Radioluminescence Sensitization in Scintillators and Phosphors: Trap

Article pubs.acs.org/JPCC

Radioluminescence Sensitization in Scintillators and Phosphors: Trap Engineering and Modeling Federico Moretti,*,† Gael Patton,† Andrei Belsky,† Mauro Fasoli,‡ Anna Vedda,‡ Mattia Trevisani,§ Marco Bettinelli,§ and Christophe Dujardin*,† †

Institut Lumière Matière, UMR5306, Université Claude Bernard Lyon1-CNRS, bâtiment Kastler, 10 rue Ada Byron 69622 Villeurbanne CEDEX, France ‡ Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 55, 20125 Milan, Italy § Dipartimento di Biotecnologie, Università di Verona, and INSTM, UdR Verona, strada Le Grazie 15, 37134 Verona, Italy S Supporting Information *

ABSTRACT: The role of charge carrier trapping in determining radioluminescence (RL) efficiency increase during prolonged irradiation of scintillators has been studied by using YPO4:Ce,Nd as a model material. The Nd3+ ions act as efficient electron traps minimizing the role of intrinsic defects. Different Nd contents were considered in order to point out the correlation between the trap concentration and the detected RL efficiency dose dependence. RL measurements as a function of temperature clarified the role of the trap thermal stability in determining the shape and the magnitude of such effect. We propose also a model based on trap filling which is able to describe accurately the complex processes which are involved. rarely been studied in a detailed manner,12 mainly because of the critical dependence on material reproducibility in term of traps which are related to uncontrolled defects in standard scintillators. The understanding of the relation between traps and bright burn is thus a critical issue. In this contribution we investigate a material for which the trap involved in the memory effect is fully under control: YPO4:Ce,Nd. Indeed, this material appears to be a good candidate for a thorough study, since the main electronic traps are due to Nd3+ ions,13−15 whose content can be selected as desired during the sample synthesis, and not by intrinsic defects or uncontrolled impurities. On the basis of the experimental results a population rate model is also proposed. The combination of luminescence sensitization measurements and modeling for various trapping conditions presented here allows precise description of the bright burn effect.

1. INTRODUCTION Scintillating materials absorb ionizing radiation and emit photons in the visible or ultraviolet range which can thus be revealed by standard photodetectors. They are widely used for radiation detection in many applications such as medical X-ray or nuclear imaging, homeland security, and high energy calorimetry.1,2 Apart from the specific required performances (light yield, timing, etc.), which can vary from one application to another, the common requirement is the response stability under irradiation. Whatever the use, the number of emitted photons and the time response has to be identical for identical incoming γ-ray photons or particle beams. In many cases, the scintillation response is unstable and this effect may occur in different ways. One source of instability is related to radiation damage, resulting in optical transmission losses and thus to a scintillation yield decrease; this effect is widely known and has been extensively studied mainly in the frame of high energy calorimetry.3,4 Apart from radiation damage, increase of scintillation yield following previous irradiation has also been observed.5−10 This effect, also known as “memory effect”, “bright burn”, or “luminescence hysteresis”, is particularly critical in X-ray imaging and dosimetry even at a few percent level, since it leads to the formation of ghost images11 or to dose misevaluations. The physical processes involved in the bright burn effect are substantially related to competition phenomena among traps and radiative recombination centers in the trapping of free carriers created during irradiation. Despite the relevant role in many detection or imaging devices, it has © 2014 American Chemical Society

2. EXPERIMENTAL DETAILS YPO4 single crystals doped with Ce and codoped with Nd were grown by spontaneous nucleation from a PbO−P2O5 flux (1:1 molar ratio). The reagents used for the growths were (NH4)2HPO4 (purity ≥99.0%, Fluka), PbO (≥99.9%, Aldrich), Y2O3 (99.99%, Aldrich), CeO2 (99.9%, Aldrich), and Nd2O3 (99.99%, Aldrich). Ce concentration was nominally 0.1 mol %, while three different Nd contents (0.01, 0.1, and 0.5 mol %) Received: February 18, 2014 Revised: April 14, 2014 Published: April 15, 2014 9670

dx.doi.org/10.1021/jp501717z | J. Phys. Chem. C 2014, 118, 9670−9676

The Journal of Physical Chemistry C

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were considered for the codoped samples. The batches were put in Pt crucibles closed by tightly fitting lids and heated up to 1300 °C inside a horizontal furnace. After a soaking time of about 15 h, the temperature was lowered to 800 °C at a rate of about 1.8 °C/h; the crucibles were then drawn out of the furnace and quickly inverted to separate the flux from the crystals grown on the bottom of the crucible. The flux was then dissolved using hot, diluted nitric acid. Single crystals with mean dimensions of about 2 × 1 × 0.1 mm3 and with good optical properties were obtained. Thermally stimulated luminescence (TSL) measurements above room temperature (RT) were performed up to 570 K with a linear heating rate of 1 K/s after irradiation at room temperature by a Machlett OEG50 X-ray tube operated at 30 kV. The emitted light was detected by an EMI 9635QB photomultiplier tube working in photon counting mode. Low temperature (10−320 K) wavelength resolved TSL measurements were performed after irradiation at 10 K by a Philips 2274 X-ray tube having a tungsten anode and operated at 30 kV. We applied a heating rate of 0.1 K/s. The emitted light was collected by a Jobin Yvon CCD (Spectrum One 3000) coupled to a Triax 180 monochromator (Jobin Yvon) and operating in the 190−1100 nm interval. RT radioluminescence measurements (RL) were obtained by irradiating the samples with a Philips X-ray tube with tungsten anode set at 35 kV; the light was collected via an optical fiber and detected by an Andor Newton 920 CCD camera coupled to a monochromator (Andor Shamrock 163) working in the 200−1000 nm interval. Radioluminescence sensitization measurements as a function of temperature were performed with the same experimental apparatus used for RT ones, employing an Advanced Research System closed cycle He cryocooler. The measurements were performed, after a preirradiation of the samples at 320 K in order to fill the stable traps, in the 250−310 K temperature interval every 10 K, from the highest to the lowest temperature. Each measurement is composed by a 8 min irradiation (dose rate 1 mGy/s) followed by a 3 min phosphorescence decay, after which the sample was heated up to 320 K and kept at this temperature for 10 min. Few measurements were also done by using the low temperature TSL setup (dose rate 70 mGy/s) and following the same scheme. RL spectra were not corrected for the instrumental response. Both TSL curves reported in Figure 2 and RL data shown in Figure 4, 5, and 7 were obtained after integration of the wavelength resolved results over the 310− 370 nm wavelength interval corresponding to the Ce3+ emissions.

Figure 1. Normalized radioluminescence spectra of YPO4: 0.1 mol % Ce,Nd for different neodymium concentrations. The spectra have been shifted along the ordinate axis for clarity.

suggest that both Ce3+ and Nd3+ behave as recombination centers for carriers created during irradiation. However, as will be shown later, Nd3+ acts as well as a trapping center for electrons, giving rise to evident Nd content dependent TSL glow peaks. The weak emissions at about 590, 610, and 700 nm are related to Eu3+ contamination. The spectrum of the sample doped only with cerium also shows a band at about 230 nm, whose origin is unclear and possibly related to defects. As we will see later, these contaminations do not induce significant traps as compared to Nd3+. The low temperature (10−320 K) TSL glow curves, obtained after X-ray irradiation at 10 K, of YPO4:Ce,Nd samples for different neodymium concentrations are presented in Figure 2. The sample doped only with Ce has a relatively

3. RESULTS AND DISCUSSION 3.1. Experimental Results. RT radioluminescence spectra of YPO4:Ce,Nd as a function of Nd content are reported in Figure 1. All the spectra are characterized by the well-known Ce3+ 5d-4f radiative transitions in the 310−370 nm region.16−18 Nd-codoped sample spectra are characterized also by the presence in the near-infrared region of the typical Nd3+ emissions, due to radiative transitions among the 4F3/2 and 4 I9/2 levels,19 as well as by the presence of two structures between 225 and 300 nm related to radiative recombinations between Nd3+ 5d and 4f levels.19,20 Other Nd3+-related 4f-4f transitions are detected in the whole visible region of the spectrum for sufficiently high neodymium concentrations. As expected, the intensity of all the Nd-related emissions increases evidently by increasing the Nd content. These results clearly

Figure 2. TSL glow curves of YPO4: 0.1 mol % Ce,Nd after irradiation at 10 K for different Nd contents. The curves have been obtained after integration of the wavelength resolved measurements on the Ce3+ emission (310−370 nm).

simple glow curve characterized by the presence of two main peaks at about 90 and 183 K; as the Nd concentration is increased other two peaks appear at about 130 and 280 K. The latter, in particular, quickly becomes the dominant feature of the glow curves, and it is strictly related to electron trapping at Nd3+ sites.14 The 130 K peak is most likely related to hole trapping, since the related emission spectrum (see Figures S1 and S2 in Supporting Information) shows mainly Nd3+ radiative transitions. The Nd concentration increase also causes a strong 9671



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reduction of the 90 K glow peak intensity. Indeed, this peak has been previously related to the formation of Ce2+/Ce4+ pairs during irradiation at 10 K;21 the introduction of Nd3+ ions inside the matrix is likely the cause of a reduced formation probability of such pairs. The glow curve of the sample codoped with 0.5 mol % Nd is also characterized by relatively high background level in the 10−80 K and the 200−260 K temperature intervals. These contributions have been previously ascribed to athermal and thermally assisted tunneling processes, respectively, occurring between Nd2+ and Ce4+ ions.21 A high temperature shift of the maximum position of the Ndrelated peak (from 281 to 287 K) is observed as the Nd concentration in the sample is increased, although the trap depth evaluated by the initial rise method keeps the constant value of 0.85 eV (±10%) in rather good agreement with the values already reported in the literature.14,21 Such an effect could be due to a progressive increase of retrapping probability, which gives rise to a second order recombination kinetic. The Nd-related peak also appears in TSL measurements performed above RT (Figure 3) after irradiation at 293 K,

Figure 4. RL and phosphorescence intensity, evaluated on the Ce3+ emissions (310−370 nm), of YPO4:Ce, Nd as a function of time and of the Nd concentration. The measurements were performed at 290 K and normalized to their maximum value.

The influence of temperature on the RL sensitization for YPO4: 0.1 mol % Ce, 0.5 mol % Nd is reported in Figure 5.

Figure 5. RL and phosphorescence intensity of YPO4:0.1 mol % Ce, 0.5 mol % Nd evaluated on the Ce3+ emission (310−370 nm) as a function of time and temperature. The measurements are normalized to the 310 K maximum value.

Figure 3. TSL measurements of YPO4: 0.1 mol % Ce,Nd obtained after X-ray irradiation at room temperature for different Nd concentrations.

Again, the measurement performed at 310 K shows a fast increase of the RL signal followed by its stabilization; as the temperature is lowered, the initial rising component becomes progressively slower, until it remains the only component visible in the irradiation time frame. Poolton et al.22 presented RL intensity measurements as a function of time for YPO4:Ce,Sm performed at a low fixed temperature also considering an optical stimulation during the irradiation; this optical stimulation was able to free electrons stored on the Smrelated traps. Interestingly, the results obtained with and without this optical excitation closely resemble those we obtained at 310 K and at 250 K, respectively. It also has to be noted that all the curves are characterized by an initial abrupt jump of the RL signal from the background level. The other two Nd codoped samples show a similar temperature dependence of the RL, though its importance is less and less pronounced as the Nd content is reduced. On the other hand, the sample doped only with Ce does not show any meaningful modifications in the RL intensity shape and value as a function of the temperature, clearly suggesting that the intrinsic traps have a practically negligible role in the RL sensitization process, and that the Ce3+ emission is not evidently affected by thermal

particularly for the two highest Nd concentrations. In this case, however, its maximum position is shifted to about 310 K because of the higher heating rate (1 K/s) used in these measurements. The glow curves are characterized also by the presence of at least four other peaks (at about 320, 370, 480, and 520 K). Despite the fact that the Nd-related trap is substantially unstable during irradiation at room temperature, its glow peak still remains the dominant characteristic of the sample with the highest Nd concentration. The RL intensity dependence on irradiation time and the subsequent phosphorescence decay measurements performed at 290 K are reported in Figure 4 for all the Nd contents. The results clearly evidence the role of Nd concentration in affecting the RL intensity profile versus irradiation time: In fact, the RL intensity of the sample containing only cerium does not show any meaningful variation, while the Nd3+ codoped ones are characterized by a clear increase toward saturation. This effect becomes more and more pronounced as the Nd concentration is increased. The same concentration-related trend is also well evident for the phosphorescence portion of the measurements. 9672



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imposes that the thermal release of trapped electrons must be explicitly taken into account. Other possible mechanisms like band-to-band transition and hole recombination on trapped electrons have not been considered. On this basis, the time dependence of the electron and hole concentrations in the delocalized bands and those on traps and recombination centers can be described by the following set of differential equations:

quenching in the considered T interval. As expected, the temperature also plays a crucial role on the phosphorescence decay tail detected after the end of the irradiation; indeed, the phosphorescence time decay becomes longer as the temperature is decreased, reflecting well the increasing thermal stability of trapped electrons on Nd-related traps. These results clearly confirm that the Nd codoping strategy used here can be considered effective for a thorough study on the bright burn effect. In fact, it enables easy control on the overall magnitude of the memory effect by simply changing the codopant concentration during sample synthesis. Moreover, the codopant-related glow peak is the dominant TSL feature for sufficiently high concentration, completely eliminating (or at least minimizing) the possible effect that the other carrier trapping sites in the material might have in shaping the RL sensitization process. In addition, the Nd-related trap characteristics work in a temperature region in which neither competitive intrinsic recombination channels for free carriers19 nor evident thermal quenching of the Ce3+ luminescence (as already mentioned few lines above) are expected. None of these characteristics can be achieved simultaneously in commonly used scintillators or phosphors, since their traps are usually due either to intrinsic defects or to impurities in the raw materials. The concentration and nature of such defects is often unknown and, in many cases, difficult to control in an easy, predictable, and reliable way. Moreover, several predominant traps are usually present in the TSL glow curves of these materials. The resulting picture required for modeling is thus simpler than that of most scintillating materials. Therefore, the RL intensity dependence on irradiation time and on temperature has been described on the basis of a one-trap/one-recombination center model.23 3.2. Model Description and Simulation Results. Figure 6 displays a sketch of the trapping-recombination processes involved. Since the goal of our work is to study the effect of trap stability on memory effect, the investigated temperature range

dnc = f (1 − α) − nc(N − n)Ae + ns exp(−E /kT ) dt − ncA r m

(1)

dn = nc(N − n)Ae − ns exp( −E /kT ) dt

(2)

dm v = f (1 − α) − m v (M − m)Ah dt

(3)

dm = m v (M − m)Ah − ncA r m dt

(4)

nc + n = m v + m

(5)

Equation 1 describes the concentration rate for electrons in the conduction band and its right-hand side contains the following terms: (i) the creation rate f of free electrons/holes in the conduction/valence band, and the direct recombination coefficient α; (ii) the electron trapping rate nc(N − n)Ae; (iii) the electron thermal detrapping rate ns exp(−E/kT), where E and s are the trap energy and frequency factor, respectively, T is the absolute temperature, and k is the Boltzmann constant; and (iv) the recombination rate ncArm. Equation 2 describes the concentration time dependence of electrons on the trap; eqs 3 and 4 represent the concentration rate of holes in the valence band and on the recombination centers, respectively, and they contain the rate of hole capture by the recombination centers mv(M − m)Ah. Equation 5 guarantees the charge neutrality during the simulations. The RL intensity at any time can be evaluated by IRL ∝ ncA r m + αf

(6)

The direct recombination coefficient α is related to the abrupt RL jump detected in the experimental results; a possible way to explain this phenomenon is to consider that, at the end of the thermalization process, part of the electrons can be so close to holes trapped on luminescence centers that the only effective Coulombic field they experience is that of the holes themselves, thus overriding the competition process. Moreover, energy transfer from excitons created during irradiation to Ce3+ ions cannot be ruled out. In fact, vacuum ultraviolet excitation spectra of Ce3+ emission in YPO4 clearly show a peak at about 8.5 eV which has been related to exciton creation.15,24 Both these excitation channels of the luminescence centers result in a reduction of electron and hole pairs that can effectively participate in the complex equilibrium between electron trapping and radiative recombination. Another explanation is related to the possibility of having an impact excitation of Ce3+ ions by partly thermalized carriers with energy below the ionization threshold:25 this interpretation has been used by Savon et al.26 in their simulations of ZnMO4 luminescence at cryogenic temperatures. However, this excitation pathway appears rather unlikely considering that it is dependent on the luminescence center concentration and starts to be effective

Figure 6. Sketch representing the energy levels and transitions involved in the mechanism of RL sensitization process in a one trap, one recombination center model. In the diagram, N and M are the total concentration of traps and recombination centers, respectively; n and m are the instantaneous concentration of electrons in the traps and of holes in the recombination centers, respectively; nc and mv are the instantaneous concentrations of electrons and holes in the delocalized bands, respectively; f is the electron hole pair creation rate, and α is the direct recombination coefficient. Ae and Ar are the transition coefficients for electrons from the conduction band to traps and recombination centers, respectively; Ah is the transition coefficient for holes from the valence band to the recombination centers. PT = s exp(−E/kT) is the thermal ionization probability for the electron on traps, where s is the frequency factor, E is the trap depth, and k is the Boltzmann constant. 9673



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above 1 mol %.27 On the other hand, Polf et al.28 considered a nonzero concentration of trapped holes on recombination centers for Al2O3:C samples. Although in their case this approach may make sense considering that the material is highly defective, in our case it is not sufficiently backed up by experimental results. In fact, no appreciable difference was observed in the RL jump obtained at the end of the measurement set portrayed in Figure 5, and that obtained after having emptied the traps at 320 and 370 K by heating the sample up to 373 K. This suggests that the concentration of electrons stored in high temperature traps is sufficiently small to be considered irrelevant in the determination of the equilibrium process occurring during irradiation. The RL intensity time dependence was calculated by numerically solving eqs 1−4 using MATLAB for the different experimental temperatures. The four equation set contains a large number of parameters, many of which are known or can be easily estimated. The trap and the recombination center concentrations (N and M, respectively) have been set according to the Nd and Ce nominal contents. The Nd-related trap depth (E) has been calculated, as already mentioned in the previous section, via the partial cleaning and initial rise method; while the frequency factor (s) has been obtained from the trap energy value, the heating rate (β) and the temperature of the TSL peak maximum (Tm) by using the following equation:23 s=

⎛ E ⎞ βE exp⎜ ⎟ 2 kTm ⎝ kTm ⎠

transition coefficient values used in the simulations (see Table 1), the probability of electron recombination (Arm) remains for Table 1. Fixed Parameter Value Used in the Solution of eqs 1−4 parameter

value

f E s N M α

1013 cm−3 0.85 eV 1013 s−1 5 × 1019 cm−3 1019 cm−3 0.39

an extended period of time small compared to the trapping one ((N − n)Ae). The resulting RL intensity profile is thus characterized by a gradual increase as a function of time in contrast to the abrupt one detected in the low temperature RL measurement (see Figure S3 of Supporting Information for a comparison between the experimental results at 250 K and simulations done with α = 0 and 0.39). The simulation results are reported, together with the experimental ones, in Figure 7 for four selected temperatures.

(7)

This formula is strictly valid only for first order recombination processes. For higher order recombinations it should give in any case the correct order of magnitude of this parameter.23 The pair creation rate (f) has been estimated from the dose rate in air, the mean energy of the X-ray beam, the YPO4 density (4.27 g/cm3) and energy gap (Eg, 10 eV), and considering 3Eg as threshold energy for e−h pair creation. The direct recombination coefficient α was evaluated from the low temperature RL sensitization measurement intensity value at 10 s. The transition coefficients (Ae, Ar, and Ah) have been optimized by using a least mean squares fit routine starting from arbitrarily set values in order to have a good reconstruction of the experimental results. The arbitrariness of the transition coefficient initial value is not a relevant issue; in fact, the RL intensity equation (eq 6) can be rewritten, by using the semi-equilibrium approximation (|dnc/dt| ≪ |dn/dt| and |dmv/dt| ≪ |dm/dt|), in the following way:

(8)

Figure 7. Comparisons, for selected temperatures, between the Ce3+ RL and phosphorescence experimental intensity time evolution of YPO4:0.1 mol % Ce, 0.5 mol % Nd (red empty circles), and the simulation results (black full lines).

This new equation clearly shows that, in the framework of this approximation and model, the luminescence intensity does not depend on the hole transition coefficient Ah. Moreover, for given trap and recombination center concentration, only the ratio between Ae and Ar is relevant for the bright burn shaping, and not their absolute values. This equation allows us to discuss also the role of the direct recombination coefficient α in the simulation results reported below. As it clearly appears form eq 8, this coefficient adds a time independent contribution to the RL intensity as a function of time. At sufficiently low temperatures and for α equal to zero, only the second term is responsible for shaping the RL time dependence. Considering the concentration of the traps (N), and the

All the fixed parameter values used to obtain these simulation results are reported in Table 1. As is clearly visible, the simulations very well reflect the experimental tendencies, and particularly so for the lowest and the highest temperature. The reconstruction, though, appears to be slightly less accurate for intermediate−high temperatures (see, e.g., that at 290 K). Also, the calculated phosphorescence decays seem to be slightly longer than the experimental ones. However, it has to be noted that the model does not consider that the experimental phosphorescence portion obtained at the lowest temperatures (250 and 260 K, not shown) also contains the contribution related to thermally assisted tunneling of electrons between

IRL

f (1 − α)A r m A m ns exp( −E /kT ) ∝ αf + + r (N − n)Ae + A r m (N − n)Ae + A r m

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Nd2+ and Ce4+ ions.21 In the case of higher temperatures, the longer phosphorescence decay calculated with the model might suggest a higher probability of electron retrapping and, thus, a nonoptimal ratio between the transition coefficients which govern the trapping and the recombination processes. The ratio of the optimized Ae and Ar values (Figure 8) decreases rather evidently by about 1 order of magnitude by

The presented results indicate that the codoping strategy used here represents a good way to reach a better fundamental comprehension of the RL efficiency increase phenomenon, helping to clarify which are the relevant parameters involved in it, and clearly demonstrating the trap role in the phenomenon. The use of other codopants (like Pr3+ or Dy3+, for instance) and experimental techniques (like optically stimulated luminescence, during and after X-ray irradiation) will give further insight for a more comprehensive understanding of this phenomenon. Better comprehension of this phenomenon allows us to predict and correct the observed luminescence sensitization in the case of scintillators used in those applications which require a stable and reproducible light signal, such as in computed tomography. Finally, this work also puts in further evidence the complex role played by traps in the scintillation process: in fact, they give rise not only to long scintillation decay tails and delayed recombination phenomena, but also to scintillation efficiency modifications which depend in a complex manner on trap characteristics, measurement temperature, and sample irradiation history.

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Figure 8. Ratio of the Ae and Ar transition coefficient calculated values as a function of the temperature.

ASSOCIATED CONTENT

S Supporting Information *

Contour plot and emission spectra of wavelength resolved TSL measurement, comparison among experimental RL sensitization measurement results and two simulation results obtained for different direct recombination coefficients. This material is available free of charge via the Internet at http://pubs.acs.org.

increasing the temperature. Even though a temperature dependence of the transition coefficients should be expected,23 this appears to be much more related to the shortcomings of the model used than to a real effect. Considering the RL spectra presented above, hole recombination on trapped electrons is likely to occur, and this may be another factor responsible for the imperfect reconstruction of the experimental results. Also, the thermally assisted tunneling process between Nd2+ and Ce4+ ions might have a role. In fact, the presence of these recombination channels leads to a higher concentration of Nd3+ sites available for electron trapping affecting, in turn, the detected RL intensity by lengthening the time taken to arrive at a stationary value. This is ultimately described by the model as a change in the Ae and Ar calculated values as a function of the temperature. The role of competitive processes which result in a lower trap filling rate has been invoked in a very recent study on Pr doped Lu2Si2O7 crystal12 in order to explain the discrepancy detected between the RL sensitization measured on the Pr3+ 4f-4f transitions and that on the 5d-4f ones.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +33(0)47 24 32 970. E-mail: [email protected]. *Phone: +33(0)47 24 78 336. E-mail: christophe.dujardin@ univ-lyon1.fr. 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 The authors are grateful to Direction Générale de la Compétitivité de l’Industrie et des Services (dgcis), Oséo, and Pays Voironnais for the financial support. They are also grateful to Erica Viviani (Univ. Verona) for expert technical assistance in the crystal growth.

4. CONCLUSIONS The Nd3+ tendency of behaving as an electron trap in YPO4:Ce,Nd crystals has been used as a tool for the study of radioluminescence (RL) efficiency increase in scintillators. The easy control on the Nd content during sample synthesis allowed us to tailor the material electron trapping characteristics in order to minimize the role of intrinsic defects and at the same time give rise to a well evident, Nd content-dependent RL efficiency increase during X-ray irradiation. RL measurements as a function of temperature put in evidence the relevant role of the Nd-related trap stability in determining the shape and the magnitude of the RL efficiency modifications. On the basis of these experimental results, a simple model describing the competition phenomena occurring during irradiation between trapping and recombination on the luminescence centers was proposed; in spite of the simplicity of such a model, the obtained simulation results are in good agreement with the experimental ones.

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REFERENCES

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Radioluminescence Sensitization in Scintillators and Phosphors: Trap

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