Thin-film-based scintillators for hard x-ray microimaging detectors: the ScinTAX Project

Thin film based scintillators for hard X-ray microimaging detectors: the ScinTAX project

A. Rack*a, A. Ceciliab, P.-A. Douissarda, K. Dupréc, V. Wesemannc, T. Baumbachb, M. Couchaudd,

X. Rochete, M. Radtkef, H. Riesemeierf, T. Martina

aEuropean Synchrotron Radiation Facility, BP 220, 38043 Grenoble, France; bKarlsruhe Institute of Technology, Institute for Photon Science and Synchrotron Radiation / ANKA, Pf. 3640, 76021

Karlsruhe, Germany; cFEE GmbH, Struthstr. 2, 55743 Idar-Oberstein, Germany; dCEA/LETI, 38054 Grenoble Cedex, France; eOptique Peter, Allée Romaine, ZA du Charpenay, 69210 Lentilly, France.

fBundesanstalt für Materialforschung und -prüfung, Unter den Eichen 80, 12205 Berlin, Germany

ABSTRACT

The project ScinTAX developed novel thin scintillating films for the application in high performance X-ray imaging and subsequent introduced new X-ray detectors to the market. To achieve this aim lutetium orthosilicate (LSO) scintillators doped with different activators were grown successfully by liquid phase epitaxy. The high density of LSO (7.4 g/cm3), the effective atomic number (65.2) and the high light yield make this scintillator highly applicable for indirect X-ray detection in which the ionizing radiation is converted into visible light and then registered by a digital detector. A modular indirect detection system has been developed to fully exploit the potential of this thin film scintillator for radiographic and tomographic imaging. The system is compatible for high-resolution imaging with moderate dose as well as adaptable to intense high-dose applications where radiation hard microimaging detectors are required. This proceedings article shall review the achieved performances and technical details on this high-resolution detector system which is now available. A selected example application demonstrates the great potential of the optimized detector system for hard X-ray microimaging, i.e. either to improve image contrast due to the availability of efficient thin crystal films or to reduce the dose to the sample.

Keywords: indirect detection, scintillator, synchrotron radiation, LSO, thin film, liquid phase epitaxy, X-ray imaging, microtomography

1. INTRODUCTION The first indirect detection schemes were reported in the 1970s to perform so-called live topography1. Here, a scintillator screen converts the X-rays into visible light while the corresponding luminescence image is captured by a camera via visible light optics. Indirect detectors allow one to reach for high spatial resolution while being rather radiation hard as all electronic components can be kept out of the X-ray beam path when periscope-like designs are used. The latter commonly employ a mirror between the scintillator and the objective to form a folded optical beam path. The spatial resolution reachable is directly connected with the numerical aperture (NA) of the front objective and the wavelength of maximum emission of the scintillator screen by the Abbe diffraction limit. While high numerical apertures are required for spatial resolutions up to the (sub-)micrometer range the correspondingly reduced depth of field of the objectives limits the scintillator thickness and hence the detectors efficiency2. This can lead to rather long exposure times especially when laboratory-based sources are used. Consequently the success story of high-resolution indirect X-ray imaging detectors is closely related to the third generation synchrotron light sources starting their operation. Especially in combination with microtomography indirect detectors are an essential tool which allows for the technique to be used nowadays at synchrotron light sources in a routinely manner3. Another advantage of indirect detectors is that they consist mostly of components which are widely commercially available such as microscope objectives and CCD cameras. Therefore the technically developments in the past years already reached a high level of maturity4. For the sake of completeness it needs to be mentioned that X-ray topography in combination with synchrotron light sources is highly benefitted from these developments as well which allows for example real time semiconductor wafer metrology in situ under high temperature5,6.

*[email protected]; phone +33 476 88-1781; fax +33 476 88-2785; www.esrf.eu

Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XVI, edited by Arnold Burger, Larry Franks, Ralph B. James, Michael Fiederle, Proc. of SPIE Vol. 9213, 921312

© 2014 SPIE · CCC code: 0277-786X/14/$18 · doi: 10.1117/12.2060599

Proc. of SPIE Vol. 9213 921312-1

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The success of the ScinTAX project builts on this existing knowledge on indirect detection schemes in a two-fold manner: novel thin film scintillators were developed to push the performance limit in terms of efficiency and reachable spatial resolution while the visible light part of the detector was trimmed to reach a maximum in terms of versatility7,8. The LSO-based thin films produced and characterized within the project have proven to significantly increase the efficiency of common detector designs, suppress tails in the detector’s point spread function and even allow for combining several thin films in a multilayer-like approach9,10.

2. LSO-BASED THIN FILM SCINTILLATOR The key element which drives the performance of an indirect detector in terms of efficiency, speed and spatial resolution is the scintillator screen. For X-ray microimaging the quest for the ideal scintillator is defined by the following features:

• high X-ray absorption: to maximize the X-ray stopping power by coupling a high density (>5 g/cm3) with a large atomic number (>50),

• light emission: high conversion efficiency (>20 visible light photons/keV), emission wavelength well matched to the CCD read out (550 – 650 nm) and low afterglow (<4 decades of magnitude after 2 ms) and high linearity of the light output with the X-ray flux,

• optical properties: high transmittance and no scattering,

• technical aspects: machining, non-toxicity, radiation hardness and mechanical strength,

• thickness available from 1 to 500 μm.

The Lu2SiO5 scintillator was chosen because of its high density (7.4 g/cm3) and high effective atomic number (65.2) following an idea by Koch et al.2. Its absorption gain compared to a standard Y3Al5O12:Ce (YAG) scintillator can be as high as a factor of 2.4 below 10 keV, 10.5 between 10 keV and 17 keV, 2.2 between 17 keV and 63 keV and from 63 keV to 100 keV. Within the project a protocol was established to grow thin films of Tb-doped LSO on dedicated substrates by the liquid phase epitaxy technique (LPE). LPE is a technique for growing single crystal layers on oriented single crystal substrates11. Both the substrate and the epi-layer growths were developed and demonstrated by CEA/LETI11. High grade quality substrates of Yb2SiO5 were grown by the Czochralski pulling technique, characterized in terms of radiation damage, X-ray fluorescence and to verify the possible presence of unwanted substrate radioluminescence. Very often the substrates used to deposit the scintillating films emit undesired luminescence and such effect could deteriorate the spatial resolution performance reachable with the later scintillating film12-15. The radiation damage turned out to be a reversible process and the analysis of the X-ray fluorescence spectra evidenced a presence of impurities with concentration below 1% (concentrations of lead down to below 300 ppm were measured in LSO:Tb films). The substrate radio-luminescence spectrum is completely free from any parasitic component. The absence of luminescence from the substrate strongly improves the spatial resolution of the X-ray images, especially for film thickness below 20 μm and at high X-ray energies above 20 keV, at which only a small part of the X-rays are absorbed in the thin films.

Different dopants were tested to optically activate the LSO crystal matrices. Solely Eu3+, Tm3+ and Tb3+ could be added as main dopants in the LSO lattice. However best optical quality was obtained with LSO:Tb layers. Several codopants (Ce, Pr, Gd, Ge, Tm, Eu) were also evaluated in the LSO:Tb lattice. The lattice mismatch between the substrate used and the film was too high or the segregation coefficient was too low, resulting in the growth of bad optical quality layers. Tb rare earth ion was finally chosen as main dopant because it is the only rare earth ion that could be properly adjusted both to fit the substrate lattice parameters and to provide a very high conversion efficiency for a range of dopant concentration (up to 130% YAG:Ce light yield). In addition its green emission is efficiently detected by the front-illuminated CCD cameras commonly used. Ce co-doping of LSO layers with Tb ions turned out to be even more efficient. The ion adds a contribution peaked at 420 nm in the emission spectrum (without quenching the emission due to Tb). Most probably an efficient energy transfer exists from Ce3+ ions to Tb3+ ions16. This is translated in the emission spectrum by the superposition of the fast (~20 ns) Ce3+ luminescence in the UV range (peaked at ~420 nm) and the slow (~10 ms) green emission of Tb3+ ions in the visible range (peaked at ~550 nm). Codoping with Eu3+, Pr3+ or Tm3+ was found to strongly decrease the conversion efficiency of LSO:Tb films. Ge and Gd were quite neutral on the efficiency of the LSO:Tb layers, but LSO:Tb,Ge displayed higher level of afterglow than LSO:Tb and LSO:Tb,Ce films.



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For the quality and performance of the active doped layer the growth rate during the LPE process plays a crucial role as the latter strongly influences Pb contamination of the former. X-ray fluorescence analysis of selected samples was carried out at the BAMline of the BESSY-II light source (Helmholtz-Zentrum Berlin, Germany)17-19 using monochromatic irradiation by 23 keV to excite the Lβ line of Pb which was used to determine the Pb contamination for different LSO-layers grown within the project (an example spectrum is shown in Fig. 1 (left)). The results are plotted in Fig. 1 (right). It turns out that higher incorporation of Pb2+ ions occurs in the LPE layers grown at low temperatures and higher growth rates. For a melt of given composition, lower growth temperature results in higher growth rate (because one is further from the saturation temperature of the melt). For the melts with different compositions (Ge 5% and Ge 18% co-doping) and layer grown at equivalent growth rate, Pb incorporation will be higher for the layer grown at lower temperature. More generally it turns out that the incorporation in the layer of atoms with large ionic radii is favored when one grows the layer far from the saturation temperature. One explanation could be that the force driving the incorporation of the elements in the LPE layer is super saturation (the difference between the growth temperature and the saturation temperature). When performing the growth at low temperature (i.e., far from the saturation temperature), the force increases the probability of the lead atoms to diffuse through the boundary layer (a kind of dead layer at the film-melt interface and due to the rotation of the substrate) and therefore to be incorporated into the film. (Similar effects are observed if the rotation speed of the substrate is increased (at a given growth temperature): the elements are drawn towards the surface with much higher rate, which again favors the probability of incorporation of unwanted elements such as Pb or Pt.)

0,0 0,2 0,4 0,6 0,8 1,0 1,2

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T=1055 oC

T=1058 oC

T=1048 oCGe 18%

Pb

cont

ent (

g/cm

2 )

growth rate (µm/min)

T = 1043 oCGe 18%

T=1061 oC

Figure 1. Left: example X-ray fluorescence spectrum acquired at the BAMline of BESSY II. Right: Pb content found in selected layers with respect to their applied growth rate.

3. VERSATILE INDIRECT X-RAY IMAGE DETECTOR Since the first OptiquePeter microscope for indirect detection of high-resolution X-ray images was installed at the ID22 beamline of the European Synchrotron Radiation Facility a numerous number of similar systems were installed at synchrotron light sources around the globe17-19. Hence, it was decided to bundle that gained experience in a modular system offering a high level of versatility for the end-user. The result is depicted in Fig. 2: the backbone of the system is an Olympus lens tube. It can be equipped with a triple-revolver head offering highest spatial resolution and efficiency by mounting standard Olympus objectives directly behind the scintillator. For shorter X-ray wavelengths a more radiation hard head available as single or twin-objective system can be installed. The so-called white beam head has a Mitutoyo objective placed above the mirror (made from glassy carbon), i.e. outside the X-ray optical bath. Hence, dose issues like darkening of the front lens are minimized while highest spatial resolution up to photon energies of 80 keV have been reached due to the effect that no backscattering material is placed closely downstream to the scintillator. Dedicated scintillator mounts with optional tilt correction are available for both type of heads as well. For the revolver the scintillator exchange has been motorized. Various glass filters can be inserted into the optical beam path in order to suppress parasitic luminescence from a potential scintillator substrate which can deteriorate the spatial resolution or in order to trim the ratio between converted X-rays and detected visible light photons. In order to adapt the system for



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different camera sensor dimensions, eye-pieces can be installed. Standard Olympus eye-pieces can be mounted in a single or multiple manners. Dedicated eye-pieces have been developed for large-format sensor. Besides the standard camera mounts like C-, T- or F-mount specific solutions are available on request. For easy installation the complete system can be mounted to standard X95 aluminum profiles.

Figure 2. A modular and highly versatile indirect X-ray microimaging detector system developed in the frame of the ScinTAX project: single or revolver-like arrangements of eye-pieces can be used or not, three types of heads are available offering either highest resolution by a triple-revolver system with objectives mounted directly behind the scintillator or so-called white beam heads where extreme radiation hardness is reached by mounting the objective above the X-ray beam path.

An example data set acquired with the high-resolution configuration is shown in Fig. 3: a microtomographic slice of bamboo. The tomographic data was acquired using the ESRF beamline ID1923. The detector configuration used consisted of a 10× objective (0.3 numerical aperture) and 2× eye-piece which project the luminescence image of an 8.8 μm-thick LSO:Tb layer (on top of a nominal 150 μm-thick YbSO substrate) onto the CCD chip of the ESRF in-house-developed FReLoN camera (type: 2k / F_A7899, 2048 × 2048 pixels)25, i.e. an effective pixel size of 0.7 μm is utilized. Hence and with respect to Fig. 2 the detector consisted of the triple-revolver head with the lens tube and an eye-piece. For such a configuration a spatial resolving of 2 µm or better can typically be reached. For highest sensitivity ID19 was operated in pink-beam mode with the u17.6 single-harmonic undulator at gap 13.5 mm and with a 1 mm diamond absorber and a Be exit window as the only mandatory optical elements in the X-ray beam path. As a result the beamline can operate with a high photon flux density and a homogeneous wave front excellently suited to image weakly attenuating samples with high spatial resolution (the nominal peak energy in this configuration is 17.9 keV). For the tomographic scan 4000 angular projection images were recorded over 360 degree rotation of the specimen. In order to



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111=Mitil

laterally increase the field of view the axis of rotation was shifted off the center of the image. This allows during pre-processing to combine two projections acquired with 180 degree angular distance, i.e. for the reconstruction a new data set with 2000 projections, 3546 × 2048 pixels covering 180 degree rotation is created. Per real projection an exposure time of 0.05 s was applied, including approx 110 ms for read-out of the CCD chip the overall scan time was around 11 min. A sample-detector distance of 37 mm was set for inline phase contrast imaging25. Besides reconstruction of the raw data with standard filtered back-projection via the ESRF in-house software PyHST_2 also a single-distance phase-retrieval using Paganin’s approach was used in order to increase the contrast26,27. The picture reveals different filling states of the cells thanks to the high sensitivity of synchrotron-based microtomography in combination with hard X-ray phase contrast. Such imaging techniques are highly beneficial in the study of water transport in plants in order to understand plant behavior such as embolism repair. The high spatial resolution in combination with short exposure times is reached thanks to the efficient scintillator and the optimal integration of the detection scheme.

4. SUMMARY Thanks to the success of the ScinTAX project high performance indirect detection system are now available in a commercially manner, i.e. the thin crystal film scintillator as well as the versatile microscope via the industrial project partners. Quantitative measures of the detector performance have been published widely7-10,12-15,22. An example for a system nowadays in routine operation at the ESRF beamline NINA28: a 10× objective with an 0.4NA is combined with a 2× eye-piece to project the luminescence image of a 24 µm-thick LSO:Tb layer onto the Kodak KAF-4320 chip of the inhouse developed camera FReLoN (type: 4m / K4320)24. The system operates with an effective pixel size of 1.0 µm. At an X-ray photon energy of 17.5 keV a true spatial resolution of 2.3 µm was measured via a knife-edge (GaAs cut along the cleavage) using the ImageJ plugin ‘Slanted Edge MTF’38. The measured DQE (detective quantum efficiency) is above 40%. Such detectors are highly suited for all kinds of application using microtomography and radiography and are more and more frequently also used with laboratory-based sources29-31. High-resolution X-ray imaging techniques are less and less considered an academic tool as they have already been proven to be well-suited and beneficial to target highly applied questions such as micromechanical behavior of commercial dental implants or bio-ceramic supported bone regeneration32,33. Finally another growing field are time-resolved studies at high spatial resolution were indirect detectors are absolutely required due to their radiation hardness. Recently, temporal resolutions down to the 160 ps regime have been demonstrated34-37.

Figure 3. Cross-sectional views of a bamboo piece, approximately 5 mm in diameter. Left: X-ray inline phase contrast, i.e. the contrast is dominated by edge enhancement, right: the same data after phase retrieval using Paganin’s approach: a direct connection between the grey-level of a voxel and its associated material is established as the contrast is now related to the density variations in the sample.



ScinTAX was funded by the European Commission as part of the Sixth Framework Programme - STRP 033 427.

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Thin-film-based scintillators for hard x-ray microimaging detectors: the ScinTAX Project

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