- Email: [email protected]
Radiation sensor based on inorganic scintillator with fibre readout for high energy radiography
Sensors and Actuators A, 32 (1992) 464469
464
Radiation sensor based on inorganic for high energy radiography
scintillator
with fibre readout
M. Laguesse Centre d’Etudes a’e Saclay STIPE-B&r. 123, 91191 Gif SW Yvetle (France) and UniversirP de Litige, Institut d’ElectricitP Montefore, Measurements and Instrumentation Dept. B28, 4000 Likge (Belgium)
Electrical
M. Bourdinaud Centre d’Etudes de Saclay STIPE-B&.
123, 91191
Gif sur Yuette (France)
Abstract This paper discusses the development of a new detector for high energy photon radiography. It is based on an inorganic scintillator optically coupled to a ribbon of fluorescent optical fibres. The basic configuration is first briefly described. An investigation of the principal properties of the scintillators used for the detector is reported next: the emission spectra, the optical transparency, the radiation resistance tested by means of a 6oCo source and an 18 MV linear accelerator, etc. Detailed characterizations of these scintillators have been realized and complete experimental results are presented.
1. Introduction The basic configuration has been described in great detail in ref. 1; it is shown in Fig. 1. A thin layer of inorganic scintillator is put laterally on the end of a fluorescent fibre ribbon. Typically, this scintillating plate is several centimetres in length and in depth and a fraction of a millimetre in thickness. X- or y-rays coming from the patient or the object to be analysed reach the scintillator in a direction parallel to the fibre axis. Thanks to the great scintillator depth, a high X-ray conversion efficiency may thus be achieved. The absorption of an X-ray produces the excitation of the scintillator, and a fraction of the resulting visible photons may reach the fluorescent fibre ribbon. In turn, these photons excite the fluorescence of the fibres. This principle allows one to realize a linear X-ray detector. As shown on Fig. 2, a scanning device is thus needed in order to reconstruct a two-dimensional image. Since collimating slits can be used, this scanning system will have the advantage over bidimensional X-ray detectors in that diffused X-photons coming from the patient or object to be analysed are not detected. Moreover, the use of optical fibres allows one to put the detection
0924-4247/92/$5.00
electronics completely outside the radiation beam, which is an advantage for high energy X-ray imaging. Like fibres used for telecommunication purposes, a fluorescent optical fibre is made up of a cladding and of a higher refractive index core where light may be guided. This core, however, is very special since it has been doped during manufacturing with fluorescent dyes [2, 31. Provided they are emitted in the right wavelength range, optical rays which reach a fluorescent fibre transversally excite the core dopants. The de-excitation of the fibre induces accordingly an emission of photons with a longer wavelength. A fraction of this fluorescent light remains trapped in the fibre, and is guided towards both ends, where a detection system may be placed to convert the optical energy into an electrical signal. Such fluorescent fibres were developed several Yeats ago using plastic materials for the core and cladding, by the “Centre d’Etudes NuclCaires” of the CEA at Saclay (France), for the detection of elementary particles [4-61. The emission and excitation spectra depend on the dyes used to dope the fibre. Two kinds of fluorescent fibres were considered. The first is excited by UV or blue light and produces green light, while the second @ 1992 - Elsevier Sequoia. All tights reserved
465
Y Fig. 1. X-ray detector scintillator; Su, support.
basic configuration:
Ri, fibre ribbon;
SC,
Fig. 2. Integration of the detection head in a scanning system: LA, X-ray generator; Ob, object to be analysed; SC, scintillator; FOFR, fluorescent optical fibre ribbon; D, detection electronics; Pr, signal processing; V, visualization unit.
gives a red light emission when excited by green light. No special study of these fluorescent fibres will be given here, but further information may be found in ref. 7.
2. Scintillator general properties Several scintillator types could be used for the plate of the Fig. 1 configuration. An inorganic scintillator has been chosen since it shows a greater conversion efficiency, a greater density than plastic ones, and therefore a higher detection efficiency. Decay times of inorganic scintillators
are also longer, but this characteristic is not a drawback here, provided it remains within limits compatible with the detector speed. Scintillators like NaI:Tl, CsI:Na, or CsI:Tl are thus well suited since they are transparent and present a good scintillation efficiency. Moreover, some special vapour deposition techniques make it possible to obtain layers of these scintillators with a fine crystal pillar structure [8,9]. Oriented in the direction of the layer thickness, this structure presents the advantage over an isotropic scintillator of a reduced crosstalk (and, therefore, an improved spatial resolution). In addition, the light yield is also improved since each pillar acts like an elementary guide for the photons coming from the scintillation process. In addition, we also considered Gdz02S:Tb (usually named ‘gadox’) for the realization of the scintillating plate. This scintillator is less transparent than activated CsI, but its quantum efficiency is a bit greater (15% for gadox [ 10, 111, versus 12% for CsI:Tl [ 12, 131). The effective number of visible photons leaving the scintillator therefore depends on the thickness of the plate. Gadox has another drawback compared with CsI since it has a slightly lower detection efficiency. For example, the stopping power of 40-mm-thick vapour-deposited CsI in an 18 MV X-beam is roughly 50% versus 43% for gadox with the same thickness. Indeed, gadox is available in the form of a powder and must be used with a binding material. Although ‘pure’ gadox has a greater density than CsI, the compound presents a lower one. Moreover, gadox is completely structureless and cannot be obtained with a pillar structure. This explains why the gadox spatial resolution is known to be worse than that of vapour-deposited CsI. However, gadox presents an advantage since it may be moulded directly on the fluorescent fibre ribbon in order to realize the scintillating plate. Activated CsI, for its part, cannot be deposited directly and must therefore be glued. Since light emitted by NaI:Tl and CsI:Na has a blue colour, one should use a green fluorescent, optical fibre. On the other hand, CsI:Tl and gadox emit in the green, so red fluorescent fibres are suitable. However, the first two crystals are very hygroscopic; this explains why we finally decided to study CsI:Tl and Gd202S:Tb in more detail for this detector.
466
400
450
500
550
600 A, "Ill
Fig. 3. Typical emission and excitation spectra of the detector scintillators and fluorescent fibres: (--) gadox emission; (- - -_) CsI:TI emission; (” - ..) red fluorescent optical fibre emission; (- - -) red fluorescent fibre excitation spectra.
Fig. 4. Experimental set-up for the measurement of the scintillator optical transparency: S, optical source; C, collimating hole; Sc, scintillater; D, photodetector.
3. Emission spectra
The emission spectra of gadox and CsI:Tl are shown in Fig. 3. Both spectra have been measured with a Perkin Elmer LS-5B spectrofluorimeter. The excitation wavelength was chosen to be 300 nm; all emission spectra are corrected for the spectral response of the spectrofluorimeter photodetector. This Figure also shows the excitation and emission spectra of the red fluorescent optical fibre. The excitation spectrum has been measured for an emission fixed at 630 nm, and the emission spectrum has been determined under an excitation at 550 nm. It is clearly seen that the colour of the light emitted by both scintillators is rather well matched to the%fibre excitation spectrum. Moreover, the fibre emission spectrum is also well matched to the spectral response of a Si photodetector.
/ Fig. 5. CsI:TI optical transparency: fitting line.
(0)
experimental
1.“.
so.-
.
‘a.-
:
6
.4e.-
% : : :
.10.-
.
La’..
.
,a..
,a*.
41*.
THICKNESS
4. Optical transparency The transparency of several scintillator samples with different thicknesses has been determined using the experimental setup shown in Fig. 4. A green LED with a l-mm-diameter collimating hole is used as the optical source and a Si photodiode as the detector. Care is taken to choose an LED whose colour roughly matches the scintillator emission spectrum, and a photodiode whose surface is great enough to collect the maximum light coming from the scintillator. The results are shown on a logarithmic scale in Fig. 5 for vapourdeposited CsI:Tl and in Fig. 6 for gadox. The best fitting line is also shown in each Figure. It can be
results; (-)
Fig. 6. Gadox optical transparency: fitting line.
(0)
s... x
experimental
’
em
results; (-)
seen that pillar CsI:TI is more transparent than gadox. In the range of the scintillator thickness we consider, the transparency T may be modelled by the following expression. T=Cexp(-crx)
(1)
where x is the scintillator thickness, TVis the linear attenuation coefficient, and C is a coefficient which takes into account the diffusion process in the scintillator.
467
From Fig. 5, we find that C, = 0.38 and (x, = 0.77 mm-’ for CsI:Tl, and from Fig. 6, C, = 0.34 and ag = 2.1 mm-’ for gadox. The coefficient C must be introduced in eqn. (1) because gadox and vapour-deposited CsI:Tl are diffusive materials. Indeed, it has already been shown (see, for example ref. 14) that the transparency of a diffusive scintillator may be modelled with a decreasing exponential function such as eqn. (I), provided that the scintillator thickness is not too small. For smaller thicknesses, the transparency increases more quickly than that exponential law, in order to reach 100% when the thickness tends to zero.
o-
400
500
600
A, nm
700
Fig. 7. Influence of @‘Co irradiations on Csl:TI transmission spectrum: (-) before irradiation; (~ ~ -) 1.8 kGy irradiation dose at 2.8 x 1O-3 Gy/s dose rate; (- - -) 24 kGy at 28 x lD~‘Gy/s.
5. Radiation damage The influence of irradiation on the properties of both scintillators is an important point to consider since the detector has been developed for high energy radiography, and will therefore have to support large radiation doses. CsI:Tl radiation damage has already been investigated [ 15 - 171, but for crystals with larger thicknesses than that of our samples. Moreover, the behaviour under radiation has been shown to depend on the impurity concentration in the scintillator which varies with the manufacturer [ 18, 191. For these reasons, we decided to undertake some measurements on our samples. We analysed the scintillators degradation for two different radiation types: y from a high activity 6oCo source and X-rays from an 18 MV therapy linear accelerator. This accelerator delivers a burst of about 0.03 Gy/s during 60 s when . . . a patient 1s n-radiated. The irradiation of our samples occurred during normal patient treatments. The mean dose integrated by the scintillators during one month may be estimated at 800 Gy. For both types of scintillators, 0.4 mm thick samples were irradiated in air at room pressure and temperature. The samples were measured with a delay of one day after the irradiation was completed, if not specified otherwise below. The absorbed doses were determined using Alanine dosimeters. It should be noted that the doses indicated below were deduced from the dosimeter indications; they could be greater than the doses effectively absorbed by the thin scintillators sam-
0
’
400
500
600
A. "In
700
specFig. 8. Influence of Wo irradiation on gadox transmission trum: (--) before irradiation; (- -) 1.8 kGy irradiation dose at 2.8 x lo-‘Gy/s dose rate; (-) 24 kGy at 28 x 10-3Gy/s measured one day after irradiation; (.. - ..) measured 3 weeks after.
ples due to a possible lack of charged equilibrium in the samples. 5.1.
“Co source irradiation
particle
results
The transmission spectra of C&T1 and gadox determined with a Perkin Elmer Lambda 2 spectrophotometer are shown for some different irradiation doses delivered by the ‘j°Co source in Figs. 7 and 8, respectively. Obviously, the transparency is decreasing as the dose increases. Transmission degradation appears to be larger at shorter wavelengths than in the spectrum range of the scintillator emission, where the loss remains small. A similar behaviour was reported earlier [ 18, 191. Figure 8 also shows the measured spectrum for gadox three weeks after an irradiation of 24 kGy was completed; a partial but rather large recovery may be clearly seen. No significant degradation of CsI:Tl scintillation efficiency was measured for irradiations up to 24 kGy. For gadox, however, a decrease of the
468
Figs. 7 and 10, respectively. As the transmission losses are nearly the same, CsI:Tl appears to deteriorate less under low dose rates than under high ones (for the same integrated dose delivered to the scintillator). No significant degradation was observed for the scintillation efficiency of CsI:TI for irradiations up to 3.5 kGy. Fig. 9. Influence of Yh irradiation on gadox excitation spectrum: (-) before irradiation; (- - -) 1.8kGy irradiation dose at 2.8 x 10m3 Gy/s dose rate; (-) 24 kGy at 28 x IO-‘Gy/s measured one day after irradiation; (‘. - ‘,) measured 3 weeks after.
6. CsI:TI hygroscopy According to the literature, Csl:Tl is claimed to be a little or not hygroscopic at all [8, 15,20,21]. We didn’t find any significant loss of performance for the transparency, nor for the scintillation efficiency after our samples remained six months in air at normal pressure, temperature ( 18-22 “C) and relative humidity (55-65 r.h.).
7. Summary Fig. 10. Influence of X-rays for a 18 MV linear accelerator on CsI:TI transmission spectrum: (-) before irradiation; (- - -) 1.4 kGy irradiation dose; (- - -) 3.5 kGy irradiation dose.
scintillation efficiency with the dose has been found as shown on the excitation spectra in Fig. 9. This Figure shows that gadox scintillation efficiency is also subject to some recovery with time. Radiation damage for CsI:Tl and gadox appears to be quite different. Significant damage is visible for CsI:Tl even with a rather low dose ( 1.8 kGy), but the deterioration seems to saturate since it is roughly only twice as important with a much larger dose (24 kGy). For gadox, on the other hand, damage is small at 1.8 kGy, but more important at 24 kGy, and is roughly comparable to that of CsI:Tl at that dose. 5.2. Linear accelerator irradiation results The transmission spectra of CsI:Tl measured after different irradiations by the linear accelerator are shown in Fig. 10. As previously noted for Fig. 7 results, the damage is more important in the blue than in the green region of the spectrum. The effect of different dose rates may be seen if 1.8 kGy and 3.5 kGy dose spectra are compared in
The basic configuration of a high energy X- or y-ray detector has been presented. Some important properties of two scintillators likely to be used for this sensor have been investigated. The major experimental results have been shown and discussed.
Acknowledgements
It is a pleasure to thank R. Turlay, A. Patoux, P. Micolon and Ph. Rebourgeard from the Department d’Astrophysique et de Physique des Particules Elementaire of the CEN at Saclay for their support, and H. Blumenfeld for many exciting and fruitful discussions. We also appreciated the collaboration of D. Gauthereau (CEN/S) for the realization of the gadox samples, J. Ernwein and F. Rondeaux (CEN/S) for the irradiations with the 6oCo source, J. Chavaudra and H. Bouhnik (Institut de Cancerologie G. Roussy, Villejuif, France) for the irradiations with the linear accelerator, and J. Chabbal, B. Munier and G. Roziere (Thomson Tubes Electroniques, Moirans, France). One of the authors (M.L.) received a grant from the CEC. He wishes to thank this organization and
469
Professor H. W. Vanderschuren for his encouragement. He also acknowledges support of part of this work by the Legs de Bay of the University of Likge.
References 1 M. Bourdinaud and M. Laguesse, Monte Carlo simulations and first experimental results of a new X-ray detector using fluorescent optical fibres, Nucl. Instrum. Methods, A299 (1990) 210. 2 L. Reiffel and N. Kapany, Some considerations of luminescent fiber chambers and intensifier screens, Rev. Sci. Instrum., 31 (1960) 1136. 3 N. Kapany, Fiber Optics: Principles and Applications, Academic Press, New York, 1967, Chs. 2 and 12. 4 H. Blumenfeld et al., Plastic fibers in high energy physics, Nucl. Inswum.Method.s, A287 (1987) 603. 5 J. C. Thevenin et al., Scintillating and fluorescent plastic optical fibers for sensor applications, IEEE Trans. Nucl. Sci., 33 (1986) 133. 6 L. Allemand et al., Optical scintillating fibers for particle detectors, Nucl. Instrum. Methods, A225 (1984) 522. 7 M. Laguesse and M. Bourdinaud. Characterization of plastic optical fibers for X-ray beam detection, Proc. SPIE, Vol. 1592, 1991, p. 134. 8 A. Stevels and A. Schrama de Pauw, Vapour deposited Csl-Na layers, Philips Res. Rep., 29 (1974) 340.
9 H. Washida and T. Sonoda, High resolution phosphor screen for X-ray image intensifiers, Ado. Electron. Electron Phys., 52 (1979) 201. IO G. Hay, X-ray imaging, J. Phys. E, II (1978) 377. 11 G. Ludwig and J. Prener, Evaluation of Gd,O,S:Tb as a phosphor for the input screen of X-ray image intensifier, NSG, 4 (1972) 3. 12 I. Ho11 et al., A measurement of the light yield of common inorganic scintillators, IEEE Trans. Nucl. Sci., 35 (1988) 105. 13 E. Sakai, Recent measurements on scintillator-photodetector systems, IEEE Trans. Nucl. Sci., 34 (1987) 418. 14 G. Giakoumakis and D. Miliatis, Light angular distribution of fluorescent screens excited by X-rays, Phys. Med. Biol., 30 (1985) 21. 15 E. Blucher et al., Tests of cesium iodide crystals for an electromagnetic calorimeter, Nucl. Instrum.MethoL, A249 (1986) 201. 16 Ch. Bieler et al., Radiation damage of BGO and CsI (TI) crystals, Nucl. Instrum. Methods, A234 (1985) 435. 17 S. Schlogl et al., Radiation damage of CsI (Tl) crystals in a long term exposure at Petra, Nucl. Instrum. Methods, A242 (1985) 89. 18 M. Kobayashi and S. Sakuragi, Radiation damage of CsI (TI) crystals above 10’ rad, Nuci. Insrrutn. Methods, A254 (1987) 275. 19 D. Renker, Radiation damage of Csl (Tl) crystals, Rep. 89-10, CERN, Geneva, 1989, p. 60 I. 20 H. Grassmann et al.. Improvements in photodiode readout for small CsI (Tl) crystals, Nucl. Znsrrum. Methods, A234 (1985) 122. 21 S. Zang and D. Ramsden, A triple phoswich detector for hard X-ray astronomy, Nucl. Instrum. Methods, A273 (1988) 673.
464
Radiation sensor based on inorganic for high energy radiography
scintillator
with fibre readout
M. Laguesse Centre d’Etudes a’e Saclay STIPE-B&r. 123, 91191 Gif SW Yvetle (France) and UniversirP de Litige, Institut d’ElectricitP Montefore, Measurements and Instrumentation Dept. B28, 4000 Likge (Belgium)
Electrical
M. Bourdinaud Centre d’Etudes de Saclay STIPE-B&.
123, 91191
Gif sur Yuette (France)
Abstract This paper discusses the development of a new detector for high energy photon radiography. It is based on an inorganic scintillator optically coupled to a ribbon of fluorescent optical fibres. The basic configuration is first briefly described. An investigation of the principal properties of the scintillators used for the detector is reported next: the emission spectra, the optical transparency, the radiation resistance tested by means of a 6oCo source and an 18 MV linear accelerator, etc. Detailed characterizations of these scintillators have been realized and complete experimental results are presented.
1. Introduction The basic configuration has been described in great detail in ref. 1; it is shown in Fig. 1. A thin layer of inorganic scintillator is put laterally on the end of a fluorescent fibre ribbon. Typically, this scintillating plate is several centimetres in length and in depth and a fraction of a millimetre in thickness. X- or y-rays coming from the patient or the object to be analysed reach the scintillator in a direction parallel to the fibre axis. Thanks to the great scintillator depth, a high X-ray conversion efficiency may thus be achieved. The absorption of an X-ray produces the excitation of the scintillator, and a fraction of the resulting visible photons may reach the fluorescent fibre ribbon. In turn, these photons excite the fluorescence of the fibres. This principle allows one to realize a linear X-ray detector. As shown on Fig. 2, a scanning device is thus needed in order to reconstruct a two-dimensional image. Since collimating slits can be used, this scanning system will have the advantage over bidimensional X-ray detectors in that diffused X-photons coming from the patient or object to be analysed are not detected. Moreover, the use of optical fibres allows one to put the detection
0924-4247/92/$5.00
electronics completely outside the radiation beam, which is an advantage for high energy X-ray imaging. Like fibres used for telecommunication purposes, a fluorescent optical fibre is made up of a cladding and of a higher refractive index core where light may be guided. This core, however, is very special since it has been doped during manufacturing with fluorescent dyes [2, 31. Provided they are emitted in the right wavelength range, optical rays which reach a fluorescent fibre transversally excite the core dopants. The de-excitation of the fibre induces accordingly an emission of photons with a longer wavelength. A fraction of this fluorescent light remains trapped in the fibre, and is guided towards both ends, where a detection system may be placed to convert the optical energy into an electrical signal. Such fluorescent fibres were developed several Yeats ago using plastic materials for the core and cladding, by the “Centre d’Etudes NuclCaires” of the CEA at Saclay (France), for the detection of elementary particles [4-61. The emission and excitation spectra depend on the dyes used to dope the fibre. Two kinds of fluorescent fibres were considered. The first is excited by UV or blue light and produces green light, while the second @ 1992 - Elsevier Sequoia. All tights reserved
465
Y Fig. 1. X-ray detector scintillator; Su, support.
basic configuration:
Ri, fibre ribbon;
SC,
Fig. 2. Integration of the detection head in a scanning system: LA, X-ray generator; Ob, object to be analysed; SC, scintillator; FOFR, fluorescent optical fibre ribbon; D, detection electronics; Pr, signal processing; V, visualization unit.
gives a red light emission when excited by green light. No special study of these fluorescent fibres will be given here, but further information may be found in ref. 7.
2. Scintillator general properties Several scintillator types could be used for the plate of the Fig. 1 configuration. An inorganic scintillator has been chosen since it shows a greater conversion efficiency, a greater density than plastic ones, and therefore a higher detection efficiency. Decay times of inorganic scintillators
are also longer, but this characteristic is not a drawback here, provided it remains within limits compatible with the detector speed. Scintillators like NaI:Tl, CsI:Na, or CsI:Tl are thus well suited since they are transparent and present a good scintillation efficiency. Moreover, some special vapour deposition techniques make it possible to obtain layers of these scintillators with a fine crystal pillar structure [8,9]. Oriented in the direction of the layer thickness, this structure presents the advantage over an isotropic scintillator of a reduced crosstalk (and, therefore, an improved spatial resolution). In addition, the light yield is also improved since each pillar acts like an elementary guide for the photons coming from the scintillation process. In addition, we also considered Gdz02S:Tb (usually named ‘gadox’) for the realization of the scintillating plate. This scintillator is less transparent than activated CsI, but its quantum efficiency is a bit greater (15% for gadox [ 10, 111, versus 12% for CsI:Tl [ 12, 131). The effective number of visible photons leaving the scintillator therefore depends on the thickness of the plate. Gadox has another drawback compared with CsI since it has a slightly lower detection efficiency. For example, the stopping power of 40-mm-thick vapour-deposited CsI in an 18 MV X-beam is roughly 50% versus 43% for gadox with the same thickness. Indeed, gadox is available in the form of a powder and must be used with a binding material. Although ‘pure’ gadox has a greater density than CsI, the compound presents a lower one. Moreover, gadox is completely structureless and cannot be obtained with a pillar structure. This explains why the gadox spatial resolution is known to be worse than that of vapour-deposited CsI. However, gadox presents an advantage since it may be moulded directly on the fluorescent fibre ribbon in order to realize the scintillating plate. Activated CsI, for its part, cannot be deposited directly and must therefore be glued. Since light emitted by NaI:Tl and CsI:Na has a blue colour, one should use a green fluorescent, optical fibre. On the other hand, CsI:Tl and gadox emit in the green, so red fluorescent fibres are suitable. However, the first two crystals are very hygroscopic; this explains why we finally decided to study CsI:Tl and Gd202S:Tb in more detail for this detector.
466
400
450
500
550
600 A, "Ill
Fig. 3. Typical emission and excitation spectra of the detector scintillators and fluorescent fibres: (--) gadox emission; (- - -_) CsI:TI emission; (” - ..) red fluorescent optical fibre emission; (- - -) red fluorescent fibre excitation spectra.
Fig. 4. Experimental set-up for the measurement of the scintillator optical transparency: S, optical source; C, collimating hole; Sc, scintillater; D, photodetector.
3. Emission spectra
The emission spectra of gadox and CsI:Tl are shown in Fig. 3. Both spectra have been measured with a Perkin Elmer LS-5B spectrofluorimeter. The excitation wavelength was chosen to be 300 nm; all emission spectra are corrected for the spectral response of the spectrofluorimeter photodetector. This Figure also shows the excitation and emission spectra of the red fluorescent optical fibre. The excitation spectrum has been measured for an emission fixed at 630 nm, and the emission spectrum has been determined under an excitation at 550 nm. It is clearly seen that the colour of the light emitted by both scintillators is rather well matched to the%fibre excitation spectrum. Moreover, the fibre emission spectrum is also well matched to the spectral response of a Si photodetector.
/ Fig. 5. CsI:TI optical transparency: fitting line.
(0)
experimental
1.“.
so.-
.
‘a.-
:
6
.4e.-
% : : :
.10.-
.
La’..
.
,a..
,a*.
41*.
THICKNESS
4. Optical transparency The transparency of several scintillator samples with different thicknesses has been determined using the experimental setup shown in Fig. 4. A green LED with a l-mm-diameter collimating hole is used as the optical source and a Si photodiode as the detector. Care is taken to choose an LED whose colour roughly matches the scintillator emission spectrum, and a photodiode whose surface is great enough to collect the maximum light coming from the scintillator. The results are shown on a logarithmic scale in Fig. 5 for vapourdeposited CsI:Tl and in Fig. 6 for gadox. The best fitting line is also shown in each Figure. It can be
results; (-)
Fig. 6. Gadox optical transparency: fitting line.
(0)
s... x
experimental
’
em
results; (-)
seen that pillar CsI:TI is more transparent than gadox. In the range of the scintillator thickness we consider, the transparency T may be modelled by the following expression. T=Cexp(-crx)
(1)
where x is the scintillator thickness, TVis the linear attenuation coefficient, and C is a coefficient which takes into account the diffusion process in the scintillator.
467
From Fig. 5, we find that C, = 0.38 and (x, = 0.77 mm-’ for CsI:Tl, and from Fig. 6, C, = 0.34 and ag = 2.1 mm-’ for gadox. The coefficient C must be introduced in eqn. (1) because gadox and vapour-deposited CsI:Tl are diffusive materials. Indeed, it has already been shown (see, for example ref. 14) that the transparency of a diffusive scintillator may be modelled with a decreasing exponential function such as eqn. (I), provided that the scintillator thickness is not too small. For smaller thicknesses, the transparency increases more quickly than that exponential law, in order to reach 100% when the thickness tends to zero.
o-
400
500
600
A, nm
700
Fig. 7. Influence of @‘Co irradiations on Csl:TI transmission spectrum: (-) before irradiation; (~ ~ -) 1.8 kGy irradiation dose at 2.8 x 1O-3 Gy/s dose rate; (- - -) 24 kGy at 28 x lD~‘Gy/s.
5. Radiation damage The influence of irradiation on the properties of both scintillators is an important point to consider since the detector has been developed for high energy radiography, and will therefore have to support large radiation doses. CsI:Tl radiation damage has already been investigated [ 15 - 171, but for crystals with larger thicknesses than that of our samples. Moreover, the behaviour under radiation has been shown to depend on the impurity concentration in the scintillator which varies with the manufacturer [ 18, 191. For these reasons, we decided to undertake some measurements on our samples. We analysed the scintillators degradation for two different radiation types: y from a high activity 6oCo source and X-rays from an 18 MV therapy linear accelerator. This accelerator delivers a burst of about 0.03 Gy/s during 60 s when . . . a patient 1s n-radiated. The irradiation of our samples occurred during normal patient treatments. The mean dose integrated by the scintillators during one month may be estimated at 800 Gy. For both types of scintillators, 0.4 mm thick samples were irradiated in air at room pressure and temperature. The samples were measured with a delay of one day after the irradiation was completed, if not specified otherwise below. The absorbed doses were determined using Alanine dosimeters. It should be noted that the doses indicated below were deduced from the dosimeter indications; they could be greater than the doses effectively absorbed by the thin scintillators sam-
0
’
400
500
600
A. "In
700
specFig. 8. Influence of Wo irradiation on gadox transmission trum: (--) before irradiation; (- -) 1.8 kGy irradiation dose at 2.8 x lo-‘Gy/s dose rate; (-) 24 kGy at 28 x 10-3Gy/s measured one day after irradiation; (.. - ..) measured 3 weeks after.
ples due to a possible lack of charged equilibrium in the samples. 5.1.
“Co source irradiation
particle
results
The transmission spectra of C&T1 and gadox determined with a Perkin Elmer Lambda 2 spectrophotometer are shown for some different irradiation doses delivered by the ‘j°Co source in Figs. 7 and 8, respectively. Obviously, the transparency is decreasing as the dose increases. Transmission degradation appears to be larger at shorter wavelengths than in the spectrum range of the scintillator emission, where the loss remains small. A similar behaviour was reported earlier [ 18, 191. Figure 8 also shows the measured spectrum for gadox three weeks after an irradiation of 24 kGy was completed; a partial but rather large recovery may be clearly seen. No significant degradation of CsI:Tl scintillation efficiency was measured for irradiations up to 24 kGy. For gadox, however, a decrease of the
468
Figs. 7 and 10, respectively. As the transmission losses are nearly the same, CsI:Tl appears to deteriorate less under low dose rates than under high ones (for the same integrated dose delivered to the scintillator). No significant degradation was observed for the scintillation efficiency of CsI:TI for irradiations up to 3.5 kGy. Fig. 9. Influence of Yh irradiation on gadox excitation spectrum: (-) before irradiation; (- - -) 1.8kGy irradiation dose at 2.8 x 10m3 Gy/s dose rate; (-) 24 kGy at 28 x IO-‘Gy/s measured one day after irradiation; (‘. - ‘,) measured 3 weeks after.
6. CsI:TI hygroscopy According to the literature, Csl:Tl is claimed to be a little or not hygroscopic at all [8, 15,20,21]. We didn’t find any significant loss of performance for the transparency, nor for the scintillation efficiency after our samples remained six months in air at normal pressure, temperature ( 18-22 “C) and relative humidity (55-65 r.h.).
7. Summary Fig. 10. Influence of X-rays for a 18 MV linear accelerator on CsI:TI transmission spectrum: (-) before irradiation; (- - -) 1.4 kGy irradiation dose; (- - -) 3.5 kGy irradiation dose.
scintillation efficiency with the dose has been found as shown on the excitation spectra in Fig. 9. This Figure shows that gadox scintillation efficiency is also subject to some recovery with time. Radiation damage for CsI:Tl and gadox appears to be quite different. Significant damage is visible for CsI:Tl even with a rather low dose ( 1.8 kGy), but the deterioration seems to saturate since it is roughly only twice as important with a much larger dose (24 kGy). For gadox, on the other hand, damage is small at 1.8 kGy, but more important at 24 kGy, and is roughly comparable to that of CsI:Tl at that dose. 5.2. Linear accelerator irradiation results The transmission spectra of CsI:Tl measured after different irradiations by the linear accelerator are shown in Fig. 10. As previously noted for Fig. 7 results, the damage is more important in the blue than in the green region of the spectrum. The effect of different dose rates may be seen if 1.8 kGy and 3.5 kGy dose spectra are compared in
The basic configuration of a high energy X- or y-ray detector has been presented. Some important properties of two scintillators likely to be used for this sensor have been investigated. The major experimental results have been shown and discussed.
Acknowledgements
It is a pleasure to thank R. Turlay, A. Patoux, P. Micolon and Ph. Rebourgeard from the Department d’Astrophysique et de Physique des Particules Elementaire of the CEN at Saclay for their support, and H. Blumenfeld for many exciting and fruitful discussions. We also appreciated the collaboration of D. Gauthereau (CEN/S) for the realization of the gadox samples, J. Ernwein and F. Rondeaux (CEN/S) for the irradiations with the 6oCo source, J. Chavaudra and H. Bouhnik (Institut de Cancerologie G. Roussy, Villejuif, France) for the irradiations with the linear accelerator, and J. Chabbal, B. Munier and G. Roziere (Thomson Tubes Electroniques, Moirans, France). One of the authors (M.L.) received a grant from the CEC. He wishes to thank this organization and
469
Professor H. W. Vanderschuren for his encouragement. He also acknowledges support of part of this work by the Legs de Bay of the University of Likge.
References 1 M. Bourdinaud and M. Laguesse, Monte Carlo simulations and first experimental results of a new X-ray detector using fluorescent optical fibres, Nucl. Instrum. Methods, A299 (1990) 210. 2 L. Reiffel and N. Kapany, Some considerations of luminescent fiber chambers and intensifier screens, Rev. Sci. Instrum., 31 (1960) 1136. 3 N. Kapany, Fiber Optics: Principles and Applications, Academic Press, New York, 1967, Chs. 2 and 12. 4 H. Blumenfeld et al., Plastic fibers in high energy physics, Nucl. Inswum.Method.s, A287 (1987) 603. 5 J. C. Thevenin et al., Scintillating and fluorescent plastic optical fibers for sensor applications, IEEE Trans. Nucl. Sci., 33 (1986) 133. 6 L. Allemand et al., Optical scintillating fibers for particle detectors, Nucl. Instrum. Methods, A225 (1984) 522. 7 M. Laguesse and M. Bourdinaud. Characterization of plastic optical fibers for X-ray beam detection, Proc. SPIE, Vol. 1592, 1991, p. 134. 8 A. Stevels and A. Schrama de Pauw, Vapour deposited Csl-Na layers, Philips Res. Rep., 29 (1974) 340.
9 H. Washida and T. Sonoda, High resolution phosphor screen for X-ray image intensifiers, Ado. Electron. Electron Phys., 52 (1979) 201. IO G. Hay, X-ray imaging, J. Phys. E, II (1978) 377. 11 G. Ludwig and J. Prener, Evaluation of Gd,O,S:Tb as a phosphor for the input screen of X-ray image intensifier, NSG, 4 (1972) 3. 12 I. Ho11 et al., A measurement of the light yield of common inorganic scintillators, IEEE Trans. Nucl. Sci., 35 (1988) 105. 13 E. Sakai, Recent measurements on scintillator-photodetector systems, IEEE Trans. Nucl. Sci., 34 (1987) 418. 14 G. Giakoumakis and D. Miliatis, Light angular distribution of fluorescent screens excited by X-rays, Phys. Med. Biol., 30 (1985) 21. 15 E. Blucher et al., Tests of cesium iodide crystals for an electromagnetic calorimeter, Nucl. Instrum.MethoL, A249 (1986) 201. 16 Ch. Bieler et al., Radiation damage of BGO and CsI (TI) crystals, Nucl. Instrum. Methods, A234 (1985) 435. 17 S. Schlogl et al., Radiation damage of CsI (Tl) crystals in a long term exposure at Petra, Nucl. Instrum. Methods, A242 (1985) 89. 18 M. Kobayashi and S. Sakuragi, Radiation damage of CsI (TI) crystals above 10’ rad, Nuci. Insrrutn. Methods, A254 (1987) 275. 19 D. Renker, Radiation damage of Csl (Tl) crystals, Rep. 89-10, CERN, Geneva, 1989, p. 60 I. 20 H. Grassmann et al.. Improvements in photodiode readout for small CsI (Tl) crystals, Nucl. Znsrrum. Methods, A234 (1985) 122. 21 S. Zang and D. Ramsden, A triple phoswich detector for hard X-ray astronomy, Nucl. Instrum. Methods, A273 (1988) 673.