The present status of the LuAlO3:Ce scintillator is reviewed. Scintillation mechanism of this material is based on capture by Ce3+ of holes and then electrons from their respective bands. Results of spectroscopic and thermoluminescence experiments are presented to support this model

Bos, Adrie J.J.

de Haas, Johan T.M.

Dorenbos, Pieter

Drozdowski, Winicjusz

van Eijk, Carel W.E.

Wiśniewski, Dariusz

Wojtowicz, Andrzej J.

Łempicki, Aleksander

Acta Physica Polonica A

English

Repository of Nicolaus Copernicus University

Vol. 90 (1996) 	 ACTA PHYSICA POLONICA Α 	 No. 2Proceedings of the 2nd Winter Workshop S&SRΕS'96, Polanica Zdrój 1996SPECTROSCOPY AND THERMOLUMINESCENCEOF LuAlO 3 : CeD. WIŚNIEWSKIα , b, W. DROZDOWSKIa, A.J. WOJTOWICZa,b, A. LEMPICKΙb,P. DORENBOSc, J.Τ.M. DE HAASc, C.W.E. VAN EIJKc AND A.J.J. BOSd° Inst. of Physics, N. Copernicus University, Grudziądzka 5, 87- 100 Toruń, PolandbDept. of Chemistry, Boston University590 Commonwealth Ave., Boston, MΑ 02215, USADelft Univ. of Tech., Faculty of Applied Physics, IRIMekelweg 15, 2629 JB Delft, The NetherlandsdDelft Univ. of Tech., Interfaculty Reactor Inst., c.o. IRIMekelweg 15, 2629 JB Delft, Tle NetherlandsTle present status of the LuΑ1O3:Ce scintillator is reviewed. Scintilla-tion mechanism of this material is based on capture by Ce 3+ of holes andthen electrons from their respective bands. Results of spectroscopic and ther-moluminescence experiments are presented to support this model.PACS numbers: 78.90.+t, 78.55.-m, 61.80.Εd, 29.40.-n1. IntroductionScintillation properties of LuAlO3:Ce (LuAP) crystals were first reported byLempicki et al. [1]. The interest in this material is derived from the high lightyield (above 20 000 photons/MeV), high density (8.34 g/cmι3 ) and short decaytime (about 18 ns). These properties result in one of the highest figures of meritof any known scintillator and put LuAP in the forefront when high counting rates,fast timing, good stopping power, and energy resolution are of importance [1, 2].Taking into consideration that this material is not fully optimized yet, and thereis evidence that its performance may be highly improved [3, 4], it becomes excep-tionally desirable to understand the physics which governs its behavior.2. Materials and experimentsLuAP samples were cut from boules grown by Litton Airtron by the Czochral-ski method. Detailed description of the growth process was presented in [1]. Crys-tals were optically clear and single-phase. Ce doped specimens were colorless withCe concentrations 0.035 and 0.11 mole% (as measured by spark source mass spec-trometry). Color of nominally undoped specimens was slightly orange with levelof unintended Ce contamination below 0.4 x 10 -4 mole%.(377)378 	 D. Wiśniewski et al.Excitation spectra were acquired using synchrotron radiation at BrookhavenNational Synchrotron Light Source (BNSLS). Emission spectra were taken underoptical (at BNSLS), γ-ray (100 μC Ru-Rh source, 0.5-2.9 MeV), and X-ray (8 keV)excitation. X-ray excited luminescence spectra were acquired using "transmission"geometry; sample was excited from one side, and emission gathered from the otherone. Decays were obtained under pulsed synchrotron light at BNSLS. ThermolumineScence measurements were performed at Delft University, using facilitiesdescribed in detail in [5] and [6]. Various sources were used for irradiation: 60 Coγ-source (1.2 kGy/h), X-ray tube with Cu anode (3.6 kGy/h) and Hg standardlamp.3. Experimental results and discussionRoom temperature emission spectra of undoped and Ce-doped specimens,taken under γ and X-ray excitation, are presented in Fig. 1. Trace a shows theγ-excited luminescence of Ce doped (0.035 mole%) sample. This spectrum is dom-inated by a single band centered at about 365 nm, which is a clear case of Ce 3+d—f emission [1]. However, after more detailed examination, one can find an addi-tional (about two orders of magnitude weaker) band at about 280 nm. This bandis somewhat stronger in case of nominally undoped sample b, but yet scintillationSpectroscopy and Thermoluminescence of LuΑ1O3:Ce 379of this sample also consists mostly of Ce3+ emission. X-ray excited luminescence ofthe same sample c, on the contrary, does not demonstrate distinguishable ceriumemission, showing instead an intense, wide, asymmetrical band peaking at about300 nm (host emission), Gd3+ line at 314 nm and some Τb 3+ lines. It is worthnoting that Gd and Tb contaminations in this material are only 0.7 and 0.2 weightppm, respectively, which suggest efIicient energy transfer to these ions. X-ray ex-cited luminescence spectrum of Ce-doped sample d shows intense Ce3+ emissionand some remains of 300 nm band. One can observe dips in host emission bandat 275, 292 and 310 nm which clearly correspond to bands in Ce 3+ luminescenceexcitation spectrum (presented in Fig. 3). Most emission diminishes with increas-ing sample thickness (e) and Ce content; it vanishes almost completely for sampledoped with 0.11 mole% of Ce (f). These observations leave no doubt that ceriumabsorption is responsible for deformation of 300 nm band. Although it may indi-cate of nonradiatiwe energy transfer from Iost emission to Ce 3+ ions, we will showlater that scintillation mechanism of LuAP:Ce is not based on this process.Luminescence spectra under Selective optical excitation are presented inFig. 2. Traces u and b present low temperature spectra of 0.11 mole% Ce dopedsample excited by 160 and 153 nm light, respectively. Both spectra display host aswell as Ce3+ emission, but their relative intensities depend on the excitation wave-length, Despite of surface excitation, an effect of deformation of host emission bycerium absorption is also evident, particularly in the furst spectrum. Under 153 nmexcitation a Gd3+ emission line at 314 nm is also observable. Room temperature(RT) spectra of the same sample will not be presented here since they do notreveal any additional features, except that splitting of Ce3+ doublet (2F5/2 and2F7 / 2 ) is no longer observed. Luminescence spectra of undoped sample let us Seean undistorted shape of host emission, which in fact appears to be superposition ofa few different ones overlapping each other. Excitation at around 160 nm revealsthe strongest of these emissions peaking at about 280 nm (13 K) and at 285 nm(RT) (c). Decreasing the excitation wavelength to 152 nm results in admixture ofan additional emission on the long-wavelength side of 285 nm band (d). At 13 K,using 217 nm excitation light, it becomes possible to isolate this new emissionas a band centered at about 330 nm (e). Furthermore, at low temperature onecan obtain two extra emissions which are not visible otherwise. An excitation at180 nm exposes an emission peaking at about 260  nm (f) and at 148 nm results inail emission band at 220 nm (g). Both emissions are about an order of magnitudeweaker than 280 urm band, but while the 220 nm band is observable in undopedas well as in intentionally Ce doped samples, the 260 nm luminescence Seems todisappear in doped crystals. It is worth noting that in undoped YAlO3, an yttriumanalog of LuAP, bands corresponding to our 220 and 280 nm emissions have beenobserved and investigated quite intensively in the past [7-9].In Fig. 3 we present RT excitation spectrum of Ce3+ luminescence in0.11 mole% Ce doped LuAP. Bands at 215 and 230 nm in addition to those men-tioned previously (at 275, 292 and 310 nm) are typical of the f-tod transitionsof the Ce3+ ion [1]. The intense excitation peak at 153 nm can be associated ei-ther with creation of band excitons or free electron-hole pairs, and consequentlygives us an approximate evaluation of LuAP band gap as 8.1 eV. The broad band3δ0 	 D. Wiśniewski el a1.centered at about 180 mm, partially overlapping both 153 nm and higher energyCe3+ f-d bands, is most likely due to photoionization of Ce3+ ions.Excitation spectra in the vicinity of band gap energies of Ce-doped andnominally undoped specimens are compared in Fig. 4. Traces a and b presentexcitation spectra of the 280 nm emission of undoped LuAP at 13 K and RT, re-spectively. Although the low-temperature (LT) spectrum consists of three bands,only 160 and 153 nm bands are in fact attributable to the 280 nm luminescence.The band centered at about 175 nm on the contrary belongs to 260 nm lumines-cence overlapping the 280 nm emission. The room temperature spectrum exhibitsa strongly increased contribution of 153 nm band and new higher energy bands atabout 142 and 125 nm, which suggests presence of thermally activated processes.Low-temperature excitation spectrum of 280 nm emission in 0.11 mole% Ce dopedsample (c) is nearly identical to that of undoped sample. The only difference is lackof the 175 nm band (and consequently the 260 nm emission). A corresponding  RTspectrum d consists of similar bands observed previously for undoped sample, butratios of their intensities are different. In general the intensities of higher energybands are decreased in comparison with the 160 mm band. Assuming that absorp-tion of the 160 nm light creates band excitons, and the 153 nm light generates freee-h pairs, it becomes evident that presence of Ce in the LuAP lattice does notSpectroscopy and Thermoluminescence of LuΑlO3:Ce 381affect direct creation of free excitons, but to some extent prevents their formationfrom free band charge carriers. The mechanism of this "prevention" is most prob-ably a competition for holes and electrons by Ce3+ ions which strongly supportsscintillation model based on consecutive carrier trapping proposed by Wojtowiczin [10].Traces e and f present excitation spectra of 330 nm emission in undopedsample at 13 K and RT, respectively. Since the 330 nm luminescence overlapsthe bng-wavelength slope of 280 nm band, the measured spectra exhibit and mixfeatures characteristic of both emissions. Some evaluations are however possible.The LT spectrum shows an excitation band centered at 217 nm which definitelybelongs only to 330 nm emission (217 nm band is beyond the wavelength range ofFig. 4, but its existence is confirmed by Fig. 2 e). The 160 nm light probably doesnot excite 330 nm emission (or at least does it insignificantly in comparison withexcitation of 280 nm band), but shorter wavelengths again do it — the 148 nmexcitation (Fig. 2 g) gives the 280 and 330 nm bands at similar intensity level (veryweak yet). The RT excitation spectrum of 330 nm emission consists exclusivelyof bands observed previously for 280 nm emission, but shorter wavelength bandsare more intense in comparison with this at 160 nm. In fact, if we subtract fromthis spectrum that measured for 280 mη emission, the result will not contain160 nm band. This suggests that the origin of 330 nm luminescence may be entirelydifferent from that of 280 nm one. While a large Stokes shift ( 3.3 eV) affirmtrapped excitons as a source of the latter, the former may be caused by defect orimpurity centers which can be excited themselves (at 217 nm) or act as radiativerecombination centers.The LT excitation spectrum of 220 nm emission of undoped LuAP (g) con-sists of two weak bands at 160 and 128 nm and an intense narrow band peaking at148 nm. The 160 nm band is most probably artificial as a result of slight overlap-ping of emissions 220 and 280 nm. The slim shape of the 148 nm peak, togetherwith a large Stokes shift (:.. 2.7 eV) proves the excitonic origin of the 220 nmluminescence. At RT the 220 nm band becomes unmeasurable. It is hard to be-lieve that thermal quenching of the emission is responsible for that, more likely alow dissociation energy of excitons (before relaxation) is the reason. In Ce dopedsamples both 220 nm emission and its excitation spectra behave in the same way.In order to complete a comparison we show the short wavelength parts ofCe3+ emission excitation spectra at 13 K (h) and RT (i). Both spectra have asimilar shape with major band at about 153 nm, some contribution from photoionization, 160 nm, and higher energy bands. Note that an admixture of the160 nm band is greater at 13 K, while the 153 nm band is much more intenseat RT. An explanation of this behavior can be presumably found in prolongedlifetime of excitons and reduced mobility of band holes at low temperature.In Fig. 5 we present Selected luminescence pulse shapes under a synchrotronlight excitation. Previous experiments with γ excitation resulted in scintillationpulse shape with an unmeasurable rise time (below 1 ns) followed by promptdecay with main time constant about 18 ns for both Ce-doped and nominallyundoped samples of LuAP. The contribution of longer components was below5% of zero-time amplitude. No faster components have been found. The decay382 D. Wiśniewski et al.curve for Ce-doped sample has been published in [1]. An optical excitation inCe3+ d—f absorption bands (α) gives a pulse shape with no rise time and a singleexponential decay with time constant equal (within an experimental error) to thatmeasured under γ excitation. For 0.11 mole% Ce doped sample this remains trueeven for excitation in band gap absorption region. Traces b and c represent pulseshapes of 280 nm luminescence in undoped specimen at 13 K and RT, respectively.Both are characterized by single exponential decays; there is, however, a slightpossibility that very long components may be hidden in the background. Thedifference of decay times (4.0 ns at 13 K and 3.1 ns at RT) indicates some thermalquenching of 280 nm emission. Room temperature decay times of the same emissionin 0.035 (d) and 0.11 mole% Ce doped (e) samples are 2.9 and 2.6 ns, respectively,which indicate some quenching of the excitonic emission in presence of significantamounts of Ce in the LuAP lattice. Although this quenching most likely happensby nonradiative energy transfer to Ce3+ ions, it is too weak to be of importancefor Ce3+ excitation.In Fig. 6 glow-curves of Ce-doped u and undoped b LuAP are presented.At a heating rate of 6 K/s, following γ-ray irradiation, both glow-curves havethree peaks, two common at about 380 and 530 K, and third at 640 K (α) or580 K (b). The thermoluminescence (TL) signal is however much weaker for theundoped sample, which suggests a correlation between presence of Ce and theSpectroscopy and Thermoluminescence of LuΑlO3:Ce 383number of traps. The series of experiments employing an X-ray tube and a Hg lampas irradiation was also performed. Regardless of the irradiation source similar glowpeaks were observed. Yet, after the Hg lamp irradiation, the TL signal was muchweaker. Using filters the 250 nm Hg line was recognized as responsible for sampleexcitation. While energy of absorbed photons is too low to stimulate band-to-bandtransitions, it is sufficient to release electrons from deeper traps. These electronscan then be retrapped into shallower traps. The shapes of TL emission spectrapresented in Fig. 7 resemble those of X-ray excited luminescences, except that therelation of intensities is different. The TL signal of undoped sample (b) consists ofa very weak host emission while the spectrum of doped one (α) is dominated by astrong emission of Ce3+ ions. Assuming that the simple classic model of TL [11]is true for LuAP, one can assert that Ce3+ ions effectively trap holes during theirradiation stage and then, gaining a positive potential, easily attract conductionband electrons. A more detailed description of TL processes in LuAP, includingestimations of trap depths and frequency faction, will be published elsewhere.4. ConclusionsAlthough there was no entirely Ce-free sample available, the LuAP latticeemission has been identified. The excitonic 280 nm band was recognized as a maincomponent of this emission. The comparison of decay times of 280 nm luminescenceat various Ce doping levels demonstrated only slight quenching of host emission,definitely not sufficient to account for nonradiative energy transfer as a significantsource of cerium excitation. Despite of obvious signs of reabsorption of host emis-sion by Ce3+ ions, no vital consequences of that were found in scintillation pulseshapes. Excitation spectra indicate competition for free charge carriers betweenCe and host emissions, Ce3+ being a winner of that contest.Theoretical speculations presented in [10] lead to the conclusion that Ce 3+ions in wide band gap materials will act as hole traps much more likely than as384 D. Wiśniewski et al.electron traps. It was confirmed by thermoluminescence spectra which showed thatCe3+ ion is a very efficient hole trap and works perfectly as radiative recombinationcenter. All the above observations, together with an astonishing Ce capability tocollect excitation deposited in the crystal lattice by γ photons in the form of hotband charge carriers, let us conclude that scintillation of Ce : LuAP is accomplishedby consecntive trapping first band holes and then electrons by Ce3+ ions.AcknowledgmentsWe acknowledge the support of the European Economic Community (Con-tract No. ERBCIPDCT940037), the US Dept. of Energy (grant no. DE-FG-O2-9OER61033) and the Netherlands Technology Foundation (STW). We thankDr. J.C. Sutherland and ħis group for their hospitality and assistance in per-forming experiments at the BNSLS.References[1]A. Lempicki, M.H. Randles, D. Wisniewski, M. Balcerzyk, C. Brecher, A.J. Woj-towicz, IEEE Nucl. Sci. 42, 280 (1995).[2] M. Moszynski, D. Wolski, T. Ludziejewski, A. Lempicki, C. Brecher,D. Wisniewski,A.J. Wojtowicz, in: Proc. Int. Conf. on Inorganic Scintillators and their Applica-tions, SCINΤ95, Delft 1995, Eds. P. Dorenbos, C.W.E. van Eijk, DeJft UniversityPress, Delft (The Netherlands) 1996, p. 348.[3] A. Lempicki, C. Brecher, D. Wisniewski, E. Zych, in: Proc. hit. Conf. on Inor-ganic Scintillators and their Applications, SCINΤ95, Delft 1995, Eds. P. Dorenbos,C.W.E. van Eijk, Delf University Press, Delf (The Netherlands) 1996, p. 340.[4] A. Lempicki, C. Brecher, D. Wisniewski, E. Zych, A.J. Wojtowicz, Nucl. Sci. Symp.felled. Imag. Conf., San Francisco 1995, to be published in Conference Record andin IEEE Trans. Nucl. Sci.[5] T.M. Piters, Ph.D. Thesis, Delft University of Technology, Delft 1993, p. 49.[6] B. van den Burg, Report no. 95, Delft University of Technology, Delft 1995, p. 11.[7] R.L. Wood, W. Hayes, J. Phys. C, Solid State Phys. 15, 7209 (1982).[8] A.I. Kuznetsov, V.N. Abramov, B.R. Narnozov, T.V. Uibo, Trudy Instituta FizikiAN Estonskoj SSR 53, 83 (1995).[9] A.I. Kuznetsov, B.R. Narnozov, V.V. Myurk, Sov. Phys. Solid State 27, 1819 (1985).[10] A.J. Wojtowicz, in: Proc. Int. Conf. on Inorganic Scintillators and their Applica-tions, SCINΤ95, Delft 1995, Eds. P. Dorenbos, C.W.E. van Eijk, Delft UniversityPress, Delft (The Netherlands) 1996, p. 95.[11] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge University Press,Cambridge 1985.

Spectroscopy and Thermoluminescence of LuAlO3:Ce

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Spectroscopy and Thermoluminescence of LuAlO3:Ce

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