N19-3 Intrinsic Luminescence and Luminescence of Inadvertent ... - Ipen

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around 750 nm was detected, its origin and effect on the scintillator performance is ... PET scanners together with the well-known LSO crystals. Though ...
Intrinsic Luminescence and Luminescence of Inadvertent Impurities in LuAP and LuYAP crystals I. A. Kamenskikh, N. V. Guerassimova, V. V. Mikhailin, A.N. Vasil’ev, C. Dujardin, C. Pedrini, A. G. Petrosyan

Abstract-- To clarify the potential of LuAP:Ce and LuYAP:Ce scintillators of new generation the mechanisms of the energy transfer from the matrix to the cerium ions and competing relaxation channels were investigated using VUV luminescence spectroscopy for the crystals provided by different producers using different growth techniques. With temperature being one of the ways to control the light yield of these crystals, effect of temperature was studied for different excitation energies. Anomalous temperature dependence of the luminescence light yield under VUV excitation was analyzed. New intrinsic luminescence band was observed in all of the crystals and energy transfer from it to cerium ions was demonstrated. New luminescence band in the IR peaking around 750 nm was detected, its origin and effect on the scintillator performance is discussed. I.

INTRODUCTION

and LuYAP crystals were chosen by the TworkingLuAP groups of Crystal Clear Collaboration for the HE

development of phoswich detectors applied in small animal PET scanners together with the well-known LSO crystals. Though scintillation properties of LuAP and LuYAP crystals allow them to compete with many other modern scintillators both for medical imaging and for nuclear sciences purposes, all these applications, and especially phoswich detectors would benefit from further improvement of their light yield. In order to clarify the potential of these scintillators the mechanisms of the energy transfer from the matrix to the cerium ions and competing relaxation channels were investigated for these crystals provided by different producers using different growth Manuscript received October 28, 2004. This work was in part supported by the DFG grant No 436 RUS 113/437 and RFBR grant 03-02-17346a. I.A.K. thanks INTAS for the travel grant. I. A. Kamenskikh, N. V. Guerassimova, V. V. Mikhailin and A.N. Vasil’ev are with the Department of Physics, M.V. Lomonosov Moscow State University, 119992 Moscow, Russia (e-mail: [email protected]). C. Dujardin and C. Pedrini are with LPCML, UMR 5620 CNRS & Université Lyon 1, 69622 Villeurbanne Cedex, France (e-mail: [email protected]). A. G. Petrosyan is with the Laboratory of Crystal Growth of Luminescent Materials, Institute for Physical Research, 378410 Ashtarak-2, Armenia (email: [email protected]).

techniques. With the temperature being one of the ways to control the light yield of these crystals, its effect was analyzed for different excitation energies. In LuAP and LuYAP at elevated temperature trapped charge carriers are released [1][3], resulting in the increase of the light yield. Indeed, under gamma ray and X-ray excitation the light yield of LuAP increases. On the other hand the yield of LSO is known to degrade with temperature, thus by tuning the temperature a suitable balance between the two yields can be achieved. However, timing characteristics of the crystals should stay within reasonable limits. The evolution of luminescence kinetics with temperature was not studied in detail for the crystals of new generation; here we present the results for the VUV excitation by synchrotron radiation. Besides, detailed study of luminescence spectra in a wide spectral range from UV to IR revealed additional luminescence bands never observed before that represent other channels of radiative relaxation of excitations in these crystals, which can compete with the main one. Their effect on the performance of LuAP and LuYAP is analyzed. II. EXPERIMENTAL The crystals for the measurements were real pixels produced for PET detector of 0.2cm × 0.2 cm × 1.0 cm size from the Armenian Institute for Physical Research (LuAP and LuYAP grown by Bridgman technique) and 0.2cm × 0.2 cm × 0.8 cm crystals from Bogoroditzk Technical Plant (LuYAP, Czochralski technique). Since the VUV light used here for the luminescence excitation is absorbed in a very thin surface layer due to high absorption coefficient, the spectra were taken both from the polished and freshly cleaved surfaces to distinguish between the surface and bulk effects. The measurements of the luminescence spectra and of the luminescence quantum yield in the excitation region 4 to 25 eV and of fluorescence decay kinetics in the nanosecond range were performed using synchrotron radiation at the SUPERLUMI station of the Doris positron storage ring, HASYLAB, DESY, Hamburg [4]. Similar measurements in

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A. Temperature Dependence of the Light Yield Under gamma excitation the scintillation light yield of LuAP and LuYAP crystals is known to undulate with temperature (see, e.g., [2]), it increases in the range 300 to 400 K. This has been verified and confirmed for the crystals studied. However, under 15 eV excitation by synchrotron radiation pulses an inverse tendency was observed: the luminescence light yield was decreasing with temperature (Fig. 1, red dots). The dots in the figure represent the area underneath decay curves measured in the maximum of Ce emission (370 nm). As was expected decay curves measured under intracenter cerium excitation (300 nm) did not change with temperature, indicating that the observed dependence was not due to some experimental artifacts. The difference between this type of measurements and a typical scintillation light yield measurement is that in the latter case the response of the crystal to a single gamma quantum is detected, while in the former case the excitation pulses follow with the period of 192 ns in our case, thus the decay curve contains information on the crystal response to all previous pulses within the duration of the measurement. By this we see not only the fast component defined by the radiative lifetime of cerium centers but also the component representing detrapping process. In the scintillation light yield measurements using gamma sources the components due to detrapping contributes as well, however the time of the measurement is defined by the shaping time and is normally in the range of microseconds. In our case each measurement was for 3 min. If we consider only the fast component in our decay curves we get a different temperature dependence (blue dots in fig. 1), which is in a good agreement with published scintillation light yield measurements. The rising parts of the temperature dependence were well described in [2] by the model assuming the presence of a trap and consecutive capture of carriers by the cerium center. Addition of a barrier for the trapping process allows to account for the dips in the temperature dependence as well [3]. However, all published models do not consider a sink for the carriers and in the limit of infinite accumulation time predict a yield not changing with temperature. In our case it was decreasing. This implies that there exists an additional loss

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III. RESULTS AND DISCUSSION

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Temperature, K Fig. 1 Temperature dependence of the luminescence light yield of Lu80Y20AP:Ce crystal measured at 370 nm excited by 15.5 eV photons: red dots – integrated yield; blue dots – fast component.

channel for the carriers, its identification could help to increase the yield of the crystals. B. Intrinsic Luminescence of LuAP:Ce and LuYAP:Ce Crystals In [5] the properties of intrinsic luminescence of pure and Ce-doped LuAP crystals were described. A luminescence band peaking at 300 nm in pure crystal and indented by cerium absoprtion in cerium-doped ones was ascribed to self-trapped exciton. In our crystals this band was observed as well and its temperature was studied, the results are presented in fig. 2 and 3.

Intensity, arb.un.

the range 70 to 600 eV were taken at the BW3 station also in the HASYLAB. The samples were mounted on the cold finger of a LHeT flow type cryostat, temperature range was 7 to 370 K. Temperature dependence for standard light yield measurements using 137Cs source was investigated for comparison.

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Fig. 2. Intrinsic luminescence of LuAP:Ce excited by 7.8 eV photons.

Decay kinetics of this band has a fast component with characteristic decay time of the order of nanoseconds and a long one (longer than in the microsecond range). Fig. 3 shows that its yield starts to decrease at the temperatures above 225 K, the curve corresponds to a Mott-type quenching.

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Fig. 3. Temperature dependence of intrinsic luminescence of LuAP:Ce measured at 270 nm.

Since in doped crystals this temperature dependence can be affected by the energy transfer to cerium, no detailed analysis of the curve was performed and activation energy was not determined. However, one can notice a qualitative correspondence between the temperature dependence of this intrinsic band and that of cerium. Excitation energy of 15.5 eV, for which the energy dependence of cerium emission was measured, corresponds to the creation of separated electronhole pairs and the kinetic energy of the primary electron in the conduction band is not sufficient for the creation of secondary pairs. Nevertheless, it can create secondary excitons, that is why temperature dependence of the intrinsic emission can be visualized via cerium emission. Clear indication of the energy transfer from this intrinsic luminescence band to cerium centers is provided by the excitation spectra presented in fig. 4. TI stands for timeintegrated measurement, fast –for the time window of ~10 ns immediately after the excitation pulse, and slow –for the time window of 50 ns shifted by 100ns relative to the bunch.

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We observed also another intrinsic luminescence band peaking around 210 nm (fig. 5). This luminescence band is present both in LuAP and LuYAP crystals from different producers. Similarly to another intrinsic band at 300 nm its kinetics (fig. 6) has a fast nanosecond component and a long one, in our curves manifested as a flat background. Fast component is strongly non-exponential indicative of quenching, which could be caused by the energy transfer. As in many other complex oxide crystals (see e.g., [6]-[8]) in LuAP and LuYAP there are two types of intrinsic emission, which can be attributed to self-trapped excitons (STE) (a shorter wavelength band) and to the recombination of selftrapped holes (STH) with electrons (longer wavelength band), respectively. Comparison of the excitation spectra of Ceemission and of two types of intrinsic luminescence presented in fig. 7 demonstrates that the features of the Ce spectrum at the threshold of fundamental absorption region (around 8.0 eV) are defined by the energy transfer from two different types of intrinsic excitations to cerium ions. It can also be noted that the hump in

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Fig. 4. Comparison of the excitation spectra of intrinsic luminescence measured at 275 and cerium emission (370 nm), 8K.

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Fig.6. Decay kinetics of 210 nm intrinsic luminescence band measured at the excitation energy of 130 eV, 8K.

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Quantum Yield, arb.un.

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Fig. 8. Luminescence spectrum of LuAP:Ce at 10 K and 6.5 eV excitation.

C. Inadvertent Impurities It was quite unusual to observe another new band in these well-studied crystals peaking around 750 nm, it is shown in Figure 8. It was observed in all the crystals studied from 4 different producers, temperature dependence suggests that its origin is different from known 600 nm emission. In the figure its yield is comparable to that of Ce. However, this is true only for a particular excitation energy of 7.0 eV (i.e. below the fundamental absorption threshold), where its excitation spectrum has a maximum. It should be noted that at excitation energies of hundreds of eV it is present as well but with a substantially lower yield. The lifetime of the emission was long, outside the limits we could define (longer than tens of microseconds). Choosing between trace impurities, which might cause it, we should not rule out chromium, which is a quite common inadvertent impurity in Al2O3 used for the growth of crystals. IV. ACKNOWLEDGMENT

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the excitation spectrum of the long wavelength intrinsic band, or STH, coincides in position with the first excitation maximum of the short wavelength band, or STE. This feature demonstrates another STE relaxation channel which exists (in addition to the radiative relaxation and energy transfer to Ce centers), namely decay into a STH and electron. In fact the excitation spectrum of the slow component of STH emission (fig. 4) has its main maximum coinciding with that of STE. Thus, we have quite a complicated picture of the energy transfer to cerium ions in LuAP and LuYAP crystals under high energy excitation: (i) consecutive capture of holes and electrons by cerium ions; (ii) energy transfer from STE (STE emission band overlaps with the absorption of cerium ions, so such transfer exists); (iii) energy transfer from the recombination luminescence of STH and electrons; in their turn STH can be created from valence band holes and from the decay of STE. It should be noted that intrinsic excitations in complex oxides are far less understood than in alkali halides. At the moment it is not possible to say whether we deal with purely “intrinsic” excitations or these excitations are related to some inherent defects of the studied crystals. All we can state now is that the slow component of cerium luminescence is defined not only by detrapping processes but also by the energy transfer from STE and STH. Substantial part of this emission does not participate in the formation of the experimentally observed scintillation light yield (due to finite measurement time). On the other hand the traps incorporated in the models of scintillation process in [2],[3] can be related to intrinsic excitations. To establish the real role of intrinsic excitations in the scintillation process of LuAP and LuYAP:Ce crystals detailed study of pure crystals is required.

We thank Prof. G. Zimmerer for providing access to the exceptional facilities of the SUPERLUMI station and fruitful discussions and Dr. M. Kirm for his kind assistance during the measurements. V. REFERENCES [1]

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Fig. 7. Luminescence excitation spectra of LuAP:Ce at 8K: cerium emission – 370 nm, black curve; STH – 275 nm, blue curve; STE – 220 nm, red curve; inadvertent impurity (Cr?) – 700 nm, magenta curve.

[2] [3] [4] [5]

D. Wisniewski, A.J. Wojtowicz, and A. Lempicki, “Spectrpscopy and scintillation mechanism in LuAlO3:Ce,” J. Lumin., vol. 72-74, pp. 789791, 1997. A.J. Wojtowicz, J. Glodo, W. Drozdowski, and K.R. Przegietka, “Electroon traps and scintillation mechanism in YAlO3:Ce and LuAlO3:Ce scintillators,” J. Luminescence, vol. 79, pp. 275-291, 1998. A. Lempicki, R.H. Bartram, “Effect of shallow traps on scintillation,” vol. 81, pp. 13-20, 1999. G. Zimmerer, “Status report on luminescence investigations with synchrotron radiation at HASYLAB”, Nucl. Instrum. Methods, vol. A308, pp. 178-186, 1991. A. J. Wojtowicz, “Scintillation mechanism: the significance of variable valence and electron-lattice coupling in R.E.-activated scintillators,” Proc. Int. Conf. On Inorganic Scintillators and Their Application SCINT95, Ed. P. Dorenbos and C.W.E. van Eijk, Delft University Press, 1995, pp. 95-102.

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[6] [7] [8]

A. I. Kuznetsov, B. R. Namosov, and V. V. Murk, “Relaxed electronic states in Al O , Y Al O , YAlO ,” Sov. J. Phys. Solid State, vol. 27, pp. 3030–3035, 1985. P. Lecoq and M. Korzhik, “New inorganic scintillation materials developed for medical imaging,” IEEE Trans. Nucl. Sci., vol. 49, no. 4, pp. 1651-1654, aug. 2002. D.W. Cooke, B.L. Bennett, R.E. Muenchausen, J.-K. Lee, and M.A. Nastasi, “Intrinsic ultraviolet luminescence from Lu2O3, Lu2SiO5 and Lu2SiO5:Ce3+,” J. Luminescence, vol. 105, pp. 125-132, 2004.

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