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Scintillation properties of CsPrP4O12 and RbPrP4O12
Nuclear Instruments and Methods in Physics Research A 486 (2002) 283–287
Scintillation properties of CsPrP4O12 and RbPrP4O12 K. Horchania, J.-C. Ga# cona,*, C. Dujardina, N. Garniera, M. Feridb, M. Trabelsi-Ayedic a
Laboratoire de Physico-Chimie des Mat!eriaux Luminescents, UMR No. 5620, CNRS-Universit!e Claude Bernard Lyon-1, 69622 Villeurbanne, France b Laboratoire des Proc!ed!es Chimiques, Institut National de Recherche Scientifique et Technique, B.P. 95 Hammam-Lif, 2050 Tunis, Tunisia c Laboratoire de Physico-Chimie Min!erale, Universit!e du 7 Novembre, Facult!e des Sciences de Bizerte, 7021 Zarzouna, Bizerte, Tunisia
Abstract Single crystals of CsPrP4O12 and RbPrP4O12 cyclotetraphosphates have been grown using the flux method. Selective excitations in the Pr3+ (4f2) 3P0 and 3P2 levels using an excimer-pumped dye laser mainly result in intense line emissions in both materials. Though originating from transitions within the Pr3+ 4f2 ground configuration, these emissions have very short decay times, on the order of 50 ns. Both compounds also exhibit intense ultraviolet band emissions in the 200–300 nm spectral range when using either a xenon lamp or a X-ray source for optical excitation. The decays of these emissions under pulsed X-ray excitation were measured using the single-photon counting technique. They exhibit a main fast component (decay time tf ) and a minor slower one (decay time ts ), the decay times of which being tf D9 and 5 ns, ts D30 and 20 ns, in the Cs and Rb compounds, respectively. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.40; 78.47; 78.55; 78.70.E; 81.05 Keywords: Praseodymium; Cyclotetraphoshates; 4f–5d-4f2 emission; Scintillation
1. Introduction 4fN1–5d-4fN fluorescent emissions of trivalent rare-earth ions in crystalline hosts received at present a special interest due to their potential applications in both fast scintillator and ultraviolet (UV) tunable solid-state laser devices [1–3]. Investigations were first focused on cerium compounds, which still receive a considerable interest. More recently, trivalent praseodymium (Pr3+) materials have gained interest [4–6]. They are indeed promising scintillating materials for medi*Corresponding author. Fax: +33-72-431130. E-mail address: [email protected] (J.-C. G#acon).
cal imaging devices, such as positron emission tomographs, due to the strong intensity and the short decay-time (typically of the order of 10 ns) of the parity-allowed electric-dipole 4f–5d-4f2 fluorescent emissions [6,7]. Indeed, a large number of works have been devoted to the spectroscopy of Pr3+ in phosphates, including stoichiometric compounds [8–13]. The present paper aims to report on primary results concerning both 4f2-4f2 and 4f–5d-4f2 transitions of Pr3+ in two concentrated materials, CsPrP4O12 and RbPrP4O12, which have been recently synthesized in our laboratories. Experimental details concerning the synthesis of the materials and the spectroscopic measurements appear in Section 2. The
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 7 1 9 - 2
284
K. Horchani et al. / Nuclear Instruments and Methods in Physics Research A 486 (2002) 283–287
results are presented and discussed in Section 3 which is followed by a conclusion.
2. Experimental details MPrP4O12 (M=Cs, Rb) single crystals were grown using the flux method described elsewhere [14,15]. Monovalent carbonate M2CO3 and praseodymium oxide Pr6O11 were dissolved in phosphoric acid H3PO4 using molar ratios M:Pr:P=5:1:15. The resulting solution was then heated in a vitreous graphite crucible at 473 K for 12 h, then at 573 K for 6 days. After that, the excess of phosphoric acid was removed by hot water. Single crystals of dimensions on the order of 102 mm3 were obtained. X-ray diffraction pattern measurements have shown that CsPrP4O12 is isostructural to RbNdP4O12 with the cubic space group I43d (Td6 ) and point group symmetry S4 at % 3+ the Pr sites [14], while RbPrP4O12 is isostructural to NH4PrP4O12 with the monoclinic space 6 group C2=c (C2h ) and point group symmetry C2 3+ at the Pr sites [16]. Due to their small sizes, the crystals were ground to obtain powders for all the spectroscopic measurements. Fluorescence spectra and decays under pulsed laser excitation in the 3P0 and 3P2 levels were measured using an excimer-pumped dye laser delivering pulses of 10 ns duration and 0.1 cm1 spectral width with a 10 Hz repetition rate. Courmarine 480 and 440 dyes were used for excitations in the 3P0 and 3P2 levels, respectively. The dye laser output beam was focused onto the powder holder under glancing incidence, its energy being always kept below 250 mJ per pulse. A 1-m Monospek 1000 Hilger and Watts monochromator was used either for scanning fluorescence spectra or selecting a given line emission for fluorescence decay measurements. Appropriate filters were introduced before the entrance slit in order to reduce the diffused laser light. A Peltier effect cooled GaAs photomultiplier tube (PMT) detected the fluorescence signal which was processed using a SR 250 gated integrator and boxcar averager providing an integrated signal to a DAC card coupled to a computer for the emission and excitation spectra measurements, while a LeCroy
9410 oscilloscope coupled to the computer was used for recording the fluorescence decays. Fluorescence spectra and decays under pulsed X-ray excitation were measured using a tungsten anode X-ray source delivering pulses of 1 ns duration with a 22 kHz repetition rate, the high voltage being set at 40 kV. The UV fluorescences were collected using an optical fiber attached to a Jobin–Yvon Triax 320 monochromator and detected by a 9789B EMI PMT. Due to their very short decay times, the fluorescence decays of the observed UV emissions were measured using the single-photon counting technique. These emissions were also observed using a cw xenon lamp. In this case, a H10D Jobin–Yvon monochromator was used for selecting the appropriate UV interval in the xenon lamp spectrum.
3. Results and discussion Generally speaking, CsPrP4O12 and RbPrP4O12 exhibit very similar optical properties, as shown from the spectra displayed below. In both CsPrP4O12 and RbPrP4O12 materials, laser excitation in the 3P0 state results at room temperature in an intense orange fluorescence visible to the naked eye. The spectrum of this fluorescence exhibits two main lines peaking around 609 and 638 nm originating from 3P0-3H6 and 3P0-3F2 transitions, respectively. These emissions decay exponentially with decay times of the order of 53 and 55 ns for CsPrP4O12 and RbPrP4O12, respectively. These values are to be compared with the 80 ns measured for the 3P0 fluorescence decay time in KPrP4O12 [10]. In this latter compound, the calculated 3P0 radiative lifetime was found to be on the order of 41.6 ms and a fluorescence quenching process involving 3P0-1D2 nonradiative transitions was assumed to be the main contribution to the nonradiative decay. The very fast decays of the 3P0 fluorescences in MPrP4O12 (M=K, Cs, Pr) may also originate from energy transfers between Pr3+ ions since concentrated materials are under consideration. However, the observed 3P0 emissions should be much weaker since interactions between Pr3+ ions usually have fluorescence quenching effects.
K. Horchani et al. / Nuclear Instruments and Methods in Physics Research A 486 (2002) 283–287
P0 → F2
3
P0 → H6
Fluorescence intensity (a.u.)
3
3
285
3
P0→ H4
3
3
P0 → F4
3
P1 → H5 + P0→ H5
3
3
3
3
3
RbPrP4O12
P0→ H5
3
3
CsPrP4O12
450
500
550
600
650
700
750
Wavelength (nm) Fig. 1. Room temperature emission spectra of CsPrP4O12 and RbPrP4O12 under excitation in the 3P2 multiplet.
The CsPrP4O12 and RbPrP4O12 room temperature emission spectra obtained under laser excitation in the 3P2 multiplet are shown in Fig. 1. The main lines in these spectra correspond to transitions originating from the 3P0 level. The nonradiative processes coupling the 3P2 and 3P1 levels to 3P0 are indeed very efficient at room temperature, most probably due to the large values of the lattice phonon energies (up to 1330 cm1) in cyclotetraphosphates [17]. Thus, it is clear that Pr3+ ions in CsPrP4O12 and RbPrP4O12 are efficient luminescent centres which could play a role in a scintillating process if the irradiation of these materials with g- or X-rays yields excitation of these centres. It was therefore tempting to extend the domain of excitation to the X-rays. Emission spectra of CsPrP4O12 and RbPrP4O12 powdered samples under pulsed Xray excitation are displayed in Figs. 2a and b, respectively. They exhibit two main strong UV bands in the 35 000–50 000 cm1 spectral range and other weaker ones at lower energies. It is
worth noting that no 4f2-4f2 line emission is seen in these spectra. The UV bands were also observed under excitation at 210 nm using a cw xenon lamp. They may be reasonably attributed to parityallowed electric dipole transitions between the lowest 4f–5d Stark level and the lower lying 4f2 states. Further investigations at low temperature are in progress to assign these transitions more precisely. The emission spectrum of a Bi4Ge3O12 powdered sample was recorded under the same experimental conditions as those of the spectra in Figs. 2a and b for purpose of comparison (Fig. 2c). All the spectra shown in Figs. 2a–c were corrected for the spectral response of the analyzing system (monochromator+PMT). Due to the lack of efficiency of the PMT for wavelengths higher than 600 nm (D16 660 cm1), correction effects strongly affect the experimental spectra in the lower energy range. Therefore, a Gaussian fit of the Bi4Ge3O12 spectrum was used in order to reproduce the whole spectrum (Fig. 2d). Relative light yields of CsPrP4O12 and RbPrP4O12 materials were then
286
K. Horchani et al. / Nuclear Instruments and Methods in Physics Research A 486 (2002) 283–287
(a) (b) (c) (d) --------
Fluorescence intensity (a.u.)
3000000
(c) 2000000
1000000
(a) (d) (b)
0 0
5
10
15
20
25
30 3
35
40
45
50
55
-1
Energy (x10 cm ) Fig. 2. Room temperature emission spectra of (a) CsPrP4O12, (b) RbPrP4O12 and (c) Bi4Ge3O12 under X-ray excitation. The dashed curve (d) is a Gaussian fit of the Bi4Ge3O12 spectrum.
1
Intensity (a.u.)
(a)
(a)
(b)
0.1
τf = 9 ns a f = 0.81 τs = 30 ns a = 0.14 s τf = 5 ns τs = 20 ns
a f = 0.76 a s = 0.08
(b)
0.01
1E-3 0
20
40
60
time (ns) Fig. 3. Room temperature decays of the UV band emission at 250 nm (40 000 cm1) in (a) CsPrP4O12 and (b) RbPrP4O12 under X-ray excitation. The solid lines correspond to fits using an expression of the form: IðtÞ ¼ af expðt=tf Þ þ as expðt=ts Þ:
obtained from the areas under the spectra displayed in Figs. 2a, b and d. These relative light yields are of the order of 37% and 16% for CsPrP4O12 and RbPrP4O12, respectively. It is to be
noted that these values are approximate, since (i) the number of X-ray photons absorbed in the sample depends on the mass density of the material and (ii) the size of the powder grains
K. Horchani et al. / Nuclear Instruments and Methods in Physics Research A 486 (2002) 283–287
affects the flux of the resulting fluorescence photons. The decays of the two main UV band emissions observed under X-ray excitation are not purely exponential (Fig. 3). They exhibit a main (>85%) fast component (decay time tf ) and a minor (o15%) slower one (decay time ts ), the decay times of which being found to be tf D9 and 5 ns, ts D30 and 20 ns, in the CsPrP4O12 and RbPrP4O12 materials, respectively. These values are of the same order of magnitude than those measured in the Pr3+-doped lutetium orthoaluminate [6].
4. Concluding remarks Primary investigations in the optical properties of CsPrP4O12 and RbPrP4O12 have shown that these concentrated praseodymium compounds exhibit intense line emissions in the visible range with very fast decay times when excited in the Pr3+ 3 P0 and 3P2 levels. These materials also exhibit strong band emissions in the near UV domain with decay times of the order of a few ns, when irradiated by X-rays. Estimated values of the light yields of CsPrP4O12 and RbPrP4O12 were found to fall around 37% and 16% of the Bi4Ge3O12 one, respectively. As a matter of fact, these materials would be of interest for applications as fast scintillators if single crystals of appropriate size and good optical quality could be synthesized.
287
References [1] D.J. Ehrlich, P.F. Moulton, R.M. Osgood Jr., Opt. Lett. 4 (1979) 184. [2] D.J. Ehrlich, P.F. Moulton, R.M. Osgood Jr., Opt. Lett. 5 (1980) 339. [3] P. Dorenbos, R. Visser, C.W.E. van Eijk, N.M. Khaidukov, M.V. Korzhik, IEEE Trans. Nucl. Sci. 40 (1993) 388. [4] E. Sarantopoulou, A.C. Cefalas, M.A. Dubinski, C.A. Nicolaides, R.Yu. Abdulsabirov, S.L. Korableva, A.K. Naumov, V.V. Semashko, Opt. Lett. 19 (1994) 499. [5] M. Laroche, A. Braud, S. Girard, J.L. Doualan, R. Moncorg!e, M. Thuau, J. Opt. Soc. Am. B 16 (1999) 2269. [6] C. Dujardin, C. P!edrini, J.-C. G#acon, A.G. Petrosyan, A.N. Belsky, A.N. Vasil’ev, J. Phys. Condens. Matter 9 (1997) 5229. [7] L. Zhang, et al., Radiat. Effects Defects Solids 150 (1999) 47. [8] H. Dornauf, J. Heber, J. Lumin. 22 (1980) 1. [9] Z. Mazurak, E. Lukowiak, B. Jezowska-Trzebiatowska, D. Schultze, Ch. Waligora, J. Phys. Chem. Solids 45 (1984) 487. [10] M. Malinowski, R. Wolski, W. Wolinski, J. Lumin. 35 (1986) 1. [11] M. Malinowski, W. Strek, J. Phys. C Solid State Phys. 20 (1987) 2595. [12] L. Li, J. Wang, Z. Yg, H. Jiang, Y. Liu, J. Phys. Chem. 93 (1989) 6867. [13] S. Zhang, J. Lumin. 40&41 (1988) 159. [14] G.I. Dorokhova, O.S. Filipenko, L.O. Atovmyan, B.N. Litvin, Russ. J. Inorg. Chem. 33 (1988) 1581. [15] A. Durif, Crystal Chemistry of Condensed Phosphates, Plenum, New York, 1995, p. 258. [16] R. Masse, J.-C. Guittel, A. Durif, Acta Crystallogr. B 33 (1977) 630. [17] G. Foumakoye, R. Cahay, P. Tarte, Spectrochim. Acta 46A (1990) 1245.
Scintillation properties of CsPrP4O12 and RbPrP4O12 K. Horchania, J.-C. Ga# cona,*, C. Dujardina, N. Garniera, M. Feridb, M. Trabelsi-Ayedic a
Laboratoire de Physico-Chimie des Mat!eriaux Luminescents, UMR No. 5620, CNRS-Universit!e Claude Bernard Lyon-1, 69622 Villeurbanne, France b Laboratoire des Proc!ed!es Chimiques, Institut National de Recherche Scientifique et Technique, B.P. 95 Hammam-Lif, 2050 Tunis, Tunisia c Laboratoire de Physico-Chimie Min!erale, Universit!e du 7 Novembre, Facult!e des Sciences de Bizerte, 7021 Zarzouna, Bizerte, Tunisia
Abstract Single crystals of CsPrP4O12 and RbPrP4O12 cyclotetraphosphates have been grown using the flux method. Selective excitations in the Pr3+ (4f2) 3P0 and 3P2 levels using an excimer-pumped dye laser mainly result in intense line emissions in both materials. Though originating from transitions within the Pr3+ 4f2 ground configuration, these emissions have very short decay times, on the order of 50 ns. Both compounds also exhibit intense ultraviolet band emissions in the 200–300 nm spectral range when using either a xenon lamp or a X-ray source for optical excitation. The decays of these emissions under pulsed X-ray excitation were measured using the single-photon counting technique. They exhibit a main fast component (decay time tf ) and a minor slower one (decay time ts ), the decay times of which being tf D9 and 5 ns, ts D30 and 20 ns, in the Cs and Rb compounds, respectively. r 2002 Elsevier Science B.V. All rights reserved. PACS: 78.40; 78.47; 78.55; 78.70.E; 81.05 Keywords: Praseodymium; Cyclotetraphoshates; 4f–5d-4f2 emission; Scintillation
1. Introduction 4fN1–5d-4fN fluorescent emissions of trivalent rare-earth ions in crystalline hosts received at present a special interest due to their potential applications in both fast scintillator and ultraviolet (UV) tunable solid-state laser devices [1–3]. Investigations were first focused on cerium compounds, which still receive a considerable interest. More recently, trivalent praseodymium (Pr3+) materials have gained interest [4–6]. They are indeed promising scintillating materials for medi*Corresponding author. Fax: +33-72-431130. E-mail address: [email protected] (J.-C. G#acon).
cal imaging devices, such as positron emission tomographs, due to the strong intensity and the short decay-time (typically of the order of 10 ns) of the parity-allowed electric-dipole 4f–5d-4f2 fluorescent emissions [6,7]. Indeed, a large number of works have been devoted to the spectroscopy of Pr3+ in phosphates, including stoichiometric compounds [8–13]. The present paper aims to report on primary results concerning both 4f2-4f2 and 4f–5d-4f2 transitions of Pr3+ in two concentrated materials, CsPrP4O12 and RbPrP4O12, which have been recently synthesized in our laboratories. Experimental details concerning the synthesis of the materials and the spectroscopic measurements appear in Section 2. The
0168-9002/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 2 ) 0 0 7 1 9 - 2
284
K. Horchani et al. / Nuclear Instruments and Methods in Physics Research A 486 (2002) 283–287
results are presented and discussed in Section 3 which is followed by a conclusion.
2. Experimental details MPrP4O12 (M=Cs, Rb) single crystals were grown using the flux method described elsewhere [14,15]. Monovalent carbonate M2CO3 and praseodymium oxide Pr6O11 were dissolved in phosphoric acid H3PO4 using molar ratios M:Pr:P=5:1:15. The resulting solution was then heated in a vitreous graphite crucible at 473 K for 12 h, then at 573 K for 6 days. After that, the excess of phosphoric acid was removed by hot water. Single crystals of dimensions on the order of 102 mm3 were obtained. X-ray diffraction pattern measurements have shown that CsPrP4O12 is isostructural to RbNdP4O12 with the cubic space group I43d (Td6 ) and point group symmetry S4 at % 3+ the Pr sites [14], while RbPrP4O12 is isostructural to NH4PrP4O12 with the monoclinic space 6 group C2=c (C2h ) and point group symmetry C2 3+ at the Pr sites [16]. Due to their small sizes, the crystals were ground to obtain powders for all the spectroscopic measurements. Fluorescence spectra and decays under pulsed laser excitation in the 3P0 and 3P2 levels were measured using an excimer-pumped dye laser delivering pulses of 10 ns duration and 0.1 cm1 spectral width with a 10 Hz repetition rate. Courmarine 480 and 440 dyes were used for excitations in the 3P0 and 3P2 levels, respectively. The dye laser output beam was focused onto the powder holder under glancing incidence, its energy being always kept below 250 mJ per pulse. A 1-m Monospek 1000 Hilger and Watts monochromator was used either for scanning fluorescence spectra or selecting a given line emission for fluorescence decay measurements. Appropriate filters were introduced before the entrance slit in order to reduce the diffused laser light. A Peltier effect cooled GaAs photomultiplier tube (PMT) detected the fluorescence signal which was processed using a SR 250 gated integrator and boxcar averager providing an integrated signal to a DAC card coupled to a computer for the emission and excitation spectra measurements, while a LeCroy
9410 oscilloscope coupled to the computer was used for recording the fluorescence decays. Fluorescence spectra and decays under pulsed X-ray excitation were measured using a tungsten anode X-ray source delivering pulses of 1 ns duration with a 22 kHz repetition rate, the high voltage being set at 40 kV. The UV fluorescences were collected using an optical fiber attached to a Jobin–Yvon Triax 320 monochromator and detected by a 9789B EMI PMT. Due to their very short decay times, the fluorescence decays of the observed UV emissions were measured using the single-photon counting technique. These emissions were also observed using a cw xenon lamp. In this case, a H10D Jobin–Yvon monochromator was used for selecting the appropriate UV interval in the xenon lamp spectrum.
3. Results and discussion Generally speaking, CsPrP4O12 and RbPrP4O12 exhibit very similar optical properties, as shown from the spectra displayed below. In both CsPrP4O12 and RbPrP4O12 materials, laser excitation in the 3P0 state results at room temperature in an intense orange fluorescence visible to the naked eye. The spectrum of this fluorescence exhibits two main lines peaking around 609 and 638 nm originating from 3P0-3H6 and 3P0-3F2 transitions, respectively. These emissions decay exponentially with decay times of the order of 53 and 55 ns for CsPrP4O12 and RbPrP4O12, respectively. These values are to be compared with the 80 ns measured for the 3P0 fluorescence decay time in KPrP4O12 [10]. In this latter compound, the calculated 3P0 radiative lifetime was found to be on the order of 41.6 ms and a fluorescence quenching process involving 3P0-1D2 nonradiative transitions was assumed to be the main contribution to the nonradiative decay. The very fast decays of the 3P0 fluorescences in MPrP4O12 (M=K, Cs, Pr) may also originate from energy transfers between Pr3+ ions since concentrated materials are under consideration. However, the observed 3P0 emissions should be much weaker since interactions between Pr3+ ions usually have fluorescence quenching effects.
K. Horchani et al. / Nuclear Instruments and Methods in Physics Research A 486 (2002) 283–287
P0 → F2
3
P0 → H6
Fluorescence intensity (a.u.)
3
3
285
3
P0→ H4
3
3
P0 → F4
3
P1 → H5 + P0→ H5
3
3
3
3
3
RbPrP4O12
P0→ H5
3
3
CsPrP4O12
450
500
550
600
650
700
750
Wavelength (nm) Fig. 1. Room temperature emission spectra of CsPrP4O12 and RbPrP4O12 under excitation in the 3P2 multiplet.
The CsPrP4O12 and RbPrP4O12 room temperature emission spectra obtained under laser excitation in the 3P2 multiplet are shown in Fig. 1. The main lines in these spectra correspond to transitions originating from the 3P0 level. The nonradiative processes coupling the 3P2 and 3P1 levels to 3P0 are indeed very efficient at room temperature, most probably due to the large values of the lattice phonon energies (up to 1330 cm1) in cyclotetraphosphates [17]. Thus, it is clear that Pr3+ ions in CsPrP4O12 and RbPrP4O12 are efficient luminescent centres which could play a role in a scintillating process if the irradiation of these materials with g- or X-rays yields excitation of these centres. It was therefore tempting to extend the domain of excitation to the X-rays. Emission spectra of CsPrP4O12 and RbPrP4O12 powdered samples under pulsed Xray excitation are displayed in Figs. 2a and b, respectively. They exhibit two main strong UV bands in the 35 000–50 000 cm1 spectral range and other weaker ones at lower energies. It is
worth noting that no 4f2-4f2 line emission is seen in these spectra. The UV bands were also observed under excitation at 210 nm using a cw xenon lamp. They may be reasonably attributed to parityallowed electric dipole transitions between the lowest 4f–5d Stark level and the lower lying 4f2 states. Further investigations at low temperature are in progress to assign these transitions more precisely. The emission spectrum of a Bi4Ge3O12 powdered sample was recorded under the same experimental conditions as those of the spectra in Figs. 2a and b for purpose of comparison (Fig. 2c). All the spectra shown in Figs. 2a–c were corrected for the spectral response of the analyzing system (monochromator+PMT). Due to the lack of efficiency of the PMT for wavelengths higher than 600 nm (D16 660 cm1), correction effects strongly affect the experimental spectra in the lower energy range. Therefore, a Gaussian fit of the Bi4Ge3O12 spectrum was used in order to reproduce the whole spectrum (Fig. 2d). Relative light yields of CsPrP4O12 and RbPrP4O12 materials were then
286
K. Horchani et al. / Nuclear Instruments and Methods in Physics Research A 486 (2002) 283–287
(a) (b) (c) (d) --------
Fluorescence intensity (a.u.)
3000000
(c) 2000000
1000000
(a) (d) (b)
0 0
5
10
15
20
25
30 3
35
40
45
50
55
-1
Energy (x10 cm ) Fig. 2. Room temperature emission spectra of (a) CsPrP4O12, (b) RbPrP4O12 and (c) Bi4Ge3O12 under X-ray excitation. The dashed curve (d) is a Gaussian fit of the Bi4Ge3O12 spectrum.
1
Intensity (a.u.)
(a)
(a)
(b)
0.1
τf = 9 ns a f = 0.81 τs = 30 ns a = 0.14 s τf = 5 ns τs = 20 ns
a f = 0.76 a s = 0.08
(b)
0.01
1E-3 0
20
40
60
time (ns) Fig. 3. Room temperature decays of the UV band emission at 250 nm (40 000 cm1) in (a) CsPrP4O12 and (b) RbPrP4O12 under X-ray excitation. The solid lines correspond to fits using an expression of the form: IðtÞ ¼ af expðt=tf Þ þ as expðt=ts Þ:
obtained from the areas under the spectra displayed in Figs. 2a, b and d. These relative light yields are of the order of 37% and 16% for CsPrP4O12 and RbPrP4O12, respectively. It is to be
noted that these values are approximate, since (i) the number of X-ray photons absorbed in the sample depends on the mass density of the material and (ii) the size of the powder grains
K. Horchani et al. / Nuclear Instruments and Methods in Physics Research A 486 (2002) 283–287
affects the flux of the resulting fluorescence photons. The decays of the two main UV band emissions observed under X-ray excitation are not purely exponential (Fig. 3). They exhibit a main (>85%) fast component (decay time tf ) and a minor (o15%) slower one (decay time ts ), the decay times of which being found to be tf D9 and 5 ns, ts D30 and 20 ns, in the CsPrP4O12 and RbPrP4O12 materials, respectively. These values are of the same order of magnitude than those measured in the Pr3+-doped lutetium orthoaluminate [6].
4. Concluding remarks Primary investigations in the optical properties of CsPrP4O12 and RbPrP4O12 have shown that these concentrated praseodymium compounds exhibit intense line emissions in the visible range with very fast decay times when excited in the Pr3+ 3 P0 and 3P2 levels. These materials also exhibit strong band emissions in the near UV domain with decay times of the order of a few ns, when irradiated by X-rays. Estimated values of the light yields of CsPrP4O12 and RbPrP4O12 were found to fall around 37% and 16% of the Bi4Ge3O12 one, respectively. As a matter of fact, these materials would be of interest for applications as fast scintillators if single crystals of appropriate size and good optical quality could be synthesized.
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