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Luminescent properties of composite scintillators based on PPO and o-POPOP doped SiO2 xerogel matrices
Journal of Luminescence 179 (2016) 178–182
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Luminescent properties of composite scintillators based on PPO and o-POPOP doped SiO2 xerogel matrices O. Viagin a,n, A. Masalov a, I. Bespalova a, O. Zelenskaya a, V. Tarasov a, V. Seminko a, L. Voloshina a, Yu. Zorenko b, Yu. Malyukin a a b
Institute for Scintillation Materials of NAS of Ukraine, 60 Science Ave., 61001 Kharkiv, Ukraine Institute of Physics of Kazimierz Wielki University of Bydgoszcz, 2 Powstańców Wielkopolskich str., 85-090 Bydgoszcz, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 20 April 2016 Accepted 1 July 2016 Available online 7 July 2016
New composite scintillation detectors were obtained by incorporation of PPO and o-POPOP organic scintillators into porous sol–gel silica matrices. Composites possess high photoluminescence intensity and decay time in nanosecond range. The absolute light yield of composite scintillators at excitation by alpha-radiation is about 4000–5000 photons/MeV and the pulse–height resolution is about 30%. The investigations of time-resolved luminescence of composites performed under excitation by synchrotron radiation in the 3.7–25 eV range have shown that the non-radiative energy transfer between host matrix and dopant molecules occurs via singlet states of SiO2 oxygen-deficient centers. & 2016 Elsevier B.V. All rights reserved.
Keywords: SiO2 Composite scintillator Sol–gel method Luminescence Synchrotron radiation excitation
1. Introduction The search for materials for effective detection of high energy particles and radiation which started in the middle of the last century has finally led to a number of single crystal scintillators with the values of light yield and energy resolution close to the possible theoretical limits [1]. Nowadays the single crystals are considered to be indispensable as a base of scintillation detectors for security and environment control systems, as well as for radiation medicine. However, the conventional crystal scintillators have a number of unavoidable drawbacks, such as high production cost, specialized equipment required for its production as well as difficulties in obtaining large area materials. In the recent decade a search for reasonable alternatives to scintillation crystals and other types of traditional scintillators has led to composite materials represented first of all by sintered dispersed powders [2] and transparent matrices with dopants of different nature. Namely, the possibility of incorporation of different dopants, including fluorescent dyes [3–5], semiconductor quantum dots (QDs) [6–9] and organic scintillators [10], to the silica matrix was shown in the mentioned papers. Such composite materials have a wide range of applications: specifically, matrices doped by fluorescent dyes (such as oxazines [3], pyrromethenes [4] and rhodamines [5]) are perspective for creation of solid state dye lasers; matrices with QDs beyond traditional luminescent n
Corresponding author. Fax: þ38 057 340 32 07. E-mail address: [email protected] (O. Viagin).
applications were recently tested as detectors of alpha [8] and gamma radiation [9]. The studies [11,12] have shown that placing liquid organic scintillators B-PBD, p-TP [11], polystyrene [12] into silica host allows obtaining materials able to detect high energy particles. It was shown also, that such materials usually have lower scintillation characteristics as compared to crystal-based detectors, but are sufficiently cheaper and easier in production, so they can replace conventional scintillators in the fields where the values of light yield and energy resolution are not crucial. In several of the above mentioned articles, the silica xerogels obtained by sol–gel method, so-called sol–gel matrices, are used as a solid host. Sol–gel method has a number of advantages over the traditional methods of obtaining glass-type matrices: first of all it needs temperatures sufficiently lower than the melting point, by variation of synthesis parameters the materials with pores of various diameter can be obtained, also sol–gel materials are lowcost in production and do not require any additional machining such as grinding or polishing. Size and shape of the final product can be varied easily. Also these matrices have high transparency, chemical stability and sufficient mechanical durability. In our paper, new composite scintillators based on nanoporous sol–gel silica matrices with PPO and o-POPOP scintillation dopants are developed. It should be noted, that sol–gel based scintillators with PPO and o-POPOP were discussed previously [10]. But in [10] the scintillation dopants were introduced as precursors during matrix synthesis. In this case, the matrix cannot be annealed at temperature higher than 100–120 °C due to destruction of the organic molecules. This fact is a disadvantage, because it is known that unannealed sol–gel matrices are very brittle and absorb from
http://dx.doi.org/10.1016/j.jlumin.2016.07.001 0022-2313/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: O. Viagin, et al., J. Lumin. (2016), http://dx.doi.org/10.1016/j.jlumin.2016.07.001i
O. Viagin et al. / Journal of Luminescence 179 (2016) 178–182
the atmosphere contaminants of different nature acting as quenching agents [13]. Therefore, in our investigation the special method was chosen which connected with impregnation of SiO2 sol–gel matrices by solutions of organic scintillators after annealing of matrices at 750 °C. That method allowed us to obtain new composite scintillation material based on the PPO and o-POPOP doped SiO2 matrices and to determine their absolute light yield under alpha-particles excitation. For the first time we have studied also the luminescence properties of the obtained composites using VUV–UV synchrotron radiation in the 3.7–25 eV range which covered the transparency range, the exciton range and the range of interband transitions of SiO2 host.
2. Synthesis and characterization of composite scintillators and experimental technique Silica matrices were synthesized using standard sol–gel procedure with tetramethoxysilane (TMOS) as a precursor. At first, the mixture of TMOS (3.75 ml) with methanol (3.15 ml) was stirred for 5 min and again stirred for 10 min after addition of distilled water (4.5 ml) and hydrochloric acid (0.525 ml). The mixture was poured in ∅ 35 10 mm plastic Petri dishes and held for 24 h at room temperature until a gel was formed, which was then dried for 120 h at a temperature of 45 °C. Stepwise annealing of the samples at 250 °С, 500 °С and 750 °С with 60 min exposure at each
179
temperature was used to decrease open porosity and improve the mechanical strength of the matrices. Synthesized by us undoped silica matrices were transparent colorless samples in the form of the disks with sizes of ∅ 20 1 mm (Fig. 1(a)). According to XRD (Fig. 1(b)) and TEM (Fig. 1 (c)) the SiO2 matrices are amorphous and consist of sintered sphere-like nanoparticles. The density and porosity of matrices were determined by the method of hydrostatic weighing [14] and were equal to 1.40 g/cm3 and 29%, respectively. The average value of microhardness was 170 kg/mm2, which is almost two times higher than the values shown by other researchers [15,16]. Luminescence dopants 2,5-diphenyloxazole (PPO) and 1,2-bis (5-phenyl-oxazolyl-2)-benzene (o-POPOP) were introduced by impregnating the matrices in toluene solutions (10 3 mol%) of organic scintillators. The matrices were placed in solutions, held therein for different time (2–12 h) to vary concentration of dopant in the samples, and then dried at 50 °С until constant weight was achieved. Concentration of dopant was determined by the weight change of the sample before and after impregnation and drying. The most intensive photoluminescence was observed for composites concentration of about 4 mass% and these samples were chosen for further investigations. Optical transmission spectra were measured using a SPECORD 200 spectrophotometer (Analytik Jenа, Germany). The time resolved luminescence and excitation spectra, as well as the decay curves of the samples under study were measured at the
Fig. 1. Photo (a), XRD (b) and TEM (c) image of pure SiO2 matrix.
Please cite this article as: O. Viagin, et al., J. Lumin. (2016), http://dx.doi.org/10.1016/j.jlumin.2016.07.001i
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O. Viagin et al. / Journal of Luminescence 179 (2016) 178–182
SUPERLUMI station at HASYLAB, DESY (Hamburg, Germany) under excitation by pulsed (120 ps) synchrotron radiation (SR) with an energy of 3.7–25 eV at 300 K and 10 K. The excitation spectra were corrected on the sensitivity of monochromator and detector units but the emission spectra were not corrected. The pulse–height spectra were obtained using standard spectrometric setup consisting of preamplifier, linear amplifier and multi-channel analyzer AMA-03-F (Tensor, Russia). An R1307 photomultiplier tube (PMT) (Hamamatsu, Japan) was used as a photodetector. Scintillations were excited by alpha-particles with energy of 5.46 MeV using 238Рu radionuclide source. Conventional plastic scintillator (PS) based on polystyrene with size of ∅ 20 1 mm with light yield of 6000 photons/MeV was used as a reference sample. The main characteristic of any scintillator is the absolute light yield, i.e. the number of generated photons per unit of the absorbed energy of ionizing radiation, which is usually expressed in “photons/MeV”. To define the absolute light yield of composite scintillators, at first step the light output (in photons/MeV) of samples was determined by the method described in International standard IEC 62372:2006, section 5.3: “Determination of intrinsic resolution and light output of housed scintillator and PMT spectrometric constant using PMT parameters” [17]. The same 238Рu radionuclide source has been used in this procedure. The total measurement error was no more than 6.5%. It is known that light output of a scintillator does not take into account the light-collection conditions during experiment [18,19]. Therefore, in the second step, the obtained values of light output
Fig. 2. Transmission spectra of the composites. 1 – pure SiO2 matrix; 2 – SiO2:PPO, 3 – SiO2:o-POPOP.
were divided on the light-collection coefficients computed by the Monte Carlo simulation [20]. The size, optical characteristics of the samples (absorption coefficient in the range of the luminescence spectrum, as well as refraction and reflection indices) and the experimental conditions (no reflector, no immersion oil between sample and PMT, path length of alpha-particles) were used as input parameters for simulation.
3. Results and discussion 3.1. Transmittance spectra The transmission spectra of pure and doped matrices are shown in Fig. 2. The pure SiO2 matrix is transparent from 250 to 700 nm, the fundamental absorption edge corresponds to 200 nm, and the transmission at a wavelength of 450 nm is about 85% (curve 1). The absorption edges of SiO2:PPO and SiO2:o-POPOP are located at 370 nm and 420 nm, respectively, while the decrease of transmission down to 75% at 450 nm for both samples is caused by dopant absorption (curves 2 and 3). 3.2. Luminescent properties under synchrotron radiation excitation Fig. 3(a) and (b) shows the luminescence spectra of pure silica matrix and matrices doped with organic scintillators under excitation by SR in the fundamental absorption range of SiO2 host (Eex ¼6.8 eV). The intrinsic luminescence of pure SiO2 matrix (curve 1) with maximum at 390 nm (Fig. 3(a)) is determined by the S1–S0 singlet–singlet transition in oxygen-deficient centers [21]. The additional shoulder with maximum at 450 nm appears in the spectrum at 10 K (Fig. 3(b)). Most probably, the band at 450 nm is caused by the forbidden T1–S0 triplet–singlet transition [22]. The curves 2 and 3 in Fig. 3(a) and (b) represent the luminescence spectra of the composite scintillators. It should be noted here, that both spectra are red-shifted ( 30–35 nm) as compared to the spectra of PPO and o-POPOP in toluene (not shown) due to higher polarity of the matrix (bathochromic shift). From Fig. 3(a) is further seen, that under 6.8 eV excitation the spectra of SiO2:PPO and SiO2:o-POPOP composites consist of the dopant emission bands only, peaked at 440 and 470 nm, respectively. Lowering the temperature leads to the increase in the composites luminescence intensity and the slight blue-shift of the emission band maxima. The lack of the intrinsic luminescence of the matrix both at room and low temperature may indicate the efficient excitation energy transfer from the host to the dopant molecules.
Fig. 3. Luminescence spectra of the composites at 300 K (a) and 10 K (b), Eex ¼6.8 eV. 1 – pure SiO2 matrix; 2 – SiO2:PPO; 3 – SiO2:o-POPOP.
Please cite this article as: O. Viagin, et al., J. Lumin. (2016), http://dx.doi.org/10.1016/j.jlumin.2016.07.001i
O. Viagin et al. / Journal of Luminescence 179 (2016) 178–182
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Fig. 4. Decay curves of the composites at 300 K (a) and 10 K (b), Eex ¼ 6.8 eV. 1 – pure SiO2 matrix, λem ¼390 nm; 2 – SiO2:PPO, λem ¼ 440 nm; 3 – SiO2:o-POPOP, λem ¼ 470 nm.
Fig. 5. Excitation spectra of the composites at 300 K (a) and 10 K (b). 1 – pure SiO2 matrix, λem ¼ 390 nm; 2 – SiO2:PPO, λem ¼440 nm; 3 – SiO2:o-POPOP, λem ¼470 nm.
The decay curves of pure silica matrix and composites at host excitation are shown in Fig. 4(a) and (b). As was mentioned above, the luminescence spectrum of pure SiO2 matrix at 10 K consists of two overlapped bands formed probably by the S1–S0 and T1–S0 transitions of oxygen-deficient centers. Analysis of corresponding decay curves confirms clearly this supposition: while the decay curve of 390 nm band at 300 K was almost single-exponential with a lifetime of about 2 ns (Fig. 4(a)), at 10 K an additional delayed component was observed in the decay kinetics that can be ascribed to the contribution of long-lasting T1–S0 luminescence due to partial overlap of 450 nm and 390 nm bands (Fig. 4(b)). The luminescence of 450 nm band decays in the microsecond or in millisecond range and cannot be correctly measured in the conditions of our experiment due to very small repetition time of SR pulse (96 ns). The decay curve of SiO2:o-POPOP is exponential at 300 K with a lifetime of about τ0 ¼7 ns, while the decay curve of SiO2:PPO can be fitted by two exponents with τ1 ¼2.6 ns and τ2 ¼10.6 ns (curves 2 and 3 in Fig. 4(a)). The decay kinetics of doped matrices varies with decrease of temperature down to 10 K in different ways: the lifetime of SiO2:PPO increases (τ1 ¼3.4 ns and τ2 ¼ 14.2 ns), while that of SiO2:o-POPOP decreases, τ1 ¼5.2 ns (Fig. 4(b)). The absence of delayed component in the decay curves of SiO2:PPO and SiO2:oPOPOP can be considered as an evidence that energy transfer from silica matrix to scintillation dopants occurs via singlet but not triplet states of the matrix.
The excitation spectra of the samples at 300 K and 10 K are shown in Fig. 5(a) and (b). The spectra of pure SiO2 exhibit several peaks in the range of 4–8 eV. At 300 K (Fig. 5(a)), excitation spectra of SiO2:PPO and SiO2:o-POPOP composites measured at 440 nm and 470 nm bands, are similar and slightly differ from those of the host, while generally repeat their shape in the 3.7–25 eV measurement range pointing to the non-radiative energy transfer from the host to dopant. It is interesting to note, that the shape of SiO2: o-POPOP spectrum remains almost unchanged at low temperature (Fig. 5(b)), while the SiO2:PPO excitation intensity decreases in the region from 5 eV to 8 eV. This is probably due to the different mechanisms of energy transfer in composites. But unfortunately, direct investigation of the mechanism of energy transfer between host and scintillation dopants (for instance, by analysis of matrix decay curves) was impossible because the intrinsic luminescence disappears even at 10 2 mass% of dopant molecules in matrix. Moreover, the introduction of low dopant concentrations by the impregnation method is a challenging task in order to control concentration and uniform distribution of molecules within the volume of the sample. 3.3. Scintillation properties The pulse–height spectra of the composites and reference PS under excitation by alpha-radiation are shown in Fig. 6. It can be seen that both spectra of the composites are symmetric and well fitted by Gaussian function (inset in Fig. 6). However, the relative
Please cite this article as: O. Viagin, et al., J. Lumin. (2016), http://dx.doi.org/10.1016/j.jlumin.2016.07.001i
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O. Viagin et al. / Journal of Luminescence 179 (2016) 178–182
characteristics of the composites, by optimization the dopant concentration or using the secondary spectral shifters. Additionally, we have found that sol–gel matrix can be doped with ions with high thermal neutron capture cross section, such as Gd and Cd. And, importantly, up to 10 mass% of dopant concentration the transparency and mechanical characteristics of the matrices do not decrease. Therefore, in prospect these composite scintillators can be used as neutron detectors, and our further research will be focused on this point.
Acknowledgments
Fig. 6. Pulse–height spectra of the composite scintillators and the reference plastic scintillator under alpha irradiation of 238Рu source, Eα ¼ 5.46 MeV. 1 – reference PS; 2 – SiO2:PPO; 3 – SiO2:o-POPOP. The inset represents the Gaussian fit of the spectra of composites (solid lines).
The investigations with synchrotron radiation at the SUPERLUMI station were performed in the frame of I-20110938 EC project. The paper includes also the results obtained in the frame of Polish NCN No. 2012/07/B/ST5/02376 research project.
References Table 1 Luminescence and scintillation characteristics of composite scintillators and reference plastic scintillator (PS). λmax is the maximum of the luminescence spectrum, τ is the luminescence decay time, R is the pulse–height resolution, L is the relative light output, kLC is the light-collection coefficient and Nph is the absolute light yield. Scintillator
λmax (nm)
PS 430 SiO2:PPO 440 SiO2:o-POPOP 470
τ (ns) (at 300 K)
R (%) L (%) kLC
2.7 2.6, 10.6 7
18 33 31
100 15 22
Nph (photons/ MeV)
0.5 6000 0.14 4400 0.14 5100
light output of the composites, i.e. position of the photopeaks relative to PS, is not greater than 22%. We have found that the composites have high absorption coefficient over the range of luminescence spectrum, which is about 0.25 cm 1. In this case, since under irradiation by alpha-particles only the surface layer of the sample, generally of several dozens of microns, is excited, the significant part of all the generated scintillation photons is reabsorbed in the volume of composites and does not reach the input window of PMT. Using the measured value of the absorption coefficient in the simulations we have obtained the lightcollection coefficient kLC ¼ 0.14 for both samples. Simultaneously, at the same experimental conditions, kLC for the reference PS, having the absorption coefficient equal to 0.005 cm 1, is 0.5. Therefore, taking into account the light-collection coefficient, the final values of absolute light yield are 4400 and 5100 photons/MeV for SiO2:PPO and SiO2:o-POPOP composite scintillators, respectively. The measured characteristics of the composites are summarized in Table 1.
4. Conclusions The impregnation of pores of silica sol–gel matrices by organic scintillators allows to produce a new scintillation material. The number of generated photons in the obtained composites under alpha-radiation is lower than that of the conventional scintillation single crystals [23], but is comparable to the one of the commercial plastic scintillators [24]. Due to high reabsorption the relative light output of the composites is only about 20% of light output of the polystyrene scintillator. So, it is necessary to improve the optical
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Please cite this article as: O. Viagin, et al., J. Lumin. (2016), http://dx.doi.org/10.1016/j.jlumin.2016.07.001i
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Luminescent properties of composite scintillators based on PPO and o-POPOP doped SiO2 xerogel matrices O. Viagin a,n, A. Masalov a, I. Bespalova a, O. Zelenskaya a, V. Tarasov a, V. Seminko a, L. Voloshina a, Yu. Zorenko b, Yu. Malyukin a a b
Institute for Scintillation Materials of NAS of Ukraine, 60 Science Ave., 61001 Kharkiv, Ukraine Institute of Physics of Kazimierz Wielki University of Bydgoszcz, 2 Powstańców Wielkopolskich str., 85-090 Bydgoszcz, Poland
art ic l e i nf o
a b s t r a c t
Article history: Received 20 April 2016 Accepted 1 July 2016 Available online 7 July 2016
New composite scintillation detectors were obtained by incorporation of PPO and o-POPOP organic scintillators into porous sol–gel silica matrices. Composites possess high photoluminescence intensity and decay time in nanosecond range. The absolute light yield of composite scintillators at excitation by alpha-radiation is about 4000–5000 photons/MeV and the pulse–height resolution is about 30%. The investigations of time-resolved luminescence of composites performed under excitation by synchrotron radiation in the 3.7–25 eV range have shown that the non-radiative energy transfer between host matrix and dopant molecules occurs via singlet states of SiO2 oxygen-deficient centers. & 2016 Elsevier B.V. All rights reserved.
Keywords: SiO2 Composite scintillator Sol–gel method Luminescence Synchrotron radiation excitation
1. Introduction The search for materials for effective detection of high energy particles and radiation which started in the middle of the last century has finally led to a number of single crystal scintillators with the values of light yield and energy resolution close to the possible theoretical limits [1]. Nowadays the single crystals are considered to be indispensable as a base of scintillation detectors for security and environment control systems, as well as for radiation medicine. However, the conventional crystal scintillators have a number of unavoidable drawbacks, such as high production cost, specialized equipment required for its production as well as difficulties in obtaining large area materials. In the recent decade a search for reasonable alternatives to scintillation crystals and other types of traditional scintillators has led to composite materials represented first of all by sintered dispersed powders [2] and transparent matrices with dopants of different nature. Namely, the possibility of incorporation of different dopants, including fluorescent dyes [3–5], semiconductor quantum dots (QDs) [6–9] and organic scintillators [10], to the silica matrix was shown in the mentioned papers. Such composite materials have a wide range of applications: specifically, matrices doped by fluorescent dyes (such as oxazines [3], pyrromethenes [4] and rhodamines [5]) are perspective for creation of solid state dye lasers; matrices with QDs beyond traditional luminescent n
Corresponding author. Fax: þ38 057 340 32 07. E-mail address: [email protected] (O. Viagin).
applications were recently tested as detectors of alpha [8] and gamma radiation [9]. The studies [11,12] have shown that placing liquid organic scintillators B-PBD, p-TP [11], polystyrene [12] into silica host allows obtaining materials able to detect high energy particles. It was shown also, that such materials usually have lower scintillation characteristics as compared to crystal-based detectors, but are sufficiently cheaper and easier in production, so they can replace conventional scintillators in the fields where the values of light yield and energy resolution are not crucial. In several of the above mentioned articles, the silica xerogels obtained by sol–gel method, so-called sol–gel matrices, are used as a solid host. Sol–gel method has a number of advantages over the traditional methods of obtaining glass-type matrices: first of all it needs temperatures sufficiently lower than the melting point, by variation of synthesis parameters the materials with pores of various diameter can be obtained, also sol–gel materials are lowcost in production and do not require any additional machining such as grinding or polishing. Size and shape of the final product can be varied easily. Also these matrices have high transparency, chemical stability and sufficient mechanical durability. In our paper, new composite scintillators based on nanoporous sol–gel silica matrices with PPO and o-POPOP scintillation dopants are developed. It should be noted, that sol–gel based scintillators with PPO and o-POPOP were discussed previously [10]. But in [10] the scintillation dopants were introduced as precursors during matrix synthesis. In this case, the matrix cannot be annealed at temperature higher than 100–120 °C due to destruction of the organic molecules. This fact is a disadvantage, because it is known that unannealed sol–gel matrices are very brittle and absorb from
http://dx.doi.org/10.1016/j.jlumin.2016.07.001 0022-2313/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: O. Viagin, et al., J. Lumin. (2016), http://dx.doi.org/10.1016/j.jlumin.2016.07.001i
O. Viagin et al. / Journal of Luminescence 179 (2016) 178–182
the atmosphere contaminants of different nature acting as quenching agents [13]. Therefore, in our investigation the special method was chosen which connected with impregnation of SiO2 sol–gel matrices by solutions of organic scintillators after annealing of matrices at 750 °C. That method allowed us to obtain new composite scintillation material based on the PPO and o-POPOP doped SiO2 matrices and to determine their absolute light yield under alpha-particles excitation. For the first time we have studied also the luminescence properties of the obtained composites using VUV–UV synchrotron radiation in the 3.7–25 eV range which covered the transparency range, the exciton range and the range of interband transitions of SiO2 host.
2. Synthesis and characterization of composite scintillators and experimental technique Silica matrices were synthesized using standard sol–gel procedure with tetramethoxysilane (TMOS) as a precursor. At first, the mixture of TMOS (3.75 ml) with methanol (3.15 ml) was stirred for 5 min and again stirred for 10 min after addition of distilled water (4.5 ml) and hydrochloric acid (0.525 ml). The mixture was poured in ∅ 35 10 mm plastic Petri dishes and held for 24 h at room temperature until a gel was formed, which was then dried for 120 h at a temperature of 45 °C. Stepwise annealing of the samples at 250 °С, 500 °С and 750 °С with 60 min exposure at each
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temperature was used to decrease open porosity and improve the mechanical strength of the matrices. Synthesized by us undoped silica matrices were transparent colorless samples in the form of the disks with sizes of ∅ 20 1 mm (Fig. 1(a)). According to XRD (Fig. 1(b)) and TEM (Fig. 1 (c)) the SiO2 matrices are amorphous and consist of sintered sphere-like nanoparticles. The density and porosity of matrices were determined by the method of hydrostatic weighing [14] and were equal to 1.40 g/cm3 and 29%, respectively. The average value of microhardness was 170 kg/mm2, which is almost two times higher than the values shown by other researchers [15,16]. Luminescence dopants 2,5-diphenyloxazole (PPO) and 1,2-bis (5-phenyl-oxazolyl-2)-benzene (o-POPOP) were introduced by impregnating the matrices in toluene solutions (10 3 mol%) of organic scintillators. The matrices were placed in solutions, held therein for different time (2–12 h) to vary concentration of dopant in the samples, and then dried at 50 °С until constant weight was achieved. Concentration of dopant was determined by the weight change of the sample before and after impregnation and drying. The most intensive photoluminescence was observed for composites concentration of about 4 mass% and these samples were chosen for further investigations. Optical transmission spectra were measured using a SPECORD 200 spectrophotometer (Analytik Jenа, Germany). The time resolved luminescence and excitation spectra, as well as the decay curves of the samples under study were measured at the
Fig. 1. Photo (a), XRD (b) and TEM (c) image of pure SiO2 matrix.
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SUPERLUMI station at HASYLAB, DESY (Hamburg, Germany) under excitation by pulsed (120 ps) synchrotron radiation (SR) with an energy of 3.7–25 eV at 300 K and 10 K. The excitation spectra were corrected on the sensitivity of monochromator and detector units but the emission spectra were not corrected. The pulse–height spectra were obtained using standard spectrometric setup consisting of preamplifier, linear amplifier and multi-channel analyzer AMA-03-F (Tensor, Russia). An R1307 photomultiplier tube (PMT) (Hamamatsu, Japan) was used as a photodetector. Scintillations were excited by alpha-particles with energy of 5.46 MeV using 238Рu radionuclide source. Conventional plastic scintillator (PS) based on polystyrene with size of ∅ 20 1 mm with light yield of 6000 photons/MeV was used as a reference sample. The main characteristic of any scintillator is the absolute light yield, i.e. the number of generated photons per unit of the absorbed energy of ionizing radiation, which is usually expressed in “photons/MeV”. To define the absolute light yield of composite scintillators, at first step the light output (in photons/MeV) of samples was determined by the method described in International standard IEC 62372:2006, section 5.3: “Determination of intrinsic resolution and light output of housed scintillator and PMT spectrometric constant using PMT parameters” [17]. The same 238Рu radionuclide source has been used in this procedure. The total measurement error was no more than 6.5%. It is known that light output of a scintillator does not take into account the light-collection conditions during experiment [18,19]. Therefore, in the second step, the obtained values of light output
Fig. 2. Transmission spectra of the composites. 1 – pure SiO2 matrix; 2 – SiO2:PPO, 3 – SiO2:o-POPOP.
were divided on the light-collection coefficients computed by the Monte Carlo simulation [20]. The size, optical characteristics of the samples (absorption coefficient in the range of the luminescence spectrum, as well as refraction and reflection indices) and the experimental conditions (no reflector, no immersion oil between sample and PMT, path length of alpha-particles) were used as input parameters for simulation.
3. Results and discussion 3.1. Transmittance spectra The transmission spectra of pure and doped matrices are shown in Fig. 2. The pure SiO2 matrix is transparent from 250 to 700 nm, the fundamental absorption edge corresponds to 200 nm, and the transmission at a wavelength of 450 nm is about 85% (curve 1). The absorption edges of SiO2:PPO and SiO2:o-POPOP are located at 370 nm and 420 nm, respectively, while the decrease of transmission down to 75% at 450 nm for both samples is caused by dopant absorption (curves 2 and 3). 3.2. Luminescent properties under synchrotron radiation excitation Fig. 3(a) and (b) shows the luminescence spectra of pure silica matrix and matrices doped with organic scintillators under excitation by SR in the fundamental absorption range of SiO2 host (Eex ¼6.8 eV). The intrinsic luminescence of pure SiO2 matrix (curve 1) with maximum at 390 nm (Fig. 3(a)) is determined by the S1–S0 singlet–singlet transition in oxygen-deficient centers [21]. The additional shoulder with maximum at 450 nm appears in the spectrum at 10 K (Fig. 3(b)). Most probably, the band at 450 nm is caused by the forbidden T1–S0 triplet–singlet transition [22]. The curves 2 and 3 in Fig. 3(a) and (b) represent the luminescence spectra of the composite scintillators. It should be noted here, that both spectra are red-shifted ( 30–35 nm) as compared to the spectra of PPO and o-POPOP in toluene (not shown) due to higher polarity of the matrix (bathochromic shift). From Fig. 3(a) is further seen, that under 6.8 eV excitation the spectra of SiO2:PPO and SiO2:o-POPOP composites consist of the dopant emission bands only, peaked at 440 and 470 nm, respectively. Lowering the temperature leads to the increase in the composites luminescence intensity and the slight blue-shift of the emission band maxima. The lack of the intrinsic luminescence of the matrix both at room and low temperature may indicate the efficient excitation energy transfer from the host to the dopant molecules.
Fig. 3. Luminescence spectra of the composites at 300 K (a) and 10 K (b), Eex ¼6.8 eV. 1 – pure SiO2 matrix; 2 – SiO2:PPO; 3 – SiO2:o-POPOP.
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Fig. 4. Decay curves of the composites at 300 K (a) and 10 K (b), Eex ¼ 6.8 eV. 1 – pure SiO2 matrix, λem ¼390 nm; 2 – SiO2:PPO, λem ¼ 440 nm; 3 – SiO2:o-POPOP, λem ¼ 470 nm.
Fig. 5. Excitation spectra of the composites at 300 K (a) and 10 K (b). 1 – pure SiO2 matrix, λem ¼ 390 nm; 2 – SiO2:PPO, λem ¼440 nm; 3 – SiO2:o-POPOP, λem ¼470 nm.
The decay curves of pure silica matrix and composites at host excitation are shown in Fig. 4(a) and (b). As was mentioned above, the luminescence spectrum of pure SiO2 matrix at 10 K consists of two overlapped bands formed probably by the S1–S0 and T1–S0 transitions of oxygen-deficient centers. Analysis of corresponding decay curves confirms clearly this supposition: while the decay curve of 390 nm band at 300 K was almost single-exponential with a lifetime of about 2 ns (Fig. 4(a)), at 10 K an additional delayed component was observed in the decay kinetics that can be ascribed to the contribution of long-lasting T1–S0 luminescence due to partial overlap of 450 nm and 390 nm bands (Fig. 4(b)). The luminescence of 450 nm band decays in the microsecond or in millisecond range and cannot be correctly measured in the conditions of our experiment due to very small repetition time of SR pulse (96 ns). The decay curve of SiO2:o-POPOP is exponential at 300 K with a lifetime of about τ0 ¼7 ns, while the decay curve of SiO2:PPO can be fitted by two exponents with τ1 ¼2.6 ns and τ2 ¼10.6 ns (curves 2 and 3 in Fig. 4(a)). The decay kinetics of doped matrices varies with decrease of temperature down to 10 K in different ways: the lifetime of SiO2:PPO increases (τ1 ¼3.4 ns and τ2 ¼ 14.2 ns), while that of SiO2:o-POPOP decreases, τ1 ¼5.2 ns (Fig. 4(b)). The absence of delayed component in the decay curves of SiO2:PPO and SiO2:oPOPOP can be considered as an evidence that energy transfer from silica matrix to scintillation dopants occurs via singlet but not triplet states of the matrix.
The excitation spectra of the samples at 300 K and 10 K are shown in Fig. 5(a) and (b). The spectra of pure SiO2 exhibit several peaks in the range of 4–8 eV. At 300 K (Fig. 5(a)), excitation spectra of SiO2:PPO and SiO2:o-POPOP composites measured at 440 nm and 470 nm bands, are similar and slightly differ from those of the host, while generally repeat their shape in the 3.7–25 eV measurement range pointing to the non-radiative energy transfer from the host to dopant. It is interesting to note, that the shape of SiO2: o-POPOP spectrum remains almost unchanged at low temperature (Fig. 5(b)), while the SiO2:PPO excitation intensity decreases in the region from 5 eV to 8 eV. This is probably due to the different mechanisms of energy transfer in composites. But unfortunately, direct investigation of the mechanism of energy transfer between host and scintillation dopants (for instance, by analysis of matrix decay curves) was impossible because the intrinsic luminescence disappears even at 10 2 mass% of dopant molecules in matrix. Moreover, the introduction of low dopant concentrations by the impregnation method is a challenging task in order to control concentration and uniform distribution of molecules within the volume of the sample. 3.3. Scintillation properties The pulse–height spectra of the composites and reference PS under excitation by alpha-radiation are shown in Fig. 6. It can be seen that both spectra of the composites are symmetric and well fitted by Gaussian function (inset in Fig. 6). However, the relative
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characteristics of the composites, by optimization the dopant concentration or using the secondary spectral shifters. Additionally, we have found that sol–gel matrix can be doped with ions with high thermal neutron capture cross section, such as Gd and Cd. And, importantly, up to 10 mass% of dopant concentration the transparency and mechanical characteristics of the matrices do not decrease. Therefore, in prospect these composite scintillators can be used as neutron detectors, and our further research will be focused on this point.
Acknowledgments
Fig. 6. Pulse–height spectra of the composite scintillators and the reference plastic scintillator under alpha irradiation of 238Рu source, Eα ¼ 5.46 MeV. 1 – reference PS; 2 – SiO2:PPO; 3 – SiO2:o-POPOP. The inset represents the Gaussian fit of the spectra of composites (solid lines).
The investigations with synchrotron radiation at the SUPERLUMI station were performed in the frame of I-20110938 EC project. The paper includes also the results obtained in the frame of Polish NCN No. 2012/07/B/ST5/02376 research project.
References Table 1 Luminescence and scintillation characteristics of composite scintillators and reference plastic scintillator (PS). λmax is the maximum of the luminescence spectrum, τ is the luminescence decay time, R is the pulse–height resolution, L is the relative light output, kLC is the light-collection coefficient and Nph is the absolute light yield. Scintillator
λmax (nm)
PS 430 SiO2:PPO 440 SiO2:o-POPOP 470
τ (ns) (at 300 K)
R (%) L (%) kLC
2.7 2.6, 10.6 7
18 33 31
100 15 22
Nph (photons/ MeV)
0.5 6000 0.14 4400 0.14 5100
light output of the composites, i.e. position of the photopeaks relative to PS, is not greater than 22%. We have found that the composites have high absorption coefficient over the range of luminescence spectrum, which is about 0.25 cm 1. In this case, since under irradiation by alpha-particles only the surface layer of the sample, generally of several dozens of microns, is excited, the significant part of all the generated scintillation photons is reabsorbed in the volume of composites and does not reach the input window of PMT. Using the measured value of the absorption coefficient in the simulations we have obtained the lightcollection coefficient kLC ¼ 0.14 for both samples. Simultaneously, at the same experimental conditions, kLC for the reference PS, having the absorption coefficient equal to 0.005 cm 1, is 0.5. Therefore, taking into account the light-collection coefficient, the final values of absolute light yield are 4400 and 5100 photons/MeV for SiO2:PPO and SiO2:o-POPOP composite scintillators, respectively. The measured characteristics of the composites are summarized in Table 1.
4. Conclusions The impregnation of pores of silica sol–gel matrices by organic scintillators allows to produce a new scintillation material. The number of generated photons in the obtained composites under alpha-radiation is lower than that of the conventional scintillation single crystals [23], but is comparable to the one of the commercial plastic scintillators [24]. Due to high reabsorption the relative light output of the composites is only about 20% of light output of the polystyrene scintillator. So, it is necessary to improve the optical
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