- Email: [email protected]
Ce-doped scintillators: LSO and LuAP
Nuclear Instruments and Methods in Physics Research A 416 (1998) 333—344
Ce-doped scintillators: LSO and LuAP A. Lempicki!,",*, J. Glodo!,# ! Chemistry Department, Boston University, 590 Commonwealth Ave. Boston, MA 02215, USA " ALEM Associates, 303A Commonwealth Ave. Boston, MA 02115, USA # Institute of Physics, N. Copernicus University, Grudziadzka 5, PL 87-100 Torun, Poland Received 2 February 1998; received in revised form 30 April 1998; accepted 15 May 1998
Abstract In this paper we compare the scintillator performance of the two of the leading contenders for Positron Emission Tomography, both cerium doped: Lu SiO (LSO) and LuAlO (LuAP). LSO crystals come in two categories, character2 5 3 ized by high (H) or low (L) light output. Such a distinction is not found in LuAP, but its output is consistently lower than that of LSO-H. Based on decays, thermoluminescent data and dependence of light output on temperature, we conclude that the principal reason for the superior properties of LSO-H is its virtual absence of shallow traps. LuAP also suffers from an additional handicap in the form of a parasitic absorption of unknown origin, which causes a light output dependence on the thickness of the sample. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 78.55.!m; 78.60.kn. Keywords: Scintillator; Detectors
1. Introduction During the past decade extensive effort has been directed at developing better scintillator materials for Positron Emission Tomography (PET). While there is no question that this effort has considerably advanced the understanding of the phenomenon of scintillation, the number of promising new materials has been disappointingly small. The reason for
* Corresponding author and present address: Chemistry Department, Boston University, 590 Commonwealth Ave., Boston MA 02215, USA. Tel.: #1 617 353 9581; fax: #1 617 353 6466; e-mail: [email protected].
this is that PET imposes a rather demanding combination of properties, including a density greater than 7 g/cm3, a scintillation decay time well below 100 ns and a light output substantially exceeding that of BGO (8500 photons/MeV). These requirements are peculiarly intertwined since they involve precise matching of the properties of host and activator. For instance, to achieve the necessary speed, the wavelength dependence of decay time virtually dictates that the emission be in the blue to near-UV; this requires host transparency in this optical region, which limits their choice to wide-bandgap insulators. The additional requirement of density narrows the choice even further: Since the stopping power for gamma rays
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 8 9 - 5
334
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
depends upon the effective atomic number of the scintillator and the spatial resolution of PET limits the size of the detector elements, the new materials must include substantial proportions of heavy elements. The ideal host, however, must also incorporate an efficient luminescent center, either as a dopant or as a property of the lattice itself (i.e., BGO). Whatever the nature of the center, the optical transition has to be allowed (for high speed), and take place in the appropriate wavelength range. Furthermore, the energy structure of the center has to be such that nonradiative decays are minimized, requiring large energy gaps below the excited state. Of the very limited number of activator candidates, Ce3`, with its allowed interconfigurational (d to f) transitions and the 30 000 cm~1 energy gap below its lowest 5d state is unquestionably the best. It is not too surprising that the need to satisfy all these requirements leaves very few potential material systems, especially if one adds the “trivial” aspects of crystal growth, such as congruent melting and the absence of phase transitions. At the present time the top of the pyramid seems to be occupied by only two credible candidates, both of which contain Ce as activator: Lu SiO (LSO) and 2 5 LuAlO (LuAP). These two materials exhibit im3 portant differences in behavior, each with its own specific advantages and drawbacks. In particular, the best LSO to date has a superb light output of 27 300 photons/MeV, the highest of all known oxidic systems, and a fine single-exponential decay with a characteristic time of about 47.2 ns [1]. Unfortunately, despite extensive development efforts, it is not unusual to find crystals with substantial deficiencies in both light output and decay behavior. The less developed LuAP, on the other hand, shows a superb scintillation decay time of 18 ns (the fastest among known oxidic systems), but accompanied by significantly slower components, and its light output is currently only a third of that of the best LSO specimens [2,3]. In this paper we make an in-depth study of various relevant properties of the two materials in an attempt to achieve better understanding of the mechanism of scintillation, to explain the reasons for the disparate behavior and hopefully to ascertain the directions and extent to which improvements can be made.
2. Experimental 2.1. Materials The LSO crystals were obtained from C. Melcher, then at Schlumberger-Doll Research.1 These were grown by the Czochralski method from the melt containing 0.25 mol% Ce, in an atmosphere of argon with 3000 ppm oxygen. The two specimens were typical representatives of the so-called “good” and “poor” LSO as measured by their light output (L for low and H for high). No specific differences in their fabrication were known that could account for their differences in behavior, which must consequently be attributed to uncontrolled variations in the crystal growth. Indeed, although literature reports [4] suggest that crystals grown at a slower rate (0.5 mm/h) are better than those grown at 2 mm/h, for these particular specimens the opposite was true: the better one (H) was grown 3 mm/h, the poorer (L) at 2 mm/h. The LuAP crystals were provided by M. Randles of Litton-Airtron2 where they were pulled from the melt on an iridium wire, in a growth atmosphere of N . The Ce concentrations in the melt were 0.25% 2 and 0.75% with respect to Lu, but mass spectroscopic analysis found the actual concentrations in the crystal to be on the order of 20% of these nominal values. 2.2. Emission spectra Emission spectra of the two materials have been published previously [2,3,5]. We include them here to emphasize the point, which will be repeated again in other contexts, that the emission of Ce3` in LuAP (Fig. 1) is essentially the same independent of the quality of the crystal, the value of its light output and the concentration of Ce in the range between 0.25% and 2% nominal. In this host Lu, and consequently Ce, resides in only one type of crystallographic site. LSO, on the other hand, has two Lu (and consequently two Ce3`) sites, called
1 Schlumberger-Doll Research, Ridgefield CT. 2 Synoptics Div., Litton-Airtron Corp., Charlotte NC.
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 1. Emission spectra of LuAP : Ce at room and low temperature (4 K) when excited at 152 nm.
335
Fig. 3. Emission spectra of LSO-L at room and low temperature (4 K) when excited at 188 nm.
in the region between 280 and 330 nm. Emission spectra of undoped LuAP have been described previously [7], but undoped LSO was not available for this study. No significant energy transfer takes place from lattice (excitonic) emission to Ce, whose excitation is therefore carrier-mediated, a process faster and more efficient than any mediated by excitonic transfer. 2.3. Excitation spectra
Fig. 2. Emission spectra of LSO-H at room and low temperature (4 K) when excited at 188 nm.
C1 and C2, of which the former is spectrally dominant [6]. Spectra optically excited close to the band edge (188 nm) are shown in Figs. 2 and 3 for the H and L samples, respectively. The principal features (with peaks at 393 and 423 nm), due to Ce in C1 sites split by spin—orbit interaction in the ground state, are visible in both samples. The C2 emission, much weaker and not well resolved, occurs at 500 nm. In the L sample, at low temperature, we also note the presence of lattice emissions
The excitation spectrum of the LuAP emission at wavelengths shorter than 330 nm is shown in Fig. 4. Above 210 nm we discern the five crystal field components of the 5d configuration, split into groups of two and three. The region between 210 and 165 nm is featureless but certainly far above baseline (compare with the bottoming of the excitation at 250 nm). At 154 nm we see a prominent peak which is identified with band-to-band transitions and occurs at an energy where the absorption increases very rapidly [7]. Indeed, this increase, associated with the UV absorption edge, is so rapid that it distorts the excitation into the shape of a peak. The absorption is so intense that the actual excitation is confined very close to the surface and is therefore more subject to variations and nonlinearities than e—h generation in the bulk. Nevertheless, the existence of this feature does demonstrate
336
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 4. Excitation spectra of LuAP : Ce at two temperatures (RT and 180 K). Note the near disappearance of the 154 nm “band-to-band” peak at 180 K.
that optical electron—hole generation leads to Ce excitation. The magnitude of the 154 nm peak is very temperature-dependent. As shown in Fig. 4 it decreases rapidly below room temperature and is mostly gone by 180 K. If the magnitude of this peak were a measure of energy transfer to the activator, one would expect that below 180 K scintillation should cease, which however is not the case (see Section 2.6). We have to conclude that at low temperatures either other transfer mechanisms begin to dominate or that processes near the surface no longer reflect what takes place in the bulk. Excitation spectra of LSO (Fig. 5) show rather different behavior. The band-to-band peak is located at 188 nm (RT), indicating a smaller energy bandgap (6.6 eV) than in LuAP (8.2 eV). At low temperature, the peak (although reduced in relative intensity) is still present. Note, however, that the peak is much broader than in LuAP, with a decidedly asymmetric shape. Possibly the rise of the absorption edge is less steep than in LuAP and excitation reaches somewhat deeper below the surface. 2.4. Absorption and “self-absorption” spectra An unusual feature of the emission spectra of LuAP is its dependence upon the geometrical
Fig. 5. Excitation spectrum of cerium emission in LSO-H. Note the broad “band-to-band” peak below 200 nm and its incomplete disappearance at 4 K.
Fig. 6. Emission spectra of LuAP when excited with gamma particles, for three different orientations of the same sample with respect to the PMT.
arrangement of the sample and detector. A striking example is shown in Fig. 6, where the spectrum varies drastically depending on the orientation of the same sample with respect to the PMT. At first sight this appears to be a classic case of self-absorption because the orientation resulting in the longest light path in the crystal gives the narrowest peak. Another manifestation of the same effect is the dependence of the light output on the size of the
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
crystal. Although samples with linear dimensions much greater than the attenuation length (which at 0.5 MeV is on the order of 1 cm) have poorly developed photopeaks, they do show a definite decrease of light output as the sample becomes larger. Some rather striking results were reported earlier [8], in which a sample was excited with X-rays that penetrated a small fraction of the thickness, and the light output was observed from the opposite side. In this geometry the bulk of the crystal acted like a filter for light generated near the other surface. This too was very suggestive of self-absorption caused by the relatively small Stokes shift. For this interpretation to be correct, however, the decay time would also have to show a distinct dimensional dependence, growing increasingly longer as the path length increases [9]. Since this is most decidedly not the case, we must seek an alternative explanation, that in the region of LuAP emission there is a parasitic absorption due to some foreign species, not Ce itself. The critical question then becomes whether the parasitic absorption, which after all we see only in doped crystals, is a necessary consequence of that doping or an accident of fabrication. Fig. 7 gives the absorption spectra of LuAP. The undoped crystal does not indicate the presence of any defects in the visible region. The slight rise below 300 nm can be attributed to the expected scattering increase at shorter wavelengths. Note that in the region of 250 nm, this does not exceed 5—6 cm~1, a fact whose significance will soon become apparent. The absorption spectrum of doped LuAP, given also in Fig. 7, shows the first three components of the 5d state. To the extent that absorption processes are characteristic of an activator and therefore lead to emission, the absorption spectrum should be essentially identical to the excitation spectrum. This statement is exactly true only in the limit of small absorption or (equivalently) for measurements made on very thin samples. If, however, absorption is measured on thin samples and excitation on thick ones, as is usually the case, excitation features that are associated with particularly intense absorptions can be severely distorted in shape. Comparison of Ce-doped LuAP absorption (Fig. 7) and excitation (Fig. 4) spectra
337
Fig. 7. Absorption spectra of LuAP undoped and doped with 0.25% Ce (nominal).
gives a prime illustration of this effect. The pronounced increase on the shorter wavelength side of the absorption shoulder at 306 nm distorts that feature into an apparent peak in the excitation spectrum. These are common occurrences with no major ramifications at this point. There is, however, a very serious discrepancy at around 250 nm that cannot be attributed to such distortion. Note that below 270 nm a gap develops in the excitation spectrum, whose intensity drops essentially to zero at 250 nm. Such gaps are quite common between d orbitals, and are even utilized in the making of optical filters [10]. In Fig. 7, however, the corresponding dip in absorption remains well above the baseline, retaining significant residual intensity. Moreover, the general shape of the spectrum strongly suggests an underlying parasitic absorption slope extending through the entire near-UV region to as high as 400 nm, where it could intercept some of the emission. An analogous effect has been observed in YAlO : Ce by Russian workers 3 [11], who noted that the depth of the absorption valley at 250 nm is quite dependent on the method of crystal growth. They report that crystals grown by zone melting in vacuum have much less of this parasitic absorption than Czochralski-grown crystals. The Russian workers attributed the extraneous absorption to the presence of Ce4` with the absorption being a charge transfer process
338
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
(valence band to Ce4`). The stability of the Ce4` state has been called into question before [12], and furthermore if Ce4` ions were present as an equilibrium valence state, then post growth heat treatments in oxidizing and reducing atmospheres should alter its concentration. However, heat treatment of LuAP in vacuum, air, and Ar/H mixtures did not significantly change any of its properties. While the origin of the parasitic absorption remains speculative, we can obtain a rough measure of its dependence on Ce concentration by comparing its effect on the emission at various doping levels. In earlier work [8] we were able to extract this absorption from the distortion of emission spectra excited by X-rays. One can obtain similar results by simply changing the geometrical arrangement of the sample and detector as shown in Fig. 6. While these suggest that the magnitude of the absorption in our specimens does scale with the Ce content, the fact that it can be significantly altered in YAP by modification of the growth procedure leaves the possibility and extent of its removal in LuAP an open question.
Fig. 8. Comparison of room temperature decays of LuAP when excited optically (230 nm) or by gamma irradiation.
2.5. Decays Scintillation decays under gamma excitation are usually measured by the photon correlation technique [13]. Decay traces are then fitted to a sum of as many as three exponential terms, whose time constants and amplitudes are reported as the decays of fast, intermediate and long components. Obviously, components longer than the measurement range are simply submerged in the background signal. Another type of decay is obtained under optical excitation, utilizing pulses of light at energies less than the band gap. In the case of doped materials one excites the dopant ion directly (f to d transitions in Ce), without creating charge carriers in their respective bands. This type of decay is usually quite different from gamma excited decay, being single exponential and not exhibiting the long components. In this section we shall describe both types of decays in LuAP and LSO as well as their temperature dependence. Fig. 8 gives the optical and gamma excited decays of LuAP at room temperature. We see that
Fig. 9. Comparison of room temperature decays of LSO-H and LSO-L when excited by gamma irradiation.
while the initial decay constants are essentially equal, the gamma excited decay slows down due to longer components. The optically excited decay is strictly exponential, fitted by a single exponential function plus a constant (DC) background. The decays of LSO are significantly different. In Fig. 9 we show the gamma excited decays of the H and L specimens measured at RT. Note that both are single exponential fits, and neither case shows evidence of intermediate components. The
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 10. Gamma-excited decays of LuAP : Ce at three different temperatures below RT. Note increase of intermediate component at 180 K.
decay of LSO-L is faster, indicating some degree of nonradiative quench. The smaller amplitude is consistent with the lower light output of the sample. Figs. 10 and 11 show two sets of three gamma excited decays of LuAP measured over two temperature ranges, one below and one above room temperature. In each case the decay at the middle temperature (180 and 340 K, respectively) shows a distinct enhancement of the intermediate component, just following the initial fast decay characteristic of the Ce ion in this lattice. The meaning of the appearance of this intermediate component at particular temperatures will become clear when we discuss the thermoluminescence behavior. In a general discussion of scintillator properties it is often assumed that the quantum efficiency of the luminescing ion is essentially unity; i.e., that there are no significant losses attributable to radiationless transitions from the excited to the ground state of the ion. However there is always a temperature above which such losses will no longer be negligible, and the optical decay will become progressively faster. Figs. 12 and 13 show the temperature dependence of the decays in LSO and LuAP. In two of these cases (LuAP and LSO-H) the decays reported were obtained by gamma rather than optical excitation, since our equipment for the latter measurements could not reach the necessary
339
Fig. 11. Gamma excited decays of LuAP : Ce at three different temperatures above RT. Note increase of intermediate component at 340 K.
Fig. 12. Temperature dependence of the first component of decay and light output of both LSO-H and LSO-L. LSO-L decays were excited optically, LSO-H by gamma irradiation.
higher temperatures; in these particular cases, however, the decay time of the fast component of gamma-excited decay was essentially identical to that obtained optically, and their temperature dependence should be equivalent. Since the quantum efficiency Q of the emission is proportional to q, both remain constant up to a certain temperature (approx. 300 K for LSO and 600 K for LuAP), above which thermal quenching becomes important.
340
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 13. Temperature dependence of the light output of LuAP : Ce and the first component of gamma excited decay. Fig. 14. Thermoluminescence of LuAP : Ce, LSO-H and LSOL. Details of the major glow peaks are given in Table 1.
2.6. Thermoluminescence Thermoluminescence of scintillator materials at elevated temperatures has been reported in several studies [14,15]. In general, these studies reach only qualitative (and sometimes incorrect) conclusions, indicating that while a great abundance of traps does cause poorer scintillation, the effect is of no great consequence once the traps are filled [15]. While possible in principle, this assessment fails to take into account the great magnitude of the fluxes needed for trap saturation [18]. Thermoluminescence of both LuAP and LSO, resulting from excitation by ionizing radiation at room temperature followed by heating, has been published previously [16,17]. In both cases the thermoluminescent emission was observed to be that of Ce, which acts as a radiative recombination center for both the carriers initially created and those liberated from traps at a later time. This work indicates that the traps responsible for the glow peaks situated well above room temperature do compete with scintillation by diverting carriers into long-term storage [18]. More recently, we extended thermoluminescent studies to cover the low-temperature range, from 4 to 400 K, not previously performed on these materials. The samples were irradiated at 4 K with a UV source set at the wavelength of the respective band-to-band excitation peaks (153 nm for LuAP and 188 nm for LSO). Again only Ce emission appears in the thermoluminescence process. The
Table 1 Values of parameters for the major thermoluminescence glow peaks Crystal
¹ (K) .!9
E (eV)
LuAP : Ce
182 266 334 95 345
0.51 28.9 0.64 23.7 0.78 22.2 0.147 14.5 not fittable — distorted
LSO-H LSO-L
ln s (s~1)
q at RT (s) 106]10~6 2.85 2520 149]10~6
low-temperature glow curves for LuAP are given in Fig. 14. The glow curves were analyzed by the simple method of Randall and Wilkins [19,20] and the results for major glow peaks are given in Table 1. (We include in this table the average lifetime of the trap given by the Arrhenius equation 1/q"s exp(!E/k¹), where s is the frequency factor and E the depth of the trap.) The most prominent peak is clearly at 178 K with a smaller one at 270 K. All the other peaks are at least an order of magnitude smaller. The glow peaks of LSO-H and LSO-L in the same temperature range, given in Fig. 14 and listed in Table 1, as well, provide a striking contrast. Note first that in the temperature range investigated here, the highest temperature peak in both LSO specimens is +340 K, while in LuAP a peak situated at
341
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
approximately the same temperature is barely detectable (see Fig. 15). Note further that LSO-H appears to have hardly any low-temperature glow peaks at all, whereas LuAP and LSO-L have prominent peaks at 178 and 90 K, respectively. While it is very difficult to compare the intensity of the peaks in two materials in any quantitative manner, it is quite evident that LSO-H is unique, in the sense of showing almost no thermoluminescence below 300 K. The prominent peak in both samples of LSO just above room temperature is responsible for the well known afterglow, which necessitates mild heating prior to usage. The conclusion has to be that relatively shallow traps are far less numerous than in case of LuAP and LSO-L. 2.7. Light output and its temperature dependence Of the many criteria for judging the overall performance of a scintillator, the most critical is the light output (LO), which is the number of scintillation photons emitted per MeV of energy of the gamma photon stopped in the crystal. Since absolute measurements of this quantity are difficult to make and extremely sensitive to error, this evaluation is most reliably made by comparison with a standard material (such as BGO) whose own light output is known. The simplified procedure that we use is to take an energy spectrum of BGO and determine the position of its photopeak, and then do the same for the substance under investigation. As long as the means of collecting all the light generated in both BGO and the sample being studied are similar, and proper allowance is made for the difference in emission wavelengths and hence PMT sensitivity, we can take the ratio of photopeak positions as a measure of light output relative to BGO. From this ratio we extract numerical values, using the value of 7000 photons/MeV characteristic of 3.6 mm thick BGO [21]. Unfortunately, the light output for LuAP is not uniquely defined because of the “pseudo-self-absorption” effect described in Section 2.4. This makes it necessary to state the dimensions of the specimen and to perform the measurement against a similarly shaped sample of BGO. Table 2 compares the LO measured by
Table 2 Light output of LuAP and LSO Material
Thickness (mm)
Photons/ Reference MeV
BGO LuAP : Ce 0.75% LuAP : Ce 0.75% LSO : Ce 0.22% LSO-H : Ce 0.25% LSO-L : Ce 0.25%
1 1 1.6 14.5 1 1.9
8 060 11 300 11 800 27 300 24 800 725
Moszynski [19] Moszynski [19] Present work Moszynski [19] Present work Dorenbos [3]
Moszynski et al. [21] with recent ones performed in our laboratory. It should be noted that the measurements also depend on the width of the gated detection window (in our case, typically 2.3]500 ns of shaping time). Since this is finite, any light generated by a scintillation event that appears after the measurement window has closed will escape detection, and thus will not contribute to the light output. This is a very important point, since it is essential to the understanding of the connection between LO and decay time: In the absence of nonradiative losses, whatever accelerates the escape of carriers from longlived traps will increase the fraction of detectable light, and hence the measured LO, while any increase in the probability of trapping will do the opposite. And the magnitude of the effect is not absolute, but is inextricably tied to the width of the measurement window. A detailed mathematical treatment of the kinetics is beyond the scope of this paper, and has been provided elsewhere [22—24]. Examination of these results reveals two important points. One is that, for as yet unknown reasons, the light output of LuAP at the present stage of its development is substantially lower than can be achieved from LSO. No less perplexing, however, is the fact that the light output of LSO itself is subject to great variation from one specimen to another, despite the absence of any detectable difference in its chemical make-up. Both these points will receive considerable attention in the discussion to follow. Using such measurements we can also determine the temperature dependence of the light output. The simplest case is that of LSO-H, given in
342
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 12, whose q curve we have already discussed. We see that the LO curve tracks the q curve over the entire temperature range. This means that the efficiency of scintillation depends on temperature as strictly determined by the quantum efficiency of Ce luminescence, with all other processes such as conversion and transfer (see also Section 2.4) being essentially independent of temperature. This result is to be contrasted with those from LSO-L and LuAP. In the former case (Fig. 12) the decay time has a temperature dependence similar to that of LSO-H, i.e. roughly constant up to 300 K. However, the temperature dependence of LO no longer tracks that of the decay, but shows its own unique pattern, with a minimum at 84 K and a maximum at 191 K. An even more striking example is provided by LuAP (Fig. 13). Here again the curve of LO vs. temperature shows pronounced structure, although we would not expect any temperature dependence of the optical decay time q, right up to the thermal quenching region.
3. Discussion One of our primary motivations for the present work has been to quantify and explain the differences in light output exhibited by the materials in question. For this we must turn first to the basic principles of the scintillation process. The mechanism of scintillation is described by the well-known phenomenological formula for the efficiency [25]: g"b]S]Q,
(1)
where b is the conversion efficiency (ratio of the number of electron—hole pairs actually produced to the maximum number possible), S the transfer efficiency (fraction of the produced electron—hole pairs whose energy actually excites the luminescent centers) and Q the luminescence quantum efficiency of those centers. Recent work has shown that the conversion efficiency, which has a simple relationship to the bandgap of the host material [26], can be quite high, leaving only the transfer as the major source of loss. Let us examine the implications of this hypothesis. In Table 3 we reproduce a few entries from
Table 3 Calculated creation of e—h pairs and observed light output (from Ref. [24]) Material
Calculated e—h pairs per MeV
Observed Efficiency light output g per MeV
CsI : Tl NaI : Tl BGO Lu SiO : Ce (LSO) 2 5 LuAlO : Ce (LuAP) 3
69 444 75 330 88 889 69 444 55 556
65 000 38 000 8 500 27 300 11 300
0.936 0.504 0.096 0.393 0.20
Ref. [26]. We note first that if S and Q were equal to unity, then the number of photons (LO) generated per MeV would be equal to the number of e—h pairs. This is evidently not the case, except for CsI : Tl whose LO is close to theoretical. For LuAP and LSO we have shown that over a broad temperature range the radiative decay time does not change, and therefore Q is essentially unity, thus identifying an inadequacy in transfer efficiency (S(1) as the factor primarily responsible for the lower than possible LO. From this we must infer that a substantial degree of nonradiative recombination must be taking place. The potential for improvement is quite substantial, 370% in LuAP and 250% in LSO. The extent to which such improvement is actually possible depends on the correctness of the calculated conversion and the absence of loss introduced by extrinsic factors. The remarkable agreement between theory and experiment in the case of CsI : Tl may perhaps provide some clues. It is now generally accepted that the principal mechanism of electron—hole recombination in Cedoped scintillators is through the Ce ion [12,23], although the extent of competing recombination mechanisms is presently unknown. However, at any temperature there is a finite probability of one or both carriers being trapped elsewhere. This necessarily introduces a loss, since such carriers are effectively unavailable for the generation of scintillator light. This loss would no longer be present if the traps were completely saturated, but depending on their concentration the material may become unstable. Experiments performed by Bartram et al.
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
[18] indicated for LuAP a loss of some 15% due to high-temperature traps. Recent work by Wojtowicz et al. [23] expanded consideration to include lowtemperature traps as well, approximately doubling this figure. Thus, a 30% improvement in LO can, in principle, be expected for trap-free LuAP, bringing it to a respectable 15 000 photons/MeV, for thin crystals. This is still a long way from the number predicted from the conversion value [26]. The effects of traps are not, however, limited to the light output alone. While the deep traps that give rise to high-temperature thermoluminescence may act merely as passive sinks for mobile carriers, the shallow traps associated with cryogenic thermoluminescence may become active participants in the kinetic processes. The accepted mechanism of scintillation, at least for Ce-doped materials, is radiative recombination in which Ce3` first captures a hole to become Ce4`, which upon subsequent capture of an electron returns to Ce3` but in an excited state [12]. Emission of a photon returns the system to its ground state. The principal competing processes are nonradiative recombination of e—h pairs via some deep centers and trapping. All three processes are illustrated in Fig. 15. It should be noted that as long as trapped carriers are eventually returned to their respective bands there is no loss of total light output, although some of this is diverted into the long components of decay. However for any trap depth and temperature there will be a population of occupied traps, depending on previous thermal and excitation history of the sample. Thus, over the time scale useful for scintillator measurements, traps tend to reduce the number of e—h pairs available for radiative recombination. Deep traps can hold carriers for very long times and produce a steady afterglow. The large glow peak in LSO just around room temperature is responsible for its well-known afterglow, which can only be destroyed by moderate heating. Direct evidence of the influence of traps on decay is given by Figs. 10 and 11. Note that in both cases, at temperatures of 180 and 340 K, a definite enhancement of intermediate components appears to take place, closely coinciding with the positions of the LuAP glow peaks. Above and below these temperatures the corresponding traps are relatively inert, since the retention time for the carriers
343
Fig. 15. Radiative and non-radiative (NR) recombination and trapping processes of electrons and holes. The radiative mechanism takes place through Ce emission.
becomes either too short or too long to have much effect on the scintillation kinetics. Another aspect of this link between low-temperature traps and scintillation is, of course, the clean single-exponential decay of trap-free LSO-H (see Fig. 12). Since the quantum efficiency of luminescence is essentially constant at temperatures below the onset of thermal quenching, the variations of LO encountered in LuAP and LSO-L must represent a complex temperature dependence of S (Eq. (1)). This could arise because each shallow trap introduces its own trapping and detrapping probabilities, both of which are Arrhenius-type functions of temperature [22—24]. Thus it is clear that not only shallow traps give rise to the complex temperature variation of S, but it is precisely their absence that is the reason for the unique behavior of LSOH. It is also evident that while high-temperature traps may reduce LO by diverting and holding carriers, they are stable enough to leave the decay unaffected. The evident mischief caused by low temperature traps adds to the difficult task of developing better scintillators. At the present time nothing is known about their nature. Dorenbos et al. [14] speculated that in LSO at least one of the traps is spatially associated with Ce. This was based on their observation of high-temperature thermoluminescence subsequent to excitation in the absorption bands of Ce. We have not been able to fully confirm this result; although excitation in Ce levels lying close to
344
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
the conduction band does indeed produce a weak TL, excitation at 350 nm does not. We ascribe the observation of this thermoluminescence not to a trap directly accessible from an excited Ce ion, but to photoionization from the higher excited states to the conduction band. The factors that reduce the light output of LuAP, the parasitic absorption and the shallow traps, may even be related. While the nature of the parasitic center has remained elusive, at least one kind of trap capable of such UV absorption would be the presence of a stable Ce2` state in the gap. Such a state could act both as an electron trap and also as the terminal state of a charge transfer transition. The presence of such an entity could readily be related to the method of crystal fabrication, which may be our only recourse for its elimination. For instance, it would be of great interest to compare the thermoluminescence and LO temperature dependence in LuAP crystals grown by different methods [27]. No matter how daunting, however, it is becoming abundantly clear that the tasks of identifying the nature of the traps and any competing nonradiative recombination centers, and the means for their elimination, must now be considered an essential part of scintillator science.
Acknowledgements The authors gratefully acknowledge the support of this research by the US Department of Energy, under DoE Grant DE-FG-02-90ER61033. We are also greatly indebted to Profs. C. Brecher and A. J. Wojtowicz for many helpful discussions and critical reading of the manuscript; to D. Wisniewski, who provided some of the spectroscopic data; and to W. Drozdowski for analysis of the glow curves.
References [1] M. Moszynski, M. Kapusta, D. Wolski, M. Szawlowski, W. Klamra, IEEE Trans. Nucl. Sci. 44 (1997) 436. [2] A. Lempicki, M.H. Randles, D. Wisniewski, M. Balcerzyk, C. Brecher, A.J. Wojtowicz, IEEE Trans. Nucl. Sci. NS-42 (1995) 280. [3] A. Lempicki, C. Brecher, D. Wisniewski, E. Zych, A.J. Wojtowicz, IEEE Trans. Nucl. Sci. NS-43 (1996) 1316.
[4] P. Dorenbos, C.W.E. van Eijk, A.J.J. Bos, C.L. Melcher, J. Lumin. 60/61 (1994) 979. [5] C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. NS39 (1992) 502. [6] H. Suzuki, T.A. Tombrello, C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. NS-40 (1993) 380. [7] D. Wisniewski, A.J. Wojtowicz, A. Lempicki, J. Lumin. 72/74 (1997) 789. [8] A. Lempicki, C. Brecher, D. Wisniewski, E. Zych, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT95, Delft University Press, Delft, the Netherlands, 1996, p. 340. [9] A.J. Wojtowicz, M. Balcerzyk, E. Berman, A. Lempicki, Phys. Rev. B 49 (1994) 4880. [10] L.R. Elias, R. Flach, W.M. Yen, Appl. Opt. 12 (1973) 138. [11] E.G. Gumanskaya, O.A. Egorcheva, M.V. Korzhik, S.A. Smirnova, E.B. Pavlenko, Opt. Spectrosc. 72 (1992) 215. [12] A.J. Wojtowicz, A. Lempicki, D. Wisniewski, M. Balcerzyk, C. Brecher, IEEE Trans. Nucl. Sci. NS-43 (1996) 2168. [13] L.M. Bollinger, G.E. Thomas, Rev. Sci. Instr. 32 (1961) 1044. [14] P. Dorenbos, C.W.E. van Eijk, A.J.J. Bos, C.L. Melcher, J. Phys.: Condens. Matter 6 (1994) 4167. [15] R. Visser, C.L. Melcher, J.S. Schweitzer, H. Suzuki, T.A. Tombrello, IEEE Trans. Nucl. Sci. NS-41 (1994) 689. [16] D. Wisniewski, W. Drozdowski, A.J. Wojtowicz, A. Lempicki, P. Dorenbos, J.T.M. de Haas, C.W.E. van Eijk, A.J.J. Bos, Acta Physica Polonica A 90 (1996) 377. [17] W. Drozdowski, D. Wisniewski, A.J. Wojtowicz, A. Lempicki, P. Dorenbos, J.T.M. de Haas, C.W.E. van Eijk, A.J.J. Bos, J. Lumin. 72/74 (1997) 756. [18] R.H. Bartram, D.S. Hamilton, L.A. Kappers, A. Lempicki, J. Lumin. 75 (1997) 183. [19] J.T. Randall, M.H.F. Wilkins, Proc. Roy. Soc. 184 (1945) 366. [20] J.T. Randall, M.H.F. Wilkins, Proc. Roy. Soc. 184 (1945) 390. [21] M. Moszynski, M. Kapusta, M. Mayhugh, D. Wolski, S.O. Flyckt, IEEE Trans. Nucl. Sci. NS-44 (1997) 1052. [22] A. Lempicki, R.H. Bartram, effect of shallow traps on scintillation, J. Lumin. to be published. [23] A.J. Wojtowicz, J. Glodo, A. Lempicki, C. Brecher, Recombination and scintillation processes in YAlO : Ce, J. Phys: 3 Condens. Matter, to be published. [24] A.J. Wojtowicz, J. Glodo, W. Drozdowski, K.R. Przegietka, Electron traps and scintillation mechanism in YAlO : Ce and LuAlO : Ce scintillators, J. Lumin., to be 3 3 published. [25] A. Lempicki, A.J. Wojtowicz, E. Berman, Nucl. Instr. and Meth. A 333 (1993) 304. [26] R.H. Bartram, A. Lempicki, J. Lumin. 72/74 (1997) 734. [27] A.G. Petrosyan, C. Pedrini, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT95, Delft University Press, Delft, the Netherlands, 1996, p. 498.
Ce-doped scintillators: LSO and LuAP A. Lempicki!,",*, J. Glodo!,# ! Chemistry Department, Boston University, 590 Commonwealth Ave. Boston, MA 02215, USA " ALEM Associates, 303A Commonwealth Ave. Boston, MA 02115, USA # Institute of Physics, N. Copernicus University, Grudziadzka 5, PL 87-100 Torun, Poland Received 2 February 1998; received in revised form 30 April 1998; accepted 15 May 1998
Abstract In this paper we compare the scintillator performance of the two of the leading contenders for Positron Emission Tomography, both cerium doped: Lu SiO (LSO) and LuAlO (LuAP). LSO crystals come in two categories, character2 5 3 ized by high (H) or low (L) light output. Such a distinction is not found in LuAP, but its output is consistently lower than that of LSO-H. Based on decays, thermoluminescent data and dependence of light output on temperature, we conclude that the principal reason for the superior properties of LSO-H is its virtual absence of shallow traps. LuAP also suffers from an additional handicap in the form of a parasitic absorption of unknown origin, which causes a light output dependence on the thickness of the sample. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 78.55.!m; 78.60.kn. Keywords: Scintillator; Detectors
1. Introduction During the past decade extensive effort has been directed at developing better scintillator materials for Positron Emission Tomography (PET). While there is no question that this effort has considerably advanced the understanding of the phenomenon of scintillation, the number of promising new materials has been disappointingly small. The reason for
* Corresponding author and present address: Chemistry Department, Boston University, 590 Commonwealth Ave., Boston MA 02215, USA. Tel.: #1 617 353 9581; fax: #1 617 353 6466; e-mail: [email protected].
this is that PET imposes a rather demanding combination of properties, including a density greater than 7 g/cm3, a scintillation decay time well below 100 ns and a light output substantially exceeding that of BGO (8500 photons/MeV). These requirements are peculiarly intertwined since they involve precise matching of the properties of host and activator. For instance, to achieve the necessary speed, the wavelength dependence of decay time virtually dictates that the emission be in the blue to near-UV; this requires host transparency in this optical region, which limits their choice to wide-bandgap insulators. The additional requirement of density narrows the choice even further: Since the stopping power for gamma rays
0168-9002/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 8 ) 0 0 6 8 9 - 5
334
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
depends upon the effective atomic number of the scintillator and the spatial resolution of PET limits the size of the detector elements, the new materials must include substantial proportions of heavy elements. The ideal host, however, must also incorporate an efficient luminescent center, either as a dopant or as a property of the lattice itself (i.e., BGO). Whatever the nature of the center, the optical transition has to be allowed (for high speed), and take place in the appropriate wavelength range. Furthermore, the energy structure of the center has to be such that nonradiative decays are minimized, requiring large energy gaps below the excited state. Of the very limited number of activator candidates, Ce3`, with its allowed interconfigurational (d to f) transitions and the 30 000 cm~1 energy gap below its lowest 5d state is unquestionably the best. It is not too surprising that the need to satisfy all these requirements leaves very few potential material systems, especially if one adds the “trivial” aspects of crystal growth, such as congruent melting and the absence of phase transitions. At the present time the top of the pyramid seems to be occupied by only two credible candidates, both of which contain Ce as activator: Lu SiO (LSO) and 2 5 LuAlO (LuAP). These two materials exhibit im3 portant differences in behavior, each with its own specific advantages and drawbacks. In particular, the best LSO to date has a superb light output of 27 300 photons/MeV, the highest of all known oxidic systems, and a fine single-exponential decay with a characteristic time of about 47.2 ns [1]. Unfortunately, despite extensive development efforts, it is not unusual to find crystals with substantial deficiencies in both light output and decay behavior. The less developed LuAP, on the other hand, shows a superb scintillation decay time of 18 ns (the fastest among known oxidic systems), but accompanied by significantly slower components, and its light output is currently only a third of that of the best LSO specimens [2,3]. In this paper we make an in-depth study of various relevant properties of the two materials in an attempt to achieve better understanding of the mechanism of scintillation, to explain the reasons for the disparate behavior and hopefully to ascertain the directions and extent to which improvements can be made.
2. Experimental 2.1. Materials The LSO crystals were obtained from C. Melcher, then at Schlumberger-Doll Research.1 These were grown by the Czochralski method from the melt containing 0.25 mol% Ce, in an atmosphere of argon with 3000 ppm oxygen. The two specimens were typical representatives of the so-called “good” and “poor” LSO as measured by their light output (L for low and H for high). No specific differences in their fabrication were known that could account for their differences in behavior, which must consequently be attributed to uncontrolled variations in the crystal growth. Indeed, although literature reports [4] suggest that crystals grown at a slower rate (0.5 mm/h) are better than those grown at 2 mm/h, for these particular specimens the opposite was true: the better one (H) was grown 3 mm/h, the poorer (L) at 2 mm/h. The LuAP crystals were provided by M. Randles of Litton-Airtron2 where they were pulled from the melt on an iridium wire, in a growth atmosphere of N . The Ce concentrations in the melt were 0.25% 2 and 0.75% with respect to Lu, but mass spectroscopic analysis found the actual concentrations in the crystal to be on the order of 20% of these nominal values. 2.2. Emission spectra Emission spectra of the two materials have been published previously [2,3,5]. We include them here to emphasize the point, which will be repeated again in other contexts, that the emission of Ce3` in LuAP (Fig. 1) is essentially the same independent of the quality of the crystal, the value of its light output and the concentration of Ce in the range between 0.25% and 2% nominal. In this host Lu, and consequently Ce, resides in only one type of crystallographic site. LSO, on the other hand, has two Lu (and consequently two Ce3`) sites, called
1 Schlumberger-Doll Research, Ridgefield CT. 2 Synoptics Div., Litton-Airtron Corp., Charlotte NC.
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 1. Emission spectra of LuAP : Ce at room and low temperature (4 K) when excited at 152 nm.
335
Fig. 3. Emission spectra of LSO-L at room and low temperature (4 K) when excited at 188 nm.
in the region between 280 and 330 nm. Emission spectra of undoped LuAP have been described previously [7], but undoped LSO was not available for this study. No significant energy transfer takes place from lattice (excitonic) emission to Ce, whose excitation is therefore carrier-mediated, a process faster and more efficient than any mediated by excitonic transfer. 2.3. Excitation spectra
Fig. 2. Emission spectra of LSO-H at room and low temperature (4 K) when excited at 188 nm.
C1 and C2, of which the former is spectrally dominant [6]. Spectra optically excited close to the band edge (188 nm) are shown in Figs. 2 and 3 for the H and L samples, respectively. The principal features (with peaks at 393 and 423 nm), due to Ce in C1 sites split by spin—orbit interaction in the ground state, are visible in both samples. The C2 emission, much weaker and not well resolved, occurs at 500 nm. In the L sample, at low temperature, we also note the presence of lattice emissions
The excitation spectrum of the LuAP emission at wavelengths shorter than 330 nm is shown in Fig. 4. Above 210 nm we discern the five crystal field components of the 5d configuration, split into groups of two and three. The region between 210 and 165 nm is featureless but certainly far above baseline (compare with the bottoming of the excitation at 250 nm). At 154 nm we see a prominent peak which is identified with band-to-band transitions and occurs at an energy where the absorption increases very rapidly [7]. Indeed, this increase, associated with the UV absorption edge, is so rapid that it distorts the excitation into the shape of a peak. The absorption is so intense that the actual excitation is confined very close to the surface and is therefore more subject to variations and nonlinearities than e—h generation in the bulk. Nevertheless, the existence of this feature does demonstrate
336
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 4. Excitation spectra of LuAP : Ce at two temperatures (RT and 180 K). Note the near disappearance of the 154 nm “band-to-band” peak at 180 K.
that optical electron—hole generation leads to Ce excitation. The magnitude of the 154 nm peak is very temperature-dependent. As shown in Fig. 4 it decreases rapidly below room temperature and is mostly gone by 180 K. If the magnitude of this peak were a measure of energy transfer to the activator, one would expect that below 180 K scintillation should cease, which however is not the case (see Section 2.6). We have to conclude that at low temperatures either other transfer mechanisms begin to dominate or that processes near the surface no longer reflect what takes place in the bulk. Excitation spectra of LSO (Fig. 5) show rather different behavior. The band-to-band peak is located at 188 nm (RT), indicating a smaller energy bandgap (6.6 eV) than in LuAP (8.2 eV). At low temperature, the peak (although reduced in relative intensity) is still present. Note, however, that the peak is much broader than in LuAP, with a decidedly asymmetric shape. Possibly the rise of the absorption edge is less steep than in LuAP and excitation reaches somewhat deeper below the surface. 2.4. Absorption and “self-absorption” spectra An unusual feature of the emission spectra of LuAP is its dependence upon the geometrical
Fig. 5. Excitation spectrum of cerium emission in LSO-H. Note the broad “band-to-band” peak below 200 nm and its incomplete disappearance at 4 K.
Fig. 6. Emission spectra of LuAP when excited with gamma particles, for three different orientations of the same sample with respect to the PMT.
arrangement of the sample and detector. A striking example is shown in Fig. 6, where the spectrum varies drastically depending on the orientation of the same sample with respect to the PMT. At first sight this appears to be a classic case of self-absorption because the orientation resulting in the longest light path in the crystal gives the narrowest peak. Another manifestation of the same effect is the dependence of the light output on the size of the
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
crystal. Although samples with linear dimensions much greater than the attenuation length (which at 0.5 MeV is on the order of 1 cm) have poorly developed photopeaks, they do show a definite decrease of light output as the sample becomes larger. Some rather striking results were reported earlier [8], in which a sample was excited with X-rays that penetrated a small fraction of the thickness, and the light output was observed from the opposite side. In this geometry the bulk of the crystal acted like a filter for light generated near the other surface. This too was very suggestive of self-absorption caused by the relatively small Stokes shift. For this interpretation to be correct, however, the decay time would also have to show a distinct dimensional dependence, growing increasingly longer as the path length increases [9]. Since this is most decidedly not the case, we must seek an alternative explanation, that in the region of LuAP emission there is a parasitic absorption due to some foreign species, not Ce itself. The critical question then becomes whether the parasitic absorption, which after all we see only in doped crystals, is a necessary consequence of that doping or an accident of fabrication. Fig. 7 gives the absorption spectra of LuAP. The undoped crystal does not indicate the presence of any defects in the visible region. The slight rise below 300 nm can be attributed to the expected scattering increase at shorter wavelengths. Note that in the region of 250 nm, this does not exceed 5—6 cm~1, a fact whose significance will soon become apparent. The absorption spectrum of doped LuAP, given also in Fig. 7, shows the first three components of the 5d state. To the extent that absorption processes are characteristic of an activator and therefore lead to emission, the absorption spectrum should be essentially identical to the excitation spectrum. This statement is exactly true only in the limit of small absorption or (equivalently) for measurements made on very thin samples. If, however, absorption is measured on thin samples and excitation on thick ones, as is usually the case, excitation features that are associated with particularly intense absorptions can be severely distorted in shape. Comparison of Ce-doped LuAP absorption (Fig. 7) and excitation (Fig. 4) spectra
337
Fig. 7. Absorption spectra of LuAP undoped and doped with 0.25% Ce (nominal).
gives a prime illustration of this effect. The pronounced increase on the shorter wavelength side of the absorption shoulder at 306 nm distorts that feature into an apparent peak in the excitation spectrum. These are common occurrences with no major ramifications at this point. There is, however, a very serious discrepancy at around 250 nm that cannot be attributed to such distortion. Note that below 270 nm a gap develops in the excitation spectrum, whose intensity drops essentially to zero at 250 nm. Such gaps are quite common between d orbitals, and are even utilized in the making of optical filters [10]. In Fig. 7, however, the corresponding dip in absorption remains well above the baseline, retaining significant residual intensity. Moreover, the general shape of the spectrum strongly suggests an underlying parasitic absorption slope extending through the entire near-UV region to as high as 400 nm, where it could intercept some of the emission. An analogous effect has been observed in YAlO : Ce by Russian workers 3 [11], who noted that the depth of the absorption valley at 250 nm is quite dependent on the method of crystal growth. They report that crystals grown by zone melting in vacuum have much less of this parasitic absorption than Czochralski-grown crystals. The Russian workers attributed the extraneous absorption to the presence of Ce4` with the absorption being a charge transfer process
338
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
(valence band to Ce4`). The stability of the Ce4` state has been called into question before [12], and furthermore if Ce4` ions were present as an equilibrium valence state, then post growth heat treatments in oxidizing and reducing atmospheres should alter its concentration. However, heat treatment of LuAP in vacuum, air, and Ar/H mixtures did not significantly change any of its properties. While the origin of the parasitic absorption remains speculative, we can obtain a rough measure of its dependence on Ce concentration by comparing its effect on the emission at various doping levels. In earlier work [8] we were able to extract this absorption from the distortion of emission spectra excited by X-rays. One can obtain similar results by simply changing the geometrical arrangement of the sample and detector as shown in Fig. 6. While these suggest that the magnitude of the absorption in our specimens does scale with the Ce content, the fact that it can be significantly altered in YAP by modification of the growth procedure leaves the possibility and extent of its removal in LuAP an open question.
Fig. 8. Comparison of room temperature decays of LuAP when excited optically (230 nm) or by gamma irradiation.
2.5. Decays Scintillation decays under gamma excitation are usually measured by the photon correlation technique [13]. Decay traces are then fitted to a sum of as many as three exponential terms, whose time constants and amplitudes are reported as the decays of fast, intermediate and long components. Obviously, components longer than the measurement range are simply submerged in the background signal. Another type of decay is obtained under optical excitation, utilizing pulses of light at energies less than the band gap. In the case of doped materials one excites the dopant ion directly (f to d transitions in Ce), without creating charge carriers in their respective bands. This type of decay is usually quite different from gamma excited decay, being single exponential and not exhibiting the long components. In this section we shall describe both types of decays in LuAP and LSO as well as their temperature dependence. Fig. 8 gives the optical and gamma excited decays of LuAP at room temperature. We see that
Fig. 9. Comparison of room temperature decays of LSO-H and LSO-L when excited by gamma irradiation.
while the initial decay constants are essentially equal, the gamma excited decay slows down due to longer components. The optically excited decay is strictly exponential, fitted by a single exponential function plus a constant (DC) background. The decays of LSO are significantly different. In Fig. 9 we show the gamma excited decays of the H and L specimens measured at RT. Note that both are single exponential fits, and neither case shows evidence of intermediate components. The
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 10. Gamma-excited decays of LuAP : Ce at three different temperatures below RT. Note increase of intermediate component at 180 K.
decay of LSO-L is faster, indicating some degree of nonradiative quench. The smaller amplitude is consistent with the lower light output of the sample. Figs. 10 and 11 show two sets of three gamma excited decays of LuAP measured over two temperature ranges, one below and one above room temperature. In each case the decay at the middle temperature (180 and 340 K, respectively) shows a distinct enhancement of the intermediate component, just following the initial fast decay characteristic of the Ce ion in this lattice. The meaning of the appearance of this intermediate component at particular temperatures will become clear when we discuss the thermoluminescence behavior. In a general discussion of scintillator properties it is often assumed that the quantum efficiency of the luminescing ion is essentially unity; i.e., that there are no significant losses attributable to radiationless transitions from the excited to the ground state of the ion. However there is always a temperature above which such losses will no longer be negligible, and the optical decay will become progressively faster. Figs. 12 and 13 show the temperature dependence of the decays in LSO and LuAP. In two of these cases (LuAP and LSO-H) the decays reported were obtained by gamma rather than optical excitation, since our equipment for the latter measurements could not reach the necessary
339
Fig. 11. Gamma excited decays of LuAP : Ce at three different temperatures above RT. Note increase of intermediate component at 340 K.
Fig. 12. Temperature dependence of the first component of decay and light output of both LSO-H and LSO-L. LSO-L decays were excited optically, LSO-H by gamma irradiation.
higher temperatures; in these particular cases, however, the decay time of the fast component of gamma-excited decay was essentially identical to that obtained optically, and their temperature dependence should be equivalent. Since the quantum efficiency Q of the emission is proportional to q, both remain constant up to a certain temperature (approx. 300 K for LSO and 600 K for LuAP), above which thermal quenching becomes important.
340
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 13. Temperature dependence of the light output of LuAP : Ce and the first component of gamma excited decay. Fig. 14. Thermoluminescence of LuAP : Ce, LSO-H and LSOL. Details of the major glow peaks are given in Table 1.
2.6. Thermoluminescence Thermoluminescence of scintillator materials at elevated temperatures has been reported in several studies [14,15]. In general, these studies reach only qualitative (and sometimes incorrect) conclusions, indicating that while a great abundance of traps does cause poorer scintillation, the effect is of no great consequence once the traps are filled [15]. While possible in principle, this assessment fails to take into account the great magnitude of the fluxes needed for trap saturation [18]. Thermoluminescence of both LuAP and LSO, resulting from excitation by ionizing radiation at room temperature followed by heating, has been published previously [16,17]. In both cases the thermoluminescent emission was observed to be that of Ce, which acts as a radiative recombination center for both the carriers initially created and those liberated from traps at a later time. This work indicates that the traps responsible for the glow peaks situated well above room temperature do compete with scintillation by diverting carriers into long-term storage [18]. More recently, we extended thermoluminescent studies to cover the low-temperature range, from 4 to 400 K, not previously performed on these materials. The samples were irradiated at 4 K with a UV source set at the wavelength of the respective band-to-band excitation peaks (153 nm for LuAP and 188 nm for LSO). Again only Ce emission appears in the thermoluminescence process. The
Table 1 Values of parameters for the major thermoluminescence glow peaks Crystal
¹ (K) .!9
E (eV)
LuAP : Ce
182 266 334 95 345
0.51 28.9 0.64 23.7 0.78 22.2 0.147 14.5 not fittable — distorted
LSO-H LSO-L
ln s (s~1)
q at RT (s) 106]10~6 2.85 2520 149]10~6
low-temperature glow curves for LuAP are given in Fig. 14. The glow curves were analyzed by the simple method of Randall and Wilkins [19,20] and the results for major glow peaks are given in Table 1. (We include in this table the average lifetime of the trap given by the Arrhenius equation 1/q"s exp(!E/k¹), where s is the frequency factor and E the depth of the trap.) The most prominent peak is clearly at 178 K with a smaller one at 270 K. All the other peaks are at least an order of magnitude smaller. The glow peaks of LSO-H and LSO-L in the same temperature range, given in Fig. 14 and listed in Table 1, as well, provide a striking contrast. Note first that in the temperature range investigated here, the highest temperature peak in both LSO specimens is +340 K, while in LuAP a peak situated at
341
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
approximately the same temperature is barely detectable (see Fig. 15). Note further that LSO-H appears to have hardly any low-temperature glow peaks at all, whereas LuAP and LSO-L have prominent peaks at 178 and 90 K, respectively. While it is very difficult to compare the intensity of the peaks in two materials in any quantitative manner, it is quite evident that LSO-H is unique, in the sense of showing almost no thermoluminescence below 300 K. The prominent peak in both samples of LSO just above room temperature is responsible for the well known afterglow, which necessitates mild heating prior to usage. The conclusion has to be that relatively shallow traps are far less numerous than in case of LuAP and LSO-L. 2.7. Light output and its temperature dependence Of the many criteria for judging the overall performance of a scintillator, the most critical is the light output (LO), which is the number of scintillation photons emitted per MeV of energy of the gamma photon stopped in the crystal. Since absolute measurements of this quantity are difficult to make and extremely sensitive to error, this evaluation is most reliably made by comparison with a standard material (such as BGO) whose own light output is known. The simplified procedure that we use is to take an energy spectrum of BGO and determine the position of its photopeak, and then do the same for the substance under investigation. As long as the means of collecting all the light generated in both BGO and the sample being studied are similar, and proper allowance is made for the difference in emission wavelengths and hence PMT sensitivity, we can take the ratio of photopeak positions as a measure of light output relative to BGO. From this ratio we extract numerical values, using the value of 7000 photons/MeV characteristic of 3.6 mm thick BGO [21]. Unfortunately, the light output for LuAP is not uniquely defined because of the “pseudo-self-absorption” effect described in Section 2.4. This makes it necessary to state the dimensions of the specimen and to perform the measurement against a similarly shaped sample of BGO. Table 2 compares the LO measured by
Table 2 Light output of LuAP and LSO Material
Thickness (mm)
Photons/ Reference MeV
BGO LuAP : Ce 0.75% LuAP : Ce 0.75% LSO : Ce 0.22% LSO-H : Ce 0.25% LSO-L : Ce 0.25%
1 1 1.6 14.5 1 1.9
8 060 11 300 11 800 27 300 24 800 725
Moszynski [19] Moszynski [19] Present work Moszynski [19] Present work Dorenbos [3]
Moszynski et al. [21] with recent ones performed in our laboratory. It should be noted that the measurements also depend on the width of the gated detection window (in our case, typically 2.3]500 ns of shaping time). Since this is finite, any light generated by a scintillation event that appears after the measurement window has closed will escape detection, and thus will not contribute to the light output. This is a very important point, since it is essential to the understanding of the connection between LO and decay time: In the absence of nonradiative losses, whatever accelerates the escape of carriers from longlived traps will increase the fraction of detectable light, and hence the measured LO, while any increase in the probability of trapping will do the opposite. And the magnitude of the effect is not absolute, but is inextricably tied to the width of the measurement window. A detailed mathematical treatment of the kinetics is beyond the scope of this paper, and has been provided elsewhere [22—24]. Examination of these results reveals two important points. One is that, for as yet unknown reasons, the light output of LuAP at the present stage of its development is substantially lower than can be achieved from LSO. No less perplexing, however, is the fact that the light output of LSO itself is subject to great variation from one specimen to another, despite the absence of any detectable difference in its chemical make-up. Both these points will receive considerable attention in the discussion to follow. Using such measurements we can also determine the temperature dependence of the light output. The simplest case is that of LSO-H, given in
342
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
Fig. 12, whose q curve we have already discussed. We see that the LO curve tracks the q curve over the entire temperature range. This means that the efficiency of scintillation depends on temperature as strictly determined by the quantum efficiency of Ce luminescence, with all other processes such as conversion and transfer (see also Section 2.4) being essentially independent of temperature. This result is to be contrasted with those from LSO-L and LuAP. In the former case (Fig. 12) the decay time has a temperature dependence similar to that of LSO-H, i.e. roughly constant up to 300 K. However, the temperature dependence of LO no longer tracks that of the decay, but shows its own unique pattern, with a minimum at 84 K and a maximum at 191 K. An even more striking example is provided by LuAP (Fig. 13). Here again the curve of LO vs. temperature shows pronounced structure, although we would not expect any temperature dependence of the optical decay time q, right up to the thermal quenching region.
3. Discussion One of our primary motivations for the present work has been to quantify and explain the differences in light output exhibited by the materials in question. For this we must turn first to the basic principles of the scintillation process. The mechanism of scintillation is described by the well-known phenomenological formula for the efficiency [25]: g"b]S]Q,
(1)
where b is the conversion efficiency (ratio of the number of electron—hole pairs actually produced to the maximum number possible), S the transfer efficiency (fraction of the produced electron—hole pairs whose energy actually excites the luminescent centers) and Q the luminescence quantum efficiency of those centers. Recent work has shown that the conversion efficiency, which has a simple relationship to the bandgap of the host material [26], can be quite high, leaving only the transfer as the major source of loss. Let us examine the implications of this hypothesis. In Table 3 we reproduce a few entries from
Table 3 Calculated creation of e—h pairs and observed light output (from Ref. [24]) Material
Calculated e—h pairs per MeV
Observed Efficiency light output g per MeV
CsI : Tl NaI : Tl BGO Lu SiO : Ce (LSO) 2 5 LuAlO : Ce (LuAP) 3
69 444 75 330 88 889 69 444 55 556
65 000 38 000 8 500 27 300 11 300
0.936 0.504 0.096 0.393 0.20
Ref. [26]. We note first that if S and Q were equal to unity, then the number of photons (LO) generated per MeV would be equal to the number of e—h pairs. This is evidently not the case, except for CsI : Tl whose LO is close to theoretical. For LuAP and LSO we have shown that over a broad temperature range the radiative decay time does not change, and therefore Q is essentially unity, thus identifying an inadequacy in transfer efficiency (S(1) as the factor primarily responsible for the lower than possible LO. From this we must infer that a substantial degree of nonradiative recombination must be taking place. The potential for improvement is quite substantial, 370% in LuAP and 250% in LSO. The extent to which such improvement is actually possible depends on the correctness of the calculated conversion and the absence of loss introduced by extrinsic factors. The remarkable agreement between theory and experiment in the case of CsI : Tl may perhaps provide some clues. It is now generally accepted that the principal mechanism of electron—hole recombination in Cedoped scintillators is through the Ce ion [12,23], although the extent of competing recombination mechanisms is presently unknown. However, at any temperature there is a finite probability of one or both carriers being trapped elsewhere. This necessarily introduces a loss, since such carriers are effectively unavailable for the generation of scintillator light. This loss would no longer be present if the traps were completely saturated, but depending on their concentration the material may become unstable. Experiments performed by Bartram et al.
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
[18] indicated for LuAP a loss of some 15% due to high-temperature traps. Recent work by Wojtowicz et al. [23] expanded consideration to include lowtemperature traps as well, approximately doubling this figure. Thus, a 30% improvement in LO can, in principle, be expected for trap-free LuAP, bringing it to a respectable 15 000 photons/MeV, for thin crystals. This is still a long way from the number predicted from the conversion value [26]. The effects of traps are not, however, limited to the light output alone. While the deep traps that give rise to high-temperature thermoluminescence may act merely as passive sinks for mobile carriers, the shallow traps associated with cryogenic thermoluminescence may become active participants in the kinetic processes. The accepted mechanism of scintillation, at least for Ce-doped materials, is radiative recombination in which Ce3` first captures a hole to become Ce4`, which upon subsequent capture of an electron returns to Ce3` but in an excited state [12]. Emission of a photon returns the system to its ground state. The principal competing processes are nonradiative recombination of e—h pairs via some deep centers and trapping. All three processes are illustrated in Fig. 15. It should be noted that as long as trapped carriers are eventually returned to their respective bands there is no loss of total light output, although some of this is diverted into the long components of decay. However for any trap depth and temperature there will be a population of occupied traps, depending on previous thermal and excitation history of the sample. Thus, over the time scale useful for scintillator measurements, traps tend to reduce the number of e—h pairs available for radiative recombination. Deep traps can hold carriers for very long times and produce a steady afterglow. The large glow peak in LSO just around room temperature is responsible for its well-known afterglow, which can only be destroyed by moderate heating. Direct evidence of the influence of traps on decay is given by Figs. 10 and 11. Note that in both cases, at temperatures of 180 and 340 K, a definite enhancement of intermediate components appears to take place, closely coinciding with the positions of the LuAP glow peaks. Above and below these temperatures the corresponding traps are relatively inert, since the retention time for the carriers
343
Fig. 15. Radiative and non-radiative (NR) recombination and trapping processes of electrons and holes. The radiative mechanism takes place through Ce emission.
becomes either too short or too long to have much effect on the scintillation kinetics. Another aspect of this link between low-temperature traps and scintillation is, of course, the clean single-exponential decay of trap-free LSO-H (see Fig. 12). Since the quantum efficiency of luminescence is essentially constant at temperatures below the onset of thermal quenching, the variations of LO encountered in LuAP and LSO-L must represent a complex temperature dependence of S (Eq. (1)). This could arise because each shallow trap introduces its own trapping and detrapping probabilities, both of which are Arrhenius-type functions of temperature [22—24]. Thus it is clear that not only shallow traps give rise to the complex temperature variation of S, but it is precisely their absence that is the reason for the unique behavior of LSOH. It is also evident that while high-temperature traps may reduce LO by diverting and holding carriers, they are stable enough to leave the decay unaffected. The evident mischief caused by low temperature traps adds to the difficult task of developing better scintillators. At the present time nothing is known about their nature. Dorenbos et al. [14] speculated that in LSO at least one of the traps is spatially associated with Ce. This was based on their observation of high-temperature thermoluminescence subsequent to excitation in the absorption bands of Ce. We have not been able to fully confirm this result; although excitation in Ce levels lying close to
344
A. Lempicki, J. Glodo/Nucl. Instr. and Meth. in Phys. Res. A 416 (1998) 333—344
the conduction band does indeed produce a weak TL, excitation at 350 nm does not. We ascribe the observation of this thermoluminescence not to a trap directly accessible from an excited Ce ion, but to photoionization from the higher excited states to the conduction band. The factors that reduce the light output of LuAP, the parasitic absorption and the shallow traps, may even be related. While the nature of the parasitic center has remained elusive, at least one kind of trap capable of such UV absorption would be the presence of a stable Ce2` state in the gap. Such a state could act both as an electron trap and also as the terminal state of a charge transfer transition. The presence of such an entity could readily be related to the method of crystal fabrication, which may be our only recourse for its elimination. For instance, it would be of great interest to compare the thermoluminescence and LO temperature dependence in LuAP crystals grown by different methods [27]. No matter how daunting, however, it is becoming abundantly clear that the tasks of identifying the nature of the traps and any competing nonradiative recombination centers, and the means for their elimination, must now be considered an essential part of scintillator science.
Acknowledgements The authors gratefully acknowledge the support of this research by the US Department of Energy, under DoE Grant DE-FG-02-90ER61033. We are also greatly indebted to Profs. C. Brecher and A. J. Wojtowicz for many helpful discussions and critical reading of the manuscript; to D. Wisniewski, who provided some of the spectroscopic data; and to W. Drozdowski for analysis of the glow curves.
References [1] M. Moszynski, M. Kapusta, D. Wolski, M. Szawlowski, W. Klamra, IEEE Trans. Nucl. Sci. 44 (1997) 436. [2] A. Lempicki, M.H. Randles, D. Wisniewski, M. Balcerzyk, C. Brecher, A.J. Wojtowicz, IEEE Trans. Nucl. Sci. NS-42 (1995) 280. [3] A. Lempicki, C. Brecher, D. Wisniewski, E. Zych, A.J. Wojtowicz, IEEE Trans. Nucl. Sci. NS-43 (1996) 1316.
[4] P. Dorenbos, C.W.E. van Eijk, A.J.J. Bos, C.L. Melcher, J. Lumin. 60/61 (1994) 979. [5] C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. NS39 (1992) 502. [6] H. Suzuki, T.A. Tombrello, C.L. Melcher, J.S. Schweitzer, IEEE Trans. Nucl. Sci. NS-40 (1993) 380. [7] D. Wisniewski, A.J. Wojtowicz, A. Lempicki, J. Lumin. 72/74 (1997) 789. [8] A. Lempicki, C. Brecher, D. Wisniewski, E. Zych, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT95, Delft University Press, Delft, the Netherlands, 1996, p. 340. [9] A.J. Wojtowicz, M. Balcerzyk, E. Berman, A. Lempicki, Phys. Rev. B 49 (1994) 4880. [10] L.R. Elias, R. Flach, W.M. Yen, Appl. Opt. 12 (1973) 138. [11] E.G. Gumanskaya, O.A. Egorcheva, M.V. Korzhik, S.A. Smirnova, E.B. Pavlenko, Opt. Spectrosc. 72 (1992) 215. [12] A.J. Wojtowicz, A. Lempicki, D. Wisniewski, M. Balcerzyk, C. Brecher, IEEE Trans. Nucl. Sci. NS-43 (1996) 2168. [13] L.M. Bollinger, G.E. Thomas, Rev. Sci. Instr. 32 (1961) 1044. [14] P. Dorenbos, C.W.E. van Eijk, A.J.J. Bos, C.L. Melcher, J. Phys.: Condens. Matter 6 (1994) 4167. [15] R. Visser, C.L. Melcher, J.S. Schweitzer, H. Suzuki, T.A. Tombrello, IEEE Trans. Nucl. Sci. NS-41 (1994) 689. [16] D. Wisniewski, W. Drozdowski, A.J. Wojtowicz, A. Lempicki, P. Dorenbos, J.T.M. de Haas, C.W.E. van Eijk, A.J.J. Bos, Acta Physica Polonica A 90 (1996) 377. [17] W. Drozdowski, D. Wisniewski, A.J. Wojtowicz, A. Lempicki, P. Dorenbos, J.T.M. de Haas, C.W.E. van Eijk, A.J.J. Bos, J. Lumin. 72/74 (1997) 756. [18] R.H. Bartram, D.S. Hamilton, L.A. Kappers, A. Lempicki, J. Lumin. 75 (1997) 183. [19] J.T. Randall, M.H.F. Wilkins, Proc. Roy. Soc. 184 (1945) 366. [20] J.T. Randall, M.H.F. Wilkins, Proc. Roy. Soc. 184 (1945) 390. [21] M. Moszynski, M. Kapusta, M. Mayhugh, D. Wolski, S.O. Flyckt, IEEE Trans. Nucl. Sci. NS-44 (1997) 1052. [22] A. Lempicki, R.H. Bartram, effect of shallow traps on scintillation, J. Lumin. to be published. [23] A.J. Wojtowicz, J. Glodo, A. Lempicki, C. Brecher, Recombination and scintillation processes in YAlO : Ce, J. Phys: 3 Condens. Matter, to be published. [24] A.J. Wojtowicz, J. Glodo, W. Drozdowski, K.R. Przegietka, Electron traps and scintillation mechanism in YAlO : Ce and LuAlO : Ce scintillators, J. Lumin., to be 3 3 published. [25] A. Lempicki, A.J. Wojtowicz, E. Berman, Nucl. Instr. and Meth. A 333 (1993) 304. [26] R.H. Bartram, A. Lempicki, J. Lumin. 72/74 (1997) 734. [27] A.G. Petrosyan, C. Pedrini, in: P. Dorenbos, C.W.E. van Eijk (Eds.), Proc. Int. Conf. on Inorganic Scintillators and their Applications, SCINT95, Delft University Press, Delft, the Netherlands, 1996, p. 498.