Defect-Driven Radioluminescence Sensitization in Scintillators: The

Article pubs.acs.org/JPCC

Defect-Driven Radioluminescence Sensitization in Scintillators: The Case of Lu2Si2O7:Pr Elisa Dell’Orto,† Mauro Fasoli,† Guohao Ren,‡ and Anna Vedda*,† †

Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, Via Cozzi 53, 20125 Milano, Italy Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, China

‡

ABSTRACT: A prompt transport of radiation induced free carriers toward luminescence centers is a key factor for an efficient conversion of high energy radiation into light in scintillator materials. However, the transport stage of the scintillation process can be hampered by the presence of lattice imperfections. In this study, we investigated the increase of radioluminescence (RL) efficiency after prolonged material irradiation. The general character of the phenomenon was revealed by its occurrence in several scintillator crystals like Lu2Si2O7:Pr, Bi3Ge4O12, Lu3Al5O12:Ce, and Lu0.3Y0.7AlO3:Ce. A detailed investigation was carried out for Lu2Si2O7:Pr. We demonstrated that the RL efficiency increase is due to the progressive filling of traps responsible for the thermoluminescence glow peaks at 460 and 515 K, which compete with Pr3+ centers in free carrier trapping during irradiation. Correspondingly, their emptying results in the recovery of the initial lowest RL efficiency. Spatial correlation between traps and Pr3+ ions was evidenced by the detection of an a-thermal tunneling afterglow between trap levels and 4f excited states of Pr3+. Such hysteresis phenomenon represents a memory of previous irradiations that remains stored in the crystal. It is a further effect caused by traps, which deserves attention like other manifestations of defects in scintillators like the reduction of light yield and the occurrence of slow tails in the scintillation time decay.

1. INTRODUCTION Scintillators are optical materials characterized by high luminescence efficiency under irradiation with ionizing radiation. Their ability to convert high energy photons or particles into ultraviolet−visible photons allows the detection of ionizing fields by optical systems.1 Scintillation originates from radiative transitions of intrinsic centers or dopants used as activators, the most common of which are rare earths or (ns2) ions.2 Numerous crystalline systems are being investigated whose variety can be even significantly increased by composition engineering strategies recently under development.3,4 Their applications extend in several fields like, for example, high energy physics, medical diagnostic, industrial controls, and security. Moreover, scintillators are also being used for real-time dosimetry especially in medicine.5 In addition to the luminescence efficiency, the main characteristics of a good scintillator are also a high density, fast scintillation decay time, energy resolution, and radiation hardness. Radiation hardness is the ability of a scintillator to maintain its optical response invariant after irradiation with ionizing radiation. Constancy of the response regards both luminescence efficiency and time decay since a variation of these properties upon absorbed dose may seriously affect the use of a material in a radiation detector. Radiation damage appears frequently as a reduction of light yield. In this case it is caused by the formation of radiation-induced defects that absorb © 2013 American Chemical Society

scintillation light and reduce the optical transmittance and the light output, or act as trapping centers for free carriers.1,6 However, an increase of the optical response of some materials due to prolonged radiation dose exposure can also occur. Radiation induced efficiency increase is observed in several luminescence processes and is sometimes exploited also for dosimetry applications. The physical mechanisms underlying such effects are related to an interplay between different defects and are often unclear. One example is the “pre-dose” effect, observed in crystalline quartz and exploited for dating and retrospective dosimetry. It consists in an increase of thermally stimulated luminescence (TSL) sensitivity after irradiation and heating treatments, and its microscopic origin is a matter of debate.7,8 In the field of scintillators, the increase of light yield in relation to irradiation history is known as hysteresis or “bright burn.” In medical imaging it is particularly detrimental because it causes the superimposition of ghost images to the real one.9 Until now, the microscopic origin of bright burn was investigated in just few materials. One of these is thalliumdoped cesium iodide (CsI:Tl), a well-known scintillator for Xray imaging application,10,11 that presents light yield dependReceived: July 22, 2013 Revised: September 6, 2013 Published: September 9, 2013 20201

dx.doi.org/10.1021/jp407248q | J. Phys. Chem. C 2013, 117, 20201−20208

The Journal of Physical Chemistry C

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method at IMRAM, Tohoku University (Japan), with a Ce concentration of 0.7% in the melt; (c) LuYAP:Ce, grown by Crytur (Czech Republic), using the Czochralski method and with a Ce concentration in the crystal of 0.19%. Thermally stimulated luminescence (TSL) measurements were performed from room temperature (RT) up to 750 K with a linear heating rate of 1 K/s, after RT X-ray irradiation by a Machlett OEG 50 X-ray tube operated at 20 kV. The emitted light was detected as a function of temperature by an EMI 9635QB photomultiplier tube (with quantum efficiency peaking at 380 nm) in photon counting mode. RL measurements were performed at RT using a homemade apparatus. Irradiation was made by a Philips 2274 X-ray tube operated at 20 kV and 5 mA. RL detection was carried out by a Jobin-Yvon Spectrum One 3000 CCD detector operating in the 200−1100 nm interval, coupled to a Jobin-Yvon Triax 180 monochromator with a 100 grooves/mm grating (the spectral resolution was 13 nm). For selected measurements, a 300 grooves/mm grating was used (in this case the spectral resolution was 1.5 nm). For spectra integrations, the following wavelength regions corresponding to dopant emission ranges were considered: 420−650 nm for LuAG:Ce; 350−750 nm for BGO; 300−450 nm for LuYAP:Ce; 230−350 nm and 570− 1000 nm for 5d−4f and 4f−4f transitions in LPS:Pr, respectively. The RL efficiency was defined, in an operative way, as the RL signal obtained for an irradiation lasting 9 s, corresponding to a dose of 0.3 Gy. The dose was evaluated in air. To analyze the increase of RL efficiency due to irradiation, 180 similar consecutive measurements were performed, and the accumulated dose corresponding to each measurement was taken as the sum of all previous irradiations. To obtain RL measurements after heating steps (Figure 4 of section 3.2), the sample was heated after irradiation with a heating rate of 4 K/s. Then, in order to test the RL efficiency, one RL measurement was executed with an integration time of just 1 s, to avoid additional sensitization of the sample. For the investigation of the tunneling amplitude dependence on heating temperature (Figure 5), the sample was first irradiated by X-rays, then it was heated at different temperatures (Ti) with a heating rate of 4 K/ s, and after cooling to RT a TSL measurement was performed with the usual heating rate of 1 K/s. In this sequence, each measurement started exactly 390 s after X-ray irradiation. Afterglow measurements reported in Figure 3b were performed immediately after an X-ray irradiation of 54 Gy, by detecting the emitted light as a function of time with the same detection system used for RL. Each data point represents the integral of a CCD spectrum (collected for 9 s) from 230 to 350 nm and from 570 to 1000 nm for the 5d−4f and 4f−4f transitions, respectively.

ence on irradiation history. Wieczorek et al. studied hysteresis in this material:12 they observed that the light yield increase shows a saturation behavior versus dose, and they suggested a role of defects trapping carriers during irradiation. In their interpretation, under radiation exposure a progressive filling of deep traps occurs, causing their neutralization as competitors with the luminescent recombination centers in free carrier capture. The result is an increase of light yield during irradiation. More recently Nagarkar et al. proposed to reduce the hysteresis effect by codoping the crystal with Sm2+,13 while Snoeren et al. proposed a method based on traps filling by UV irradiation.14 The bright burn phenomenon was also observed in Al2O3:C, that is of particular interest in medical application for optical stimulated luminescence (OSL) dosimetry.15 Some further interesting results were very recently obtained in Lu1.8Y0.2SiO5:Ce (LYSO:Ce), also codoped with Ca2+: emission spectra modifications and light yield increase following prolonged X-ray irradiation were found to occur and were related to the progressive filling of oxygen vacancies located close to Ce3+ ions and competing with them in free charge carrier capture.16 In more detail, the phenomenon can have a temporary or a permanent character, according to the temporal stability of traps responsible for it. A temporary effect is produced by rather shallow traps, which are emptied in a relatively short time and give rise to an afterglow signal. An example is the case of radioluminescence (RL) from Ce-doped scintillating fibers.17 However, if traps are very deep and stable, no afterglow can be observed in the laboratory measurement time scale, but their filling gives rise to a permanent increase of the RL efficiency, due to the progressive reduction of trapping probability for free carriers with respect to radiative recombination. In this case, the phenomenon produces a memory of previous irradiations, which remains stored in the material. RL efficiency increase due to very deep traps was observed in Tb-doped silica.18 In spite of the importance that this phenomenon assumes in modern scintillators applications, especially for X-ray imaging and real time dosimetry, until now it was not extensively investigated. In this work, we first make a survey of the room temperature RL modifications under prolonged X-ray irradiation of well-known scintillator crystals like Lu2Si2O7:Pr (LPS:Pr), Bi3Ge4O12 (BGO), Lu3Al5O12:Ce (LuAG:Ce), and Lu0.3Y0.7AlO3:Ce (LuYAP:Ce). Then, we focus on the peculiar behavior of LPS:Pr. Our choice is motivated by the fact that this scintillator is presently widely studied due to its very promising luminescence features when doped with luminescent centers like Ce3+ and Pr3+.19−24 Moreover, in accordance with previous preliminary findings obtained at low temperature,25 it displays an important hysteresis effect, which is the strongest among those displayed by the crystals here investigated; therefore, the study of its hysteresis properties is of interest both for the fundamental comprehension of the phenomenon and for its impact on the future application potential of the material.

3. RESULTS AND DISCUSSION 3.1. RL Hysteresis in Different Scintillators. Sequences of repeated RL efficiency measurements as a function of X-ray accumulated dose were performed for all crystals, and the results are reported in Figure 1. Curves are normalized to the respective initial value. After the highest imparted dose corresponding to 54 Gy, BGO and LuYAP:Ce display an RL efficiency increase of about 1%. While for BGO the RL efficiency has a fast increase at the beginning of irradiation and then it remains stable, LuYAP:Ce shows a rather linear RL intensity increase. LuAG:Ce sample displays an increase of RL efficiency of about 30%, with a trend suggesting that the process approaches a saturation state. Unlike other samples,

2. EXPERIMENTAL METHODS The LPS:Pr3+ single crystal was grown at the Shanghai Institute of Ceramics of the Chinese Academy of Sciences (SICCAS, Shanghai, China) by the Czochralski method using an Ir crucible. The Pr3+ concentration in the melt was 0.5%.26 Samples measured for comparisons were (a) BGO, grown by Bridgman technique at the Shonan Institute of Technology (Japan); (b) LuAG:Ce, grown by the micropulling-down 20202



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Figure 2. Room temperature RL emission spectrum of LPS:Pr3+ obtained under 20 kV X-ray irradiation.

Figure 1. (a) Room temperature RL efficiency of all samples versus irradiation dose. Data are normalized to the respective initial value. Panels b and c are enlargements showing the curves of BGO, LuYAP:Ce, and LuAG:Ce. BGO and LuYAP:Ce samples show an increase of about 1%. The increase is of about 30% for LuAG:Ce and of almost a factor of 14 for LPS:Pr.

LPS:Pr exhibits a much more remarkable RL increase, of about a factor of 14 by integrating RL spectra on both the 5d−4f and 4f−4f emission ranges. Also in this case the curve exhibits a saturation behavior. Such a comparison demonstrates that the enhancement of RL efficiency as a function of irradiation is a common characteristic of several scintillators, whose dose dependence and overall relevance are peculiar of each material and might reflect the different concentrations and stabilities and capture cross-sections of carrier traps as well as their spatial correlation with the recombination centers. In the following, we focus our attention on LPS:Pr, that exhibits the strongest hysteresis effect. 3.2. Role of Traps in the RL Hysteresis of LPS:Pr. The RL spectrum of LPS:Pr is reported in Figure 2, in which both the 5d−4f and the much weaker 4f−4f emission transitions of Pr3+ are displayed. Only a few lines (peaking at 627, 654, 882, 893, and 897 nm) could not be identified with Pr3+ transitions and are probably due to other rare earth ions present as trace impurities. The contributions to RL efficiency increase from both kinds of transitions were investigated. Figure 3a displays an example of RL efficiency versus irradiation dose, split into the two parts due to 5d−4f and 4f− 4f transitions respectively. Two subsequent RL measurement sequences are reported. After the first one, a TSL measurement was performed to empty traps. A complete recovery is evidenced by trap emptying since it was verified that the RL response was restored to its initial and lowest value; the process could be repeated obtaining the same result (measurement no. 2). Considering a set of 11 independent measurements, it

Figure 3. (a) Room temperature RL efficiency of LPS:Pr versus irradiation dose for 5d−4f and 4f−4f Pr3+ emission regions. Two RL measurements are reported. Data are normalized to the initial values. The points report efficiency tests after 1 h and 3 days following the measurement sequence. (b) Time t = 0 corresponds to irradiation stop. Afterglow is evident in the 4f−4f emission region. The inset shows that afterglow follows a t−1 law, testifying that it is due to tunneling recombination from electrons freed from spatially distributed traps to Pr3+ recombination centers.

turned out that the 5d−4f component shows an increase of a factor of 14.3 ± 0.5, while the 4f−4f one has a lower increase, namely, of a factor of 3.4 ± 0.3. This suggests that traps affect more the radiative recombination from the 5d lowest excited level with respect to that by 4f levels. Moreover, the data points in the graph represent the RL sensitivities measured 1 h and 3 days after the end of the irradiation sequence, during which the crystal was held at RT. The values coincide and show a 20203



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allow to interpret the RL radiation-induced efficiency increase as due to the progressive filling of traps responsible for the 460 and 515 K TSL peaks, which play the role of competitors with Pr3+ centers in free carrier trapping during irradiation. Correspondingly, their emptying results in the recovery of the initial RL efficiency. The long RT decay times of such traps are in accordance with the RT stability of the hysteresis phenomenon. However, no influence on RL is evidenced for the TSL peak occurring at about 570 K. Apart its weaker intensity with respect to the other ones, we remind that wavelength resolved TSL measurements revealed that in this case the spectral composition of the emitted light corresponds to a defect related band peaking at about 400 nm, while no emission from Pr3+ was evidenced.28 Therefore, a specific spatial correlation between the trap responsible for the 570 K peak and a recombination center of defect origin can be suggested, making the competition process between these traps and Pr3+ centers of poor relevance. 3.3. A-Thermal Tunneling Emission and RL Hysteresis. In the previous section, the evidence for the occurrence of athermal tunneling from traps to 4f levels of Pr3+ ions was given by the observation of an afterglow signal after the end of irradiation related only to the 4f−4f transitions, featuring a hyperbolic time dependence. The tunneling probability between a trap and a recombination center is governed by the transmission coefficient of the potential barrier between them; accordingly, it should follow an exponential dependence upon distance and give rise also to an exponential afterglow time decay. However, it was demonstrated that, if a random distribution of trap-center couples exists, the sum of their different exponential tunneling processes produces a hyperbolic decay law.27 It must be noted that in principle also a continuous distribution of trap depths would give rise to the same hyperbolic decay law.29 However, such continuous distributions are characteristic of disordered materials and are characterized also by anomalously broad glow peaks; in our case, the good crystalline quality of the material, the presence of discrete trap levels, and their stability at RT revealed by previous TSL studies25,28 allow to rule out this possibility. In the following, we better investigate the phenomenon and discuss its influence on the hysteresis effect. The presence of an a-thermal afterglow contribution in TSL glow curves was investigated. To reveal whether tunneling recombination could be originated by deep traps, we collected glow curves after 200 Gy irradiation and heating up to progressively higher temperatures after which the complete glow curve was measured to empty all traps (partial cleaning of glow curves). The procedure was similar to that adopted for RL hysteresis (Figure 4) and care was taken to begin each TSL measurement exactly after the same time delay following irradiation (390 s). Indeed, it was not possible to acquire the whole TSL curves because of signal saturation occurring above 350 K due to the high dose employed. The results are reported in Figure 5. The first part of each curve is nearly flat and it is clearly related to tunneling from deep traps, which is temperature independent. The weakness of the afterglow signal with respect to TSL peaks is due to a low tunneling contribution in our experimental procedure, as testified by the similar intensities of glow curves measured 60 s after irradiation (the minimum possible time interval between irradiation and TSL measurement) and after the time delay of 390 s used in the sequence of

decrease of RL relative efficiency of about 5% occurring only within the first hour; this result demonstrates the long-term stability of the effect at RT. Figure 3b displays an afterglow measurement carried out after irradiation interruption. Emissions related to the 5d−4f transitions do not manifest afterglow, while those related to the 4f−4f transitions exhibit a clear afterglow signal. The inset shows that the afterglow features a hyperbolic decay as a function of time, that suggests the occurrence of an a-thermal tunneling recombination of electrons from randomly distributed traps to 4f levels of closely lying Pr3+ recombination centers.27 Therefore, at RT afterglow does not originate from thermal release of trapped carriers since in that case an exponential decay should be observed. The presence of tunneling recombination reveals a spatial correlation between traps and Pr3+ centers. The presence of afterglow and its relation with hysteresis will be discussed in the next section. To relate the hysteresis phenomenon to traps, RL measurements were executed after several heating steps and compared to the TSL glow curve. First, the LPS:Pr sample received a 54 Gy X irradiation (corresponding to the maximum dose accumulated in the RL sequences); then, it was heated with a heating rate of 4 K/s at different temperatures (Ti) varying from 373 K until 743 K and then immediately cooled to RT. After each step the RL efficiency was evaluated. In Figure 4,

Figure 4. Room temperature RL efficiency of LPS:Pr after different heating steps performed with a heating rate of 4 K/s, normalized to the minimum. After each step, the sample was brought to RT and its RL efficiency was measured. The TSL glow curve obtained after RT 4 mGy X-ray irradiation is also superimposed. The initial efficiency is restored by the emptying of the TSL structure at 475 K. Dotted lines are guides for eyes.

relative RL efficiencies versus Ti are superimposed to the TSL glow curve obtained after X irradiation. The dependencies of 5d−4f and 4f−4f emissions are reported separately. Above Ti = 373 K, we observed a progressive decrease of both curves until Ti = 573 K, where the RL efficiency was restored to its initial value. The temperature interval in which efficiency recovery is observed corresponds to that of the main TSL structure of the crystal, peaking at about 475 K. Actually, this is composed of two distinct peaks, one at 460 K and another less intense at 515 K, whose thermal energies were found to be 1.27 and 1.10 eV, respectively, corresponding to RT decay times both of the order of 104 days.28 Recombination of electrons freed from such traps was found to occur at Pr3+ centers, with a major contribution coming from 5d−4f transitions.28 These results 20204



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Figure 5. TSL glow curves performed after heating procedures at different Ti reported in the box. The constant amplitudes at the beginning of the curves are the evidence of the a-thermal tunneling emission, which gradually decreases and totally disappears after heating at 573 K.

Figure 6. Tunneling emission amplitudes after different partial cleaning temperatures (blue filled squares). The TSL glow curve obtained after RT 4 mGy X-ray irradiation is also superimposed. Tunneling emission disappears when the TSL structure peaking at 475 K is completely empty. The continuous red line represents the numerical fit obtained using eq 4. The green dashed line represents the fit obtained using eq 5, while the black dash−dot lines represent the individual components.

Figure 5 (data not shown). Moreover, it can be considered that a portion of the tunneling signal, coming from traps lying very closely to Pr, is quickly emitted during the very early stages after irradiation (see Figure 3b) and could not be monitored in a TSL measurement, which starts at least after 60 s. Additionally, the weakness of tunneling emission could be due to the fact that the TSL signal is dominated by the principal 5d−4f emission better matching the efficiency of the photomultiplier used. The lack of observation of a hyperbolic decay could be due to the relatively small time window available for tunneling emission detection during the TSL measurement (approximately 50 s prior to that T is high enough for thermally activated processes to dominate the measurement) and to some superposition of the afterglow signal to the TSL one increasing with temperature; moreover, also a slight increase of tunneling probability by increasing temperature could be suggested, possibly related to a better matching between tunneling levels thanks to phonon absorption by trapped carriers. Interestingly, by increasing Ti we observed a decrease of the tunneling emission amplitude, which disappeared completely after heating at 573 K. To interpret such finding, we suggest that carriers in traps responsible for TSL peaks at 460 and 515 K possess two distinct emptying mechanisms, namely, the classical thermal release to the conduction band and an a-thermal process due to tunneling toward 4f levels of Pr. In Figure 6, the tunneling amplitudes, taken from the flat part of the glow curves in Figure 5, are plotted versus heating temperature and superimposed to the glow curve. The graph is very similar to that obtained in Figure 4 and points out that it is possible to relate also the tunneling emission to the main TSL structure. Therefore, the traps that compete with luminescent centers in capturing charges are the same ones accountable for tunneling. In this framework, a simple numerical analysis of tunneling amplitudes versus heating temperature was attempted. After an initial transient time, the tunneling emission amplitude can be written as27

I (t ) =

Cn0 t

An assumption related to the heating cycles was made. In fact, the procedure implies a monotonic heating of the sample at 4 K/s up to different Ti temperatures. However, considering that the probability for thermal detrapping exhibits an exponential dependence on temperature (see below), we made the hypothesis to be in the condition of an isothermal heating for a fixed time interval Δt. In other words, we assumed that only the very final part of the heating process corresponding to the highest temperature intervals influenced trap population. Therefore, in the case of a single kind of trap with depth E and frequency factor s, the initial concentration n0 of filled traps at the starting of each TSL measurement in Figure 5 depends on the annealing temperature as n0(T ) = N0 exp( −P(T )Δt )

(2)

where N0 is the concentration of filled traps prior to heating, while the thermal detrapping probability P(T) is P(T ) = s exp( −E /kT )

(3)

where k is the Boltzmann’s constant. Accordingly, the tunneling amplitude can be written I (t ) =

CN0 exp[−sΔt exp(−E /kT )] t

= I0 exp[−sΔt exp(−E /kT )]

(4)

In Figure 6, the numerical fit of the tunneling amplitudes (obtained for the same t = 390 s after every irradiation) obtained using eq 4 is reported (continuous line). The parameters turned out to be E = 1.3 eV and sΔt = 1014, consistent with previous analyses28 for the main peak at 460 K if an effective Δt of few seconds is considered. The disagreement with data observed at around 500 K could be related to a small contribution to the tunneling emission by the trap responsible for the peak at 515 K. In this case, eq 4 becomes

(1)

where C is a constant and n0 is the initial concentration of filled traps (corresponding to the initial concentration of trap-center couples). 20205



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irradiation, Pr3+ ions are supposed to trap holes becoming tetravalent. During the TSL process, the following recombination process is expected to occur:

I = I01 exp[−s1Δt exp(−E1/kT )] + I02 exp[−s2Δt exp(−E2 /kT )]

(5)

Because of the high numbers of parameters, we fixed reasonable values for I01 and I02 from experimental data, to be approximately 9 × 103 and 3 × 103, respectively. The numerical fit taking into account two traps is also reported in Figure 6 (dashed line), together with separate component contributions (dash−dot lines). The trap parameters turned out to be: E1 = 1.3 eV, s1Δt = 1014 and E2 = 1.1 eV, s2Δt = 1010. Also in this case there is a good agreement with the values reported in the literature. This analysis confirms the role of the composite structure at 475 K in the tunneling effect, with the predominant contribution of the peak at 460 K. It is so demonstrated that, during the TSL process, the carriers in trap levels can recombine at Pr3+ both thanks to a thermally activated process and through a tunneling mechanism. However, this second path involves only the 4f levels of the rare earth ion, which are at similar depths as the traps in the forbidden gap. Such finding provides also a hint for a qualitative interpretation of the lower RL hysteresis displayed by 4f−4f transitions with respect to 5d−4f ones as shown in Figure 3. In fact, it could be tentatively explained by the continuous selective population of excited 4f levels during irradiation thanks to an effective and fast tunneling process from very closely lying traps; in this case, an efficient RL recombination is promoted; however, a stable filling of such close traps is prevented, and the overall effect is that the RL sensitization becomes less evident in the 4f−4f spectral region. Our results suggest some speculations about the influence of spatial correlation between traps and luminescent centers in the hysteresis phenomenon. In general, a close proximity between traps and centers is expected to enhance the effect because they directly compete in carrier capture during the very final stage of their path, when their energy is already too small to produce additional lattice ionizations and excitations. However, if the trap-center distance is really small (of the order of a few Angstroms) and if a correspondence between their energy levels exists (like in our case for the Pr3+ 4f excited levels), then the hysteresis phenomenon can be made less evident thanks to the occurrence of a direct tunneling recombination. In summary, the experimental picture points out that the hysteresis phenomenon originates from the filling of traps during irradiation. Such progressive filling lowers their competition with luminescent centers in free carrier capture, and for this reason, the RL efficiency progressively increases. This phenomenon is not necessarily accompanied by afterglow. In fact, if traps are deep enough to be thermally stable at RT and if there are no a-thermal tunneling processes, no afterglow is expected. In our case, traps are indeed thermally stable; therefore, no afterglow due to the thermal release of trapped electrons in the conduction band is expected. However, afterglow by a-thermal tunneling is observed, and its emission spectrum displays only the 4f−4f emissions. This means that the trap levels are in similar positions in the forbidden gap as the excited 4f levels, allowing a-thermal tunneling of trapped electrons toward such levels. Such an a-thermal mechanism does not involve the 5d excited state, and for this reason, no 5d−4f emission bands are observed in the afterglow spectrum. 3.4. Considerations on the Origin of Traps. TSL active defects are reasonably electron traps, due to the observed TSL spectrum featuring emissions of Pr3+ centers; in fact, under

Pr 4 + + e(freed from trap) → Pr 3 + *(excited state) → Pr 3 + + hν(TSL emitted light)

(6)

However, no direct information is available about the nature of such electron traps responsible for hysteresis. A reasonable hypothesis is that they could be related to oxygen vacancies, similarly to Lu oxy-orthosilicate (LSO), for which several experimental and theoretical investigations pointed out the role of such kinds of defects in TSL and afterglow phenomena.30−33 It must, however, be remarked that LPS was supposed to have a lower tendency to host oxygen vacancies with respect to LSO because of its different crystalline structure (monoclinic C2/m and C2/c for LPS and LSO, respectively) leading to specific differences in the oxygen surroundings.34 Anyway a relation with oxygen vacancies was suggested for a TSL peak at 543 K in LPS:Ce,24 probably corresponding to the 570 K peak observed in our measurements considering the shape of the glow curve because of its disappearance after annealing in air. At variance, the TSL structure at lower temperature, which is responsible for hysteresis as demonstrated in this article, was not found to be influenced by annealing in air. Therefore, from the presently available data, it can be supposed that the defects responsible for TSL peaks at 460 and 515 K are not related to oxygen vacancies or that they are due to variants of oxygen-deficient centers with a high stability so that they cannot be easily recovered by annealing treatments. In any case, the defects responsible for such TSL peaks are likely to be of intrinsic origin since peaks in the temperature region from 400 to 600 K were systematically observed in samples prepared by various techniques and doped with either Ce3+ or Pr3+.21,24,34

4. CONCLUSIONS An increase of radioluminescence (RL) efficiency under irradiation was detected in several scintillator materials and was deeply investigated for the case of lutetium pyrosilicate doped with praseodymium in which such phenomenon was found to be particularly relevant. The link between this effect and the presence of traps in the crystal was clearly established by parallel thermally stimulated luminescence measurements. Spatial correlation between traps and Pr3+ ions was also evidenced by the detection of an a-thermal tunneling luminescence emission. Probably, both a high concentration of traps and their spatial correlation with recombination centers contribute to the highlighting of the RL sensitization in this crystal. The phenomenon represents a memory of previous irradiations, which remains stored in the material and could possibly be exploited for dosimetry purposes. However, its occurrence is problematic for the use of a material in scintillator applications. It could be eliminated by the reduction of deep traps and/or by their spatial decoupling with respect to Pr3+ ions. The presented results help to increase the fundamental comprehension of such defect-driven phenomenon; moreover, they could stimulate the improvement of synthesis technologies aimed at the control of material defectiveness. To this purpose, future work should focus on the use of high purity raw 20206



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materials, postgrowth thermal treatments, and possibly also codoping procedures with optically inactive ions aimed at changing the concentration of trapping defects.

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AUTHOR INFORMATION

Corresponding Author

*(A.V.) E-mail: [email protected]. Phone: +39 02 64485162. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A. Yoshikawa of Tohoku University, Japan, M. Ishii of Daichi Kiden Co, Japan, and CRYTUR, Czech Republic, are gratefully acknowledged for providing the LuAG:Ce, BGO, and LuYAP:Ce samples, respectively.

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Defect-Driven Radioluminescence Sensitization in Scintillators: The

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