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Structured alkali halides for medical applications
Nuclear Instruments and Methods in Physics Research B 191 (2002) 800–804 www.elsevier.com/locate/nimb
Structured alkali halides for medical applications B. Schmitt a
a,*
, M. Fuchs a, E. Hell a, W. Kn€ upfer a, P. Hackenschmied b, A. Winnacker b
Siemens AG, Medical Solutions, Vacuum Technology, Guenther-Scharowsky-Strasse 21, D-91058 Erlangen, Germany Institute of Material Science VI, University of Erlangen-Nuernberg, Martensstrasse 7, D-91058 Erlangen, Germany
b
Abstract Image plates based on storage phosphors are a major application of radiation defects in insulators. Storage phosphors absorb X-ray quanta creating trapped electron–hole pairs in the material. Optical stimulation of the electron causes recombination leading to light emission. Application of image plates requires an optimal compromise between resolution (represented by the modulation transfer function (MTF)) and sensitivity. In our paper we present a new solution of the problem of combining a high MTF with a high sensitivity by structuring the image plates in form of thin needles acting as light guides. This suppresses the lateral spread of light which is detrimental to resolution. As doped CsBr, e.g. CsBr:Ga [Physica Medica XV (1999) 301], can pose a good storage phosphor evaporated layers are of interest in computed radiography. Needle structured CsI:Tl is used as scintillator in direct radiography [IEEE Trans. Nucl. Sci. 45 (3) (1998)]. CsBr layers have been produced by evaporation in vacuum and in inert gas atmosphere varying pressure and temperature. The resulting structures are of fibrous or columnar nature being in good agreement with the zone model of Thornton [Ann. Rev. Mater. Sci. 7 (1977) 239]. A zone model for CsBr has been developed. Measurements on doped alkali halide image plates having needle structure show good MTF at high sensitivity making a significant progress in image plate technology. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Storage phosphors; Alkali halides; Image plates; Cesium bromide
1. Introduction The first general model to explain the morphology of evaporated films in vacuum was developed by Movchan and Demchishin [4]. Depending on the ratio of substrate temperature and melting point TS =TM three zones were found. Zone 1 appears for TS =TM < 0:25 and consists
* Corresponding author. Tel.: +49-9131-731097; fax: +499131-732427. E-mail address: [email protected] (B. Schmitt).
of a porous layer of low density. In the range 0:25 < TS =TM < 0:45 a columnar structure (zone 2) is observed. Higher substrate temperatures lead to zone 3, a dense structure of grains. Thornton [3,5] extended the model to sputtered thick films. He additionally found a zone T which he described as a dense zone of fibrous grains. Srolovitz [6,7] introduced a theoretical model for columnar growth as well as studies done by Monte-Carlo simulations. He describes the columnar growth from an initial surface as result of surface diffusion and atomic shadowing. Anisotropy in the surface energy leads to fast growth of grains with low surface energy. This generates growth texture and
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 6 5 6 - 0
B. Schmitt et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 800–804
is observed in deposited films [5,7,8]. Needle structured evaporated films show outstanding properties in direct radiography [2] and are subject of research in computed radiography.
2. Experimental CsBr films of 0.5 mm thickness were grown at various substrate temperatures in vacuum and in inert gas atmosphere. Experimental setup is described in [9]. The vacuum quality achieved 5 103 Pa, growth rate ranged between 5 and 30 lm/min. Characterisation was done by taking pictures of the layer surface and the edge of a broken layer piece in a raster electron microscope (REM). X-ray-diffraction pictures were taken for each layer type. A Siemens D5000 apparatus providing CuKa radiation suitable for powder diffractometry was used. Furthermore porosity was calculated from the volume and weight of the layers in the following way: P ¼1
m qAh
ð1Þ
m is the layer mass, A the sample area, h the layer height and q the bulk density of CsBr. Layer height was measured in a light microscope. To measure MTF irradiation of a resolution target at a dose of 1.5 mGy was recorded by the image plate. Radiation was generated at 75 kV using 7 mm Al filter to remove low energy quanta. Readout was then done with a He–Ne laser. A Schott BG 3/5 mm filter prevented the detection of the stimulation laser. The contrast of the pattern (square wave response (SWR)) was evaluated and MTF was calculated from [10]: p 1 MTFðmÞ ¼ SWRðmÞ þ SWRð3mÞ 4 3 1 ð2Þ SWRð5mÞ 5 Doped CsBr becomes a good storage phosphor [1,11]. MTF measurements were done at a series of zone 2b structures of doped CsBr with equal morphology but different layer thickness, BaFBr:Eu powder layers of comparable thickness and
801
a commercial powder image plate. Sensitivity was taken as the signal intensity measured by the scanner under constant conditions for all plates. A correction was then done to the filter absorption of the emission light which has different wavelengths for the two phosphors. Furthermore an estimate 2 for the squared signal to noise ratio ðS=N Þ was done for all plates by the following formula with Sm being the mean value and r2 the variance of several points.
S N
2 ¼
Sm2 r2
ð3Þ
3. Results and discussion Schematically Fig. 1 shows a comparison of the Thornton model for sputtered films and the zones found for evaporated CsBr films. In Fig. 2 the corresponding morphologies are shown. Zone 3 structure can be seen in Fig. 2(a) and (b). Layers look opaque, porosity was found to be less than 10%. Grains only occurred at the layer bottom, the top remained columnar. The needles found for
Fig. 1. Comparison of the structure zones found by Thornton (dotted lines) and own zones (solid lines). The smaller region of zone T results from the different conditions of evaporation and sputtering. Zone 2 is split in round columns (2b) and squareshaped columns (2a).
802
B. Schmitt et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 800–804
Fig. 2. REM pictures of surfaces (right hand) and broken edges (left hand) of CsBr. (a and b): recrystallized needles, zone 3 (c and d): round columnar needles, zone 2b (e and f): square-shaped columnar needles, zone 2a (g and h): fibrous, dense structure, zone T (i and j): highly voided brittle structure, zone 1.
zone 2 are usually round but can also appear square shaped. So, zone 2 was split into zone 2a (square-shaped needles, see Fig. 2(f)) and zone 2b (round needles see Fig. 2(d)). Porosities of zone 2a and 2b range between 29–33% and 12–22%, respectively. Fibrous grains can only be found at high gas pressures (zone T, see Fig. 1). Zone T is enhanced when highly accelerated gas atoms (10– 40 eV) hit the surface layer [3]. Evaporation in gas atmosphere provides gas ions that may carry the thermal energy of the crucible (0.125 eV). Only a high number of gas atoms leads to sufficient bombarding supporting zone T. Porosity of zone T
is 30–38%. Figs. 2(g) and (h) show one sample. Zone 1 was found at 80 °C and 3 Pa. It is a white brittle layer. The structure is shown in Fig. 2(i) and (j). 36% porosity is observed. X-ray-diffraction pictures show h1 0 0i texture in growth direction for all samples. The texture is best evolved in the needles of zone 2b and shown in Fig. 3. In zone 3 structures recrystallisation diminishes the texture. Zones 1 and 2 show a reduced texture as result of the poorly defined internal structure. Amorphous behaviour was not observed. To depict the conflict between sensitivity and resolution the sensitivity of several plates was plotted versus the limiting res-
B. Schmitt et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 800–804
803
Fig. 2 (continued).
Fig. 3. Diffraction pattern of a zone 2b layer. The pronounced peaks prove the strict orientation of the needle crystals.
olution frequency in Fig. 4. As limiting frequency the line frequency was taken for which the MTF is 0.1. Needle image plates show higher sensitivity at equal resolution. Structural noise is detrimental to the detective quantum efficiency (DQE) which is defined as the squared signal-to-noise ratio of a system output divided by the squared signal-tonoise ratio of the system input. The signal-to-noise ratio shows higher values for the needle image
Fig. 4. Sensitivity of different image plates vs. resolution frequency limit which is set as MTFðmlim Þ ¼ 0:1. Needle image plates show much higher sensitivity at equal resolution.
plates (see Table 1). So a higher DQE of needle image plates can be expected. 4. Conclusion Deposition of evaporated CsBr in inert gas atmosphere leads to different mainly columnar
804
B. Schmitt et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 800–804
Table 1 Data of needle image plates and powder iamge plates S (a.U.)
ðS=N Þ2 (103 )
Needle image plates 50 6.8 120 6.1 170 5.7 260 5.2 310 5.0 390 4.7 490 4.7 550 4.4 720 4.1
61 147 238 341 314 469 496 715 733
3.3 7.1 8.1 9.2 14.1 10.0 8.8 5.5 5.7
Powder image plates 50 7.8 130 6.2 170 5.6 240 5.3 340 4.8 410 4.5 450 4.4 550 4.4 710 4.0 250a 4.1
14 40 48 62 105 90 94 121 107 168
1.0 2.4 1.6 1.9 1.7 1.7 2.5 1.7 1.3 5.4
d (lm)
mlim (lp/mm)
d is the layer thickness, mlim is the line frequency with MTF ðmlim Þ ¼ 0:1, S is the signal measured during a scan. ðS=N Þ2 is evaluated from Eq. (3). a Commercial image plate.
structures. Taking into account the existing models for vacuum evaporated [4] and sputtered films [3,5] the observed structures can be classified. Doped needle structures of zone 2b type show a high sensitivity keeping considerable MTF especially when layer thickness exceeds 250 lm. This makes needle layers suitable for medical applications. Signal to noise ratio in needle image plates
was found to be high compared to powder image plates, where grain structure noise harmfully affects DQE. Acknowledgements The support by the Bavarian Research foundation (FOROB, project II.1) is gratefully acknowledged. We also thank Mr. Kohler and Prof. Dr. M. Goebbels from the institute for Mineralogy for the support in providing the X-ray-diffraction pictures. References [1] P.J.R. Leblans, L. Struye, H. Gebele, Physica Medica XV (4) (1999) 301. [2] V.V. Nagarkar, T.K. Gupta, S.R. Miller, Y. Klugerman, M.R. Squillante, G. Entine, IEEE Trans. Nucl. Sci. 45 (3) (1998) 492. [3] J.A. Thornton, Ann. Rev. Mater. Sci. 7 (1977) 239. [4] B.A. Movchan, A.V. Demchishin, Fiz. metal. metalloved. 28 (4) (1969) 653. [5] J.A. Thornton, J. Vac. Sci. Technol. 11 (4) (1974) 666. [6] D.J. Srolovitz, J. Vac. Sci. Technol. A 4 (6) (1986) 2925. [7] D.J. Srolowitz, J. Vac. Sci. Technol. A 6 (4) (1988) 2371. [8] D.N. Lee, J. Mater. Sci. 34 (1999) 2575. [9] Erich Hell, Manfred Fuchs, Detlef Mattern, Bernhard Schmitt, Paul Leblans, U.S. Patent Application No. US 2001/0007352 A1, 12 July 2001. [10] J.C. Dainty, R. Shaw, Image Science, Academic Press, New York, 1974, p. 243. [11] P. Hackenschmied, G. Zeitler, M. Batentschuk, A. Winnacker, B. Schmitt, M. Fuchs, E. Hell, W. Kn€ upfer, Nucl. Instr. and Meth. B 191 (2002) 163.
Structured alkali halides for medical applications B. Schmitt a
a,*
, M. Fuchs a, E. Hell a, W. Kn€ upfer a, P. Hackenschmied b, A. Winnacker b
Siemens AG, Medical Solutions, Vacuum Technology, Guenther-Scharowsky-Strasse 21, D-91058 Erlangen, Germany Institute of Material Science VI, University of Erlangen-Nuernberg, Martensstrasse 7, D-91058 Erlangen, Germany
b
Abstract Image plates based on storage phosphors are a major application of radiation defects in insulators. Storage phosphors absorb X-ray quanta creating trapped electron–hole pairs in the material. Optical stimulation of the electron causes recombination leading to light emission. Application of image plates requires an optimal compromise between resolution (represented by the modulation transfer function (MTF)) and sensitivity. In our paper we present a new solution of the problem of combining a high MTF with a high sensitivity by structuring the image plates in form of thin needles acting as light guides. This suppresses the lateral spread of light which is detrimental to resolution. As doped CsBr, e.g. CsBr:Ga [Physica Medica XV (1999) 301], can pose a good storage phosphor evaporated layers are of interest in computed radiography. Needle structured CsI:Tl is used as scintillator in direct radiography [IEEE Trans. Nucl. Sci. 45 (3) (1998)]. CsBr layers have been produced by evaporation in vacuum and in inert gas atmosphere varying pressure and temperature. The resulting structures are of fibrous or columnar nature being in good agreement with the zone model of Thornton [Ann. Rev. Mater. Sci. 7 (1977) 239]. A zone model for CsBr has been developed. Measurements on doped alkali halide image plates having needle structure show good MTF at high sensitivity making a significant progress in image plate technology. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Storage phosphors; Alkali halides; Image plates; Cesium bromide
1. Introduction The first general model to explain the morphology of evaporated films in vacuum was developed by Movchan and Demchishin [4]. Depending on the ratio of substrate temperature and melting point TS =TM three zones were found. Zone 1 appears for TS =TM < 0:25 and consists
* Corresponding author. Tel.: +49-9131-731097; fax: +499131-732427. E-mail address: [email protected] (B. Schmitt).
of a porous layer of low density. In the range 0:25 < TS =TM < 0:45 a columnar structure (zone 2) is observed. Higher substrate temperatures lead to zone 3, a dense structure of grains. Thornton [3,5] extended the model to sputtered thick films. He additionally found a zone T which he described as a dense zone of fibrous grains. Srolovitz [6,7] introduced a theoretical model for columnar growth as well as studies done by Monte-Carlo simulations. He describes the columnar growth from an initial surface as result of surface diffusion and atomic shadowing. Anisotropy in the surface energy leads to fast growth of grains with low surface energy. This generates growth texture and
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 6 5 6 - 0
B. Schmitt et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 800–804
is observed in deposited films [5,7,8]. Needle structured evaporated films show outstanding properties in direct radiography [2] and are subject of research in computed radiography.
2. Experimental CsBr films of 0.5 mm thickness were grown at various substrate temperatures in vacuum and in inert gas atmosphere. Experimental setup is described in [9]. The vacuum quality achieved 5 103 Pa, growth rate ranged between 5 and 30 lm/min. Characterisation was done by taking pictures of the layer surface and the edge of a broken layer piece in a raster electron microscope (REM). X-ray-diffraction pictures were taken for each layer type. A Siemens D5000 apparatus providing CuKa radiation suitable for powder diffractometry was used. Furthermore porosity was calculated from the volume and weight of the layers in the following way: P ¼1
m qAh
ð1Þ
m is the layer mass, A the sample area, h the layer height and q the bulk density of CsBr. Layer height was measured in a light microscope. To measure MTF irradiation of a resolution target at a dose of 1.5 mGy was recorded by the image plate. Radiation was generated at 75 kV using 7 mm Al filter to remove low energy quanta. Readout was then done with a He–Ne laser. A Schott BG 3/5 mm filter prevented the detection of the stimulation laser. The contrast of the pattern (square wave response (SWR)) was evaluated and MTF was calculated from [10]: p 1 MTFðmÞ ¼ SWRðmÞ þ SWRð3mÞ 4 3 1 ð2Þ SWRð5mÞ 5 Doped CsBr becomes a good storage phosphor [1,11]. MTF measurements were done at a series of zone 2b structures of doped CsBr with equal morphology but different layer thickness, BaFBr:Eu powder layers of comparable thickness and
801
a commercial powder image plate. Sensitivity was taken as the signal intensity measured by the scanner under constant conditions for all plates. A correction was then done to the filter absorption of the emission light which has different wavelengths for the two phosphors. Furthermore an estimate 2 for the squared signal to noise ratio ðS=N Þ was done for all plates by the following formula with Sm being the mean value and r2 the variance of several points.
S N
2 ¼
Sm2 r2
ð3Þ
3. Results and discussion Schematically Fig. 1 shows a comparison of the Thornton model for sputtered films and the zones found for evaporated CsBr films. In Fig. 2 the corresponding morphologies are shown. Zone 3 structure can be seen in Fig. 2(a) and (b). Layers look opaque, porosity was found to be less than 10%. Grains only occurred at the layer bottom, the top remained columnar. The needles found for
Fig. 1. Comparison of the structure zones found by Thornton (dotted lines) and own zones (solid lines). The smaller region of zone T results from the different conditions of evaporation and sputtering. Zone 2 is split in round columns (2b) and squareshaped columns (2a).
802
B. Schmitt et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 800–804
Fig. 2. REM pictures of surfaces (right hand) and broken edges (left hand) of CsBr. (a and b): recrystallized needles, zone 3 (c and d): round columnar needles, zone 2b (e and f): square-shaped columnar needles, zone 2a (g and h): fibrous, dense structure, zone T (i and j): highly voided brittle structure, zone 1.
zone 2 are usually round but can also appear square shaped. So, zone 2 was split into zone 2a (square-shaped needles, see Fig. 2(f)) and zone 2b (round needles see Fig. 2(d)). Porosities of zone 2a and 2b range between 29–33% and 12–22%, respectively. Fibrous grains can only be found at high gas pressures (zone T, see Fig. 1). Zone T is enhanced when highly accelerated gas atoms (10– 40 eV) hit the surface layer [3]. Evaporation in gas atmosphere provides gas ions that may carry the thermal energy of the crucible (0.125 eV). Only a high number of gas atoms leads to sufficient bombarding supporting zone T. Porosity of zone T
is 30–38%. Figs. 2(g) and (h) show one sample. Zone 1 was found at 80 °C and 3 Pa. It is a white brittle layer. The structure is shown in Fig. 2(i) and (j). 36% porosity is observed. X-ray-diffraction pictures show h1 0 0i texture in growth direction for all samples. The texture is best evolved in the needles of zone 2b and shown in Fig. 3. In zone 3 structures recrystallisation diminishes the texture. Zones 1 and 2 show a reduced texture as result of the poorly defined internal structure. Amorphous behaviour was not observed. To depict the conflict between sensitivity and resolution the sensitivity of several plates was plotted versus the limiting res-
B. Schmitt et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 800–804
803
Fig. 2 (continued).
Fig. 3. Diffraction pattern of a zone 2b layer. The pronounced peaks prove the strict orientation of the needle crystals.
olution frequency in Fig. 4. As limiting frequency the line frequency was taken for which the MTF is 0.1. Needle image plates show higher sensitivity at equal resolution. Structural noise is detrimental to the detective quantum efficiency (DQE) which is defined as the squared signal-to-noise ratio of a system output divided by the squared signal-tonoise ratio of the system input. The signal-to-noise ratio shows higher values for the needle image
Fig. 4. Sensitivity of different image plates vs. resolution frequency limit which is set as MTFðmlim Þ ¼ 0:1. Needle image plates show much higher sensitivity at equal resolution.
plates (see Table 1). So a higher DQE of needle image plates can be expected. 4. Conclusion Deposition of evaporated CsBr in inert gas atmosphere leads to different mainly columnar
804
B. Schmitt et al. / Nucl. Instr. and Meth. in Phys. Res. B 191 (2002) 800–804
Table 1 Data of needle image plates and powder iamge plates S (a.U.)
ðS=N Þ2 (103 )
Needle image plates 50 6.8 120 6.1 170 5.7 260 5.2 310 5.0 390 4.7 490 4.7 550 4.4 720 4.1
61 147 238 341 314 469 496 715 733
3.3 7.1 8.1 9.2 14.1 10.0 8.8 5.5 5.7
Powder image plates 50 7.8 130 6.2 170 5.6 240 5.3 340 4.8 410 4.5 450 4.4 550 4.4 710 4.0 250a 4.1
14 40 48 62 105 90 94 121 107 168
1.0 2.4 1.6 1.9 1.7 1.7 2.5 1.7 1.3 5.4
d (lm)
mlim (lp/mm)
d is the layer thickness, mlim is the line frequency with MTF ðmlim Þ ¼ 0:1, S is the signal measured during a scan. ðS=N Þ2 is evaluated from Eq. (3). a Commercial image plate.
structures. Taking into account the existing models for vacuum evaporated [4] and sputtered films [3,5] the observed structures can be classified. Doped needle structures of zone 2b type show a high sensitivity keeping considerable MTF especially when layer thickness exceeds 250 lm. This makes needle layers suitable for medical applications. Signal to noise ratio in needle image plates
was found to be high compared to powder image plates, where grain structure noise harmfully affects DQE. Acknowledgements The support by the Bavarian Research foundation (FOROB, project II.1) is gratefully acknowledged. We also thank Mr. Kohler and Prof. Dr. M. Goebbels from the institute for Mineralogy for the support in providing the X-ray-diffraction pictures. References [1] P.J.R. Leblans, L. Struye, H. Gebele, Physica Medica XV (4) (1999) 301. [2] V.V. Nagarkar, T.K. Gupta, S.R. Miller, Y. Klugerman, M.R. Squillante, G. Entine, IEEE Trans. Nucl. Sci. 45 (3) (1998) 492. [3] J.A. Thornton, Ann. Rev. Mater. Sci. 7 (1977) 239. [4] B.A. Movchan, A.V. Demchishin, Fiz. metal. metalloved. 28 (4) (1969) 653. [5] J.A. Thornton, J. Vac. Sci. Technol. 11 (4) (1974) 666. [6] D.J. Srolovitz, J. Vac. Sci. Technol. A 4 (6) (1986) 2925. [7] D.J. Srolowitz, J. Vac. Sci. Technol. A 6 (4) (1988) 2371. [8] D.N. Lee, J. Mater. Sci. 34 (1999) 2575. [9] Erich Hell, Manfred Fuchs, Detlef Mattern, Bernhard Schmitt, Paul Leblans, U.S. Patent Application No. US 2001/0007352 A1, 12 July 2001. [10] J.C. Dainty, R. Shaw, Image Science, Academic Press, New York, 1974, p. 243. [11] P. Hackenschmied, G. Zeitler, M. Batentschuk, A. Winnacker, B. Schmitt, M. Fuchs, E. Hell, W. Kn€ upfer, Nucl. Instr. and Meth. B 191 (2002) 163.