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Scintillation properties of Ce3+-doped YCl3 and YBr3
Radiation Measurements 47 (2012) 917e920
Contents lists available at SciVerse ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Scintillation properties of Ce3þ-doped YCl3 and YBr3 V.L. Cherginets a, b, *, T.P. Rebrova a, T.V. Ponomarenko a, O.A. Tarasenko a, N.N. Kosinov a, O.V. Zelenskaya a a b
Institute for Scintillation Materials, National Acad. Sci. Ukraine, Lenin Avenue 60, Kharkov 61001, Ukraine National Technical University, Kharkiv Polytechnical Institute, 21 Frunze St., 61002 Kharkov, Ukraine
h i g h l i g h t s < Maximal light yield value of YCl3:Ce and YBr3:Ce is 8700 and 20,600 photons per MeV. < Scintillation pulse decay of YCl3:Ce is described by two components e 37 and 640 ns. < Scintillation pulse decay of YBr3:Ce is described by two components e 36 and 450 ns. < For YCl3:Ce and YBr3:Ce fractions of faster component are 86 and 79%, respectively.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2011 Received in revised form 3 September 2012 Accepted 5 September 2012
The dependence of the scintillation properties of Ce3þ-doped YCl3 and YBr3 on activator concentration (0.5, 1 and 2 mol%) has been studied. The radioluminescence spectra of both materials contain asymmetric bands with maxima located at 3.13 eV (383 nm) for YCl3:Ce3þ and 2.84 eV (422 nm) for YBr3:Ce3þ. The scintillation pulse decay curves for both materials are described by two components with decay constants of 37 and 640 ns for YCl3:Ce3þ and 36 and 450 ns for YBr3:Ce3þ, the fractions of the faster component being 86 and 79 per cent, respectively. The dependences of the light yield of the studied materials on Ce3þ concentration pass through a maximum near 1 mol% of the activator, and the maximum light yields (relative to NaI:Tl) are 8700 photons per MeV for YCl3:Ce3þ and 20,600 photons per MeV for YBr3:Ce3þ. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Scintillation Bridgman Light yield Scintillation pulse decay
1. Introduction The recent discovery of high-performance scintillators based on Ce3þ-doped lanthanum halides (van Loef et al., 2001a, 2002) caused a revival in the development of new promising scintillation materials. Within a short time, similar compounds of gadolinium (van Loef et al., 2001b; Glodo et al., 2008; Grippa et al., 2010), praseodymium (Birowosuto et al., 2006) and lutetium (van Loef et al., 2003; Glodo et al., 2005) were studied to find other bright scintillation materials. Except lanthanum compounds, rare-earth metal iodides possess the highest light yields compared with the corresponding chlorides and bromides, and sometimes this parameter reaches up to 100,000 photons per MeV (Glodo et al., 2008). However, the synthesis of pure rare-earth iodides is extremely difficult. The best results are observed for materials obtained by
* Corresponding author. Institute for Scintillation Materials, National Acad. Sci. Ukraine, Lenin Avenue 60, Kharkov 61001, Ukraine. Tel.: þ380 57 3410218; fax: þ380 57 3404474. E-mail address: [email protected] (V.L. Cherginets). 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2012.09.002
direct synthesis from a rare-earth metal and iodine with subsequent vacuum distillation of the synthesized product (Kramer et al., 2006). This difficulty does favour wide industrial production of rare-earth iodide scintillators, whereas rare-earth halides formed by lighter halide ions (chlorides and bromides) may be more promising. For instance, LaCl3:10 mol% Ce3þ (BrilLanCeÔ 350) and LaBr3:5 mol% Ce3þ (BrilLanCeÔ 380) are now produced by Saint-Gobain (see http://www.detectors.saint-gobain.com/MaterialsGasTubes.aspx). Therefore, other Ce3þ-doped rare-earth chlorides and bromides may be of interest for material scientists, but the properties of these substances cannot as yet be predicted theoretically and this necessitates an empirical approach to the study of each new material (‘cook-and-look’). The purpose of this work is to study luminescence and scintillation properties of YCl3 and YBr3 doped by different concentrations of Ce3þ. It is well known that the light yield of Ce-doped LaCl3 and LaBr3 exceeds that of chlorides and bromides formed by heavier lanthanides and it is of interest to study the properties of the yttrium compounds as formed by a lighter analog of lanthanum. In this paper we report on the further development of our initial investigation of YCl3:Ce3þ (Cherginets et al., 2009) and also investigate a new scintillation material, YBr3:Ce3þ.
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2. Experimental Anhydrous YCl3 and YBr3 were obtained by dissolution of a weight of Y2O3 (extra pure, 4 N) in the corresponding acid (HCl, extra pure, 4 N; HBr, reagent quality, 3 N) taken with excess of 5%. Separately an amount of CeO2 (reagent grade, 3 N) providing a preliminary chosen concentration of Ce3þ in the final crystal was dissolved in HCl or HBr; in the latter case formic acid was added for the reduction of CeIV to CeIII. The concentrations of Ce3þ in the growth charge were 0.5, 1 and 2 mol %. The solutions were mixed, and a weight of corresponding ammonium halide (NH4Cl or NH4Br, chemically pure, 3 N) providing NH4X:YX3 molar ratio not less than 6 (here ‘X’ is Cl or Br) was added to the mixed solution. This suspension was dried in air at 120e150 C, crushed and dried in vacuum (p 10 Pa) applying a slow increase in temperature from 100 to 400 C, and an 8-h pause at 200 C. The charge obtained was placed in an 18 mm dia. quartz ampoule and melted. The chloride melts were treated for 8 h in CCl4 vapour obtained by passing dry argon through liquid tetrachloromethane at room temperature (the pressure of the saturated vapour was approximately 12 kPa) to decrease the concentration of oxide admixtures. Before growth the melts were kept for a day in vacuum (p 10 Pa). The crystal growth was performed by the Bridgman method; the temperature at the diaphragm was 721 C (YCl3) and 905 C (YBr3), the temperature gradient was 6e8 C cm1 and the velocity of the ampoule descent was varied in the range 0.5e1.3 mm h1. The ingot obtained was cooled to room temperature at a rate of 2e3 C h1. To measure the functional characteristics of the materials, detectors of 12 mm dia. and 2 mm height were prepared. The radioluminescence spectra were measured under 241Am source excitation (g, 59.6 keV) and recorded using a MDR-23 monochromator. The pulse-height spectra were measured under gamma irradiation from 137Cs and 241Am sources (PMT Hamamatsu R1307). The scintillation pulse shape of the YCl3:Ce3þ and YBr3:Ce3þ detectors were obtained using the delayed-coincidence method (Bollinger and Thomas, 1961). Two 9142B (Electron Tubes Ltd.) photomultipliers were used for the detection of scintillation photons both in ‘start’ and in ‘stop’ channels of the optical part of the set-up. The detectors were irradiated by gamma photons of a 152Eu radionuclide source (g, 41, 77.9, 122 keV). The photomultiplier in the ‘start’ channel was used to give a zero-time signal and placed in optical contact with the scintillator. A scintillation pulse was attenuated by a diaphragm to single photon mode and then detected by the photomultiplier to generate a delayed timing signal in the ‘stop’ channel. 3. Results and discussion Yttrium chloride and bromide single crystals are referred to as layered materials and the growth of high-quality single crystals is very complicated. It should be noted that 300e400 h duration of the crystal growth procedure does not provide a high quality of grown crystal. Therefore, some parameters (e.g., energy resolution) cannot be measured. The radioluminescence spectra of Ce3þ-doped YCl3 and YBr3 recorded at room temperature (Fig. 1) contain one asymmetric with maxima at 3.13 eV (383 nm) and 2.84 eV (422 nm) for YCl3:Ce3þ and YBr3:Ce3þ, respectively. These bands are typical for Ce3þ luminescence, and they are the superposition of the components corresponding to the transitions from 5d level to 2 levels of the ground state of Ce3þ e 2F5/2 and 2F7/2. To obtain information about the composition of the scintillation pulse of the grown materials the curves of scintillation pulse decay
Fig. 1. Normalized radioluminescence spectra of YCl3:1 mol% Ce3þ (1) and YBr3:1 mol% Ce3þ (2) at 298 K. The excitation source was 241Am.
were constructed (Fig. 2). They are described by two components for both materials according to,
IðtÞ ¼ I0 $ðA1 expðt=s1 Þ þ A2 expðt=s2 ÞÞ;
(1)
where I0 and I(t) are the initial and current intensities of the scintillation pulse, t, the time, A1 and A2, the initial fractions of fast and slow components in the scintillation pulse, respectively, s1 and s2, the decay constants of the fast and slow components. In the case of YCl3:Ce3þ, the fast component is characterized by s1 ¼ 37 ns, and for YBr3:Ce3þ a similar value was obtained 36 ns. However, the fractions of the fast component are different: 0.86 for the chloride and 0.79 for bromide scintillators, respectively. These parameters are close to those obtained for Ce3þ-doped rare-earth metal halide scintillators (http://scintillator.lbl.gov/). As for the slow components, their decay constants are 640 and 450 ns, respectively. So, one can conclude that both materials could be referred to fast scintillators. The light yield of YCl3:Ce3þ and YBr3:Ce3þ materials was estimated on the basis of the pulse-height spectra and the examples obtained using an 241Am excitation source are presented in Fig. 3. It is apparent that their light yield is essentially lower than that of NaI:Tl, being 8% (YCl3:Ce3þ) and 37% (YBr3:Ce3þ) of that from NaI:Tl and, using a 137Cs excitation source, 9.6% and 43%, respectively. From Fig. 3 it follows that YBr3:Ce3þ is a brighter material than YCl3:Ce3þ, usually attributed to a decrease in the band gap width with the increase in the halide ion radius and the effect of Ce3þ concentration on light yield is shown in Fig. 4. Different Ce3þ-activated rare-earth halide scintillators are known to possess maximum light yield values in an activator concentration range from 0.1 to several mol.%. Therefore, it is sufficient to obtain 3e4 concentration values to characterize the scintillation parameters with the rise of the activator concentration (we obtained 3 points for each scintillator). The light yield values for both scintillators reach their maxima at concentration of Ce3þ close to 1 mol%. The absolute light output was calculated (M. Moszynsky et al., 1997) using the following equation,
Nphe ¼ Nph $hL $Q :E$ðlÞ$ε$E;
(2)
where Nphe is the number of photoelectrons, Nph is the absolute light yield, hL is the light collection efficiency, Q.E.(l) is quantum efficiency of PMT, ε is efficiency of photoelectron collection of PMT, E is the photon energy 0.662 MeV (137Cs). The number of
V.L. Cherginets et al. / Radiation Measurements 47 (2012) 917e920
919
Fig. 2. Scintillation pulse growth and decay curves recorded under 152Eu g-excitation: a) YCl3:1 mol% Ce3þ (1, black) and YBr3:1 mol% Ce3þ (2, grey) at 298 K; b) the decay curve for YBr3:1 mol% Ce3þ (1, black), the fast component (2, dashed, A1 ¼ 0.79, s1 ¼ 36 ns), the slow component (3, light grey, A2 ¼ 0.21, s2 ¼ 640 ns).
we can see a typical concentration dependence; the initial growth of the light yield results from an increase in the number of luminescence centers and, for concentrations exceeding 1 mol%, the light yield decreases owing to concentration quenching of the luminescence which is characteristic of activated scintillators. In the vicinity of 1 mol% of Ce3þ there is a plateau, where both effects are reciprocally compensated. 4. Conclusion
Fig. 3. Pulse height spectra of YCl3:1 mol% Ce3þ, (1, solid), YBr3:1 mol% Ce3þ, (2, dashed) in comparison with NaI:Tl (3, dotted). The excitation source was 241Am.
photoelectrons for a standard NaI:Tl sample was estimated to be 4400 (hL ¼ 0.661; Nph ¼ 40,000 ph MeV1; Q.E.(l) ¼ 0.928 (for a Hamamatsu R1307 PMT); ε ¼ 0.27. For YCl3:Ce3þ Nphe was 420 photoelectrons and for YBr3:Ce, 1890 photoelectrons hL was estimated to be 0.27 10% and 0.52 5% respectively and similarly the values of Q.E.(l), 0.995 and 0.98 respectively. The absolute values of light yield were 8700 photons per MeV for YCl3:Ce3þ and 20,600 photons per MeV for YBr3:Ce3þ. In Fig. 4
In this work YBr3:Ce3þ and YCl3:Ce3þ crystals with different activator concentrations were grown and all the crystals possessed a typical layered structure. The radioluminescence spectra of these materials contain one asymmetric band formed by two overlapping peaks corresponding to 5de4f transitions in the cerium ion. The scintillation light decay curves can be described by two components, the decay constants and the fractions of the corresponding components for which have been estimated. The light yield of YBr3:Ce3þ and YCl3:Ce3þ was measured and the concentration dependence passes through a maximum at 1 mol % of Ce3þ, typical of the behaviour of other activated scintillation materials. Acknowledgement The authors are very indebted to Dr. V.A. Tarasov (Institute for Scintillation Materials) for the discussion of some principal questions concerning the absolute light yield of the obtained materials. References
Fig. 4. The dependence of light yield of YBr3:Ce3þ (1, 2) and YCl3:Ce3þ (3, 4), relative to NaI:Tl on Ce3þ concentration in the charge. The excitation sources were 137Cs (1, 3) and 241 Am (2, 4).
Birowosuto, M.D., Dorenbos, P., van Eijk, C.W.E., Kramer, K.W., Gudel, H.U., 2006. PrBr3:Ce3þ: a new fast lanthanide trihalide scintillator. IEEE Trans. Nucl. Sci. 53, 3028e3030. Bollinger, L.M., Thomas, G.E., 1961. Measurement of the time dependence of scintillation intensity by a delayed-coincidence method. Rev. Sci. Instrum. 32, 1045e1050. Cherginets, V.L., Ponomarenko, T.V., Trefilova, L.N., Rebrova, N.V., Kosinov, N.N., Alekseev, V.D., Zelenskaya, O.V., 2009. Preparation and scintillation properties of YCl3:Ce crystals. Inorg. Mater. 45, 946e948. Glodo, J., Shah, K.S., Klugerman, M., Wong, P., Higgins, B., Dorenbos, P., 2005. Scintillation properties of LuI3:Ce. Nucl. Instrum. Methods Phys. Res. Sect. A 537, 279e281. Glodo, J., van Loef, E.V.D., Higgins, W.M., Shah, K.S., 2008. Mixed lutetium iodide compounds. IEEE Trans. Nucl. Sci. NS-55, 1496e1500. Grippa, A.Y., Datsko, Y.N., Zelenskaya, O.V., Kosinov, N.N., 2010. Synthesis and scintillation properties of GdCl3:Ce3þ (Gd1-xCexCl3, x¼0.005e0.08). Radiat. Meas. 45, 398e399. Kramer, K.W., Dorenbos, P., Gudel, H.U., van Eijk, C.W.E., 2006. Development and characterization of highly efficient new cerium doped rare earth halide scintillator materials. J. Mater. Chem. 16, 2773e2780. Moszynsky, M., Kapusta, M., Mayhugh, M., Wolski, D., Flyckt, S.O., 1997. Absolute light output of scintillators. IEEE Trans. Nucl. Sci. 44, 1052e1061.
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van Loef, E.V.D., Dorenbos, P., van Eijk, C.W.E., Kramer, K., Gudel, H.U., 2001a. Scintillation properties of LaCl3:Ce3þ crystals: fast, efficient, and high-energy resolution scintillators. IEEE Trans. Nucl. Sci. 48, 341e345. van Loef, E.V.D., Dorenbos, P., van Eijk, C.W.E., Kramer, K., Gudel, H.U., 2001b. Optical and scintillation properties of pure and Ce3þ doped GdBr3. Opt. Commun. 189, 297e304.
van Loef, E.V.D., Dorenbos, P., van Eijk, C.W.E., Kramer, K., Gudel, H.U., 2002. Scintillation properties of LaBr3:Ce3þ crystals: fast, efficient and high-energyresolution scintillators. Nucl. Instrum. Methods Phys. Res. Sect. A 486, 254e258. van Loef, E.V.D., Dorenbos, P., van Eijk, C.W.E., Kramer, K., Gudel, H.U., 2003. Properties and mechanism of scintillation in LuCl3:Ce3þ and LuBr3:Ce3þ crystals. Nucl. Instrum. Methods Phys. Res. Sect. A 496, 138e145.
Contents lists available at SciVerse ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Scintillation properties of Ce3þ-doped YCl3 and YBr3 V.L. Cherginets a, b, *, T.P. Rebrova a, T.V. Ponomarenko a, O.A. Tarasenko a, N.N. Kosinov a, O.V. Zelenskaya a a b
Institute for Scintillation Materials, National Acad. Sci. Ukraine, Lenin Avenue 60, Kharkov 61001, Ukraine National Technical University, Kharkiv Polytechnical Institute, 21 Frunze St., 61002 Kharkov, Ukraine
h i g h l i g h t s < Maximal light yield value of YCl3:Ce and YBr3:Ce is 8700 and 20,600 photons per MeV. < Scintillation pulse decay of YCl3:Ce is described by two components e 37 and 640 ns. < Scintillation pulse decay of YBr3:Ce is described by two components e 36 and 450 ns. < For YCl3:Ce and YBr3:Ce fractions of faster component are 86 and 79%, respectively.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2011 Received in revised form 3 September 2012 Accepted 5 September 2012
The dependence of the scintillation properties of Ce3þ-doped YCl3 and YBr3 on activator concentration (0.5, 1 and 2 mol%) has been studied. The radioluminescence spectra of both materials contain asymmetric bands with maxima located at 3.13 eV (383 nm) for YCl3:Ce3þ and 2.84 eV (422 nm) for YBr3:Ce3þ. The scintillation pulse decay curves for both materials are described by two components with decay constants of 37 and 640 ns for YCl3:Ce3þ and 36 and 450 ns for YBr3:Ce3þ, the fractions of the faster component being 86 and 79 per cent, respectively. The dependences of the light yield of the studied materials on Ce3þ concentration pass through a maximum near 1 mol% of the activator, and the maximum light yields (relative to NaI:Tl) are 8700 photons per MeV for YCl3:Ce3þ and 20,600 photons per MeV for YBr3:Ce3þ. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Scintillation Bridgman Light yield Scintillation pulse decay
1. Introduction The recent discovery of high-performance scintillators based on Ce3þ-doped lanthanum halides (van Loef et al., 2001a, 2002) caused a revival in the development of new promising scintillation materials. Within a short time, similar compounds of gadolinium (van Loef et al., 2001b; Glodo et al., 2008; Grippa et al., 2010), praseodymium (Birowosuto et al., 2006) and lutetium (van Loef et al., 2003; Glodo et al., 2005) were studied to find other bright scintillation materials. Except lanthanum compounds, rare-earth metal iodides possess the highest light yields compared with the corresponding chlorides and bromides, and sometimes this parameter reaches up to 100,000 photons per MeV (Glodo et al., 2008). However, the synthesis of pure rare-earth iodides is extremely difficult. The best results are observed for materials obtained by
* Corresponding author. Institute for Scintillation Materials, National Acad. Sci. Ukraine, Lenin Avenue 60, Kharkov 61001, Ukraine. Tel.: þ380 57 3410218; fax: þ380 57 3404474. E-mail address: [email protected] (V.L. Cherginets). 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2012.09.002
direct synthesis from a rare-earth metal and iodine with subsequent vacuum distillation of the synthesized product (Kramer et al., 2006). This difficulty does favour wide industrial production of rare-earth iodide scintillators, whereas rare-earth halides formed by lighter halide ions (chlorides and bromides) may be more promising. For instance, LaCl3:10 mol% Ce3þ (BrilLanCeÔ 350) and LaBr3:5 mol% Ce3þ (BrilLanCeÔ 380) are now produced by Saint-Gobain (see http://www.detectors.saint-gobain.com/MaterialsGasTubes.aspx). Therefore, other Ce3þ-doped rare-earth chlorides and bromides may be of interest for material scientists, but the properties of these substances cannot as yet be predicted theoretically and this necessitates an empirical approach to the study of each new material (‘cook-and-look’). The purpose of this work is to study luminescence and scintillation properties of YCl3 and YBr3 doped by different concentrations of Ce3þ. It is well known that the light yield of Ce-doped LaCl3 and LaBr3 exceeds that of chlorides and bromides formed by heavier lanthanides and it is of interest to study the properties of the yttrium compounds as formed by a lighter analog of lanthanum. In this paper we report on the further development of our initial investigation of YCl3:Ce3þ (Cherginets et al., 2009) and also investigate a new scintillation material, YBr3:Ce3þ.
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V.L. Cherginets et al. / Radiation Measurements 47 (2012) 917e920
2. Experimental Anhydrous YCl3 and YBr3 were obtained by dissolution of a weight of Y2O3 (extra pure, 4 N) in the corresponding acid (HCl, extra pure, 4 N; HBr, reagent quality, 3 N) taken with excess of 5%. Separately an amount of CeO2 (reagent grade, 3 N) providing a preliminary chosen concentration of Ce3þ in the final crystal was dissolved in HCl or HBr; in the latter case formic acid was added for the reduction of CeIV to CeIII. The concentrations of Ce3þ in the growth charge were 0.5, 1 and 2 mol %. The solutions were mixed, and a weight of corresponding ammonium halide (NH4Cl or NH4Br, chemically pure, 3 N) providing NH4X:YX3 molar ratio not less than 6 (here ‘X’ is Cl or Br) was added to the mixed solution. This suspension was dried in air at 120e150 C, crushed and dried in vacuum (p 10 Pa) applying a slow increase in temperature from 100 to 400 C, and an 8-h pause at 200 C. The charge obtained was placed in an 18 mm dia. quartz ampoule and melted. The chloride melts were treated for 8 h in CCl4 vapour obtained by passing dry argon through liquid tetrachloromethane at room temperature (the pressure of the saturated vapour was approximately 12 kPa) to decrease the concentration of oxide admixtures. Before growth the melts were kept for a day in vacuum (p 10 Pa). The crystal growth was performed by the Bridgman method; the temperature at the diaphragm was 721 C (YCl3) and 905 C (YBr3), the temperature gradient was 6e8 C cm1 and the velocity of the ampoule descent was varied in the range 0.5e1.3 mm h1. The ingot obtained was cooled to room temperature at a rate of 2e3 C h1. To measure the functional characteristics of the materials, detectors of 12 mm dia. and 2 mm height were prepared. The radioluminescence spectra were measured under 241Am source excitation (g, 59.6 keV) and recorded using a MDR-23 monochromator. The pulse-height spectra were measured under gamma irradiation from 137Cs and 241Am sources (PMT Hamamatsu R1307). The scintillation pulse shape of the YCl3:Ce3þ and YBr3:Ce3þ detectors were obtained using the delayed-coincidence method (Bollinger and Thomas, 1961). Two 9142B (Electron Tubes Ltd.) photomultipliers were used for the detection of scintillation photons both in ‘start’ and in ‘stop’ channels of the optical part of the set-up. The detectors were irradiated by gamma photons of a 152Eu radionuclide source (g, 41, 77.9, 122 keV). The photomultiplier in the ‘start’ channel was used to give a zero-time signal and placed in optical contact with the scintillator. A scintillation pulse was attenuated by a diaphragm to single photon mode and then detected by the photomultiplier to generate a delayed timing signal in the ‘stop’ channel. 3. Results and discussion Yttrium chloride and bromide single crystals are referred to as layered materials and the growth of high-quality single crystals is very complicated. It should be noted that 300e400 h duration of the crystal growth procedure does not provide a high quality of grown crystal. Therefore, some parameters (e.g., energy resolution) cannot be measured. The radioluminescence spectra of Ce3þ-doped YCl3 and YBr3 recorded at room temperature (Fig. 1) contain one asymmetric with maxima at 3.13 eV (383 nm) and 2.84 eV (422 nm) for YCl3:Ce3þ and YBr3:Ce3þ, respectively. These bands are typical for Ce3þ luminescence, and they are the superposition of the components corresponding to the transitions from 5d level to 2 levels of the ground state of Ce3þ e 2F5/2 and 2F7/2. To obtain information about the composition of the scintillation pulse of the grown materials the curves of scintillation pulse decay
Fig. 1. Normalized radioluminescence spectra of YCl3:1 mol% Ce3þ (1) and YBr3:1 mol% Ce3þ (2) at 298 K. The excitation source was 241Am.
were constructed (Fig. 2). They are described by two components for both materials according to,
IðtÞ ¼ I0 $ðA1 expðt=s1 Þ þ A2 expðt=s2 ÞÞ;
(1)
where I0 and I(t) are the initial and current intensities of the scintillation pulse, t, the time, A1 and A2, the initial fractions of fast and slow components in the scintillation pulse, respectively, s1 and s2, the decay constants of the fast and slow components. In the case of YCl3:Ce3þ, the fast component is characterized by s1 ¼ 37 ns, and for YBr3:Ce3þ a similar value was obtained 36 ns. However, the fractions of the fast component are different: 0.86 for the chloride and 0.79 for bromide scintillators, respectively. These parameters are close to those obtained for Ce3þ-doped rare-earth metal halide scintillators (http://scintillator.lbl.gov/). As for the slow components, their decay constants are 640 and 450 ns, respectively. So, one can conclude that both materials could be referred to fast scintillators. The light yield of YCl3:Ce3þ and YBr3:Ce3þ materials was estimated on the basis of the pulse-height spectra and the examples obtained using an 241Am excitation source are presented in Fig. 3. It is apparent that their light yield is essentially lower than that of NaI:Tl, being 8% (YCl3:Ce3þ) and 37% (YBr3:Ce3þ) of that from NaI:Tl and, using a 137Cs excitation source, 9.6% and 43%, respectively. From Fig. 3 it follows that YBr3:Ce3þ is a brighter material than YCl3:Ce3þ, usually attributed to a decrease in the band gap width with the increase in the halide ion radius and the effect of Ce3þ concentration on light yield is shown in Fig. 4. Different Ce3þ-activated rare-earth halide scintillators are known to possess maximum light yield values in an activator concentration range from 0.1 to several mol.%. Therefore, it is sufficient to obtain 3e4 concentration values to characterize the scintillation parameters with the rise of the activator concentration (we obtained 3 points for each scintillator). The light yield values for both scintillators reach their maxima at concentration of Ce3þ close to 1 mol%. The absolute light output was calculated (M. Moszynsky et al., 1997) using the following equation,
Nphe ¼ Nph $hL $Q :E$ðlÞ$ε$E;
(2)
where Nphe is the number of photoelectrons, Nph is the absolute light yield, hL is the light collection efficiency, Q.E.(l) is quantum efficiency of PMT, ε is efficiency of photoelectron collection of PMT, E is the photon energy 0.662 MeV (137Cs). The number of
V.L. Cherginets et al. / Radiation Measurements 47 (2012) 917e920
919
Fig. 2. Scintillation pulse growth and decay curves recorded under 152Eu g-excitation: a) YCl3:1 mol% Ce3þ (1, black) and YBr3:1 mol% Ce3þ (2, grey) at 298 K; b) the decay curve for YBr3:1 mol% Ce3þ (1, black), the fast component (2, dashed, A1 ¼ 0.79, s1 ¼ 36 ns), the slow component (3, light grey, A2 ¼ 0.21, s2 ¼ 640 ns).
we can see a typical concentration dependence; the initial growth of the light yield results from an increase in the number of luminescence centers and, for concentrations exceeding 1 mol%, the light yield decreases owing to concentration quenching of the luminescence which is characteristic of activated scintillators. In the vicinity of 1 mol% of Ce3þ there is a plateau, where both effects are reciprocally compensated. 4. Conclusion
Fig. 3. Pulse height spectra of YCl3:1 mol% Ce3þ, (1, solid), YBr3:1 mol% Ce3þ, (2, dashed) in comparison with NaI:Tl (3, dotted). The excitation source was 241Am.
photoelectrons for a standard NaI:Tl sample was estimated to be 4400 (hL ¼ 0.661; Nph ¼ 40,000 ph MeV1; Q.E.(l) ¼ 0.928 (for a Hamamatsu R1307 PMT); ε ¼ 0.27. For YCl3:Ce3þ Nphe was 420 photoelectrons and for YBr3:Ce, 1890 photoelectrons hL was estimated to be 0.27 10% and 0.52 5% respectively and similarly the values of Q.E.(l), 0.995 and 0.98 respectively. The absolute values of light yield were 8700 photons per MeV for YCl3:Ce3þ and 20,600 photons per MeV for YBr3:Ce3þ. In Fig. 4
In this work YBr3:Ce3þ and YCl3:Ce3þ crystals with different activator concentrations were grown and all the crystals possessed a typical layered structure. The radioluminescence spectra of these materials contain one asymmetric band formed by two overlapping peaks corresponding to 5de4f transitions in the cerium ion. The scintillation light decay curves can be described by two components, the decay constants and the fractions of the corresponding components for which have been estimated. The light yield of YBr3:Ce3þ and YCl3:Ce3þ was measured and the concentration dependence passes through a maximum at 1 mol % of Ce3þ, typical of the behaviour of other activated scintillation materials. Acknowledgement The authors are very indebted to Dr. V.A. Tarasov (Institute for Scintillation Materials) for the discussion of some principal questions concerning the absolute light yield of the obtained materials. References
Fig. 4. The dependence of light yield of YBr3:Ce3þ (1, 2) and YCl3:Ce3þ (3, 4), relative to NaI:Tl on Ce3þ concentration in the charge. The excitation sources were 137Cs (1, 3) and 241 Am (2, 4).
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