SCINTILLATION PROPERTIES OF SELECTED OXIDE MONOCRYSTALS
ACTIVATED WITH Ce AND Pr
Andrzej J. Wojtowicz1,*), Winicjusz Drozdowski1), and Dariusz Wisniewski1)
Jean-Luc Lefaucheur2), Zbigniew Galazka2), and Zhenhui Gou2)
Tadeusz Lukasiewicz3), and Jaroslaw Kisielewski3)
1)
Institute of Physics, N. Copernicus University, ul. Grudziadzka 5, 87-100 Torun, Poland
2)
Photonic Materials, Ltd, Strathclyde Business Park, Bellshill, ML4 3BF, Scotland
3)
Institute of Electronic Materials Technology, ul. Wolczynska 133, 01-919 Warszawa,
Poland
ABSTRACT
In the last 10-15 years there has been a significant effort toward development of new, more
efficient and faster materials for detection of ionizing radiation. A growing demand for better
scintillator crystals for detection of 511 keV gamma particles has been due mostly to recent
advances in modern imaging systems employing positron emitting radionuclides for medical
diagnostics in neurology, oncology and cardiology.
While older imaging systems were
almost exclusively based on BGO and NaI:Tl crystals the new systems, e.g. ECAT Accel,
developed by Siemens/CTI, are based on recently discovered and developed LSO
(Lu2SiO5:Ce, Ce-activated lutetium oxyorthosilicate) crystals.
Interestingly, despite very
good properties of LSO, there still is a strong drive toward development of new scintillator
crystals that would show even better performance and characteristics.
In this presentation we shall review spectroscopic and scintillator characterization of new
complex oxide crystals, namely LSO, LYSO, YAG, LuAP (LuAlO3, lutetium aluminate
perovskite) and LuYAP activated with Ce and Pr. The LSO:Ce crystals have been grown by
CTI Inc (USA), LYSO:Ce, LuAP:Ce and LuYAP:Ce crystals have been grown by Photonic
Materials Ltd, Scotland (PML is the only company providing large LuAP:Ce crystals on a
commercial scale), while YAG:Pr and LuAP:Pr crystals have been grown by Institute of
Electronic Materials Technology (Poland). All these crystals have been characterized at
Institute of Physics, N. Copernicus University (Poland). We will review and compare results
of measurements of radioluminescence, VUV spectroscopy, scintillation light yields,
scintillation time profiles and low temperature thermoluminescence performed on these
crystals. We will demonstrate that all experiments clearly indicate that there is a significant
* Corresponding author: Tel.: +48-56-6113239; fax: +48-56-6226397; e-mail: andywojt@phys.uni.torun.pl,
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room for improvement of LuAP, LuYAP and YAG. While both Ce-activated LSO and LYSO
perform very well, we also note that LuYAP:Ce, LuAP:Ce and YAG:Pr offer some
advantages and, after a likely improvement of some parameters, may also present a viable and
desired alternative in applications that require high counting rates or better time resolution.
Unfortunately, LuAP:Pr, although the fastest among all the materials studied, may be
seriously limited in its achievable light yield by inherent physical processes that are
responsible for nonradiative quenching of scintillation light in this material.
PACS codes: 29.40.Mc; 72.20.Jv; 78.55.Hx
Key words: scintillators, LSO, LYSO, LuAP, LuYAP, YAG, Ce, Pr, VUV spectroscopy,
thermoluminescence
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1. Introduction
Although Ce-activated lutetium oxyorthosilicate (LSO, Lu2SiO5:Ce) with its superior
scintillator characteristics has been clearly established as a leading scintillator material in the
area of medical imaging [1] many researchers believe that it is possible to develop a material
that would display even better scintillation properties. It is now fairly well established that
scintillation light in LSO is produced at two different sites (Ce1 and Ce2) one of which (Ce2)
is strongly quenched at room temperature [2,3].
The dominant Ce1 site emission
(unquenched at ambient temperatures) decays with the time constant of about 35 ns and any
faster components in the scintillation time profiles (STP) of LSO are due to the quenched
emission from the Ce2 site. Clearly part of the energy deposited in the LSO host by ionizing
radiation is lost to nonradiative processes introduced by inefficient Ce2 sites.
It is likely that in the mixed crystal, (Lu1-x,Yx)2SiO5 (LYSO), in which some of Lu cations
are replaced by Y cations, the distribution of energy between the two sites may change and,
consequently, the scintillation light yield of LYSO may increase as well. On the other hand
disorder typical of the mixed crystal may be responsible for additional losses due e.g. to
additional traps, scintillation light scattering and so on.
Another interesting possibility to extend the pool of available dense oxide scintillators
offers another Lu-based scintillator material, LuAlO3:Ce (LuAP), an isostructural analog of
the well known laser and scintillator material YAlO3 (YAP). Scintillation properties of Ceactivated LuAP, recommended earlier by a number of groups, in a pure perovskite
monocrystal phase were first studied and reported by Lempicki and coworkers at the 1994
IEEE Nuclear Science Symposium [4]. Importantly, LuAP features only one Ce site and the
radiative lifetime of the excited Ce3+ ion in this site and, consequently, the decay time of the
dominant scintillation component, is only about 17 ns, the shortest scintillation decay time in
any known Ce-activated scintillator material. This is mostly due to relatively short emission
wavelength (the Ce emissions in LuAP and YAP peak at 360 nm, in LSO at about 420 nm).
Since the two isostructural materials, LuAP and YAP, are closely related, their
spectroscopic properties upon Ce-activation, such as emission wavelengths and radiative
lifetimes, are nearly the same. This is also true of the mixed material, Lu1-xYxAlO3 (LuYAP,
typically at 30% of Y), which is easier to grow than LuAP. Yet the scintillation properties of
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LuAP and LuYAP differ which comes as no surprise since the scintillation properties of
LuAP and YAP differ as well.
Shorter decay times can be obtained only from the Pr- and Nd-activated materials since the
4fn-15d - 4fn emissions from these ions are characterized by even shorter wavelengths. The Pr
and Nd emissions typically peak at about 250 and 190 nm, hence, from the wavelength
dependence of the electric dipole spontaneous transition rate [5], the estimated radiative
lifetimes are 8 and 5 ns for Pr and Nd, respectively. A scintillator material that would possess
such a short scintillation decay time would be of major interest in a number of applications
that require high counting rates or a high timing resolution, such as PET and time-of-flight
PET.
Although transition energies between Nd3+ d and f levels usually fit fluoride bandgaps
(typically 10 eV), in oxide scintillators, which have lower bandgaps (6-8 eV), the only viable
alternative, with only a few notable exceptions (e.g. Nd-activated phosphates, [6]), to Ceactivation, is Pr-activation.
In this paper we shall review results of a number of R&D projects aimed at development of
Ce-activated LYSO, LuAP and LuYAP scintillator crystals at Photonic Materials Ltd,
Bellshill, Scotland (PML), and development of Ce- and Pr-activated LuAP, YAP and YAG
scintillator crystals at Institute of Electronic Materials Technology, Warsaw, Poland (ITME),
in collaboration with N. Copernicus University, Torun, Poland (UMK).
2. Crystals and experimental set-ups
All the samples were cut as pixels (2x2x10 mm) or plates (5x5x1 mm) from larger boules
grown by Czochralski method at Photonic Materials Ltd, Bellshill, Scotland (PML) and at
Institute of Electronic Materials Technology, Warsaw, Poland (ITME).
PML is the only company growing and providing large LuAP:Ce crystals on a commercial
scale (diameter up to 50 mm, length of a cylindrical body up to 120 mm). More information
on growth of LYSO, LuAP and LuYAP by PML has already been presented (LYSO, [7,],
LuAP and LuYAP, [8]).
At ITME, the LuAP, YAP and YAG single crystals doped with Ce and Pr ions were grown
with use of the oxypuller 05-03 (Cyberstar France). The system consisted of the Ir crucible,
50 mm in diameter, embedded in zirconia grog, the passive iridium afterheater and alumina
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insulation around a crucible and the afterheater. Raw materials Y2O3, Al2O3 and dopants
CeO2 and Pr6O11 (4N) were used. The growth process, carried on in nitrogen atmosphere,
was driven by a computer program controlling the weight of a growing crystal. The rate of
growth was kept constant at 1 mm/h and the rotation rate was 12-15 rpm. The crystals
measured 20 mm in diameter and 50-60 mm length.
The samples were of high optical quality, clear and displayed no visible color and no
inclusions.
The radioluminescence spectra were measured using a modified Dron X-ray set-up with an
X-ray tube employing a copper anode operated at 35 kV, and the Advanced Research System
closed-cycle He cooler equipped with a heater driven by a temperature controller (Lakeshore).
The light emitted by the sample was collected by quartz optics and passed through the 0.5 m
SpectraPro Acton monochromator with the 1200 g/mm grating working in the 1-th order.
The luminescence spectra, luminescence excitation spectra and emission time profiles
under pulsed VUV and UV synchrotron excitation in the wavelength range 50-335 nm were
measured using experimental facilities of Superlumi station at Hasylab in Hamburg, Germany
[9].
Scintillation light yield (LY) was measured at Institute of Physics, N. Copernicus
University in Torun, Poland using a typical set-up consisting of radioactive source (Na22, 511
and 1274 keV), a photomultiplier tube (Hamamatsu R2059), Canberra modules, and BGO
(Bi4Ge3O12) standard crystal (2x2x10 mm pixel).
Since the current drawn from the
photocathode of the photomultiplier tube (PMT) is, at the time of scintillation, quite large
(one to six thousand photoelectrons in 10-100 ns) it is not surprising that the standard method
of measuring the relative scintillation light yield by comparing photopeak positions in the
energy spectra of a given scintillator material and, say, a BGO standard crystal, is likely to
provide results with unacceptably large systematic errors. We have developed a method that
allows to extract a correct value of the relative scintillation light yield from a number of
spectra measured for different amplifier gains (to exclude the effect of the offset in energy
spectra) and for a number of PMT voltages (to correct distortions introduced by a PMT
caused, most likely, by space charge in the PMT itself). The method will be described in
details later [10].
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The scintillation time profiles were measured by a classical synchronous photon counting
method [11] using a Time-to-Pulse Height Converter and other relevant Canberra modules.
Thermoluminescence glow curves were measured using a modified method originally
proposed by Bartram et al [12] and a Dron based X-ray set-up used for radioluminescence.
The temperature controller was programmed to run a closed-cycle He cooler and the heater to
accomodate an irradiation step and the TL run. During the irradiation step the temperature of
the sample was kept constant at about 10 K while the sample was irradiated by an X-ray tube
for some time (usually 10 minutes). The light emitted by the sample was collected by quartz
optics and passed through a 0.5 m SpectraPro Acton monochromator with the 1200 g/mm
grating working in the 0-th order producing, at the PMT, a steady state radioluminescence
signal which was then sent to computer and recorded (a flat part of the curve). After
irradiation the X-ray tube was switched off and the TL run started. During the TL run the
temperature of the sample was raised by controller with constant heating rate at about
9 K/min, up to 300 K. Consequently, the emission released from the sample by heating was
recorded by the same set-up and under the same geometrical conditions as in the irradiation
step. This method allows to directly measure the fraction of the total energy deposited in the
sample that is intercepted by traps.
3. Experimental results and discussion
3.1. Radioluminescence
Unlike photoluminescence spectra which may vary strongly with the excitation wavelength,
radioluminescence spectra are determined by a dominant host-to-ion energy transfer
mechanism and as such constitute a first basic step in characterization of any scintillator
material. Since radioluminescence spectra of most of the materials reported in this paper have
already been published, we have chosen to present here only two cases, namely LuAP:Pr and
YAG:Pr. As we will demonstrate shortly, the spectra of these two materials show some
interesting untrivial differences that are likely to be reflected in their, significantly different,
scintillation performance.
In Figs. 1 and 2 we present steady state, X-ray excited radioluminescence spectra of five
pixel samples (2x2x10 mm) of LuAP:Pr (0.003, 0.04 and 0.08 mol%Pr), and YAG:Pr (0.1 and
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0.3 mol%Pr) measured at 15 K (LuAP) and at room temperature (RT, YAG), respectively.
The spectra show “host” emissions at the shorter wavelength side of the d-f emission bands
that are strongly suppressed for higher Pr concentrations, as observed in Ce-activated
materials. The Pr3+ d-f emission bands peak at about 245 nm in LuAP and at 320 nm in YAG
which reflects a higher crystal field in YAG pushing down the lower emitting d-level.
Since in YAG Pr3+ 5d emission lifetimes and intensities at RT are only marginally affected
by nonradiative d-f transitions (see [13]) and there is no direct energy transfer from host to flevels, the contribution of f-f emissions to the spectra should be low, as observed in Fig. 2.
We also note that the relative intensities of f-f transition lines in two YAG samples (0.1 and
0.3 mol%Pr) are almost the same. The lines are slightly broader in the 0.3mol%Pr sample
which is not unusual and may explain slightly lower intensities in this sample by means of
energy migration between Pr ions leading to concentration quenching of the f-f emission.
However the effect is certainly marginal and the final conclusion is that there is practically no
interaction between Pr ions in YAG.
Since the energy gap between d and f Pr3+ levels in LuAP is larger one would expect even
less nonradiative d-f relaxation and, consequently, even smaller contribution from f-f emission
lines to the total steady state radioluminescence spectrum. As shown in Fig. 1, this is
consistent with experimental observations only for the lowest Pr concentration (0.003
mol%Pr). For higher Pr concentrations we clearly observe higher relative intensities of f-f
emission lines. Although the effect is hard to quantify (there possibly still is energy migration
between Pr ions in the 3P0,1,2 state) it is, nevertheless, obvious that there must be a mechanism
by which, in LuAP, but not in YAG, energy is transferred from d levels to f levels for higher
Pr concentrations. We note that the d-f transition energy in LuAP, corresponding roughly to
250 nm wavelength, is enough to excite the neighboring Pr ion to the 3P0 state assuming that
the final state of a given Pr ion is also 3P0. The same is not true of the d-f transition in
YAG:Pr as its transition energy corresponds to emission wavelength of 320 nm which is not
short enough to have the two interacting Pr ions even in the lower 1D2 final state (the emission
from 1D2 peaks at about 620 nm).
The deexcitation process we propose can be described shortly as:
 

  
  
Pr 3 5d4f   Pr 3 4f 2 3 H 4  Pr 3 4f 2 3 P0  Pr 3 4f 2 3 P0
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which is a reversed well known process of energy up-conversion. Interestingly such a process
of the two-step excitation of d levels has been demonstrated in Pr-doped YAP [14].
3.2. VUV spectroscopy
Selective VUV (vacuum ultraviolet) excitation is a powerful tool to study large bandgap
scintillation materials as it allows to identify all the different channels of host-to-ion energy
transfer that can potentially contribute to scintillation. At the VUV station Superlumi at
Hasylab, Hamburg, Germany, it is possible to simultaneously measure at least three different
spectra under the wide range of exciting wavelengths between 50 and 335 nm. In each
spectral run we have chosen to measure a time-integrated spectrum and two different timegated spectra. In the time-integrated spectrum the signal was accumulated during the time
between two consecutive synchrotron pulses (separated by 192 ns) while the time-gated
spectra were measured within a 40 ns time window triggered by a synchrotron pulse with
almost no delay (2-3 ns to reduce stray light) or 150 ns delay.
In Figs 3 and 4 we present selected time resolved excitation spectra of different emissions
from the pixel (2x2x10 mm) sample of LYSO grown at PML. The spectra shown in Fig. 3
were measured with the emission wavelength set at 400 nm to select the Ce3+ emission from
the Ce1 site. These are the time-gated spectra with a zero delay gate (“fast” emission, thick
solid line) and 150 ns delay (“slow” emission, thin solid line). The spectra shown in Fig. 4
were measured with emission wavelengths set at 480 nm favoring the emission from the Ce2
site (thick solid line) and 320 nm corresponding to the so-called “host” emission,
characteristic of the undoped crystal but still present to some extent in activated crystals (thin
solid line). Both spectra shown in Fig. 4 are time-integrated with the signal accumulated
within the 192 ns time window between the consecutive synchrotron pulses.
Clearly the excitation spectra of the Ce1 emission strongly change with the delay of the
time gate. The “slow” emission, represented by a “delayed” spectrum, is excited more
efficiently in a narrow wavelength range around the bandgap energy (160 to 180 nm) while
excitation efficiency at the wavelengths corresponding to VUV (50 to 150 nm) and Ce3+ f-d
absorption bands (240 to 335 nm) is clearly lower than in the “zero-delay” spectrum
representing the “fast” emission. Nevertheless, some slow Ce3+ emission produced (mostly at
Ce1 sites) under the VUV excitation (compare also Fig. 7) strongly suggests that there are
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some slow energy transfer channels from the host to the Ce3+ ions most likely related to traps
that intercept and then release charge carriers with some, trap dependent delay. Delayed
recombination of carriers released from traps is likely to produce slower components in VUV
(or X-, or gamma-ray) excited pulses of emission light.
The structure in VUV below 140 nm reflects the spectral characteristics of the primary
monochromator equipped with the Al grating, and does not correspond to any real physical
processes. Nevertheless the high signal at these wavelengths is indicative of strong sensitivity
of a given emission to the VUV excitation. Since the VUV photons provide over the bandgap
excitation of the host material generating free electron-hole pairs, it also points out that Ceions must be efficient recombination centers.
Note that because of the upper wavelength limit (335 nm) imposed by the detection
monochromator the lowest energy f-d absorption band in the Ce1 excitation spectrum is
missing and there are only two bands at 260 and 286 nm corresponding to the higher energy
d-levels split by a lower symmetry crystal field component.
The time-integrated excitation spectrum measured at 320 nm emission wavelength
corresponding to the “host” emission band and shown in Fig. 4, shows no contribution from
the Ce3+ f-d bands. A steep increase at wavelengths corresponding to the energy bandgap of
the material and, consequently, the large Stokes shift between emission and excitation
wavelengths, are typical of emissions due to decay of self- or defect-trapped excitons (“host”
emissions).
Finally the time-integrated excitation spectrum measured at 480 nm shows the structure
that was previously assigned to the Ce2 center [2,3]. Note in particular that the 325 nm peak
is very well suited to maximize the contribution of the Ce2 emission since the Ce1 excitation
spectrum at this wavelength shows a local minimum.
In Figs. 5 and 6 we present time-integrated emission spectra of LYSO under VUV and UV
excitations, at room temperature (RT) and 11 K, respectively. The over the bandgap VUV
excitations (70 and 109 nm) mimick very well gamma or X-ray excitations and produce
spectra that clearly point to Ce1 sites as being responsible for the major contribution to the
emission under ionizing excitation. Although at RT the emission from the Ce2 sites is
strongly quenched, nevertheless, under the 325 or 317 nm excitations (both of which
correspond to the minimum in the Ce1 excitation spectrum and maximum in the Ce2
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excitation spectrum), there is a clearly visible contribution from Ce2 sites at RT, on the longer
wavelength side of the Ce1 emission. At lower temperatures (11 K, Fig. 6) the contribution
from Ce2 sites is responsible for a well resolved band peaking at about 460 nm. Note that at
11 K VUV excitation produces, in addition to Ce-emission, also the “host” emission
characteristic of the unactivated crystal. Interestingly the “host” emission in Ce-activated
LuAP and LuYAP is not visible under VUV excitations and at low temperatures. This may
suggest that Ce hole capture cross sections are higher in LuAP and LuYAP than in LSO.
Obviously in any of those materials the “host” emission is absent under excitations into Ce 3+
f-d absorption bands at any temperature.
Finally, in Fig. 7, we present emission time profiles of LYSO at 11 K, for two different
emission wavelengths (390 nm and 500 nm), and for three different excitation wavelengths.
Note that for 293 and 317 nm wavelengths representing a direct excitations into absorption
bands of the Ce ions in the Ce1 and Ce2 sites, the profiles show single exponential decays
with decay times, 35.4 ns and 56 ns, that are likely to correspond to pure emissions from Ce3+
ions in these two sites. It is also very instructive to note that the VUV excited emission
profile (109 nm excitation, 390 nm emission) shows a decay time of 39 ns (longer than the
radiative decay time of Ce1 but shorter than the radiative lifetime of Ce2) and background
that must be due to slow components likely to originate from the trap contribution.
The general features observed in the excitation and emission spectra of LSO are similar as
in LYSO. The spectra of Ce-activated perovskite and garnet samples (LuAP, YAP, LuYAP
and YAG) are simpler since there is only one Ce-site in these materials. The VUV spectra of
Pr-activated LuAP, YAP and YAG have already been published (see e.g. [15]).
3.3. Scintillation light yields
The so-called scintillation energy spectrum depicts the number of scintillations (y-axis)
plotted against the amount of light in the scintillation (x-axis) obtained from a given
scintillator material under irradiation by X- or gamma particles. The complexity of the
simplest possible spectrum obtained under irradiation by monoenergetic X- or gamma
photons reflects all the possible processes in which high energy photons transfer a part
(Compton scattering) or all of its energy (photoabsorption) to a scintillator material.
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Energy spectra are measured by transforming a scintillation, generated by each single event
(Compton scattering or photoabsorption), into charge at the photocathode of the
photomultiplier tube (PMT) which is then properly amplified and formed into a standard
width and variable height voltage pulse by electronic modules of the experimental set-up.
The height of each of the standard pulses is proportional to the amount of scintillation light in
a given scintillation. All the pulses are collected and assigned to different channels by a
multichannel analyzer working in a PHA (pulse height analyzer) mode, for a predetermined
collection time. Since the number of the channel in which a given pulse has been counted
depends on the height of the pulse, which in turn depends on the amount of light in a given
scintillation, all the pulses collected and assigned to different channels, form the characteristic
energy spectrum.
The position of the photoabsorption peak (photopeak) in such a spectrum, under
assumption of a linear response of the system, depends on the impinging gamma photon
energy and scintillation efficiency of a given scintillator material. Consequently, the ratio of
positions of photopeaks in two energy spectra measured using the same set-up and the same
gamma source, and two scintillator materials, one of which is a reference material, represents
a measure of the relative scintillation efficiency or scintillation light yield (LY) of the second
material. As we have mentioned before, this approach is likely to provide only approximate
results. To avoid systematic errors we have employed the method to be described in details
later [10].
In Fig. 8 we present energy spectra of representative BGO, LSO:Ce, LYSO:Ce, LuAP:Ce,
LuYAP:Ce and YAG:Pr crystals. All the samples were pixels (2x2x10 mm) placed vertically
on the photomultiplier window. It is important to note that LY of the pixel sample measured
in vertical geometry is usually 60 to 30% of the LY of the same sample in horizontal
geometry, or the plate sample cut from the same boule. The BGO pixel served as a standard
crystal. The LSO:Ce crystal was grown at CTI, LYSO:Ce, LuYAP:Ce and LuAP:Ce were
grown at PML and YAG:Pr was grown at ITME. We do not show the spectrum of LuAP:Pr
as its LY was too low to produce a well resolved photopeak needed to calculate the LY. The
PMT voltage was 1500 V. The spectroscopy amplifier gains used to measure the spectra and
the relative scintillation efficiencies, scaled in BGO units and calculated using the method
described in Ref. [10], are indicated in the figure (note that the calculated efficiencies may not
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reflect exactly photopeak positions in the spectra). The different appearance of the YAG
spectrum is due to lower density of the YAG crystal (lower photofraction) and may cause
some confusion and problems with the proper identification of photopeaks (at 80, for 511 keV
and 230, for 1274 keV).
Both LSO and LYSO have light yields that are much higher than the light yield of BGO but
we also note that the LYSO pixel shows light yield that is about 20% higher from that of the
LSO crystal. The best LYSO pixel from PML had LY of 9.7 BGO, almost 50% higher than
LY of the best LSO pixel at 6.6 BGO (this particular sample, labeled 1050 and grown at CTI,
was made availalable to us for comparison by Dr Lecoq and Dr Kuntner, CERN, in Nov.
2002).
Note also the high light yield of LuAP:Ce, LuYAP:Ce and YAG:Pr (at 1.98, 1.93 and 3.1
BGO, respectively). Of these three crystals, YAG is clearly the best. This observation is
consistent with earlier reports claiming high scinitllation efficiency of YAG:Pr [16], but,
unfortunately, because of its low density, YAG is not a viable alternative for gamma
detection.
3.4. Scintillation time profiles
Scintillation time profiles of BGO, LSO:Ce, LYSO:Ce, LuAP:Ce, LuYAP:Ce, YAG:Pr and
LuAP:Pr, under gamma excitation from Na22 radioactive source at RT are shown in Fig. 9.
The dynamic ranges of all the y axes in this figure are the same; the maximum y value is
always 150 times larger than the minimum y value, to aid comparison of different profiles.
Experimental points were fitted to two- or three-exponential expressions with background
(background may contain some contributions from very slow components). Fit parameters
obtained from the fitting procedure are shown in the figure. All the materials studied are
much faster than BGO. Note the similarity of time profiles characterizing LSO and LYSO.
The fastest material is, as expected, LuAP:Pr, with its fast component decay time at 9.4 ns .
Unfortunately the time profile of LuAP:Pr has also some contribution from slower
components. The fast components in LuAP:Ce, LuYAP:Ce and YAG:Pr have decay times of
16 to 18 ns but all three materials have also some part of scintillation light emitted in slow
components. The contribution of slow components is probably smaller in LuAP (both Ceand Pr-activated) and somewhat higher in LuYAP and YAG.
- 12 -
3.5. Low temperature thermoluminescence
Low temperature thermoluminescence glow curves were measured after the sample had
been irradiated by X-rays for about 10 min at 10 K. Both steady state radioluminescence
emitted by the sample during the irradiation cycle and thermoluminescence intensity, emitted
during the heating cycle were recorded successively and relevant plots for LYSO:Ce,
LuAP:Ce, LuYAP:Ce and YAG:Pr pixel samples are shown in Fig. 10. The heating rate was
kept constant at about 0.14 K/s. The maximum temperature was 300 K. Since the same setup was used to record both parts of the total curve it is possible to compare them and to find
the ratio of the time integrated thermoluminescence intensity to the total integrated intensity
(steady state radioluminescence and thermoluminescence). This ratio is directly related to the
fraction of the energy intercepted by relatively shallow traps (glowing at temperatures below
300 K) to the total energy deposited in the crystal by X-ray irradiation.
Unless the integration time is too long and, consequently, traps are saturated, this fraction
should also give the information of how much of energy deposited in the crystal by a single
gamma particle is lost to traps and therefore to slower components in the scintillation time
profile. We should note, however, that loss to deep traps, glowing at higher temperatures
(above RT), is not included in the fraction measured by this method and shown in the figure
for each of the crystal measured. Nevertheless it is reasonable to expect that materials having
larger fraction (like LuAP, LuYAP, YAG) should have also larger contribution from the
slower components in the scintillation time profiles. By the same token one can expect that
these materials have lower scintillation light yields.
These predictions are confirmed by experimental observations. We note that LSO (not
shown) and LYSO have very low fractions (on the order of 0.001), scintillation time profiles
dominated by fast components and high scintillation light yields. The same is not true for
other materials, studied in this paper.
4. Summary
We have characterized a number of new materials that are being developed at PML and
ITME, using radioluminescence, VUV-spectroscopy, scintillation energy spectra, scintillation
time profiles and low temperature thermoluminescence.
- 13 -
We have identified two main loss mechanisms in all the studied materials. The first one is
due to some unidentified absorption centers responsible for loss of scintillation light in pixel
samples. We have observed that a vertically positioned pixel is characterized by LY, which is
at best 60% of the LY characterizing the same pixel in horizontal position (for good LSO and
LYSO) but in other cases (LuAP, LuYAP and YAG) it is even lower at 30 to 50%.
The second loss mechanism is due to traps. Although the scintillation light is not truly lost
since part of the energy deposited in the crystal and intercepted by traps appears as delayed
scintillation, nevertheless, in applications where high counting rate and/or short time gate (to
measure true coincidence rate) is required, this effect may have to be counted as loss. We
have developed and applied the method which allows to measure fraction of the energy lost to
traps by means of the low temperature thermoluminescence experiment described in this
paper. The fractions found in thermoluminescence experiments suggest that, while good LSO
and LYSO can not be improved any further, the trap free perovskites and garnets might have
LYs higher by 20 to 40%.
We conclude that some of materials studied in this paper, including LSO developed at CTI,
have already reached a mature stage. In particular LYSO developed by PML has at least 20%
higher LY than LSO. Other materials still have potential to be improved and to achieve
higher LY and faster scintillation time profiles.
Unfortunately, Pr-doped materials, for higher Pr-concentrations, develop problems, due,
most likely, to Pr-specific processes of nonradiative Pr-Pr quenching of scintillation light.
The only interesting exception is Pr-activated YAG, which is faster than LSO:Ce and
LYSO:Ce, as fast as LuAP:Ce and already more efficient than LuYAP and LuAP activated
with Ce. Obviously YAG is not dense enough to be used in 511 keV gamma detection (PET)
but it is worth pointing out that Pr-activated lutetium analog of YAG (LuAG), may provide a
new interesting addition to the pool of available dense oxide scintillators.
Acknowledgments
We thank Dr Charles Melcher of CTI, Prof. Stefaan Tavernier of Brussels University, Dr
Paul Lecoq and Dr Claudia Kuntner of CERN, for providing LSO crystals used in this study.
The ITME and UMK teams gratefully acknowledge the support of Polish Government (KBN)
under the grant no 4T08D 044 24. The VUV and UV experiments at Hasylab were supported
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by the IHP-Contract HPRI-CT-1999-00040 of the European Community. The UMK team
gratefully acknowledges the support and hospitality of Prof. Georg Zimmerer and Dr.
Sebastian Vielhauer at Hasylab.
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Figure captions
Fig. 1. Uncorrected steady state radioluminescence spectra of the three pixel (2x2x10 mm)
samples of LuAP:Pr, ITME, at 15 K. Pr concentrations in the three samples were 0.003, 0.04
and 0.08 mol%Pr in the melt. Resolution was 1.7 nm.
Fig. 2. Uncorrected steady state radioluminescence spectra of the two pixel (2x2x10 mm)
samples of YAG:Pr, ITME, at RT. Pr concentrations were 0.1 and 0.3 mol%Pr in the melt.
Resolution was 1.7 nm.
Fig. 3. Uncorrected excitation spectra of LYSO:Ce, PML, at 11 K. Resolution was 0.25 nm.
The emission wavelength was set at 400 nm (Ce1). Two different time gates were employed;
a thick solid line corresponds to gate set at 0-40 ns, a thin solid line corresponds to gate set at
150-190 ns.
Fig. 4. Uncorrected excitation spectra of LYSO:Ce, PML at 11 K. Resolution was 0.25 nm.
The emission wavelengths were set at 320 nm (thin solid line, “host” emission) and 480 nm
(thick solid line, Ce2-emission). Time gate was set at 0 to 192 ns (time-integrated).
Fig. 5. Uncorrected time-integrated emission spectra of LYSO:Ce at room temperature (RT).
Resolution was 4.32 nm. The excitation wavelength was set at 70 nm (VUV, thick solid line)
and 325 nm (Ce2 characteristic peak, thin dashed line).
Fig. 6. Uncorrected time integrated emission spectra of LYSO:Ce at 11 K. Resolution was
4.32 nm. The excitation wavelength was set at 109 nm (VUV, thick solid line) and 317 nm
(Ce2 characteristic peak, thin dashed line).
Fig. 7. Time profiles of the LYSO:Ce emissions, measured at 390 and 500 nm, excited by
synchrotron pulses of different wavelengths: 109 nm (thick solid line), 293 nm (Ce-1
absorption band, thin solid line), and 317 nm (Ce-2 absorption band, thin dashed line).
Temperature was 11 K. The profiles were shifted vertically to aid presentation.
Fig. 8. Energy spectra of BGO, LSO:Ce, LYSO:Ce, LuAP:Ce, LuYAP:Ce and YAG:Pr
samples. All the samples are pixels (2x2x10 mm). The amplifier gains and light yields (in
BGO units) of each of the crystal are indicated in the figure. Note the different gain values
for BGO, LuYAP:Ce and LuAP:Ce. The YAG:Pr photopeaks are positioned at channels 80
(511 keV) and 230 (1274 keV) while the relevant Compton edges at channels 45 and 180.
The LY value of the YAG: 0.1% Pr sample was 2.7 BGO.
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Fig. 9. Scintillation time profiles at RT of BGO, LSO:Ce, LYSO:Ce, LuAP:Ce, LuYAP:Ce,
YAG:Pr and LuAP:Pr under gamma excitation from Na22 source. Time constants from twoor three-exponential fits are indicated in the figure.
Fig. 10.
Steady state radioluminescence (flat part of the curve) and low temperature
thermoluminescence glow curve for LYSO:Ce, LuAP:Ce, LuYAP:Ce and YAG:Pr pixel
crystals. During irradiation by X-rays the temperature of the sample was 10 K. At zero time,
after irradiation, the temperature was raised from 10 K to 300 K in 35 min. (heating rate was
0.14 K/s). The value of fraction defined as the ratio of the area under TL glow curve to the
total area is given in the figure.
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References
[1]. C.L. Melcher, The J. of Nucl. Med. 41 (2000), p. 1051.
[2]. H. Suzuki, T.A. Tombrello, C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. 40
(1993), p. 380.
[3]. D.W. Cooke, B.L. Bennett, K.J. McClellan, J.M. Roper, M.T. Whitttaker and A.M.
Portis, Phys. Rev. B, 61 (2000), p. 11973.
[4]. A. Lempicki, M.H. Randles, D. Wisniewski, M. Balcerzyk, C. Brecher and A.J.
Wojtowicz, IEEE Trans. Nucl. Sci. 42 (1995), p. 280.
[5]. B. Henderson and G.F. Imbush, Optical Spectroscopy of Inorganic Solids, Clarendon
Press, Oxford, 1989, p. 173.
[6]. D. Wiśniewski, S. Tavernier, A.J. Wojtowicz, M. Wiśniewska, P. Bruyndonckx,
P. Dorenbos, E. van Loef, C.W.E. van Eijk, L.A. Boatner, Nucl. Instr. & Meth. A486 (2002),
p. 239; D. Wiśniewski, S. Tavernier, P. Dorenbos, M. Wiśniewska, A.J. Wojtowicz,
P. Bruyndonckx, E. van Loef, C.W.E. van Eijk, L.A. Boatner, IEEE Trans. Nucl. Sci. 49 (3)
(2002). p. 937.
[7]. J. L. Lefaucheur, Z. Galazka, A. Caramanian, Z. Gou, P. McDade; "Growth of large
(Lu,Y)2SiO5:Ce crystals", ICCG-14, France-Grenoble (2004).
[8]. Z. Galazka, J. L. Lefaucheur, "Recent development in growth of large and high quality
LuAP:Ce and LuYAP:Ce crystals", ICCG-14, France-Grenoble (2004).
[9]. G. Zimmerer, Nucl. Instr. Meth. Phys.Res., A308 (1991), p. 178.
[10]. A.J. Wojtowicz et al, Nucl. Instr. Meth. Phys. Res, to be published.
[11]. L.M. Bollinger and G.E. Thomas, Rev. Sci. Instr. 32 (1961) p. 1044.
[12]. R.H. Bartram, D.S. Hamilton, L.A. Kappers and A. Lempicki, J. Lumin. 75 (1997). p.
183.
[13]. M.J. Weber, Sol. State Comm. 12 (1973), p. 741.
[14]. S. Nicolas, M. Laroche, S. Girard, R. Moncorge, Y. Guyot, M.-F. Joubert, E. Descroix,
A.G. Petrosyan, J. Phys. 11 (1999), p. 7937.
[15]. M. Wisniewska, A.J. Wojtowicz, T. Lukasiewicz, Z. Frukacz, Z. Galazka, M.
Malinowski, Proc. SPIE 4412 (2001), p. 351; see also contributions of the UMK team (under
Wojtowicz, Wisniewska) to Hasylab annual reports 1998, 1999, 2000, at http://wwwhasylab.desy.de/science/annual_reports/main.htm.
[16]. M. Wisniewska, D. Wisniewski, A.J. Wojtowicz, S. Tavernier, T. Lukasiewicz, Z.
Frukacz, Z. Galazka, M. Malinowski, IEEE Trans. Nucl. Sci., 49 (2002), p. 926.
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