology, The Netherlands.
He started as a nuclear physicist and shifted his interest via
intermediate-energy physics to
instrumentation for detection
of radiation in general. He is
professor of applied sciences at
the Delft University of
Technology and head of the
Radiation Detection & Matter
department.
This journal is ß The Royal Society of Chemistry 2006
bromides are formed by dissolving the rare-earth oxides in
hydrochloric or hydrobromic acid, respectively, adding an
excess of ammonium halide and drying the mixture. The
net reaction in given by eqn (1), with M = Sc, Y, La–Lu, and
X = Cl, Br. Some of the lighter rare earths with bigger ionic
radii tend to form (NH4)2MX5 instead of (NH4)3MX6.
M2O3 + 6 HX + 6 NH4X A 2 (NH4)3MX6 + 3 H2O
(1)
Next the ternary salt is heated to 400uC and decomposed
under vacuum to obtain the rare earth halide according to
eqn (2).
(NH4)3MX6 A MX3 + 3 NH4X
Fig. 1 Schematic diagram of the relevant multiplets and the optical
excitation and emission transitions of Ce3+ in an Oh coordination in
a configurational coordinate diagram. Straight and curly arrows
designate radiative and non-radiative processes, respectively. Crystalfield interactions in the 2F5/2 and 2F7/2 states and spin–orbit
interactions in the 2T2g and 2Eg states are neglected.
Non-radiative relaxation processes within the Tb3+ levels are
then followed by the green emission. In modern lighting
devices Ce3+ has shifted its part from a sensitizer to an
activator. One way to produce white light in an all-solid-state
lamp is based on a combination of a blue light emitting diode
(LED) around 470 nm with a phosphor such as YAG : Ce3+
with a broad yellow emission centered around 560 nm.8
The 5d–4f light emission process of Ce3+ described above is
often highly efficient. Multiphonon relaxation between the 5d
and 4f states is usually unimportant because of the large energy
separation between the states. Current beliefs are that thermal
quenching is due to ionization processes of the 5d electron to
conduction band states followed by non-radiative recombination. When the 5d state is sufficiently far below the bottom of
the conduction band, the 5d–4f emission is purely radiative at
room temperature. The transition is dipole allowed, and the
lifetime varies from as short as 15 ns for LaBr3 : Ce3+ up to
around 60 ns for YAG : Ce3+. The highly efficient and very
fast transition together with an energy near the sensitivity
maximum of photomultiplier tubes makes Ce3+ a unique
activator for scintillator applications. Indeed, several
scintillators based on Ce3+ already existed before we started
our research, e.g. Y3Al5O12 : Ce3+, YAlO3 : Ce3+, Gd2SiO5 :
Ce3+, Lu2SiO5 : Ce3+.9
2.2. Synthesis and crystal growth of Ce3+ doped halide crystals
Rare earth halides are conveniently synthesized from mixtures
of the respective binary halides as starting materials. Since the
hygroscopicity of such compounds strongly increases from
chlorides to iodides, their preparation needs special precautions to avoid oxide contamination. Anhydrous chlorides and
bromides are obtained via the ammonium halide route.10,11 In
a first step ternary ammonium rare-earth chlorides or
This journal is ß The Royal Society of Chemistry 2006
(2)
The preparation of anhydrous rare earth iodides is best done
directly from the elements.10 The metal together with a slight
excess of iodine is sealed under vacuum in a silica ampoule and
carefully heated close to the melting point of the respective
iodide. One end of the ampoule has to protrude out of the
furnace to keep the iodine in the solid state during the reaction.
Uncontrolled heating will result in the explosion of the
ampoule!
In a final step the binary halides are sublimed for purification under high vacuum. In this step the oxide impurities
remain as involatile oxyhalides MOX in the residue. Optical
grade rare earth oxides or metals must be used as starting
materials to minimize product contamination by other
optically active rare earth elements. All handling of the
anhydrous halides MX3 requires sealed inert gas or vacuum
systems, e.g. glove boxes with less than 1 ppm of water
or oxygen.
For use as a scintillator the material needs to be optically
transparent at least in the spectral region of its emission. Thus
most scintillator materials are used in the form of single
crystals, and crystal growth is an important issue with respect
to upscaling and commercialization. On the lab scale batches
of 5 to 50 g starting material are used. The appropriate mixture
of binary halides is sealed under vacuum in a silica ampoule,
and crystals are grown from the melt by the vertical Bridgman
technique, see Fig. 2(a). Other crystallization methods such as
Czochralski growth appear to be less suitable due to the
vapour pressure of rare earth halide melts and their high
reactivity with gaseous impurities. The crystals are handled in
a dry box with a built in microscope with polarizers, a
diamond saw, and polishing equipment. Samples for optical
investigations are sealed in silica ampoules or measured
directly within the dry box.
The step from the lab scale to industrial production is an
important one. Apart from the scintillation properties of a
material, this step will determine whether a new scintillator
makes its way into an application. Starting materials are now
required in a 1 to 100 kg scale and for a commercially
acceptable price. The crystal growth has to be scaled up from
1 cm3 to at least several 100 cm3 size. Also the crystal
processing has to be adapted to hygroscopic materials. For
LaCl3 : Ce3+ this process was developed at St. Gobain Crystals
and Detectors, Nemours, France, and took the remarkably
short period of only 3 years from the lab to the production of
J. Mater. Chem., 2006, 16, 2773–2780 | 2775
Fig. 3 Excitation spectrum (a) monitoring the Ce3+ emission at lem =
360 nm and emission spectrum (b) excited at lex = 295 nm of LaBr3 :
0.5% Ce3+ at 10 K. The excitation spectrum of the STE emission
in pure LaBr3 (c) is shown for comparison (dotted line). Reproduced
with permission from ref. 12.
Fig. 2 Scintillator crystals grown at the University of Bern (a) and by
St. Gobain Crystals and Detectors (b). The lab-scale LaBr3 : Ce3+
crystal (a) has dimensions of 10 mm diameter and 60 mm length. The
commercial-scale, big LaCl3 : Ce3+ crystal (b) has dimensions of 75 mm
in diameter and height. Right next to it a 25 mm crystal in a gas-tight
metal container with optical window is placed, ready to use.
crystal cylinders 25 mm in diameter and height. Meanwhile
crystals of more than 75 mm diameter and height were
produced, see Fig. 2(b). This process was accelerated by the
experience at St. Gobain with NaI : Tl+ scintillator crystals
which are also hygroscopic. Industrial scale Bridgman crystal
growth facilities for halides were already available, and the
crystal processing of LaCl3 : Ce3+ and LaBr3 : Ce3+ could be
done in the same dry rooms. However, problems like the
anisotropic thermal expansion of the hexagonal LaCl3 : Ce3+
and LaBr3 : Ce3+ crystals, which are not present in the cubic
NaI : Tl+, turned out to be formidable in the upscaling process.
2.3. Experimental characterization of the new materials
In the following the various types of experiments used to
characterize the light emission and scintillator properties will
be illustrated by examples.
Fig. 3 shows the optical excitation (a) and emission (b)
spectra of LaBr3 : Ce3+ at 10 K.12 The spectra were recorded at
the SUPERLUMI station of the HASYLAB synchrotron
facilities at DESY, Hamburg, Germany. The samples were
mounted on the cold finger of a He-flow cryostat and excited
by the ns synchrotron radiation pulses through a vacuum UV
monochromator. For further details we refer the reader to
ref. 12. The emission, centered in the near UV, is clearly
resolved into two components separated by about 2000 cm21.
This splitting is always observed in Ce3+ emissions. It is
independent of the coordination environment and due to the
splitting of the 4f1 configuration into the two terms 2F5/2 and
2
F7/2, see Fig. 1. The excitation spectrum (a) shows a rich
structure, and this structure does depend on the coordination
environment. It results from the combined action of the crystal
2776 | J. Mater. Chem., 2006, 16, 2773–2780
field and the spin–orbit coupling of the 5d electron. The Ce3+
coordination in LaBr3 : Ce3+ is a tri-capped trigonal prism with
coordination number (CN) nine. It splits the 5d1 state into
5 bands, i.e. the 2T2g and 2Eg states as shown in Fig. 1 are
further split by the lower symmetry. The total spread of the 5d
excitation bands is of the order of 6000 cm21. Note that the
Stokes shift between absorption and emission is very large
and there is no overlap between the absorption and emission
bands of Ce3+. In octahedrally coordinated systems, such as
the elpasolite Cs2LiYCl6 : Ce3+, the crystal field splitting is
much higher with a total spread of about 18500 cm21 and two
well separated groups which can be labeled 2T2g and 2Eg,
according to Fig. 1.13 Included in Fig. 3 is the excitation
spectrum of the self-trapped exciton (STE) emission in pure
LaBr3, which will be described in section 2.4. The excitation
band centered at about 220 nm in LaBr3 : Ce3+ can clearly be
attributed to this host lattice excitation.
Fig. 4 compares the X-ray excited emission spectra of Ce3+
doped K2LaCl5 (a), K2LaBr5 (b), and K2LaI5 (c).12,14,15 The
samples, sealed in silica ampoules to prevent moistening, were
mounted on the cold finger of a liquid nitrogen cryostat and
exposed to X-rays from an X-ray tube with copper anode
operating at 25 mA and 35 kV. The radioluminescence from
the crystal is focused on the entrance slit of a monochromator.
The recorded luminescence spectra were corrected for the
transmission of the monochromator and the quantum
efficiency of the photomultiplier detector. In this isostructural
series the Ce3+ has a mono-capped trigonal prismatic
coordination with CN seven. The 5d–4f Ce3+ doublet
dominates the emission. It is accompanied at lower energies
by a broad emission band attributed to self-trapped excitons.
Fig. 4 nicely illustrates the effect of the chemical variation
along the halide series. The observed redshift of the doublet
Ce3+ emission is the result of three competing effects: the
energy of the 5d orbital decreases along the series as a result of
increasing covalency of the Ce3+–X2 bond, thus reducing the
5d–4f energy gap and providing a redshift. The crystal-field
splitting of the 5d orbital, on the other hand, decreases along
the series because of the increasing size of the anion and thus
produces a blue shift. And finally, the spin–orbit splitting of
5d decreases because of covalency and produces a redshift.
This journal is ß The Royal Society of Chemistry 2006
Fig. 4 X-Ray excited luminescence spectra of (a) K2LaCl5 : 0.1%
Ce3+, (b) K2LaBr5 : 0.7% Ce3+, and (c) K2LaI5 : 0.7% Ce3+. The spectra
were measured at 80 K (solid trace) and at 300 K (dotted trace).
Reproduced with permission from ref. 12.
Fig. 5 Pulse height spectrum of LaBr3 : 5% Ce3+ excited by 662 keV
c-rays and measured with an avalanche photodiode. The energy
resolution (FWHM) at 662 keV is 2.6%.
The net result is the observed redshift by about 4000 cm21
between the chloride and iodide.
Fig. 5 shows a so-called scintillation pulse-height spectrum
measured with a 6 6 6 6 6 mm3 large LaBr3 : 5% Ce3+ crystal.
The scintillation light pulse produced in the crystal by 662 keV
c-rays is detected by means of an avalanche photodiode. The
photodiode delivers a charge pulse that is electronically
processed into a pulse with a height proportional to the total
amount of detected photons. The pulse height spectrum then
shows a histogram of the scintillation pulses emitted by the
scintillator. The channel number can be calibrated in terms of
the detected amount of energy or the detected number of
scintillation photons. The peak at 662 keV is due to the full
absorption of a 662 keV c-ray photon. Some events may
produce slightly more light than others due to statistical
processes or due to crystal inhomogeneities. The result is a
Gaussian shaped profile, see also ref. 16 and references therein.
The full width of the peak at half maximum height (FWHM) is
an important parameter because it defines the energy resolution of the scintillator. Fig. 5 demonstrates a FWHM value
of 2.6% at 662 keV which is a record low value for a
scintillation crystal.
Fig. 6 shows the scintillation decay with time after a 662 keV
c-ray excitation pulse for pure (a) and Ce3+ doped (b) K2LaX5
(X = Cl, Br, I) in a semi-logarithmic representation.12 In the
pure host materials the emission is thought to be due to selftrapped excitons that also produce the broad emission bands
at low energies in Fig. 4. As seen in Fig. 6(a), it decays
exponentially with time constants of 3.7 ms, 2.2 ms and 350 ns
for the chloride, bromide, and iodide, respectively. This
decrease has been ascribed to a combination of a decreasing
intrinsic lifetime and an increasing efficiency of non-radiative
loss channels in the heavier halides.12 Such a decrease in
lifetime from chlorides to iodides is also well known from the
alkali halides.17,18 The scintillation decay in the Ce3+ doped
crystals (Fig. 6(b)) is shorter and no longer single exponential.
A deviation from single exponential scintillation decay is often
observed for Ce3+ doped scintillators. One may observe a
multiple exponential decay due to the presence of different
luminescence centers either because of different Ce3+ sites in
the lattice or because of a mix of host related emission with
Ce3+ emission. In addition there might be many more reasons
Fig. 6 (a) Scintillation decay time spectra of K2LaCl5 (1), K2LaBr5 (2), and K2LaI5 (3) at room temperature. (b) Scintillation decay time spectra of
K2LaCl5 : 0.1% Ce3+ (1), K2LaBr5 : 0.7% Ce3+ (2), and K2LaI5 : 0.7% Ce3+ (3) at room temperature. Reproduced with permission from ref. 12.
This journal is ß The Royal Society of Chemistry 2006
J. Mater. Chem., 2006, 16, 2773–2780 | 2777
for non-exponential decay. Most important is the transport of
free charge carriers to the luminescence centers. That transport
is usually thermally activated leading to slow scintillation
components. A recent review on scintillation mechanisms in
Ce3+ doped halide compounds can be found in ref. 19. In the
case of K2LaX5 : Ce3+ compounds the reduction of the
scintillation decay time is very pronounced when X changes
from Cl, to Br, to I. Since the scintillation emission at room
temperature is mainly Ce3+ related, see Fig. 4, and because the
intrinsic lifetime of the emitting Ce3+ 5d state is always
between 15 and 60 ns we conclude that the effective
scintillation decay is controlled by slow charge and energy
transfer processes from the host lattice (the ionization track) to
Ce3+. Apparently the transfer speed increases when the halide
ion changes from Cl, to Br, to I. The fast scintillation
component of K2LaI5 : Ce3+ has a decay time of 24 ns
indicating that the transfer is almost instantaneous.
2.4. Scintillation mechanisms and properties of the new Ce3+
doped materials
The physical processes following the absorption of a c-ray and
resulting in the emission of visible light can be extremely
complex. The interaction of a c-ray photon with energy Ec
creates a fast electron that is slowed down in the scintillator by
producing a multitude of secondary electrons in the ionization
track. Down to secondary electron energies of about 100 eV
the whole process can be simulated very well with Monte Carlo
techniques based on atomic and free electron interactions
alone. Below 100 eV energy, the physical and chemical
properties of the host crystal start to play a role. The value
for the energy between the conduction and the valence band
Egap, collective excitations (plasmons), and non-radiative
losses determine the total number of ionizations in the track.
Roughly, the number of ionizations is close to Ec/2.5Egap.
The second phase in the scintillation process is the migration
of the free thermalized electrons in the conduction band and
free thermalized holes in the valence band to Ce3+. This is a
very crucial and complex phase that has been reviewed for
Ce3+ doped halides in ref. 19. Electrons and holes can be selftrapped in the lattice to form a so-called self-trapped exciton
(STE). The STE may luminesce by itself but can also slowly
migrate towards Ce3+ resulting in slow scintillation decay
components. Alternatively, electrons and holes can be trapped
promptly by Ce3+, leading to fast scintillation decay components equal to the intrinsic lifetime of the Ce3+ emitting state.
Also a hole can be trapped first and an electron at a later stage.
All these processes occur in competition and in parallel. Their
relative importance depends on Ce3+ concentration, temperature, and host lattice.
For example, Fig. 4(b) clearly shows the broad exciton
emission of K2LaBr5 : Ce3+ around 440 nm at 80 K. This
emission disappears at room temperature due to the energy
transfer from the STE to Ce3+. The STE emission at 80 K is
much weaker for K2LaI5 : Ce3+, see Fig. 4(c). At the same time
Fig. 6(b) shows a much faster scintillation response of K2LaI5 :
Ce3+ compared to K2LaBr5 : Ce3+. Similar features are
observed when the scintillation properties of LaBr3 : Ce3+
are compared with those of LaCl3 : Ce3+. The whole
2778 | J. Mater. Chem., 2006, 16, 2773–2780
scintillation process can be very complex and seemingly similar
types of compounds often display totally different scintillation
behaviors. This makes the prediction of scintillation properties
very difficult, if not impossible. On the other hand, we
occasionally observe clear trends which guide the way to select
new, possibly interesting compounds. For example, hole and
exciton migration times and also the exciton lifetime often
shorten when the halide ions are changed from Cl, to Br, to I.
Charge and energy migration times also decrease with the
increase of Ce3+ concentration or temperature.
At the start of our collaboration more than a decade ago,
Ce3+ was already known to be an efficient and fast activator.
We started to study K2LaCl5 : Ce3+ as the first lanthanide
halide scintillator.14,20 K2LaCl5 : 10% Ce3+ showed surprisingly high light output with very good energy resolution for the
detection of 662 keV c-ray photons. The energy resolution of
5.1% (FWHM) was unequalled at that time. However, the
scintillation decay was relatively slow and, in addition, the host
material contains the naturally occurring radioactive 40K
isotope that is unwanted for scintillator applications. Ce3+
5d–4f emission together with broad band host lattice STE
emission was observed at low Ce3+ concentration. With higher
Ce3+ concentration (up to 10%) the STE emission disappears
with the simultaneous increase of Ce3+ emission. This together
with an increase in scintillation speed was explained by a
higher probability of direct capture of free electrons and free
holes with increasing Ce3+ concentration and also by a faster
transfer of excitation energy from STE like defects to Ce3+. A
10% Ce3+ doping in K2LaCl5 actually provided a good
scintillator. However, due to the small Stokes shift between
emission and absorption, losses due to self absorption are
important, and eventually this material was commercially not
of sufficient interest. Nevertheless, much knowledge on
scintillation processes was gained that proved to be of
importance for later scintillator developments.
Years later we studied21 LaCl3 : 0.5%Ce3+ and noticed much
similarity with the scintillation response of K2LaCl5 : Ce3+;
again a fast Ce3+ 5d–4f emission together with a relatively slow
STE emission was observed. There were two very crucial
differences: i) the absence of radioactive 40K isotopes and ii) a
uniquely large Stokes shift between absorption and emission.
We decided to increase the Ce3+ concentration and found that
at 10% doping almost all slow scintillation components had
disappeared in favor of the desired fast 5d–4f emission of Ce3+
and without any deteriorating effects of self absorption.22,23
The step to LaBr3 : Ce3+, that has the same crystal structure
as LaCl3, was made soon afterwards.24,25 LaBr3 : Ce3+ appears
to be an even better scintillator. It has a higher density,
a higher light output, better energy resolution, and better
timing properties.
Table 1 summarizes the scintillator characteristics of LaCl3 :
Ce3+ and LaBr3 : Ce3+ and compares them with those of the
most widely used classical materials NaI : Tl+ and Lu2SiO5 :
Ce3+. Included in Table 1 are the key properties of the very
recently found new scintillator material LuI3 : Ce3+.26 NaI : Tl+
is the most widely used scintillator material and has dominated
the market for more than 50 years. Lu2SiO5 : Ce3+ is used for
medical imaging applications such as positron emission
tomography (PET). All the new Ce3+ doped halide materials
This journal is ß The Royal Society of Chemistry 2006
Table 1 Scintillator characteristics of Ce3+ doped rare-earth halides
in comparison to Lu2SiO5 : Ce3+ and NaI : Tl+. The main decay
component (ns) and its % contribution to the total light yield are given.
Energy resolution is the FWHM of the 662 keV photopeak in pulse
height spectra
Material
Decay
time/ns
LaCl3 : 10% Ce3+ 24
16
LaBr3 : 5% Ce3+
24
LuI3 : Ce3+
3+
42
Lu2SiO5 : Ce
+
230
NaI : Tl
(60%)
(100%)
(60%)
(100%)
(100%)
Light yield/
photon MeV21
Energy
resolution
at 662 keV Density/
(%)
g cm23
50000
70000
95000
30000
48000
3.1
2.6
3.3
9
6.5
3.9
5.1
5.6
7.4
3.7
are superior to the existing ones with respect to decay time,
light yield and energy resolution. The latter is particularly
important for c-ray spectroscopy. The faster decay time makes
the new materials, particularly LaBr3 : Ce3+, outstanding
candidates for PET applications, which are based on
coincidence measurements. And the higher light yields are a
bonus in any application.
3. Conclusions and outlook
Fig. 7 compiles the scintillation light yield expressed in photons
emitted per MeV of absorbed c-ray energy of ‘‘traditional’’
scintillators and scintillators developed within our collaboration as a function of the band gap of the host crystal. LaBr3 :
Ce3+ is an excellent scintillator with a light output of 70000
photons per MeV. However, there is always a demand for even
better ones. There is still room for improvement regarding the
density of the host crystal, the light output and the related
energy resolution. A first step was already made very recently
with the material LuI3 : Ce3+.26 It shows a record high photon
yield approaching 95000 photons per MeV, see Fig. 7 and
Table 1. The density is also higher than that of LaBr3 : Ce3+.
The aim of higher light output scintillators may possibly be
achieved by smaller band gap materials. In theory they provide
more ionizations per unit energy and potentially more photons
Fig. 7 The scintillation light output of traditional scintillators (&)
and the Ce3+ doped scintillators developed within our collaboration
(n) as a function of the band gap Egap of the host crystal for c-ray
excitation Ec. The solid line represents the ratio Ec/2.5Egap and
corresponds to the theoretical maximum light output.
This journal is ß The Royal Society of Chemistry 2006
Fig. 8 The energy resolution (FWHM) of scintillators for 662 keV
c-ray detection as a function of the number of detected photons with
photomultiplier tube read out. Two data points are with avalanche
photo diode (APD) read-out. The solid line is the calculated
contribution according to Poisson statistics.
can be emitted. With Ce3+ doped compounds we predict that
the ultimate light output scintillators are to be found in
compounds with Egap of about 3 eV. Potential candidates
are iodides as well as selenides and tellurides. These latter
compounds could also provide higher densities (7–8 g cm23).
Fig. 8 illustrates the energy resolution for 662 keV c-ray
detection of scintillators as a function of the number of
detected scintillation photons. The best result so far is for
LaBr3 : Ce3+ displayed in Fig. 5; an energy resolution of 2.6%
was measured with an avalanche photo diode read-out. We
predict that the ultimate energy resolution for 662 keV c-rays
with Ce3+ scintillators is below 2%. Such resolution combined
with higher density and fast response is the aim for our
future efforts.
References
1 Proceedings of the Seventh International Conference on Inorganic
Scintillators and their Use in Scientific and Industrial Applications
SCINT 2003, ed. F. Ballester, J. M. Benlloch, J. Diaz, C. W.
Lerche, F. Sanchez and A. Sebastia, Nucl. Instrum. Methods Phys.
Res., Sect. A, 2005, 537.
2 C. W. E. van Eijk, Inorganic scintillators in medical imaging, Phys.
Med. Biol., 2002, 47, R85–R106.
3 G. Blasse and B. C. Grabmaier, Luminescent Materials, Springer
Verlag, Berlin, 1994.
4 Phosphor Handbook, ed. S. Shionoya and W. M. Yen, CRC Press,
Boca Raton, 1999.
5 P. Dorenbos, J. Lumin., 2000, 91, 155–176.
6 P. Dorenbos, Phys. Rev. B, 2001, 64, 1–12.
7 J. M. P. J. Verstegen, J. Electrochem. Soc., 1974, 121, 1623–1626.
8 S. Nakamura and G. Fasol, The blue laser diode, Springer Verlag,
Berlin, 1997; S. Nakamura, MRS Bull., 1997, 22, 29–35.
9 M. J. Weber, J. Lumin., 2002, 100, 35–45.
10 G. Meyer and M. S. Wickleder, in Handbook on the Physics and
Chemistry of Rare Earth, ed. K. A. Gschneidner and L. Eyring,
Elsevier, Amsterdam, 2000, vol. 28, ch. 177, pp. 53–129.
11 G. Meyer, Inorg. Synth., 1989, 25, 146–150.
12 E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. W. Krämer
and H. U. Güdel, Phys. Rev. B, 2003, 68, 1–9.
13 A. Bessiere, P. Dorenbos, C. W. E. van Eijk, L. Pidol,
K. W. Krämer and H. U. Güdel, J. Phys.: Condens. Matter,
2004, 16, 1887–1897.
14 J. C. van’t Spijker, P. Dorenbos, J. T. M. de Haas, C. W. E. van
Eijk, H. U. Güdel and K. Krämer, Radiat. Meas., 1995, 24,
379–381.
J. Mater. Chem., 2006, 16, 2773–2780 | 2779
15 E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. W. Krämer
and H. U. Güdel, Nucl. Instrum. Methods Phys. Res., Sect. A, 2005,
537, 232–236.
16 P. Dorenbos, J. T. M. de Haas and C. W. E. van Eijk, IEEE Trans.
Nucl. Sci., 2004, 51, 1289–1296.
17 L. F. Chen, K. S. Song and C. H. Leung, Nucl. Instrum. Methods
Phys. Res., Sect. B, 1990, 46, 216–219.
18 K. S. Song and R. T. Williams, Self-trapped Excitons,
Springer Series in Solid State Sciences 105, Springer Verlag,
Berlin, 1993.
19 P. Dorenbos, Phys. Status Solidi A, 2005, 202, 195–200.
20 J. C. van’t Spijker, P. Dorenbos, C. W. E. van Eijk, K. Krämer and
H. U. Güdel, J. Lumin., 1999, 85, 1–10.
2780 | J. Mater. Chem., 2006, 16, 2773–2780
21 O. Guillot-Noël, J. T. M. de Haas, P. Dorenbos, C. W. E. van Eijk,
K. Krämer and H. U. Güdel, J. Lumin., 1999, 85, 21–35.
22 E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. Krämer and
H. U. Güdel, Appl. Phys. Lett., 2000, 77, 1467–1468.
23 E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. Krämer and
H. U. Güdel, IEEE Trans. Nucl. Sci., 2001, 48, 341–345.
24 E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. Krämer and
H. U. Güdel, Appl. Phys. Lett., 2001, 79, 1573–1575.
25 E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. W. Krämer
and H. U. Güdel, Nucl. Instrum. Methods Phys. Res., Sect. A, 2002,
486, 254–258.
26 M. D. Birowosuto, P. Dorenbos, C. W. E. van Eijk, K. W. Krämer
and H. U. Güdel, IEEE Trans. Nucl. Sci., 2005, 52, 1114–1118.
This journal is ß The Royal Society of Chemistry 2006