Czochralski Growth, Optical, Scintillation, and Defect Properties of

May 22, 2019 - With 0.1 atom % Cu2+ codoping, the scintillation light yield of LSO:Ce can be significantly enhanced from 32 000 to 39 000 photons/MeV ...
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Cite This: Cryst. Growth Des. 2019, 19, 4081−4089

Czochralski Growth, Optical, Scintillation, and Defect Properties of Cu2+ Codoped Lu2SiO5:Ce3+ Single Crystals Yuntao Wu,*,†,‡ Merry Koschan,† Camera Foster,†,‡ and Charles L. Melcher†,‡,§,∥ †

Scintillation Materials Research Center, University of Tennessee, Knoxville, Tennessee 37996, United States Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States § Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, Tennessee 37996, United States ∥ Department of Nuclear Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States Downloaded via UNIV OF SOUTHERN INDIANA on July 18, 2019 at 03:10:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: Divalent cation codoping, such as with Ca2+ and Mg2+, is beneficial for the scintillation performance enhancement of Czochralski-grown Lu2SiO5:Ce (LSO:Ce) single crystals for nuclear medical imaging applications, but with that benefit comes a tendency toward acentric growth due to the reduced surface tension of the melt. Here, we present a divalent Cu codoping strategy to achieve a simultaneous improvement of light yield, energy resolution, scintillation decay, and afterglow in LSO:Ce single crystals without destabilizing the solid−liquid interface or promoting acentric growth. High-quality 32-mm-diameter and 110mm-long LSO:Ce single crystals codoped with 0.1 and 0.3 atom % Cu2+ ions in the melt were successfully grown using the Czochralski method. While the surface tension of the LSO melt does decrease with Cu2+ codoping, analogous to the effect of Ca2+ codoping, it is not reduced enough to affect the crystal growth stability or diameter control. With 0.1 atom % Cu2+ codoping, the scintillation light yield of LSO:Ce can be significantly enhanced from 32 000 to 39 000 photons/MeV with an improved enegy resolution of 9% at 662 keV, and a reduced afterglow at room temperature. A continuous shortening of scintillation decay time with Cu2+ codoping is ascribed to the combined effect of enhanced thermal ionization from the Ce3+ 5d1 state and a reduction of the emission contribution from Ce2 centers, i.e., in the site neighboring six oxygens. Thermoluminescence and afterglow measurements are utilized to study the defect structure and explain the variation in scintillation yield.

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reduction in Ce4+ concentration.4 Sr2+ codoping can improve the gamma-ray spectral resolution of LaBr3:Ce3+ to 2.0% at 662 keV, and Zr4+ codoping can enhance the spectral resolution of SrI2:Eu2+ and KCaI3:Eu2+ to 2.5% and 2.7% at 662 keV, because of a more proportional light yield response.5−7 Aluminate g a r n e t s , s u c h a s L u 3 A l 5 O 1 2 : C e (L u A G :C e ) a n d (Lu0.75Y0.25)3Al5O12:Pr (LuYAG:Pr), can approach an excellent energy resolution of 4% at 662 keV by Mg2+ and Li+ codoping, respectively, owing to an improvement in counting statistics resulting from the enhancement of light yield.8,9 The 5d-4f emission of stable Ce4+ is as efficient as stable Ce3+ under ionization irradiation, but with a faster lifetime due to the bypass of the first step of the stable Ce3+ scintillation emission, namely capturing a hole from the valence band. Aliovalent codoping was proven to shorten the decay time of Ce doped oxides by introducing stable Ce4+, for example, a 36% shortening of the fast component for Gd3Ga3Al2O12:Ce (GGAG:Ce) and 7% for LuAG:Ce by Mg2+ codoping.8,10 For bulk single-crystal scintillators grown using the Czochralski method, a codopant

INTRODUCTION Inorganic scintillators are materials that can convert ionizing radiation into light pulses that are detectable by photosensors when struck with electromagnetic radiation such as X- and γ-rays or charged particles such as α and β particles.1,2 Over past decades, inorganic scintillators as affordable, scalable, and efficient radiation detection materials have been widely used for high-energy physics, homeland security, and nuclear medical imaging applications. In recent years, there has been a great deal of activity toward improved performance due to the enhanced requirements of detection systems, for example, better timing resolution for the High Luminosity Large Hadron Collider (HLLHC) and time-of-flight positron emission tomography (TOFPET), better spatial resolution and discrimination of scattered photons for single photon emission computer tomography (SPECT), and better radionuclide identification capability for radioisotope identification devices (RIDs). Ion codoping has been regarded as a useful approach for engineering critical aspects of performance, e.g., scintillation yield, energy resolution, and scintillation decay time of inorganic scintillators, particularly in the form of bulk crystals. 3 Tetravalent cation codoping, such as Zr4+, Hf4+, and Ge4+, can enhance the scintillation yield of Gd2SiO5:Ce due to the © 2019 American Chemical Society

Received: April 10, 2019 Revised: May 2, 2019 Published: May 22, 2019 4081

DOI: 10.1021/acs.cgd.9b00479 Cryst. Growth Des. 2019, 19, 4081−4089

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may not only influence the performance, but also the crystallization behavior. The morphological stability of the solid−liquid interface can be affected by the presence of unintentional impurities from the raw materials and intentional impurities like dopants or codopants. For instance, the impurities that are concentrated in front of the solidification interface due to segregation11,12 can lead to a reduced radial temperature gradient at the interface due to the increase of radiative heat absorption by the grown boule if the absorption induced by the impurities overlaps with the emission wavelengths of the melt,13 as well as Marangoni flow caused by surface tension and/or concentration gradients.14,15 Single crystals of Ce-doped LSO are commonly used in PET scanners because of their high density of 7.4 g/cm3, high light yield of 75% that of NaI:Tl, and fast scintillation decay of about 40 ns.16,17 Looking toward better timing resolution for TOFPET applications, LSO scintillation yield, rise time, and decay time were further improved by divalent codoping, such as Ca2+ or Mg2+ ions.18,19 The increased light yield and shortened scintillation decay time were ascribed to the suppression of the defects that trap electrons and holes20 and the introduction of stable Ce4+ ions,19 respectively. Nonetheless, Ca2+ codoping was found to lower the surface tension of the LSO melt, and when the Ca2+ codoping concentration in the melt reaches 0.4 atom %, the solid−liquid interface will become unstable, resulting in an acentric (or off-axis) growth.21 The acentric growth leads to an inhomogeneous distribution of optical and scintillation performance along the radial direction22 and a decrease of crystal yield. Fortunately, it was found that by using Zn2+ as a codopant it is possible to restore the growth stability by increasing the surface tension of the melt.21 The effect of monovalent and divalent codoping of lutetiumbased orthosilicates has been reported previously.23−27 For example, Zagumennvi et al. reported that codoping with Cu can reduce crystal cracking as well as create waveguide properties.24 However, they also reported that Cu impurities are likely to introduce Ce4+ ions in lutetium-based orthosilicates, and they describe Cu as a potentially harmful and undesirable impurity that results in reduced scintillation light output. Specifically, they reported that Cu impurities should be controlled under 30 ppm.25−27 Contrary to that recommendation, we report here several beneficial effects of divalent Cu codoping at a much higher concentration of 0.1−0.2%, including simultaneous improvement of light yield, energy resolution, scintillation decay, and afterglow in LSO:Ce single crystals without destabilizing the solid−liquid interface or promoting acentric growth. In this paper, the work is organized as follows: First, the correlation between surface tension of the melt and crystallization behavior is studied by comparison with the scenario of Ca2+ codoping. Second, the Ce valence state and luminescence characteristics of the Ce1 and Ce2 centers are investigated by optical absorption, photoluminescence excitation and emission spectra, photoluminescence decays, and X-ray excited radioluminescence spectra; then, the scintillation properties, including light yield, energy resolution, and scintillation decay time of non-codoped and Cu2+ codoped LSO:Ce are examined. Finally, we study the effect of Cu2+ codoping on defect structure by using thermoluminescence and afterglow measurements and clarify the correlation between scintillation yield variation and defect structure by comparing our results with published findings for Li+ and Ca2+ codoped LSO:Ce.

Article

EXPERIMENTAL METHODS

Czochralski Crystal Growth. The Czochralski (Cz) method was used to grow LSO:0.1 atom % Ce3+ crystals codoped with 0, 0.1, and 0.3 atom % Cu2+ in the melt. The starting materials were Lu2O3, SiO2, CeO2, and CuO with at least 99.99% purity. The atomic percentage of the dopant and codopant concentrations is relative to that of lutetium in the melt. The dopant and codopant concentrations in the grown boule will be lower than the nominal concentration in the melt because of the segregation that occurs during growth. The crystals were grown in iridium crucibles which were loaded with raw materials and inductively heated by a 30 kW Hüttinger power supply. Crystal growth was initiated with a seed crystal oriented along the [100] axis. The pulling rate was 1.5 mm/h with a 10 rpm rotation rate. The growth atmosphere was nitrogen with about 0.1% oxygen. All as-grown boules are 32 mm in diameter and 110-mm-long. All 5 mm × 5 mm× 5 mm samples used for measurements were cut from the middle section of their respective boules. Optical Property Measurements. Optical absorption spectra from 200 to 800 nm were acquired with a Varian Cary 5000 UV−vis− IR spectrophotometer. Photoluminescence emission (PL) and excitation (PLE) spectra were recorded with a HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer. A 450 W continuous xenon lamp was used as an excitation source. Photoluminescence decay profiles were conducted on the same spectrofluorometer using a time-correlated single-photon counting module. The excitation sources are HORIBA Jobin Yvon NanoLEDs. The duration of the light pulse was shorter than 2 ns. The repetition rate of the NanoLED pulser was set to be 500 kHz. Scintillation Performance Measurements. The scintillation decay profile was acquired with an Agilent DSO6104A digital oscilloscope in single shot mode under a 137Cs source irradiation. The scintillation light yields (LY) were evaluated using a pulse processing chain that consists of a Hamamatsu R2059 photomultiplier tube (PMT), an Ortec 672 Amp, a Canberra model 2005 pre-Amp, and a Tukan 8k multichannel analyzer. A shaping time of 3 μs was used to achieve full light integration. Each sample was measured under irradiation with a 15 μCi 137Cs source. To maximize the light collection, mineral oil was used as a couplant between the sample and the PMT, and a 50 mm diameter PTFE-lined dome-shaped reflector was used as a top reflector. The error bar of the LY is ±5%. The energy resolutions of the samples at 662 keV were evaluated by using a Hamamatsu R6231100 PMT. X-ray radioluminescence (RL) measurements were conducted in transmission mode under excitation of a copper X-ray source operated at 35 kV and 0.1 mA. The emission signals were detected with an Acton SpectraHub monochromator with a focal length of 155 mm and a broadband PMT. The emission intensities were corrected by the Acton SpectraSense acquisition software based on the spectral sensitivity of the PMT. The room temperature afterglow profiles of the samples were acquired using a Hamamatsu R2059 PMT operated at −1500 Vbias. To enhance the light collection, a Tetratex TX3104 PTFE membrane was used as a reflector. The samples were stored in the dark for 24 h before measurements. The samples were irradiated with X-rays for 15 min while being held in the dark, and the emission signals from each sample were recorded immediately after irradiation cutoff. Thermoluminescence Measurements. Thermoluminescence (TL) glow curves were acquired in the temperature range of 275 to 550 K. After X-ray irradiation at 275 K with an X-ray tube operated at 35 kV and 0.1 mA for 15 min, the TL glow curve was recorded while heating the sample at a rate of 3 K/min. Prior to the TL measurements, the samples were individually heated to 550 K to ensure that all traps in the temperature range were empty. A Hamamatsu R2059 PMT optically coupled to a cryostat’s borosilicate window was used to record the spectrally unresolved emission from the sample. Standard NIM electronics were used to convert the PMT current signal into a voltage signal. The voltage signal was digitized by a National Instruments 6002E data acquisition card. The sample temperature and the signal intensity were correlated by software that was developed in-house. 4082

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Article

RESULTS AND DISCUSSION Stable Crystal Growth of Cu2+ Codoped LSO:Ce with Reduced Surface Tension of the Melt. We first investigate the effect of Cu2+ codoping on the surface tension of the LSO melt by using the same method used in ref 21. When a grown boule is extracted from the melt, the force exerted on the melt surface can be seen by an increase in the mass reading on the associated load cell; this continues to increase until reaching a maximum value. The difference between the initial mass reading and the maximum value can be used to make a relative comparison of the surface tension of the melt for each composition. The surface tension is proportional to the maximum force required to extract the boule from the melt.28 The maximum extraction forces for 0.1 and 0.3 atom % Cu2+ codoped LSO:Ce boules are plotted in Figure 1 as well as the

Figure 2. As-grown boules of (a) non-codoped, (b) 0.1 atom %, and (c) 0.3 atom % Cu2+ codoped LSO:Ce. Figure 1. Maximum extraction force required to separate the Cu2+ codoped LSO:Ce boule of each composition from the melt as well as the results published for non-codoped and 0.1, 0.3, and 0.4 atom % Ca2+ codoped LSO:Ce.

published values for non-codoped, 0.1, 0.3, and 0.4 atom % Ca2+ codoped LSO:Ce for comparison. The surface tension of the Cu2+ codoped LSO:Ce melt is lower than that of the noncodoped one but still higher than that of Ca2+ codoped ones with an equivalent codopant concentration in the melt. However, the growths of the crystals codoped with Cu and Ca in the 0.1% and 0.3% concentrations were both stable, and there was not a noticeable tendency toward acentric growth. This indicates that the reduced surface tension induced by Cu and Ca codoping in the 0.1% and 0.3% concentrations is not large enough to cause growth instability. As shown in Figure 2, the as-grown LSO:Ce boules codoped with 0.1 and 0.3 atom % Cu2+ are highly transparent and almost crack-free. Ce Valence State: Nonconversion of Stable Ce3+ to Ce4+. Optical absorption spectroscopy is a qualitative tool to identify the presence of stable Ce4+ ions in Ce-doped oxides due to the detectable ligand-to-metal charge transfer (CT) transition of Ce4+.19,29−32 Because of the partial conversion of stable Ce3+ into Ce4+ ions in an LSO (or LYSO) host induced by divalent Ca and Mg codoping,19,32 and the importance of stable Ce4+ for timing properties,15,30,31 it is necessary to investigate the effect of divalent Cu codoping on the Ce valence state. As seen in Figure 3, the spectrum of non-codoped LSO:Ce shows typical Ce3+ 4f− 5d absorptions. The Ce oxidation state in Cz-grown LSO:Ce single crystals has been proven to be purely trivalent by both X-

Figure 3. Optical absorption spectra of non-codoped, 0.1 atom %, and 0.3 atom % Cu2+ codoped LSO:Ce single crystals. The optical absorption spectra of non-codoped, 0.1 atom %, and 0.3 atom % Ca2+ codoped LSO:Ce single crystals are shown in the inset for comparison.

ray absorption spectroscopy and electron energy loss spectroscopy.33,34 The spectra of Cu2+ codoped LSO:Ce samples, regardless of Cu2+ concentration in the range less than or equal to 0.3%, also show Ce3+ 4f−5d absorptions without the presence of the CT absorption band of Ce4+. It indicates that the stable Ce3+ will not be converted into Ce4+ with Cu2+ codoping, dissimilar to the effect of Cu+24 and Ca2+ codoping (see Figure 3 inset).18,19 The Cu valence state in LSO is expected to maintain 4083

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+2 or reach a higher valence state (+3 or +4) rather than +1; because the Cu atoms coordinate with highly electronegative oxygen atoms in the LSO host lattice, there should be enough electron affinity and electrostatic attraction to draw off 3d valence electron(s). Luminescence Characteristics of Ce1 and Ce2 Centers: The Suppression of Ce2 Emission and an Enhanced Ionization of the Ce3+ 5d1 State. In the LSO host lattice, Ce ions are known to occupy two different Lu sites. Ce1 is usually designated to the Lu site neighboring seven oxygens, and Ce2 is designated to the Lu site neighboring six oxygens.35,36 Ce1 and Ce2 centers have distinct spectral properties and decay kinetics. The Ce3+ 5d−4f emissions of Ce1 centers are at 3.15 and 2.9 eV with a PL decay of 33 ns and 2.64 eV for Ce2 centers with a PL decay of 46 ns.37,38 The PL, PLE, and RL spectra and PL decays were used to study the effects of Cu2+ codoping on the luminescence characteristics of Ce1 and Ce2 centers. Figure 4 shows the normalized PL and PLE spectra of Ce1 and Ce2 centers in non-codoped and Cu2+ codoped LSO:Ce. All the emission spectra can be well fitted with three Gaussian peaks with maximums at 3.15 eV (393 nm), 2.91 eV (426 nm), and 2.64 eV (470 nm).37,38 The two green peaks at 2.91 and 3.15 are Ce1 emissions, and the blue peak at 2.64 eV is Ce2 emission. As the Cu2+ codoping concentration increases, the emission contribution from Ce2 centers gradually decreases. It should be noted that monovalent Li codoping can also reduce the emission contribution from Ce2 without formation of stable Ce4+.23 The reduction/suppression of Ce2 emission was also observed in Ca2+ codoped LSO:Ce, which was attributed to the occupation of Ce2 sites by optically inactive Ce4+ ions instead of Ce3+ ions, as a result of charge compensation due to Ca2+ codoping.22 However, this explanation does not apply to the case of Cu2+ codoping because no stable Ce4+ was created by Cu2+ codoping. The as-measured RL spectra of non-codoped, 0.1 atom %, and 0.3 atom % Cu2+ codoped LSO:Ce are shown in Figure 5a. A 20% increase of scintillation efficiency of LSO:Ce is achieved by 0.1 atom % Cu codoping, but it decreases when the Cu concentration is further increased to 0.3 atom %. The RL spectra were normalized for a better comparison of the emission contributions from Ce1 and Ce2. There is a reduction of emission contribution from Ce2, consistent with the PL emission results. The PL decay curves of non-codoped, 0.1 atom %, and 0.3 atom % Cu2+ codoped LSO:Ce can be well fitted by a single exponential function (see Figure 6). We note that the decay constants of both Ce1 and Ce2 emissions decrease with increased Cu2+ codoping concentration. For the Ce1 emission, the decay time decreases from 35.0 ns for non-codoped, to 34.0 ns for 0.1 atom % Cu, to 29.6 ns for 0.3 atom % Cu. For the Ce2 emission, it decreases from 46.4 ns for non-codoped, to 44.7 ns for 0.1 atom % Cu, to 31.7 ns for 0.3 atom % Cu. We also note that the background value relative to the decay amplitude increases noticeably in highly codoped samples. For LSO and LYSO, the luminescence quenching of Ce3+ at high temperature is caused by the thermal ionization process from the Ce3+ 5d1 state to the conduction band (CB).39,40 The ionization process will promote an electron into the CB, and leave a temporary Ce4+ behind. The electron left in the CB can recombine with the other temporary Ce4+ centers and then result in a delayed emission. The delayed emission spreading into much longer time scales than the time delay between two subsequent optical excitation pulses will cause the background signal to rise. The shortening of decay times of both Ce1 and Ce2 emissions also

Figure 4. Normalized PL and PLE spectra of Ce1 and Ce2 centers in (a) non-codoped, (b) 0.1 atom % Cu2+, and (c) 0.3 atom % Cu2+ codoped LSO:Ce.

implies an increased probability of nonradiative recombination through the ionization process. In addition, the strong UVinduced afterglow of the 0.3 atom % Cu2+ codoped sample could also contribute to its high PL decay background (see Figure 11 inset). 4084

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Figure 5. (a) As-measured and (b) normalized X-ray excited RL spectra of non-codoped, 0.1 atom % Cu, and 0.3 atom % Cu2+ codoped LSO:Ce.

Figure 6. PL decay curves of (a) Ce1 and (b) Ce2 centers in noncodoped, 0.1, and 0.3 atom % Cu2+ codoped LSO:Ce. The excitation and emission wavelengths are 360 and 392 nm for Ce1 and 333 and 500 nm for Ce2, respectively.

Scintillation Properties: A Simultaneous Improvement of Light Yield, Energy Resolution, and Scintillation Decay Time. Pulse height spectra of 5 mm3 non-codoped and Cu2+ codoped LSO:Ce samples acquired with a Hamamatsu R2059 PMT under 137Cs gamma-ray source irradiation are plotted in Figure 7. The absolute light yield was evaluated by using the single photoelectron method41 and the emissionweighted quantum efficiency of the PMT estimated for each sample. The light yield of the non-codoped LSO:Ce sample is 32 000 photons/MeV. The light yield can be enhanced to about 39 000 photons/MeV with 0.1 atom % Cu2+ codoping. This result is comparable to the values achieved by Ca2+18 and Li+34 codoping. As the Cu2+ codoping concentration is further increased to 0.3 atom %, the light yield drops to 18 000 photons/MeV. The pulse height spectra acquired for energy resolution evaluation are shown in Figure 8. The energy resolution (ΔE/E) is calculated as the full width at halfmaximum of a photopeak divided by the location of the peak centroid. The energy resolution at 662 keV slightly improves from 10% for non-codoped to 9% for the 0.1 atom % Cu2+ codoped sample. The scintillation decay curves of non-codoped, 0.1, and 0.3 atom % Cu2+ codoped LSO:Ce samples are presented in Figure 9. The high background of the PL decay curve of the 0.3 atom % Cu codoped sample is not present in its scintillation decay. It could be relevant to the high repetition rate of the NanoLED pulser used for the PL decay acquisition. All curves can be well fit by a single exponential function. The scintillation decay time shows a monotonically decreasing trend with the increase in

Figure 7. 137Cs pulse height spectra acquired by a Hamamatsu R2059 PMT for 5 mm3 non-codoped, 0.1, and 0.3 atom % Cu2+ codoped LSO:Ce single crystals.

Cu2+ concentration, from 45.5 ns for non-codoped, to 43.2 ns for 0.1 atom % Cu2+, to 34.0 ns for the 0.3 atom % Cu2+ codoped sample. Such a scintillation decay time shortening with codoping can be explained in a similar way as has been done in Li+ codoped LSO:Ce23 but is significantly different from the Ce4+involved mechanism for Ca2+ or Mg2+ codoped LSO:Ce.19 Specifically, the shortening of scintillation decay time in Cu2+ codoped LSO:Ce originates from the reduced PL decay time of 4085

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Figure 8. 137Cs pulse height spectra acquired by a Hamamatsu R6231 PMT for 5 mm3 non-codoped and 0.1 atom % Cu2+ codoped LSO:Ce single crystals.

Figure 10. (a) TL glow curves of non-codoped, 0.1 atom %, and 0.3 atom % Cu2+ codoped LSO:Ce single crystals. The as-measured (black line) and cumulative fitting (red dash line) curves of (b) non-codoped, (c) 0.1 atom % Cu2+, and (d) 0.3 atom % Cu2+ codoped LSO:Ce. The blue dashed line represents the individual fitting component.

codoping. The modified general-order kinetics expression describing TL intensity I as a function of temperature T is utilized to fit the glow curve:43

Figure 9. Scintillation decay profiles of 5 mm3 non-codoped, 0.1, and 0.3 atom % Cu2+ codoped LSO:Ce single crystals.

ij E yz I(T ) = sn0 expjjj− t zzz j κBT z k { l o o (l − 1)s ji E zy o ×m × T × expjjj− t zzz o j κBT z o β o k { n ÄÅ É l /(1 − l) | 3Ñ 2 ÅÅi o i κBT zy i κBT zy ÑÑÑÑ o ÅÅjj κBT zyz j j o zz + 6jj z × ÅÅÅjj z − 2jjj jj E zzz ÑÑÑÑ + 1} o j Et zz o ÅÅjk Et zz{ o t k { k { ÑÑÖ ÅÇ ~

Ce1 and Ce2 due to an enhanced thermal ionization from the Ce3+ 5d1 state and the suppression of the slow Ce2 emission. Defect Structure: Thermoluminescence and Afterglow Analysis. Because of the valence state mismatch between Cu2+ codopants and Lu3+ and Si4+ host cations, and the nonconversion of stable Ce3+ to Ce4+, the defect structure of LSO:Ce has to be altered by Cu2+ codoping in order to achieve electrical neutrality in the lattice. TL as an effective tool to detect the trap states was utilized to study the effect of Cu2+ codoping on defect structure. The TL glow curves of non-codoped and Cu2+ codoped LSO:Ce are shown in Figure 10a. For noncodoped LSO:Ce, the dominant TL peaks at 346, 404, 451, and 510 K are associated with the oxygen vacancies (VO) that are the nearest neighbors to the Ce center.42 These TL emissions result from the radiative recombination of electrons stored in VO with holes localized at Ce3+ through a thermally assisted tunneling mechanism.42 A drastic decrease of the TL peak at 346 K is observed in the 0.1 atom % Cu2+ codoped sample. For the 0.3 atom % Cu2+ codoped sample, although the TL peaks at 346 and 404 K are completely reduced, two new TL peaks at 288 and 480 K appear with high emission signals. Further discussion is based on a quantitative analysis of TL peaks and a comparison with the results of Ca2+ and Li+

(1)

where n0 is the concentration of trapped charges at t = 0, Et the energy level of the trap, κB the Boltzmann constant, l the kinetic order, s the frequency factor, and β the heating rate (3 K/min in this measurement). The TL parameters of LSO:Ce evaluated from the partial cleaning and initial rise method published in ref 42 were used as initial fitting parameters. As seen in Figure 10b− d, the fitting curves agree well with the experimental ones. The results of peak temperature, trap depth, and frequency are listed in Table 1. The derived trap depth and frequency confirm the suppression of VO (and/or dissociation of Ce and VO) by Cu2+ codoping and the introduction of new deep traps in highly codoped samples. 4086

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codoped and Cu2+ codoped LSO:Ce samples were measured (see Figure 11). A reciprocal correlation between scintillation

Table 1. TL Fitting Parameters of Non-Codoped, 0.1 Atom %, and 0.3 Atom % Cu2+ Codoped LSO:Ce Single Crystals composition

peak temperature (K)

trap depth (eV)

frequency (s−1)

detrapping time (s)

346 404 451 510 427 497 288 480

0.99 0.99 0.99 0.99 1.16 1.09 0.61 2.00

4.0 × 1012 2.5 × 1010 1.0 × 109 4.0 × 107 5.9 × 1011 9.0 × 108 7.0 × 108 1.7 × 1019

1.5 × 102 2.4 × 104 6.1 × 105 1.5 × 107 2.2 × 109 5.2 × 107 2.5 × 101 2.4 × 1014

LSO:Ce

LSO:Ce, 0.1 atom % Cu LSO:Ce, 0.3 atom % Cu

On the basis of the optical properties mentioned above, we found that Cu2+ and Li+ codoping has the same effect on the Ce valence state and the luminescence properties of the Ce1 and Ce2 center, but both are quite different from Ca2+ codoping. We ascribe the similarity between Li+ and Cu2+ codoping to their close ionic radii; for example, under 6-coordination, the ionic radii of Li+ and Cu2+ are 76 and 73 pm, respectively, much smaller than that of Ca (100 pm).44 Ca2+ with a larger ionic radius is supposed to occupy the seven-coordinated Lu3+ site and induce the stable Ce4+ ions.22 In contrast, Li+ ions are prone to occupy not only seven-coordinated Lu3+ substitution sites but also the six-coordinated interstitial spaces.23,45 The electrical neutrality of the system can be achieved by self-compensation between Lii and LiLu and the variation in type and concentration of VO, rather than by the conversion of Ce valence states.23 Thus, it is reasonable to believe that Cu2+ ions also tend to occupy the Lu3+ substitutional site and the interstitial space. Moreover, the ratio between CuLu and Cui should depend on the Cu2+ codoping concentration, analogous to that of Li+ codoping.23 Specifically, the Cui should be dominant in the lightly codoped sample, and the CuLu is dominant in highly codoped sample. This deduction can explain the TL variation induced by Cu2+ codoping: (i) the dominant and positively charged Cui in the lightly codoped sample (0.1 atom % Cu) can suppress/diminish the formation of VO, which leads to a drastic reduction of the TL peak at 346 K (see Figure 10a); (ii) the dominant CuLu in the highly codoped sample can couple with VO to form {CuLu+VO} complex defects due to the Coulomb attraction. The formation of complex defects can dissociate the spatially correlated Ce and VO and result in a suppression of the four VO-associated TL peaks (see Figure 10a), similar to the effect of the {CaLu+VO} complex as suggested in Ca2+ codoped LSO.22 However, there is a difference between Cu2+codoping and both Li+ and Ca2+ codoping in that the newly formed defects themselves, such as CuLu and {CuLu+Vo}, may serve as new electron traps with an energy depth of 0.6 and 2.0 eV. It is necessary to conduct 63Cu nuclear magnetic resonant (NMR) experiments and density functional theory (DFT) calculations to clarify the preferential site occupation of Cu2+ ions in LSO and its role in defect structure alternation, but these are beyond the scope of our current work. On the basis of the fitted trap parameters, the detrapping time τ at room temperature (RT) was calculated by using the following equation:46 τ = s−1 × e E / kT

Figure 11. X-ray induced afterglow profiles of non-codoped and Cu2+ codoped LSO:Ce single crystals. Inset are the pictures of the as-grown boule codoped with 0.3 atom % Cu2+ under UV irradiation and after UV excitation cutoff for 1 min.

light yield and afterglow is found. In the case of the 0.1 atom % Cu2+ codoped sample with improved light yield, afterglow drops by 50% compared to that of the non-codoped sample due to the removal of a VO-associated TL peak at 346 K. Because of the formation of deep traps associated with a TL peak at 288 K, the afterglow level of the 0.3 atom % Cu2+ codoped sample is 1 order of magnitude higher than that of non-codoped LSO:Ce. As seen in the Figure 11 inset, the as-grown boule codoped 0.3 atom % Cu2+ has a bright blue emission under UV excitation, and the afterglow emission is still visible after turning off the UV excitation for 1 min.

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CONCLUSIONS

High-quality 32-mm-diameter and 110-mm-long LSO:Ce single crystals codoped with 0.1 and 0.3 atom % Cu2+ ions were successfully grown by the Czcoralski method. The surface tension of the melt was not reduced enough by Cu2+ codoping to affect the crystal growth stability. Unlike the partial conversion of Ce ions from trivalent to tetravalent by Ca2+ codoping, Cu2+ codoping does not introduce stable Ce4+ into the LSO lattice. The emission contribution from Ce2 centers gradually reduces as the Cu2+ codoping conentration increases. Despite the fact that Cu2+ induces an enhanced thermal ionization effect from the Ce3+ 5d1 state, the scintillation yield of LSO:Ce is still improved from 32 000 to 39 000 photons/MeV by suppression of VO defects. The shortening of scintillation decay times is regarded as a result of both the enhanced thermal ionization effect from the Ce3+ 5d1 state and the reduction of the slow Ce2 emission. The similarity in codoping behaviors between Li+ and Cu2+ in LSO:Ce, and the dissimilarity to codoping behaviors of Ca2+ suggest that the empirical selection criteria of suitable codopants for performance enhancement in oxide scintillators consists of not only the valence state of the codopant but, very importantly, the ionic radius. We believe that the present work expands the understanding of the effect of ion codoping on oxide scintillators in general.

(2)

The calculated detrapping times at RT are listed in Table 1. Since there is a correlation between deep traps and RT afterglow, the room temperature afterglow profiles of non4087

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yuntao Wu: 0000-0002-9708-0304 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Siemens Medical Solutions. REFERENCES

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