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A new efficient X-ray phosphor: green emitting terbium activated yttrium-gadolinium-lutetium oxyorthosilicate
173
Materials Chemistry and Physics, 31 (1992) 173-374
A new efficient X-ray phosphor: green omitting terbium activated yttrium-gadolinium-lutetium oxyorthosilicate R. Morlotti 3M-Ita~~a Ricer&e, Ferrania (Sv) titan)
Abstract YC.6Gdo.8Luo.6SiOS:T(0.1) belongs to a family of new blue a/o green emitting phosphors constituted by ternary substitutional solid solutions, characterized by high X-ray efficiencies and excellent chemical stability and resistance to exposure damage. Its overall efficiency, measured at 70 KVp on single powder layers, is a factor 1.20 higher than that of a commercial green emitting Gd,O,S:Tb having similar grain size (3.5 p, approx.). A value between 0.18 and 0.20 is estimated for the intrinsic conversion efficiency.
Introduction
X-ray phosphors are used today in three main radiographic systems. In the conventional and most widely used one, a double face film is assembled in a cassette between two phosphor screens whose luminescence spectrum suitably matches the silver halide sensitivity, possibly extended to longer wavelengths. Essentially the same phosphors are used in more recent digital devices where the luminescence is picked up by electronic photodetectors and the image can be seen on a display and reproduced as hard copies. The third system is storage digital radiography, based on the storage properties of particular phosphors, emitting when photost~ulated by laser scanning after X-ray exposure. The phosphor considered here is characterized by immediate emission during exposure. Rare earth oxyorthosilicates, as single compounds or binary solid solutions, emitting during CR or UV excitation, are described in several articles and patents [l--6]. One of them [5], mentions X-ray luminescence properties of Gd,O,S:Tb. The present paper refers to some X-ray luminescent properties, i.e. R.T. emission spectra and a preliminary evaluation of the intrinsic conversion efficiency, of the green emitting Y,.~Gd,.*Lua,Si05:Tb(0.1), belonging to a family of new blue a/o green emitting phosphors recently described in a patent [7].
present ternary Y-Gd-Lu oxyorthosilicate substitutional solid solutions. 4N rare earth oxides (Rhane-Poulenc) were dissolved in diluted HN03. The solution was mixed with ethyl alcohol and Si(Oc;H,), and, finally, with NH,OH, a gel was obtained. After drying and a sequence of firings in air at different temperatures, up to 1400 “C, the material was cooled to RT and homogenized in an agate mortar. The product was a white crystalline powder with a grain size comprised between 1 and 6 ,u. It has high chemical stability and resistance to radiation damage. The chemical density is about 6.5 g/cm3. The crystal structure, obtained by X-ray powder diffraction, is monoclinic. Lattice parameters, with reference to the space group 12/c, are: a= 10.42(4) A, b=6.73(3) A, c = 12.51(5) A, /3= 102.8” ES]. Emission spectra were obtained by irradiating single binderless phosphor layers, 1 mm thick, assembled with a ‘rear’ configuration in a metal holder provided with a polyester window. The Xray tube was at 70 KVp, 30 mA. Light intensity was measured by an Intensified Detector (EG&G Part) Mod. 1455R 51 2HQ, filtering X-rays with 2.5 mm Al. Spectra were obtained by a JarrelAsh Monospec 27 Monochromator. Data elaboration was performed by an Optical Multichannel Analyzer Mod. 1460 (EG&G Part).
Experimental
Results and discussion
A gelling technique, previously used to prepare Sm activated Mg,Si04 [8], was optimized for the
The light emission spectra SiO,:Tb, for two concentrations
0254-0584/92/$5.00
0
of Y,_,GdO.sLuo.hof Tb, i.e. 0.1 and
1992 - Elsevier Sequoia.
All rights reserved
174
0.01 at per mole respectively, are shown in Fig. 1. The typical blue emission, from the 5D3 level, evident at the lower Tb content, is absent at the higher one, due to concentration quenching [lo]. Green to red emissions, from the 5D4 level, are remarkably enhanced in the latter case. Figure 2 shows the emission spectrum of a commercial Gd,O,S:Tb phosphor, having a similar grain size. The overall luminescence intensity, obtained by integration in the spectral range 360-640 nm, of the oxyorthosilicate with higher Tb content is by a factor 1.2 higher than that of the oxysulfide. Such an intensity ratio permits the estimation of the intrinsic X-ray to light conversion efficiency of the oxyorthosilicate, when the ‘true’ X-ray attenuation coefficient and the light extinction coefficient are known [ll]. In the present case, the former has been calculated using the X-ray cross sections at the various energies [12]; it is 70
kV
30
mA
33.9 cm-’ for the oxyorthosilicate and 44 cm-’ for the oxysulfide. Light extinction coefficients have been considered similar. Taking the value 0.16 for the intrinsic conversion efficiency of Gd,O,S:Tb [13], and the ratio 1.2 for the measured intensities, a value in the range 0.18-0.20 for the intrinsic conversion efficiency of Y0.6Gdo.8Lu,,6Si05:Tb(0.1) can be estimated. The result of this preliminary evaluation agrees with the predictions in [5], concerning the efficiency of an optimized Gd,SiO,:Tb X-ray phosphor. Conclusions
Green emitting Y0.6Gd,,8Lu0.6Si05:Tb(0.1) belongs to a family of ternary oxyorthosilicate compositions, Tb and, possibly, Ce coactivated, characterized by high blue or green luminescence efficiency under X-ray irradiation. They have high chemical stability and resistance to exposure damage. The overall efficiency of such a green emitting composition, measured at 70 KVp, 30 mA, is by a factor 1.2 higher than that of a commercial Gd,O$!Zl’b green emitting X-ray phosphor having similar grain size (3.5 p, approx.). A value between 0.18 and 0.20 is estimated for the intrinsic conversion efficiency. Acknowledgements
Thanks are due to Mr. S. Bruno for the efficiency measurements. Wavelength
nm
Fig. 1. X-ray light emission spectra, at room temperature (70 KVp, 30 mA), of Y0,6Gd,,.sLu,&Si05:Tb, at twoTb concentration (0.01 and 0.1 at per mole), crossed and continuous line respectively). 70
kV
30
mA
70
0
340
380
420
460 Wovelength
500
540
580
620
1
“m
Fig. 2. X-ray light emission spectrum, (70 KVp, 30 mA), of Gd,O,S:Tb.
at room temperature
References 1 W. L. Wanmaker, W. P. De Graaf and H. L. Spier, Physica, 25 (1959) 1125. 2 T. E. Peters, i. Electrochem. Sot., 116 (1969) 985. 3 K. Takagy and T. Fukazawa, A&. Phys. Lett., 42 (1983) 43. 4 J. HBlsl, K. Jyrkls and M. Leskell, J. Less-Common Metals, 126 (1986) 215. 5 M. J. J. Lammers and G. Blasse, J. Electrochem. Sot., 134 (1987) 2068. 6 J. Shmulovich, G. W. Berkstresser, C. D. Brandle and A. Valentino, J. Electrochem. Sot., 135 (1988) 3141. 7 S. Kemmler-Sack, R. Morlotti and J. Reichardt, U.S. Pat. 4 928 017 (1990). 8 R. Morlotti and G. Ottonello, Geoch. Cosmoch. Acta, 48 (1984) 1. 9 J. Reichardt, Dirsertation (Dokt, Natwwiss.), Inst, fiir An. Chemie, Univ. Tiibingen, 1987. 10 G. Blasse and A. Bril, Phi&s Res. Repts, 22 (1967) 481. 11 H. A. Klasens and W. De Groot, Rev. Tech. Philips, 11 (9) (1947) 321. 12 E. Storm and H. I. Israel, Rep. U-3753, Los Alamos Sci Lab., 1967. 13 G. Zweig and D. A. Zweig, J. of Imag. Tech., 10 (1984) 43.
Materials Chemistry and Physics, 31 (1992) 173-374
A new efficient X-ray phosphor: green omitting terbium activated yttrium-gadolinium-lutetium oxyorthosilicate R. Morlotti 3M-Ita~~a Ricer&e, Ferrania (Sv) titan)
Abstract YC.6Gdo.8Luo.6SiOS:T(0.1) belongs to a family of new blue a/o green emitting phosphors constituted by ternary substitutional solid solutions, characterized by high X-ray efficiencies and excellent chemical stability and resistance to exposure damage. Its overall efficiency, measured at 70 KVp on single powder layers, is a factor 1.20 higher than that of a commercial green emitting Gd,O,S:Tb having similar grain size (3.5 p, approx.). A value between 0.18 and 0.20 is estimated for the intrinsic conversion efficiency.
Introduction
X-ray phosphors are used today in three main radiographic systems. In the conventional and most widely used one, a double face film is assembled in a cassette between two phosphor screens whose luminescence spectrum suitably matches the silver halide sensitivity, possibly extended to longer wavelengths. Essentially the same phosphors are used in more recent digital devices where the luminescence is picked up by electronic photodetectors and the image can be seen on a display and reproduced as hard copies. The third system is storage digital radiography, based on the storage properties of particular phosphors, emitting when photost~ulated by laser scanning after X-ray exposure. The phosphor considered here is characterized by immediate emission during exposure. Rare earth oxyorthosilicates, as single compounds or binary solid solutions, emitting during CR or UV excitation, are described in several articles and patents [l--6]. One of them [5], mentions X-ray luminescence properties of Gd,O,S:Tb. The present paper refers to some X-ray luminescent properties, i.e. R.T. emission spectra and a preliminary evaluation of the intrinsic conversion efficiency, of the green emitting Y,.~Gd,.*Lua,Si05:Tb(0.1), belonging to a family of new blue a/o green emitting phosphors recently described in a patent [7].
present ternary Y-Gd-Lu oxyorthosilicate substitutional solid solutions. 4N rare earth oxides (Rhane-Poulenc) were dissolved in diluted HN03. The solution was mixed with ethyl alcohol and Si(Oc;H,), and, finally, with NH,OH, a gel was obtained. After drying and a sequence of firings in air at different temperatures, up to 1400 “C, the material was cooled to RT and homogenized in an agate mortar. The product was a white crystalline powder with a grain size comprised between 1 and 6 ,u. It has high chemical stability and resistance to radiation damage. The chemical density is about 6.5 g/cm3. The crystal structure, obtained by X-ray powder diffraction, is monoclinic. Lattice parameters, with reference to the space group 12/c, are: a= 10.42(4) A, b=6.73(3) A, c = 12.51(5) A, /3= 102.8” ES]. Emission spectra were obtained by irradiating single binderless phosphor layers, 1 mm thick, assembled with a ‘rear’ configuration in a metal holder provided with a polyester window. The Xray tube was at 70 KVp, 30 mA. Light intensity was measured by an Intensified Detector (EG&G Part) Mod. 1455R 51 2HQ, filtering X-rays with 2.5 mm Al. Spectra were obtained by a JarrelAsh Monospec 27 Monochromator. Data elaboration was performed by an Optical Multichannel Analyzer Mod. 1460 (EG&G Part).
Experimental
Results and discussion
A gelling technique, previously used to prepare Sm activated Mg,Si04 [8], was optimized for the
The light emission spectra SiO,:Tb, for two concentrations
0254-0584/92/$5.00
0
of Y,_,GdO.sLuo.hof Tb, i.e. 0.1 and
1992 - Elsevier Sequoia.
All rights reserved
174
0.01 at per mole respectively, are shown in Fig. 1. The typical blue emission, from the 5D3 level, evident at the lower Tb content, is absent at the higher one, due to concentration quenching [lo]. Green to red emissions, from the 5D4 level, are remarkably enhanced in the latter case. Figure 2 shows the emission spectrum of a commercial Gd,O,S:Tb phosphor, having a similar grain size. The overall luminescence intensity, obtained by integration in the spectral range 360-640 nm, of the oxyorthosilicate with higher Tb content is by a factor 1.2 higher than that of the oxysulfide. Such an intensity ratio permits the estimation of the intrinsic X-ray to light conversion efficiency of the oxyorthosilicate, when the ‘true’ X-ray attenuation coefficient and the light extinction coefficient are known [ll]. In the present case, the former has been calculated using the X-ray cross sections at the various energies [12]; it is 70
kV
30
mA
33.9 cm-’ for the oxyorthosilicate and 44 cm-’ for the oxysulfide. Light extinction coefficients have been considered similar. Taking the value 0.16 for the intrinsic conversion efficiency of Gd,O,S:Tb [13], and the ratio 1.2 for the measured intensities, a value in the range 0.18-0.20 for the intrinsic conversion efficiency of Y0.6Gdo.8Lu,,6Si05:Tb(0.1) can be estimated. The result of this preliminary evaluation agrees with the predictions in [5], concerning the efficiency of an optimized Gd,SiO,:Tb X-ray phosphor. Conclusions
Green emitting Y0.6Gd,,8Lu0.6Si05:Tb(0.1) belongs to a family of ternary oxyorthosilicate compositions, Tb and, possibly, Ce coactivated, characterized by high blue or green luminescence efficiency under X-ray irradiation. They have high chemical stability and resistance to exposure damage. The overall efficiency of such a green emitting composition, measured at 70 KVp, 30 mA, is by a factor 1.2 higher than that of a commercial Gd,O$!Zl’b green emitting X-ray phosphor having similar grain size (3.5 p, approx.). A value between 0.18 and 0.20 is estimated for the intrinsic conversion efficiency. Acknowledgements
Thanks are due to Mr. S. Bruno for the efficiency measurements. Wavelength
nm
Fig. 1. X-ray light emission spectra, at room temperature (70 KVp, 30 mA), of Y0,6Gd,,.sLu,&Si05:Tb, at twoTb concentration (0.01 and 0.1 at per mole), crossed and continuous line respectively). 70
kV
30
mA
70
0
340
380
420
460 Wovelength
500
540
580
620
1
“m
Fig. 2. X-ray light emission spectrum, (70 KVp, 30 mA), of Gd,O,S:Tb.
at room temperature
References 1 W. L. Wanmaker, W. P. De Graaf and H. L. Spier, Physica, 25 (1959) 1125. 2 T. E. Peters, i. Electrochem. Sot., 116 (1969) 985. 3 K. Takagy and T. Fukazawa, A&. Phys. Lett., 42 (1983) 43. 4 J. HBlsl, K. Jyrkls and M. Leskell, J. Less-Common Metals, 126 (1986) 215. 5 M. J. J. Lammers and G. Blasse, J. Electrochem. Sot., 134 (1987) 2068. 6 J. Shmulovich, G. W. Berkstresser, C. D. Brandle and A. Valentino, J. Electrochem. Sot., 135 (1988) 3141. 7 S. Kemmler-Sack, R. Morlotti and J. Reichardt, U.S. Pat. 4 928 017 (1990). 8 R. Morlotti and G. Ottonello, Geoch. Cosmoch. Acta, 48 (1984) 1. 9 J. Reichardt, Dirsertation (Dokt, Natwwiss.), Inst, fiir An. Chemie, Univ. Tiibingen, 1987. 10 G. Blasse and A. Bril, Phi&s Res. Repts, 22 (1967) 481. 11 H. A. Klasens and W. De Groot, Rev. Tech. Philips, 11 (9) (1947) 321. 12 E. Storm and H. I. Israel, Rep. U-3753, Los Alamos Sci Lab., 1967. 13 G. Zweig and D. A. Zweig, J. of Imag. Tech., 10 (1984) 43.