Luminescence Characteristics of the Ce3+-Doped Pyrosilicates

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Luminescence Characteristics of the Ce -doped Pyrosilicates: the Case of La-admixed GdSiO Single Crystals 2

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Vitezslav Jary, Martin Nikl, Shunsuke Kurosawa, Yasuhiro Shoji, Eva Mihokova, Alena Beitlerova, Gian Paolo Pazzi, and Akira Yoshikawa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5080384 • Publication Date (Web): 22 Oct 2014 Downloaded from http://pubs.acs.org on October 28, 2014

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The Journal of Physical Chemistry

Luminescence Characteristics of the Ce3+-Doped Pyrosilicates: The Case of La-Admixed Gd2Si2O7 Single Crystals Vitezslav Jary1, *Martin Nikl1, Shunsuke Kurosawa2,4, Yasuhiro Shoji2, Eva Mihokova1, Alena Beitlerova1, Gian Paolo Pazzi3, Akira Yoshikawa2,4

1

Institute of Physics AS CR, Cukrovarnicka 10, 16253 Prague, Czech Republic

2

Institute for Materials Research , Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577 Japan

3

Institute of Applied Physics ‘‘N.Carrara’’ (IFAC) of CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino (Firenze), Italy 4

New Industry Creation Hatchery Center (NICHe), Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan

1 ACS Paragon Plus Environment


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ABSTRACT We study Ce3+ luminescence and scintillation characteristics in novel, high performing host among the oxide single crystal scintillators, the La-admixed Gd2Si2O7. Obtained characteristics are systematically compared with the earlier studied Lu2Si2O7 host. We determine the barrier height of the thermal ionization of the Ce3+ excited center, we address the effect of Laadmixture on the physical characteristics and we construct an energy level diagram of the material system. High scintillation efficiency, low afterglow and the onset of Ce3+ excited state ionization appearing well above room temperature indicate a large potential of La-admixed Gd2Si2O7 in a variety of scintillating material applications. KEYWORDS :Ce3+, luminescence decay, scintillator, pyrosilicate, GPS.

1. Introduction Ce3+-doped rare earth oxy-orthosilicate, namely Gd2(SiO4)O:Ce (GSO:Ce - discovered in 1983 by Takagi and Fukazawa1 ) and Lu2(SiO4)O:Ce (LSO:Ce - introduced in 19922 ), are well known single crystal scintillating materials due to their favourable combination of high density, effective atomic number, fast room temperature decay time of several tens of nanosecond and non-hygroscopic nature. Their fundamental optical characterisation, completed by those of Y2(SiO4)O:Ce (YSO:Ce), which, due to lower effective atomic number, have been analyzed mainly for cathode-ray tube applications3, was provided by Suzuki4 and reveals two kinds of Ce3+ emission centres, so called Ce1 and Ce2, embedded at two sites of the RE3+ cation in the orthosilicate structure. Recently, they have been analyzed also by means of density functional theory ab initio calculations combined further with the wave function-based embedded cluster calculations and good agreement with experimental data was obtained5.LSO:Ce crystals, however, have an intrinsic background signal of a few hundred Hz/cm3 due to the presence of


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radioactive isotope

176

Lu, while GSO:Ce does not suffer from any similar disadvantage. Thus,

GSO:Ce is used in low-signal-count-rate environments such as gamma-ray astronomy6 or in a Compton camera in order to discriminate the background signal7,8. Moreover, owing to higher temperature stability of the Ce1 center in GSO host, the GSO:Ce is widely used in oil well industry and geophysical explorations. On the other hand the LSO:Ce and yttrium-admixed LYSO:Ce show, compared to GSO:Ce, few time higher light yield exceeding 30 000 phot/MeV and are used in the latest generation of scintillation detectors in Positron Emission Tomography (PET) especially after their optimization by divalent ion co-doping and post-growth annealing the mechanism of which has been recently clarified9. More than a decade ago lutetium pyrosilicate Lu2Si2O7 (LPS) was also found as potentially interesting scintillator host10. Comparative EPR study of the Ce3+-doped LSO and LPS showed that the Ce ion in LPS structure substitutes for Lu in its single crystallographic site while in the structure of LSO it is found in both Lu crystallographic sites11. The light yield of LPS:Ce single crystals, which were synthesized by the melting zone technique, can reach the value comparable to that of LSO:Ce, and the scintillation decay time of the Ce3+ is around 37 ns, with no observable afterglow10,12. Furthermore, similarly to LSO:Ce13 post-growth annealing in air at elevated temperatures was found efficient in increasing the scintillation efficiency14. The lack of afterglow in LPS:Ce in contrast to its observation in LSO:Ce was correlated with the significantly higher temperature maxima of thermoluminescence (TSL) glow peaks above room temperature15. Lately, Gd2Si2O7:Ce (GPS:Ce), which also belongs to the pyrosilicate group, has been introduced. It shows much higher light output and even shorter decay time compared to GSO:Ce16-18. Toropov et al. found that the composition Gd2O3●2SiO2 is not congruent in the



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Gd2O3●SiO2 system19 and Kawamura et al. reported that the single crystal GPS phase can be grown from the melt in case of heavy Ce-doping (approximately 10 mol %)16. However, at such a high Ce-concentration, the light output is significantly reduced because of the self-absorption and concentration quenching. An optimal cerium concentration in oxide hosts is usually within 0.1 - 1 at. %. The congruent crystal growth of GPS:Ce is, in fact, achieved by expansion of average ionic radius in the Gd site resulting from Ce-doping. At the same coordination number the La3+ ion has very similar ionic radius as the Ce3+ one20so that the substitution of La instead of Ce can also be applied to stabilize the pyrosilicate phase. The concentration quenching can thus be prevented by avoiding high Ce concentrations. The optical and scintillation properties of (Ce0.01, Gd0.90, La0.09)2Si2O7 were reported for the first time by Suzuki21 where these single crystals were grown by the floating zone (FZ) method under argon atmosphere. Using Siavalanche photodiode, the scintillation properties of such crystals were further measured by Kurosawa22 and excellent values of light output of 41.000 ± 1000 photons/MeV and FWHM energy resolution at 662 keV of 4.4 ± 0.1% were achieved at 23.0 ± 0.2 C. Floating zone growth and scintillation characteristics of the GPS:Ce single crystals were also described23-25. However, crystals obtained in this way are mostly cracked and contain high concentration of cerium up to 25 mol%. The impact of La and Sc admixture in GPS:Ce prepare by top seeded solution growth with SiO2 self-flux was studied as for their structure, optical and scintillation properties26. Apart from above-mentioned applications, GPS:Ce crystals also received attention due to their possible application for a thermal neutron monitoring27. Light output at thermal neutron monitoring is evaluated as twice higher in comparison with GSO:Ce. The observed rather high afterglow level (0.2% after 20 ms) and moderate energy resolution (13%) guarantees a room for improvement of these parameters by further optimization of crystal


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quality. A combined detector containing GPS:Ce crystals for separate detection of fast neutrons and thermal neutrons in the presence of gamma background radiation is introduced by Galunov28. Ce, Pr, and La-doped GPS single crystals grown by the Czochralski and Top Seeded Solution Growth (TSSG) techniques were introduced for the first time29 where formation conditions of different pyrosilicate phases were determined. La-admixed GPS:Ce with triclinic structure was also grown by another group using TSSG technique and the light yield exceeding that of commercial NaI:Tl scintillator with comparable energy resolution was achieved30 . In this study, the absorption and luminescence characteristics of Ce3+ center in LPS:Ce and heavily La-admixed GPS:Ce are reported and comprehensively compared. Temperature dependences of nanosecond decay times are approximated by a phenomenological model and those of the delayed recombination decay integrals are used to indicate the onset of the excited state ionization of Ce3+ center. Potential of La-admixed GPS:Ce in scintillator applications is discussed.

2. Experimental 2.1 Sample preparation Single crystals of (Gd0.7La0.3)2Si2O7:Ce1% (GPSLa30%:Ce), (Gd0.52La0.48)2Si2O7:Ce1% (GPSLa48%:Ce), and LPS:Ce0.5% (LPS:Ce) were grown by Czochralski technique. The growths of GPSLa30%:Ce and GPSLa45%:Ce crystals were performed in Ir crucible, containing 99.99% pure starting oxides of La2O3, SiO2, Gd2O3 and CeO2. The pulling-up and rotation rate applied were 0.5 mm/h and 10 rpm, respectively. The atmosphere in the growth chamber was Ar + O2(2%). The plates 5x5x1 mm were cut from the crystal boules and the faces 5x5 polished to an optical grade for all the optical and luminescence measurements.



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The rare earth pyrosilicates Ln2Si2O7 were found in seven crystalographic structural modifications31. The structure of the powdered crystal pieces of both compositions described above was identified by XRDP as the triclinic one, analogous to that of GPS32. Taking into account very similar ionic radius of La3+ and Ce3+ at the same coordination number it is consistent with the finding that the structure of the (Ce,Gd)2Si2O7 with 10-60 at.% of Ce is also the triclinic one32. However, due to restricted angle range32, another XRDP experiment might be necessary to reveal eventual further structural details. The growth and characteristics of LPS:Ce crystal were described by Feng14 .

2.2 Experimental techniques Absorption spectra were measured using the UV/VIS/NIR Spectrophotometer Shimadzu 3101PC. Radioluminescence (RL), photoluminescence excitation (PLE) and emission (PL) spectra, decay and afterglow curves were measured by the custom made spectrofluorometer 5000M, Horiba Jobin Yvon, using the steady state deuterium lamp (PL and PLE spectra), the Xray tube (RL spectra and afterglow curves), the microsecond xenon pulsed flash lamp (slow delayed recombination decays) or nanosecond nanoLED pulsed light sources (fast prompt decay curves) as the excitation sources. Detection part of the set-up involved a single grating monochromator and the photon counting detector TBX-04. Measured spectra were corrected for the spectral dependence of excitation energy (PLE) and spectral dependence of detection sensitivity (PL). Convolution procedure was applied to the decay curves to determine true decay times (SpectraSolve software package, Ames Photonics). Measurements of the optical characteristics within the 77 – 500(700) K temperature region were performed using the liquid nitrogen bath optical cryostat Oxford Instruments (Janis).


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3. Results and discussion Unlike orthosilicate (LSO, LYSO, YSO, LGSO and GSO) hosts accommodating two inequivalent luminescence centres when doped by the Ce3+ activator1,4,33-35, in pyrosilicates (LPS, GPS), due to different crystal structure, there is only single Ln3+ site. Consequently, a single Ce3+ luminescence centre is available. Such crucial difference results in less complex absorption and luminescence features of pyrosilicates.

3.1 Absorption spectra RT absorption spectra of LPS:Ce and GPSLa30%:Ce are shown in figures 1a and 1b, respectively. The absorption bands in the UV spectral region are attributed to fully-allowed 4f – 5dx transitions of the Ce3+ activator in these pyrosilicate hosts. Position of all five absorption transitions to the 5dx states of Ce3+ in GPSLa30%, x = 1 – 5, was obtained by decomposition of the spectrum into gaussian components, see figure 1b and Table 1. Given the absorption transitions to 5d1,2,3,4,5 levels of Ce3+ in LPS host (see table 1) at 351, 313, 278, 213, 193 nm, respectively36 , the crystal field splitting of 5d state of Ce3+ center in GPSLa30% host is apparently much smaller. In the gadolinium containing sample, typical Gd3+ absorption lines can be also seen (marked in figure 1b).



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Figure 1.RTabsorption spectra of LPS:Ce with indicated maxima of 4f-5dx transitions after35(a) and GPSLa30%:Ce and its gaussian decomposition into five G1 - G5 components matching the 4f-5dx transitions, i=1-5, respectively (b). For the absorption transition peak values see Table 1.

5d1[nm]

5d2[nm]

5d3[nm]

5d4 [nm]

5d5[nm]

LPS:Ce35

351

313

278

213

193

GPSLa30%:Ce

338 (3.67

320 (3.88

294 (4.21

242 (5.13

219 (5.66

eV)

eV)

eV)

eV)

eV)

Table 1. Position of the absorption transitions to the 5dx states, x = 1 – 5, of Ce3+ in LPS36and GPSLa30%.


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3.2 Photoluminescence spectra and decays Normalized PLE and PL spectra of LPS:Ce and GPSLa30%:Ce recorded at liquid nitrogen temperature are presented in figures 2a and 2b, respectively, for various excitation and emission wavelengths. Spectral positions of maxima in the PLE spectra given by the Ce3+ 4f 5dx transitions mostly well coincide with those in the absorption spectra (indicated by arrows) displayed in figures 1a and 1b and summarized in Table 1. The presence of the peak at 275 nm in PLE spectrum of GPSLa30%:Ce evidences the energy transfer Gd3+ → Ce3+ enabled by the overlap of the Gd3+ 6Px-8S emission line around 312 nm with a broad absorption transition 2F5/2 → 5d2 of Ce3+ positioned around 319-320 nm, see figure 1b. PL spectra reveal the presence of two sub-bands, in case of GPSLa30%:Ce with well identified separation of 1950 cm-1.This value matches the expected splitting of the ground state doublet 2F5/2 and 2F7/2 of the Ce3+ center. With increasing temperature the doublet is thermalized and at RT these two bands are considerably less resolved (not shown).

Figure 2. Normalized PLE and PL spectra of LPS:Ce (a) and GPSLa30%:Ce (b) recorded at 77 K; excitation and emission wavelengths are displayed in the legend. The arrows denote the 4f5dx absorption maxima given in Table 1.



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From the position of the 4f-5d1 transitions in absorption spectrum in figures 1a and 1b and 5d1 - 2F5/2 emission subband maximum in figures 2a and 2b, the approximate Stokes shift value of 235 meV = 1895 cm-1 (for LPS:Ce) and 262 meV = 2113 cm-1 (for GPSL30%:Ce) can be estimated. Those values are much lower compared to the Ce1 centre in both LGSO:Ce and GSO:Ce35.

Figure 3. Nanosecond decay time temperature dependence of the Ce3+ emission in LPS (a) and GPSLa48%:Ce (b) (λex = 339 nm, λem = 380 nm); Solid lines are fits of the data by the model described in the text. Values of parameters used in the fit are reported in Table 2.

Ce3+ nanosecond decay times temperature dependences for LPS:Ce and GPSLa48%:Ce are displayed in figures 3a and 3b, respectively and the latter dependence for the GPSLa48%:Ce is extended up to 700 K. The onset of nanosecond decay times shortening, regardless its origin, for all studied pyrosilicates lies well above room temperature around 380 K (LPS:Ce) and 440 K


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(GPSLa30%:Ce and GPSLa48%:Ce). In earlier studies the onset of this process in LPS:Ce was reported at 470 K37 and 400 K38. The variation in reported values might be due to actual concentration of Ce3+ in the crystals since the concentration quenching can effectively shift the onset temperature to lower values39.

Figure 4. (a) Normalized RT PL decay curves related to the Ce3+ 5d - 4f transitions in LPS:Ce (black solid line), GPSLa48%:Ce (blue solid line) and GPSLa30% (red solid line) hosts; (b) PL decay curve related to the Ce3+ 5d - 4f transition in GPSLa48%:Ce at 700 K; λex = 339 nm and λem = 380 nm.

As an example, decay curves related to the Ce3+ 5d – 4f transitions in LPS, GPSLa48% and GPSLa30% hosts recorded at RT (λex =339 nm and λem =380 nm) are shown in figure 4a. They are all perfectly single exponential through more than three orders of magnitude in intensity and corresponding decay times at RT are 37.1 ns, 30.6 ns and 32.2 ns for LPS:Ce, GPSLa48%:Ce and GPSLa30%:Ce, respectively. These values well match expected figures of the fully-allowed 5d - 4f Ce3+ transition. At the same time, radiative lifetimes in studied



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pyrosilicates are comparable to those of Ce1 in oxy-orthosilicate hosts4,34,35. There is no slower component observed in the decays which would be connected to delayed recombination process at RT. The decay curve related to GPSLa48%:Ce at 700 K is shown in figure 4b (λex =339 nm and λem =380 nm). It can be well approximated by double exponential function yielding the decay time values around 1.4 ns and 8 ns. While the shorter decay time undoubtedly belongs to the prompt decay process, the slower component might indicate the presence of the thermally induced ionization of the excited state as it will be discussed below. Pyrosilicates doped by cerium therefore exhibit much higher thermal stability as the onset of ns decay time shortening, for both Ce1 and Ce2 centres in LSO:Ce, LYSO:Ce, GSO:Ce, LGSO:Ce, lies at significantly lower temperatures33-35. We approximate the above reported nanosecond decay time temperature dependences in figures 3a and 3b by considering a thermal quenching as a simple barrier process. The decay rate therefore may be written as:

(τobserved)-1 = (τradiative)-1 + K×exp(-E/kT)

(1)

where observed, radiative, K, E, k, and T represent the PL decay time measured at temperature T, the low temperature limit of the reciprocal PL decay time k = 1/τ, frequency factor of escaping channel, energy barrier height, Boltzmann constant and absolute temperature, respectively. All temperature dependences of nanosecond decay times in LPS:Ce, GPSLa30%:Ce (not shown in figure 3) and GPSLa48%:Ce can be fit to such a single escaping channel model. The obtained parameters K, Eare listed in Table 2.Fairly similar values of frequency factors and energy


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barriers around 600 meV indicate relative insensitivity of Ce3+ center on La concentration in the GPSLa pyrosilicate host.



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krad [s-1]

K [s-1]

E [meV]

LPS:Ce

2.64×107

1.3×1014

600

GPSLa30%:Ce

3.05×107

1.4×1013

600

GPSLa48%:Ce

3.23×107

1.9×1013

610

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Table 2. Parameters of the best fits connected to the Ce3+ ns decay time temperature dependence (Eqn 1 is fit to the experimental data from fig. 3).

To better understand the origin of the temperature-induced decrease of the Ce3+ decay times, the innovative method of measuring temperature dependence of intensity of the delayed radiative recombination was employed40. In this experiment the excitation/emission wavelengths are set the same as in the nanosecond decay measurement (λex = 339 nm, λem = 380 nm), but the time window is set up to a few tens of milliseconds. The excitation is accomplished by a low frequency (10 - 30 Hz) intense microsecond xenon flash lamp and the driving software ensures that the next excitation flash comes right after the time window is closed, i.e. the dead-times between the time windows are minimized. Temperature dependence of the integrated decays can, with an excellent sensitivity, indicate the threshold of thermal ionization of the Ce3+ excited state. When an electron escapes from the Ce3+ 5d1 state to the conduction band, its later return results in the delayed radiative recombination.


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Figure 5. Temperature dependence of normalized delayed recombination integrals of the Ce centres for LPS:Ce (black full circles), GPSLa48%:Ce (red full triangles) and GPSLa30%:Ce (blue full squares); λex = 339 nm and λem = 380 nm.

Figure 5 illustrates the temperature dependence of integrated delayed recombination (DR) decays related to the Ce3+ centres in LPS:Ce, GPSLa30%:Ce and GPSLa48%:Ce (see legend). Before integrating the curves the highest intensity points (containing prompt ns luminescence) were omitted and the background was subtracted40,41. The onset of delayed recombination intensity increase, indicating the onset of thermal ionization, is positioned far above room temperature at around 380 K (LPS:Ce) and 400 K (GPSLa30%:Ce, GPSLa48%:Ce) which is in an excellent agreement with the onset of ns decay time shortening (see figures 3a and 3b). Potential discrepancy might be due to the difference in monitoring the onset of decay times shortening and the onset of delayed recombination intensity increase. While the former is monitored as the change with respect to its maximum value, the latter is monitored as the change with respect to its minimum value. As a result, the latter is more sensitive in the onset indication and shows higher overall dynamic range as well. Comparison of the data in figures 3 and 5



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implies that the nanosecond decay time shortening is indeed due to thermally induced ionization of the excited state in all hosts, LPS, GPSLa30% and GPSLa48%. It is also consistent with the profile of decay curves (see figures 4a and 4b) showing no slower decay component at RT, but evidently present at the highest temperatures. We also note that DR intensity shows a non-zero value even at the lowest temperatures, which has been explained by quantum tunnelling between the luminescence centre and a nearby defect42. However, better understanding of the DR behaviour would require an independent study of characteristics of the traps involved in the delayed recombination process43. It is also worth noting that the onset of the excited state ionization is shifted to higher temperature with higher La content, i.e. the energy separation between the Ce3+ excited state and the bottom of the conduction band is higher in case of higher La concentration. In some La-based compounds the 5d state of Ce3+ is completely located in the conduction band and Ce3+ emission is absent. Well-known examples include LaAlO344 and La2O345, hosts while in others the Ce3+ emission survives well above the room temperature as in LaCl346 , LaBr347 , K2LaX5, X= Cl,Br,I48 or in currently presented case of GPSLa:Ce. The reason for such differences consists mainly in the substituted site symmetry, chemical bonding and resulting crystal field induced splitting of the Ce3+ 5d levels in each particular compound. The detailed models have been already developed to quantitatively calculate the Ce3+ and other lanthanide ions energy levels within the host band gap49.


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Figure 6. Photoluminescence decay curve of the GPSLa48%:Ce in an extended time and dynamical scales, λex = 337 nm (N2 laser), λem = 390 nm. Solid line is a convolution of function I(t) displayed in the figure and the instrumental response.

When the nanosecond prompt component decay and μs–ms delayed recombination decay (as described above) are measured in separate experiments, one cannot estimate their relative contributions to the overall decay. Such a task can be better handled by the measurement set-up with the low-frequency ns excimer (or N2) laser excitation and detection part equipped with the fast photomultiplier in the current regime coupled with a digital scope. In fact, several orders of magnitude and time can be monitored in a single measurement50. An example of such a measurement performed in GPSLa48%:Ce sample at RT is displayed in figure 6 together with the two exponential approximation I(t). Prompt Ce3+ emission component with the decay time value of 26.4 ns and a minor weaker component with the decay time value around 50 ns are present. However, no slower decay components are present within the sensitivity limit of the experiment. This result means that the temperature independent delayed recombination processes



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observed as plateau below 350 K in figure 5 are of negligible influence at the time and dynamical scales of this measurement. Based on the 4f-5d and 4f-4f transition energies of Ce3+ and Gd3+ ions, respectively, and ionization barrier obtained for the former from the temperature dependence of nanosecond decay times, and considering also the general trends in the ground state positioning of trivalent rare earth ions in oxides matrices51, we constructed the energy level diagram of Ce3+-doped (Gd0.7La0.3)2Si2O7, presented in figure 7. Taking into account a usual band gap Eg shrinkage in compounds where Lu or Y cation is replaced by La (Eg of about 1.2 – 1.3 eV when comparing LaI3 with LuI352 or LaAlO344,53 with YAlO354 ), the value of the band gap in Lu2Si2O7 estimated as 6.95 eV at 10 K55 and the fact that the value of the band gap in (La0.3Gd0.7)2Si2O7 will be greater than 6.5 eV (it follows from fig. 1b), it can be estimated within 6.6-6.8 eV under the assumption that the bottom of conduction band is formed by La3+ 5d levels.

Figure 7. Energy level scheme in (La0.3Gd0.7)2Si2O7:Ce. ET stands for energy transfer from Gd3+ to Ce3+ center sketched in the figure and discussed in the text.


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3.3 Scintillation characteristics Normalized RT X-ray excited radioluminescence spectra of LPS:Ce, GPSLa48%:Ce and GPSLa30%:Ce are displayed in figure 8. Analogously to the above described PL spectra in figure 2 they are due to the fully allowed 5d1 – 4f transition of the Ce3+ ion, peaking at 377 nm and 372 nm in LPS and GPSLa (both 48% and 30% of La), respectively, and showing the double peak character due to the splitting of the ground state doublet 2F5/2 and 2F7/2. To quantitatively evaluate the scintillation efficiency, the absolute spectra of these samples and that of the reference BGO were integrated in the energy and radiant flux coordinates. RL intensities of LPS:Ce, GPSLa48%:Ce and GPSLa30%:Ce reach about 250 %, 1210 % and 1530 % of that of BGO reference sample, respectively. Comparing with a scintillation efficiency of high quality LYSO:Ce,Ca sample (LY of 32 000 phot/MeV)55 measured in the same way and the same setup, the GPSLa48%:Ce and GPSLa30%:Ce samples show 1.6 and 1.9 times higher values, respectively.



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Figure 8. Normalized RT radioluminescence spectra of LPS:Ce (black solid line), GPSLa48%:Ce (blue dotted line) and GPSLa30%:Ce (red dash-and-dotted line) under the X-ray (40 kV, 15 mA) excitation.

Figure9. Normalized RT afterglow curves for GPSLa48%:Ce (black full circles), GPSLa30%:Ce (blue full squares), LPS:Ce (red full triangles) and BGO (green half-full squares) excited by the X-ray (40 kV), all emission regions. Averaged afterglow level in 1-10 ms after X-ray cut-off is given in the legend.

To evaluate the afterglow effect, its dependence on the pyrosilicate composition and to compare it with the standard scintillation material Bi4Ge3O12 (BGO), known for its negligible afterglow, the afterglow curves were measured – see figure 9. The samples were continuously irradiated by the X-ray source (40 kV) for several tens of seconds and then the excitation was cut-off. The spectrally unresolved emission intensity was monitored along this process in the 0.7 s time window. Keeping the experimental and geometrical conditions the same, it is possible to quantitatively compare different samples. The average signal within 1-10 ms after the X-ray cut


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off is 0.0015, 0.00041, 0.011 and 0.000089 for GPSLa30%:Ce, GPSLa48%, LPS:Ce and BGO, respectively. Apparently, with increasing La amount in the GPSLa:Ce samples the afterglow becomes less significant. LPS:Ce, on the other hand, shows stronger afterglow effect since the intensity after the x-ray cut off is an order of magnitude higher compared to GPSLa30%. Presented afterglow values of GPSLa:Ce are comparable to those obtained for GSO:Ce and LGSO:Ce and significantly lower with respect to LSO:Ce35.

3.4 General considerations The La-admixed GPS:Ce scintillator appears another successful example of the band gap engineering approach in wide band gap scintillators57. Among oxide scintillators, this strategy appeared extremely productive in the group of garnet scintillators, where balanced admixture of Gd and Ga into the structure of classical Y3Al5O12 or Lu3Al5O12 aluminium garnets gave rise to new ultraefficient multicomponent garnet scintillators with light yield exceeding 50 000 phot/MeV58 though at the expense of their temperature stability due to Ce3+ excited state ionization early above room temperature59. In orthosilicates the Ce-doped LGSO:Ce scintillator can be also considered such a case60 with comparative advantages in scintillator characteristics around room temperature with respect to both LSO:Ce and GSO:Ce35 . Apart from specific changes in the band structure it seems from already several recent examples of cation-mixed compound scintillators61 that atomistically inhomogeneous arrangement of cations may give rise to local variation of electronic structure band edges which effectively limit the out-diffusion of charge carrier from ionization track and consequently increase the probability of their radiative recombination, i.e. increase the light yield of such a scintillator. The increase of scintillation efficiency and/or light yield was reported both in the undoped CsI-CsBr62 and ZnWO4-MgWO461



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solid solutions or in the Ce-doped solid solutions of LaBr3-LaCl363, Lu2SiO5-Y2SiO564, Lu2SiO5Gd2SiO560 and LuAlO3-YAlO365. In case of La-admixed GPS host, such inhomogenities will arise due to atomistic disorder of La and Gd cations at the RE3+ site of pyrosilicate structure. Due to the fact that La3+ energy levels are expected to provide dominant contribution to the very bottom of conduction band, such an effect is indeed expected. Due to the reduced band gap value with respect to orthosilicate scintillators based on LSO:Ce or LYSO:Ce, high intrinsic scintillation efficiency indicated from radioluminescence spectra intensity shown above, and very favourable values of light yield achieved at the early stage of La-admixed GPS:Ce development21,22 these scintillators could easily become serious competitors to all existing high performance single crystal oxide scintillators.

4. Conclusions

Absorption spectra, photoluminescence spectra as well as decays, and selected scintillation characteristics were studied for LPS:Ce, GPSLa30%:Ce and GPSLa48%:Ce single crystals. By gaussian decomposition of absorption spectrum we determined peak positions of the 4f – 5dx, x = 1 – 5, Ce3+ absorption bands in GPSLa30% at 338, 320, 294, 242 and 219nm, respectively. The 5d - 4f emission of Ce3+ is peaking at 377 nm and 372 nm in LPS and GPSLa hosts (both 48% and 30% of La), respectively. Prompt and delayed decay kinetics were measured in a broad temperature range. Prompt decay was approximated by a simple phenomenological model providing the energy-barrier heights and frequency factors governing the excitation-energy escape transition deteriorating the luminescence efficiency of Ce3+ center. The onset of ns decay times shortening appears around 380 K (LPS:Ce) and 440 K (GPSLa30%:Ce and GPSLa48%:Ce). Temperature dependence of the delayed recombination decays evidences that the nanosecond decay time reduction is caused by thermally-assisted excited state ionization.


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Such process, with an onset at around 380 K (LPS:Ce) and 400 K (GPSLa30%:Ce, GPSLa48%:Ce), will deteriorate a scintillation performance at yet higher temperatures. The Ce3+ ionization onset favourably occurring well above RT provides an opportunity to exploit LPS:Ce and particularly GPSLa:Ce in high temperature applications. Evaluated scintillation efficiency (overall RL intensity) reach about 250 %, 1210 % and 1530 % of that of BGO single crystalline standard for LPS:Ce, GPSLa48%:Ce and GPSLa30%:Ce, respectively. In the last compound the efficiency is almost doubled with respect to that of commercial high performance LYSO:Ce. Afterglow of La admixed gadolinium pyrosilicates is fairly low and tends to get less intense with increasing La concentration becoming comparable to that of BGO. Taking further into account about two orders of magnitude lower intrinsic radioactivity (due to due to

138

La isotope, 0.09% natural abundance, T1/2~1011y) compared to Lu-based scintillators,

the La-admixed GPS:Ce single crystals show a combination of characteristics highly favourable for medical imaging, oil industry and geophysical applications.

AUTHOR INFORMATION Corresponding Author *Institute of Physics AS CR, Cukrovarnicka 10, 16253 Prague Czech Republic, ph. +420220318445, fax +420 233343184, email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT



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Financial support of Czech MEYS KONTAKT project no. 14266 and joint Czech-Italian AS CR-CNR project are gratefully acknowledged. Moreover, this work is partially supported by (i) Japan Society for the Promotion of Science (JSPS) -ASCR Bilateral Joint Research Projects, (ii) Development of Systems and Technology for Advanced Measurement and Analysis, Japan Science and Technology Agency (JST), (iii) Adaptable & Seamless Technology Transfer Program through Target-driven R&D (A-STEP), JST, (iv) the Association for the Progress of New Chemical Technology, (v) The Murata Science Foundation, (vi) Nippon Sheet Glass Foundation for Materials Science and Engineering, and (vii) TonenGeneral Sekiyu Foundation. In addition, we would like to thank following persons for their support: Mr. Yoshihiro Nakamura of Institute of Multidisciplinary Research for Advanced Materials (IMRAM), and Mr. Hiroshi Uemura, Ms. Keiko Toguchi, Ms. Megumi Sasaki and Ms. Yuka Takeda of IMR, Tohoku University.


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Graphical abstract for TOC


Luminescence Characteristics of the Ce3+-Doped Pyrosilicates

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