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An improved X-ray detector for use at synchrotrons
Nuclear Instruments and Methods in Physics Research A 413 (1998) 175—184
An improved X-ray detector for use at synchrotrons M. Thoms!,",*, S. Bauchau!, D. Ha¨usermann!, M. Kunz!, T. Le Bihan!, M. Mezouar!, D. Strawbridge! ! European Synchrotron Radiation Facility, Av. des Martyrs - B.P. 220, F-38043 Grenoble cedex 09, France " University of Erlangen-Nu( rnberg, Institute of Material Science VI, Martensstr. 7, D-91058 Erlangen, Germany Received 29 September 1997
Abstract Due to the advent of synchrotrons as X-ray sources with high power and brilliance new improved X-ray detectors are needed in order to make efficient use of the unique features of the synchrotron radiation. In this article the construction and characteristics of an X-ray detector, which is based on image plates and which has been especially designed for the detection of X-ray images at synchrotrons is reported. The detector has an X-ray sensitive area of 250 mm]305 mm, and allows the acquisition of X-ray images with variable pixel sizes and readout speeds according to the requirements of the particular experiment. In the normal setup mode images can be recorded every 13 s using a pixel size of 80 lm]80 lm, which relates to 12]106 pixels per image, while in the high-resolution setup images can be recorded every 41 s with 40 lm]40 lm pixel size, which relates to 48]106 pixels per image. The relation of detector characteristics, such as readout speed, resolution and efficiency to the physical processes of image storage and readout are discussed. Examples of images taken during the first test experiment on the high-pressure beamline (ID30) of the ESRF are presented. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: BaFBr : Eu; Image plate; Image readout; Light scattering; X-ray absorption; X-ray sensitivity; Photon diffusion; Photostimulation; Photostimulated luminescence; Sensitivity; Storage phosphor
1. Introduction The advent of synchrotrons with their high brilliance and X-ray flux has created new demands for X-ray detectors. For example diffraction experiments can be carried out in much shorter time compared to experimental setups using conven-
* Correspondence address: University of Erlangen-Nu¨rnberg, Institute of Material Science VI, Martensstr. 7, D-91058 Erlangen, Germany. Tel.: #49 9131 857683; fax: #49 9131 858495.
tional X-ray tubes. This requires fast and efficient X-ray image detectors, which cover a large solid angle of the diffracted radiation and have a high precision. Although X-ray detectors, which are based on image plates have the potential to fit these requirements, commercially available image plate detectors have the drawback of low-readout speed resulting in detection cycles of X-ray images in the order of minutes and longer. Therefore, an optimized image plate detector, which is described together with its operation principle in Section 2, allowing fast readout cycles in the order of 10 s has
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been built at the ESRF. The detector has several advantages compared to X-ray detectors, which are based on CCD-detectors and are coupled to a phosphor screen: 1. The large sensitive area of 250 mm]305 mm allows the detection of X-ray radiation that is diffracted in a large solid angle without putting the detector very close to the sample. Therefore, the signal/background ratio is improved and the possibility of overlapping diffraction spots is reduced. 2. During the exposure no background signal is accumulated. This background is caused in silicon based CCD-detectors by the thermal excitation of charge carriers. Therefore, image plates have no restrictions on exposure times. 3. The dynamic range is not limited by the well capacity as in CCD-detectors. 4. The sensitivity and spatial resolution of the detector can be simply adapted to the requirements of the experiments by changing the image plate and by varying the pixel size and the amplification of the photomultiplier tubes. 5. The readout noise of the detector is low since the image signal, which results from photostimulated luminescence (PSL), is electronically amplified in photomultiplier tubes by several orders of magnitude before the analog-to digital conversion. Further, the dark count of the photomultiplier, which is about a few hundred photons/s, only influences the dose measurement during the readout of the image plate. However, the readout of a single pixel takes typically between about 1 and 100 ls and thus it is rare, that dark counts contribute to the readout of a single pixel. 6. In image plates the image is detected at the same side of the phosphor screen as the X-rays create the image. In contrast, in most CCD-based detectors the CCD is mounted opposite to the side of X-ray exposure in order to avoid damage of the sensor by X-rays. Therefore, in such systems the X-ray generated luminescence has to travel from the side of X-ray exposure through the granular phosphor layer to the side of detection, which results in a spatial broadening of the light profile and therewith in a loss of resolution.
7. The problem of charge trapping of image information in bad pixels does not exist. This effect occurs in CCD detectors because the image information carrying charges has to be sequentially shifted pixel by pixel to the output lines. 8. The pixel rate of 2]106 pixels/s allows a fast digitization of X-ray images without affecting the efficiency and readout noise of the detection. In CCD-based systems it is known that the transfer efficiency of the charge shifting from pixel to pixel degrades for increasing readout speeds. Further, the readout noise increases, resulting, together with the limited well capacity, in a decrease of the dynamic range of the CCD. 9. There are no problems with X-rays which directly hit the CCD and cause overflow in the corresponding pixels. However, physical processes also limit the performance of the image plate detector during the generation, storage and readout of the image information. Therefore, the influence of these processes on the performance of the detector and ways of its further improvement will be discussed in Section 3. In Section 4 results of the first test experiments made on the high-pressure beamline (ID30) of the ESRF will be presented. A summary is given in Section 5.
2. The optimized image plate detector and its operation principle The operation principle of X-ray detectors which are based on image plates can be subdivided into the X-ray irradiation, where the X-ray information is generated in the image plate and into the readout of this information. During the X-ray irradiation X-rays are absorbed in the image plate. A typical image plate is composed of a thin layer of storage phosphor that is embedded in an organic binder and coated onto a supporting polymer film. X-rays, which are absorbed in storage phosphor grains, which have typically a size of about 5 lm and consist of BaFBr : Eu, create electron and hole storage centers in the material [1,2]. The number of generated centers is directly proportional to the X-ray dose.
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An average energy of about 125 eV is needed in order to generate a photostimulable defect pair [3]. As a result an absorbed X-ray quantum of 10 keV energy produces about 80 photostimulable defect pairs. Since the generation of a defect pair involves the excitation of electrons from the valence to the conduction band and since the band gap energy is about 8 eV in the case of BaFBr : Eu no thermal generation of defect pairs is possible at room temperature. Therefore, the image plates can be exposed to X-rays over long time periods without accumulating thermal background signals, as observed in the case of CCD-based detectors. During the readout process the image plate is scanned point by point with a laser beam. The luminescence intensity, resulting from the photostimulated recombination of the defects is measured with a photomultiplier, which is optically coupled to the point of emission by a light collector. The intensity signal is subsequently digitized and stored in a computer. After the scan the residual information has to be bleached before the image plate can be exposed the next time. Several detector systems based on image plates have already been built [4—6] or are commercially available. They all differ in type of lasers, scanning mechanisms, techniques for the light collection and ADC converters.
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Therefore, the performance, especially the readout speeds of these detectors are different and the total time in order to scan one image is at least of the order of 1 min. In particular, detectors which use a movement of the image plate relative to a quasistationary laser or vice versa and a movement of the laser source relative to a quasi-stationary image plate in order to scan the information show long readout times because of the mechanical limitations of the speed of the movement. Here, we report on an image plate detector, which is based on a former detector design [7] that has been modified and optimized for the use in combination with a synchrotron source. The detector is schematically shown in Fig. 1. For the scan of the image information the green light (3) of a frequency doubled Nd : YAG laser (1) is first expanded to a diameter of 10 mm and then deflected by a mirror (4) onto a motor driven rotating polygon mirror (5). By the rotation of the polygon mirror each facet deflects the light which is first focused by a f-theta lens (6) and then reflected by a mirror (7) onto a scan line in the indicated direction. Thereby, it passes a slit (8) in an elliptical shaped mirror (9), which is used for the collection and projection of the PSL onto the photodetector (10). The photodetector consists of a photomultiplier
Fig. 1. Schematic drawing of the X-ray detector based on image plates.
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tube with a photocathode of 264 mm]40 mm area. An optical filter blocking the green laser light covers the photocathode. The photocathode incorporates a preamplifier with a linear amplification stage and a second amplifier with square root transfer characteristic, which increases the orders of magnitude in X-ray dose that can be measured with this detector in combination with a usual ADC. During the scan the image plate (11) is moved by a translation stage (12), which is driven by a stepping motor (13) from the start position in the indicated direction. Thereby, the image information is scanned line by line by a combination of the fast scanning speed of the laser beam (up to 200 m/s) with the slow translation of the image plate. Subsequent to the complete scan of the image plate a high power lamp (14) illuminates the image plate in order to bleach the residual image information during the movement back to the start position. To avoid a damage of the image plate by the heat of the bleaching lamp, fans (15) cool the image plate during this process. The system is mounted in a housing with an X-ray window which allows the X-ray irradiation of the image plate in the start position. Therefore, the detector can be directly attached to the experimental setup at a synchrotron beamline and no exchange of the image plates is necessary for their readout. However, the image plates can be easily replaced by others, which are more suitable for the experiment with respect to the X-ray absorption or resolution, by opening the X-ray window. The detector is operated by a PC having a Pentium processor, 256 Mb of Ram memory, which can be accessed directly via DMA transfer by a PCI analog-to digital-converter-board (ADC) that is used for the digitization of the image data. Thereby, a digitization speed of 2]106 pixels/s with 14-bit resolution is reached for the linear and square root output signals of the photodetector. The trigger signal of the ADC is generated by a self-built control electronic. This electronic can be configured with the PC by a serial interface line with variable parameters as the wanted image and pixel size and controls the polygon rotation, the movement of the translation stage, the bleaching lamp, the ventilation system and the high-voltage setting of the photomultiplier tube. After the readout of the im-
age plate and the digitization, the data can be either stored on hard disk or transferred by an ATM interface to an external computer. Since the PC is operated under Windows NT the data of a subsequent readout can be acquired while the data of the previous readout is transferred to the disk or analyzed. This important feature allows continuous data acquisition. The performance of the detector is listed in Table 1. The high power of the laser in combination with the high scan speed of the polygon make it possible to acquire X-ray images within about 10 s using 80 lm pixel size, which is a big improvement compared to the commercial image plate detectors at the ESRF. In case that a finer digitization of the X-ray images is required it is possible to scan the images using smaller pixel sizes down to 40 lm by reducing the scan speed of the polygon from 160 to 80 m/s. Since the laser focus diameter in the scan line is 20 lm or less the detector is also compatible to high-resolution image plates. Further, because the depth of focus is about 1 mm, no problems arise when image plates with different thicknesses are used. In combination with STV image plates (Fuji) it is possible to acquire diffraction data with a resolution better than 200 lm FWHM.
3. Physical processes influencing the performance of the detector 3.1. Readout speed For the use at synchrotrons the readout speed of image plates is of major importance. Often the readout systems for image plates are restricted in the readout speed by mechanical limitations. For example, in the case of rotating drum or disk scanners the image plate is fixed to a drum or disk and is rotated thereby passing a readout head. In such scanners mechanical forces, which increase with the speed of rotation, set a limit to the scan speed at the order of 10 m/s. However, in the detector design presented here, a combination of fast deflection of the laser beam by a rotating polygon mirror and a slow translation of the image plate was used. With this scan technique speeds up to 1000 m/s can be reached. This would permit together with a
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Table 1 Parameters of the improved X-ray detector based on image plates X-ray sensitive image area Spatial distortion after software correction Diameter of the focal spot of the laser beam on the scan line Laser power Excitation wavelength Readout time of the image information
Time for erasure Number of pixels per image
Scan speed of the laser beam in the image plane during the readout
Range of detectable X-ray dose Digitization Transfer characteristics of amplification stage Pixel rate
faster digitization of the photodetector signal to scan images within the order of a second. Also the photomultiplier tubes, which are known to have time constants shorter than 10 ns, in combination with appropriate ADCs would allow sampling rates of 100 Mpixels/s or more. Therefore, the main limitation in the readout speed is at present the decay time constant of 680 ns of the storage phosphor BaFBr : Eu, which is used in the commercially available image plates. Since the photodetector in the readout system is sensitive to the PSL, which is emitted from the scan line, the image information of subsequently scanned points superimposes with increasing scan speeds. The action of this decay time constant q is like applying in the scan direction a low pass filter with a filter constant of l"l q, (1) 4#!/ where l is the scan speed, on the X-ray image. 4#!/ For the detector presented here this corresponds to a low pass filtering with 110 and 55 lm for 80 lm pixel size and 40 lm pixel size settings, respectively, which is well adapted to the spatial resolution of the commercial image plates (see Section 3). Therefore, a further increase in the readout speed is only
250 mm ] 305 mm 16 lm 20 lm (FWHM) 200 mW 532 nm 9.5 s (80 lm pixels) 17 s (60 lm pixels) 37 s (40 lm pixels) 3s 3125]3813 (80 lm pixels) 4167]5084 (60 lm pixels) 6250]7625 (40 lm pixels) 160 m/s (80 lm pixels) 120 m/s (60 lm pixels) 80 m/s (40 lm pixels) 4 nGy—0.1 Gy 14 bit Linear, square root (more than 14 bit dynamic range) 2]106 pixels/s
possible, if image plates which are based on faster storage phosphors are developed. 3.2. Efficiency The efficiency of X-ray detection with image plate detectors depends on a sequence of physical processes: The X-ray absorption of the image plate, the conversion efficiency to storage centers (&8 centers per keV of absorbed X-ray energy), the degree of readout of these centers during the scan and the efficiency of the photodetector for the detection of the PSL which is emitted during the recombination of the defects [8]. As shown in Fig. 2 the X-ray absorption of the commercial image plates of type ST III and ST V (Fuji) is about 100% for energies below 20 keV and decreases for energies up to 37 keV to about 50%. At the K-edge of Ba the X-ray absorption jumps to about 80% and decreases to about 50% at 55 keV. Over the whole energy range the ST V image plate shows higher X-ray absorption than the ST III image plate. These results show that efficient X-ray detection is possible, if the absorbed X-ray quanta can be detected during the scan. The detection efficiency of
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Fig. 2. The X-ray absorption of image plates of the type ST III and ST V (Fuji) in the energy range between 18 and 57 keV.
the absorbed X-ray quanta can be roughly estimated for the example of a 30 keV X-ray quantum as follows: Every absorbed X-ray quantum generates about 30]8"240 photostimulable defects. The fraction of PSL that is read during a single scan using 80 and 40 lm pixel size was measured experimentally to be 63% and 85%, respectively. This can be done by comparing the intensity of the first scan with the sum of intensities of subsequent scans without bleaching. Therefore, about 150 or 204 PSL photons are generated during the scan, respectively. Assuming a light collection efficiency of the photodetector of 25% and a quantum efficiency of 20% for the conversion of the PSL to electrons at the photocathode of the photomultiplier tube, every absorbed X-ray quantum generates 8 or 10 photoelectrons, respectively. Since these electrons can be easily detected due to the following electron multiplication in the tube, the quantum efficiency of the detector is mainly limited by the X-ray absorption of the image plate. Therefore, taking into account all relevant physical processes a quantum efficiency of about 50% can be expected for the detector at 30 keV.
light scattering results in a resolution of 180 lm FWHM [10]. For the detector presented here in combination with a STV image plate a resolution of about 200 lm FWHM was measured. Theoretical calculations based on a diffusion-like propagation of light in the phosphor layer [9] have shown that this value could be lowered by reducing the thickness of the phosphor layer. Especially in the case of applications which use X-ray energies below 20 keV the spatial resolution could be improved without much degrading of the X-ray absorption (&100% for ST III and ST V image plates) and therefore the efficiency. Comparing the resolution of the optimized image plate detector with scintillation screen based detectors, in which the spontaneous luminescence is detected from the side opposite to the X-ray irradiation, the image plate detector has the following advantages. In the case of scintillation screen based detectors the X-ray generated luminescence has to travel from the X-ray side of the screen to the detection side which results in a broadening of the intensity profile. However, in the case of the imaging plate detector the laser beam reads the stored information from the side of the X-ray irradiation. Therefore, the path of the laser beam to the photostimulable centers is very short. Thus the broadening of the stimulating intensity profile at the site of stimulation of the image plate is smaller than the broadening of the X-ray generated luminescence at the side of detection of the scintillation screen. As a consequence, the light, which is measured by the photomultiplier of the image plate detector, represents the X-ray information of a narrower area of the image plate resulting in a higher resolution. Further, the smaller path length of PSL relative to X-ray generated luminescence in the scintillation screen also reduces the fraction of light quanta being absorbed in the phosphor layer.
3.3. Spatial resolution 3.4. Range of detectable X-ray dose The spatial resolution of the detector is influenced by three different processes. The light scattering in the phosphor layer of the image plate [9], the temporal decay of the PSL, as discussed in Section 1, and bleaching effects which are related to the irradiated laser energy per scan length [10]. In the case of the ST III image plate it was shown that
Compared to CCD-based systems the image plate detector has the advantage, that the amplification of the image signal can be easily varied over several orders of magnitude by changing the high voltage setting of the photomultiplier tube as indicated in Fig. 3. A broad range of 1 : 105 in X-ray dose is
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accessible just by varying the amplification. However, this range is extended by the 14 bit resolution of the ADC to a range of about 1 : 108. As shown in Fig. 4 it is possible to cover a range of 10 nGy to 0.1 Gy in X-ray dose. At doses above 0.1 Gy an increasing blue coloration of the image plate has been observed which reduces the depth of readout of the phosphor layer because of increasing laser absorption and causes a deviation from linearity [11]. At doses below 100 nGy the natural background of X-rays has to be estimated and subtracted from the measurements.
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Because the currently used ADC offers a 14 bit digitization, a range of 1 :16 000 in X-ray dose is accessible for a single measurement in each pixel. However, in diffraction experiments the image information of a diffraction spot is usually spread over several pixels. In this case the summed intensity of these pixels can exceed 16 000 which again improves the dynamic range. Experimentally a dynamic range of at least 1 : 100 000 was estimated from a single scan of an image, which was irradiated with the beta radiation of 12 different test sources with an area of 1 cm2 and activities ranging from 1.1]106 to 8.3 dpm/cm2. In order to access a larger range in X-ray dose it is possible to subsequently scan the image plate with different sensitivity settings without intermediate bleaching of the image information by the high power lamp. Since the amplification factors of the different sensitivity settings and the fraction of the information that is bleached during a readout are known, the intensities of the recorded images can be scaled against each other.
4. Results of the first test experiments Fig. 3. The range of amplification of the improved image plate detector. The sensitivity is given in arbitrary scale units of the control electronic.
Fig. 4. The dynamic range of X-ray dose that is accessible with the image plate detector. Open symbols indicate the experimental data after a correction for the natural background radiation which was estimated to be 4 nGy.
For the first test experiments the new detector has been used in combination with a large volume high-pressure cell (Paris Edinburgh cell) and monochromatic X-ray radiation of 50 keV energy. With this monochromatic setup it is possible to record powder diffraction pattern under pressures up to 10 GPa and temperatures of up to 1500 K within less than a minute including the exposure time. However, the sample is surrounded by material, which partly acts as pressure transmitting medium. Thus high background and even diffraction cones can be caused through diffraction of X-rays by the medium, as exemplary shown in Fig. 5. This background level can be reduced by collimation optics in formerly used energy dispersive setup, which utilizes a white X-ray beam and a single point energy dispersive Ge detector. However, the energy dispersive setup has also several disadvantages, which make the quantitative analysis of diffraction pattern by full structural refinement (Rietveld refinement) almost impossible. First, because of unknown absorption corrections for the white
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Fig. 5. Diffraction pattern of BN powder sample taken with the new X-ray detector in combination with a large volume high-pressure cell at a pressure of 2 GPa and a temperature of 600 K. Several diffraction cones are caused by graphite, which surrounds the sample in the large volume cell and is used as a pressure transmitting medium. The exposure time was 30 s and a pixel size of 80 lm was used for the readout of the image plate. The readout took additionally 12 s.
radiation no reliable intensity information can be extracted. Secondly, due to the use of a single point detector only the reflections of a few powder grains are recorded. Therefore, no corrections for preferred orientations in the sample can be made and only poor statistical information of the average intensities can be obtained. Thus the only possibility to obtain data suitable for structure refinement is to use the monochromatic setup and to overcome the problem of high background. Because the background decreases with the reciprocal of the square of the sample to detector distance, it is advantageous to increase this distance and to use a detector with large X-ray sensitive area. In the case where the same range of diffraction angles is detected and the distance is adapted to the area of the detector the background level decreases with the reciprocal of the image area. Thus the background level is already about one order lower in the case of the image plate detector presented here compared to CCD-based X-ray detectors. In addition the background can be almost completely removed by displacing the press relative to the incident beam, recording the diffraction pattern of the material, which surrounds the sample as shown exemplary in Fig. 6 and subtracting this pattern as shown in Fig. 7. As will be presented in more detail in a future publication careful analysis of the background
subtracted diffraction patterns revealed excellent quality of the data. Even in the case of the light material BN it was possible to perform a Rietveld refinement resulting in an error of only 1%. It should be noted, that in this case the total time to collect the patterns was less than 2 min and the majority of this time was used for the exposure, which can be significantly shortened in the case of the study of heavier materials. This time is about two orders of magnitudes shorter compared to previous studies [12]. However, even compared to another commercial image plate detector, which can be alternatively used at the ID30 beamline, the new detector seems to have about one order of magnitude higher sensitivity, which shortens additionally the exposure time. From a general point of view the possibility of the new detector to record rapidly 2D images of radiation scattered even by light elements under conditions of simultaneous extreme pressure and temperature opens a new field of investigations. In particular, it now becomes possible to perform in situ studies of synthesis of superhard materials of which cubic boron nitride and diamond are examples. But also the study of the kinetics of phase transitions under high pressure or single-crystal work under high pressure will become possible with the new detector.
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Fig. 6. Diffraction pattern of the material surrounding the sample taken with the new X-ray detector in combination with a large volume high-pressure cell. Compared to Fig. 5 the cell was displaced slightly during the exposure. Therefore, only diffraction cones of the surrounding material, which is mainly composed of graphite, were recorded.
which influence the performance of the detector are discussed. First promising results of powder diffraction experiments using BN as a sample in a large volume high pressure cell at the ID30 beamline of the ESRF at high pressure and temperature are reported. A Rietveld refinement of the obtained data down to an error of 1% has been possible. Compared to previously reported experiments the total time needed for the experiment has been reduced by about two orders of magnitude to about 2 min. Acknowledgements Fig. 7. Circularly integrated 1-d diffraction pattern of the BN sample and the pressure medium as shown in Fig. 5 (a), of the pressure medium surrounding the sample as shown in Fig. 6 (b) and of the final data taken by subtracting pattern b from pattern a (c).
5. Summary A new image plate X-ray detector, which combines large image area, high dynamic range and high resolution with high-readout speed, is presented. The high-readout speed of 9 s for an image of 250 mm]305 mm enables a high duty cycle in the use of the X-ray radiation, which is of special importance at the 3rd generation synchrotrons. The advantages of the detector compared to CCDbased X-ray detectors and physical processes,
The authors like to acknowledge the support of the University of Erlangen, Institut fu¨r Werkstoffwissenschaften VI and thank Prof. Ross Angel and Dr. Larry Finger for valuable discussions.
References [1] M. Thoms, H. von Seggern, A. Winnacker, Phys. Rev. B 44 (17) (1991) 9240. [2] T. Hangleiter, F.K. Koschnick, J.-M. Spaeth, R.H.D. Nuttall, R.S. Eachus, J. Phys.: Condens. Matter 2 (1990) 6837. [3] M. Thoms, H. von Seggern, J. Appl. Phys. 75 (9) (1994) 4658. [4] J. Miyahara, K. Takahashi, Y. Amemiya, N. Kamiya, Y. Satow, Nucl. Instr. and Meth. A 246 (1986) 572. [5] Y. Amemiya, T. Matsushita, A. Nakagawa, Y. Satow, J. Miyahara, J. Chikawa, Nucl. Instr. and Meth. A 266 (1988) 645.
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An improved X-ray detector for use at synchrotrons M. Thoms!,",*, S. Bauchau!, D. Ha¨usermann!, M. Kunz!, T. Le Bihan!, M. Mezouar!, D. Strawbridge! ! European Synchrotron Radiation Facility, Av. des Martyrs - B.P. 220, F-38043 Grenoble cedex 09, France " University of Erlangen-Nu( rnberg, Institute of Material Science VI, Martensstr. 7, D-91058 Erlangen, Germany Received 29 September 1997
Abstract Due to the advent of synchrotrons as X-ray sources with high power and brilliance new improved X-ray detectors are needed in order to make efficient use of the unique features of the synchrotron radiation. In this article the construction and characteristics of an X-ray detector, which is based on image plates and which has been especially designed for the detection of X-ray images at synchrotrons is reported. The detector has an X-ray sensitive area of 250 mm]305 mm, and allows the acquisition of X-ray images with variable pixel sizes and readout speeds according to the requirements of the particular experiment. In the normal setup mode images can be recorded every 13 s using a pixel size of 80 lm]80 lm, which relates to 12]106 pixels per image, while in the high-resolution setup images can be recorded every 41 s with 40 lm]40 lm pixel size, which relates to 48]106 pixels per image. The relation of detector characteristics, such as readout speed, resolution and efficiency to the physical processes of image storage and readout are discussed. Examples of images taken during the first test experiment on the high-pressure beamline (ID30) of the ESRF are presented. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: BaFBr : Eu; Image plate; Image readout; Light scattering; X-ray absorption; X-ray sensitivity; Photon diffusion; Photostimulation; Photostimulated luminescence; Sensitivity; Storage phosphor
1. Introduction The advent of synchrotrons with their high brilliance and X-ray flux has created new demands for X-ray detectors. For example diffraction experiments can be carried out in much shorter time compared to experimental setups using conven-
* Correspondence address: University of Erlangen-Nu¨rnberg, Institute of Material Science VI, Martensstr. 7, D-91058 Erlangen, Germany. Tel.: #49 9131 857683; fax: #49 9131 858495.
tional X-ray tubes. This requires fast and efficient X-ray image detectors, which cover a large solid angle of the diffracted radiation and have a high precision. Although X-ray detectors, which are based on image plates have the potential to fit these requirements, commercially available image plate detectors have the drawback of low-readout speed resulting in detection cycles of X-ray images in the order of minutes and longer. Therefore, an optimized image plate detector, which is described together with its operation principle in Section 2, allowing fast readout cycles in the order of 10 s has
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been built at the ESRF. The detector has several advantages compared to X-ray detectors, which are based on CCD-detectors and are coupled to a phosphor screen: 1. The large sensitive area of 250 mm]305 mm allows the detection of X-ray radiation that is diffracted in a large solid angle without putting the detector very close to the sample. Therefore, the signal/background ratio is improved and the possibility of overlapping diffraction spots is reduced. 2. During the exposure no background signal is accumulated. This background is caused in silicon based CCD-detectors by the thermal excitation of charge carriers. Therefore, image plates have no restrictions on exposure times. 3. The dynamic range is not limited by the well capacity as in CCD-detectors. 4. The sensitivity and spatial resolution of the detector can be simply adapted to the requirements of the experiments by changing the image plate and by varying the pixel size and the amplification of the photomultiplier tubes. 5. The readout noise of the detector is low since the image signal, which results from photostimulated luminescence (PSL), is electronically amplified in photomultiplier tubes by several orders of magnitude before the analog-to digital conversion. Further, the dark count of the photomultiplier, which is about a few hundred photons/s, only influences the dose measurement during the readout of the image plate. However, the readout of a single pixel takes typically between about 1 and 100 ls and thus it is rare, that dark counts contribute to the readout of a single pixel. 6. In image plates the image is detected at the same side of the phosphor screen as the X-rays create the image. In contrast, in most CCD-based detectors the CCD is mounted opposite to the side of X-ray exposure in order to avoid damage of the sensor by X-rays. Therefore, in such systems the X-ray generated luminescence has to travel from the side of X-ray exposure through the granular phosphor layer to the side of detection, which results in a spatial broadening of the light profile and therewith in a loss of resolution.
7. The problem of charge trapping of image information in bad pixels does not exist. This effect occurs in CCD detectors because the image information carrying charges has to be sequentially shifted pixel by pixel to the output lines. 8. The pixel rate of 2]106 pixels/s allows a fast digitization of X-ray images without affecting the efficiency and readout noise of the detection. In CCD-based systems it is known that the transfer efficiency of the charge shifting from pixel to pixel degrades for increasing readout speeds. Further, the readout noise increases, resulting, together with the limited well capacity, in a decrease of the dynamic range of the CCD. 9. There are no problems with X-rays which directly hit the CCD and cause overflow in the corresponding pixels. However, physical processes also limit the performance of the image plate detector during the generation, storage and readout of the image information. Therefore, the influence of these processes on the performance of the detector and ways of its further improvement will be discussed in Section 3. In Section 4 results of the first test experiments made on the high-pressure beamline (ID30) of the ESRF will be presented. A summary is given in Section 5.
2. The optimized image plate detector and its operation principle The operation principle of X-ray detectors which are based on image plates can be subdivided into the X-ray irradiation, where the X-ray information is generated in the image plate and into the readout of this information. During the X-ray irradiation X-rays are absorbed in the image plate. A typical image plate is composed of a thin layer of storage phosphor that is embedded in an organic binder and coated onto a supporting polymer film. X-rays, which are absorbed in storage phosphor grains, which have typically a size of about 5 lm and consist of BaFBr : Eu, create electron and hole storage centers in the material [1,2]. The number of generated centers is directly proportional to the X-ray dose.
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An average energy of about 125 eV is needed in order to generate a photostimulable defect pair [3]. As a result an absorbed X-ray quantum of 10 keV energy produces about 80 photostimulable defect pairs. Since the generation of a defect pair involves the excitation of electrons from the valence to the conduction band and since the band gap energy is about 8 eV in the case of BaFBr : Eu no thermal generation of defect pairs is possible at room temperature. Therefore, the image plates can be exposed to X-rays over long time periods without accumulating thermal background signals, as observed in the case of CCD-based detectors. During the readout process the image plate is scanned point by point with a laser beam. The luminescence intensity, resulting from the photostimulated recombination of the defects is measured with a photomultiplier, which is optically coupled to the point of emission by a light collector. The intensity signal is subsequently digitized and stored in a computer. After the scan the residual information has to be bleached before the image plate can be exposed the next time. Several detector systems based on image plates have already been built [4—6] or are commercially available. They all differ in type of lasers, scanning mechanisms, techniques for the light collection and ADC converters.
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Therefore, the performance, especially the readout speeds of these detectors are different and the total time in order to scan one image is at least of the order of 1 min. In particular, detectors which use a movement of the image plate relative to a quasistationary laser or vice versa and a movement of the laser source relative to a quasi-stationary image plate in order to scan the information show long readout times because of the mechanical limitations of the speed of the movement. Here, we report on an image plate detector, which is based on a former detector design [7] that has been modified and optimized for the use in combination with a synchrotron source. The detector is schematically shown in Fig. 1. For the scan of the image information the green light (3) of a frequency doubled Nd : YAG laser (1) is first expanded to a diameter of 10 mm and then deflected by a mirror (4) onto a motor driven rotating polygon mirror (5). By the rotation of the polygon mirror each facet deflects the light which is first focused by a f-theta lens (6) and then reflected by a mirror (7) onto a scan line in the indicated direction. Thereby, it passes a slit (8) in an elliptical shaped mirror (9), which is used for the collection and projection of the PSL onto the photodetector (10). The photodetector consists of a photomultiplier
Fig. 1. Schematic drawing of the X-ray detector based on image plates.
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tube with a photocathode of 264 mm]40 mm area. An optical filter blocking the green laser light covers the photocathode. The photocathode incorporates a preamplifier with a linear amplification stage and a second amplifier with square root transfer characteristic, which increases the orders of magnitude in X-ray dose that can be measured with this detector in combination with a usual ADC. During the scan the image plate (11) is moved by a translation stage (12), which is driven by a stepping motor (13) from the start position in the indicated direction. Thereby, the image information is scanned line by line by a combination of the fast scanning speed of the laser beam (up to 200 m/s) with the slow translation of the image plate. Subsequent to the complete scan of the image plate a high power lamp (14) illuminates the image plate in order to bleach the residual image information during the movement back to the start position. To avoid a damage of the image plate by the heat of the bleaching lamp, fans (15) cool the image plate during this process. The system is mounted in a housing with an X-ray window which allows the X-ray irradiation of the image plate in the start position. Therefore, the detector can be directly attached to the experimental setup at a synchrotron beamline and no exchange of the image plates is necessary for their readout. However, the image plates can be easily replaced by others, which are more suitable for the experiment with respect to the X-ray absorption or resolution, by opening the X-ray window. The detector is operated by a PC having a Pentium processor, 256 Mb of Ram memory, which can be accessed directly via DMA transfer by a PCI analog-to digital-converter-board (ADC) that is used for the digitization of the image data. Thereby, a digitization speed of 2]106 pixels/s with 14-bit resolution is reached for the linear and square root output signals of the photodetector. The trigger signal of the ADC is generated by a self-built control electronic. This electronic can be configured with the PC by a serial interface line with variable parameters as the wanted image and pixel size and controls the polygon rotation, the movement of the translation stage, the bleaching lamp, the ventilation system and the high-voltage setting of the photomultiplier tube. After the readout of the im-
age plate and the digitization, the data can be either stored on hard disk or transferred by an ATM interface to an external computer. Since the PC is operated under Windows NT the data of a subsequent readout can be acquired while the data of the previous readout is transferred to the disk or analyzed. This important feature allows continuous data acquisition. The performance of the detector is listed in Table 1. The high power of the laser in combination with the high scan speed of the polygon make it possible to acquire X-ray images within about 10 s using 80 lm pixel size, which is a big improvement compared to the commercial image plate detectors at the ESRF. In case that a finer digitization of the X-ray images is required it is possible to scan the images using smaller pixel sizes down to 40 lm by reducing the scan speed of the polygon from 160 to 80 m/s. Since the laser focus diameter in the scan line is 20 lm or less the detector is also compatible to high-resolution image plates. Further, because the depth of focus is about 1 mm, no problems arise when image plates with different thicknesses are used. In combination with STV image plates (Fuji) it is possible to acquire diffraction data with a resolution better than 200 lm FWHM.
3. Physical processes influencing the performance of the detector 3.1. Readout speed For the use at synchrotrons the readout speed of image plates is of major importance. Often the readout systems for image plates are restricted in the readout speed by mechanical limitations. For example, in the case of rotating drum or disk scanners the image plate is fixed to a drum or disk and is rotated thereby passing a readout head. In such scanners mechanical forces, which increase with the speed of rotation, set a limit to the scan speed at the order of 10 m/s. However, in the detector design presented here, a combination of fast deflection of the laser beam by a rotating polygon mirror and a slow translation of the image plate was used. With this scan technique speeds up to 1000 m/s can be reached. This would permit together with a
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Table 1 Parameters of the improved X-ray detector based on image plates X-ray sensitive image area Spatial distortion after software correction Diameter of the focal spot of the laser beam on the scan line Laser power Excitation wavelength Readout time of the image information
Time for erasure Number of pixels per image
Scan speed of the laser beam in the image plane during the readout
Range of detectable X-ray dose Digitization Transfer characteristics of amplification stage Pixel rate
faster digitization of the photodetector signal to scan images within the order of a second. Also the photomultiplier tubes, which are known to have time constants shorter than 10 ns, in combination with appropriate ADCs would allow sampling rates of 100 Mpixels/s or more. Therefore, the main limitation in the readout speed is at present the decay time constant of 680 ns of the storage phosphor BaFBr : Eu, which is used in the commercially available image plates. Since the photodetector in the readout system is sensitive to the PSL, which is emitted from the scan line, the image information of subsequently scanned points superimposes with increasing scan speeds. The action of this decay time constant q is like applying in the scan direction a low pass filter with a filter constant of l"l q, (1) 4#!/ where l is the scan speed, on the X-ray image. 4#!/ For the detector presented here this corresponds to a low pass filtering with 110 and 55 lm for 80 lm pixel size and 40 lm pixel size settings, respectively, which is well adapted to the spatial resolution of the commercial image plates (see Section 3). Therefore, a further increase in the readout speed is only
250 mm ] 305 mm 16 lm 20 lm (FWHM) 200 mW 532 nm 9.5 s (80 lm pixels) 17 s (60 lm pixels) 37 s (40 lm pixels) 3s 3125]3813 (80 lm pixels) 4167]5084 (60 lm pixels) 6250]7625 (40 lm pixels) 160 m/s (80 lm pixels) 120 m/s (60 lm pixels) 80 m/s (40 lm pixels) 4 nGy—0.1 Gy 14 bit Linear, square root (more than 14 bit dynamic range) 2]106 pixels/s
possible, if image plates which are based on faster storage phosphors are developed. 3.2. Efficiency The efficiency of X-ray detection with image plate detectors depends on a sequence of physical processes: The X-ray absorption of the image plate, the conversion efficiency to storage centers (&8 centers per keV of absorbed X-ray energy), the degree of readout of these centers during the scan and the efficiency of the photodetector for the detection of the PSL which is emitted during the recombination of the defects [8]. As shown in Fig. 2 the X-ray absorption of the commercial image plates of type ST III and ST V (Fuji) is about 100% for energies below 20 keV and decreases for energies up to 37 keV to about 50%. At the K-edge of Ba the X-ray absorption jumps to about 80% and decreases to about 50% at 55 keV. Over the whole energy range the ST V image plate shows higher X-ray absorption than the ST III image plate. These results show that efficient X-ray detection is possible, if the absorbed X-ray quanta can be detected during the scan. The detection efficiency of
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Fig. 2. The X-ray absorption of image plates of the type ST III and ST V (Fuji) in the energy range between 18 and 57 keV.
the absorbed X-ray quanta can be roughly estimated for the example of a 30 keV X-ray quantum as follows: Every absorbed X-ray quantum generates about 30]8"240 photostimulable defects. The fraction of PSL that is read during a single scan using 80 and 40 lm pixel size was measured experimentally to be 63% and 85%, respectively. This can be done by comparing the intensity of the first scan with the sum of intensities of subsequent scans without bleaching. Therefore, about 150 or 204 PSL photons are generated during the scan, respectively. Assuming a light collection efficiency of the photodetector of 25% and a quantum efficiency of 20% for the conversion of the PSL to electrons at the photocathode of the photomultiplier tube, every absorbed X-ray quantum generates 8 or 10 photoelectrons, respectively. Since these electrons can be easily detected due to the following electron multiplication in the tube, the quantum efficiency of the detector is mainly limited by the X-ray absorption of the image plate. Therefore, taking into account all relevant physical processes a quantum efficiency of about 50% can be expected for the detector at 30 keV.
light scattering results in a resolution of 180 lm FWHM [10]. For the detector presented here in combination with a STV image plate a resolution of about 200 lm FWHM was measured. Theoretical calculations based on a diffusion-like propagation of light in the phosphor layer [9] have shown that this value could be lowered by reducing the thickness of the phosphor layer. Especially in the case of applications which use X-ray energies below 20 keV the spatial resolution could be improved without much degrading of the X-ray absorption (&100% for ST III and ST V image plates) and therefore the efficiency. Comparing the resolution of the optimized image plate detector with scintillation screen based detectors, in which the spontaneous luminescence is detected from the side opposite to the X-ray irradiation, the image plate detector has the following advantages. In the case of scintillation screen based detectors the X-ray generated luminescence has to travel from the X-ray side of the screen to the detection side which results in a broadening of the intensity profile. However, in the case of the imaging plate detector the laser beam reads the stored information from the side of the X-ray irradiation. Therefore, the path of the laser beam to the photostimulable centers is very short. Thus the broadening of the stimulating intensity profile at the site of stimulation of the image plate is smaller than the broadening of the X-ray generated luminescence at the side of detection of the scintillation screen. As a consequence, the light, which is measured by the photomultiplier of the image plate detector, represents the X-ray information of a narrower area of the image plate resulting in a higher resolution. Further, the smaller path length of PSL relative to X-ray generated luminescence in the scintillation screen also reduces the fraction of light quanta being absorbed in the phosphor layer.
3.3. Spatial resolution 3.4. Range of detectable X-ray dose The spatial resolution of the detector is influenced by three different processes. The light scattering in the phosphor layer of the image plate [9], the temporal decay of the PSL, as discussed in Section 1, and bleaching effects which are related to the irradiated laser energy per scan length [10]. In the case of the ST III image plate it was shown that
Compared to CCD-based systems the image plate detector has the advantage, that the amplification of the image signal can be easily varied over several orders of magnitude by changing the high voltage setting of the photomultiplier tube as indicated in Fig. 3. A broad range of 1 : 105 in X-ray dose is
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accessible just by varying the amplification. However, this range is extended by the 14 bit resolution of the ADC to a range of about 1 : 108. As shown in Fig. 4 it is possible to cover a range of 10 nGy to 0.1 Gy in X-ray dose. At doses above 0.1 Gy an increasing blue coloration of the image plate has been observed which reduces the depth of readout of the phosphor layer because of increasing laser absorption and causes a deviation from linearity [11]. At doses below 100 nGy the natural background of X-rays has to be estimated and subtracted from the measurements.
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Because the currently used ADC offers a 14 bit digitization, a range of 1 :16 000 in X-ray dose is accessible for a single measurement in each pixel. However, in diffraction experiments the image information of a diffraction spot is usually spread over several pixels. In this case the summed intensity of these pixels can exceed 16 000 which again improves the dynamic range. Experimentally a dynamic range of at least 1 : 100 000 was estimated from a single scan of an image, which was irradiated with the beta radiation of 12 different test sources with an area of 1 cm2 and activities ranging from 1.1]106 to 8.3 dpm/cm2. In order to access a larger range in X-ray dose it is possible to subsequently scan the image plate with different sensitivity settings without intermediate bleaching of the image information by the high power lamp. Since the amplification factors of the different sensitivity settings and the fraction of the information that is bleached during a readout are known, the intensities of the recorded images can be scaled against each other.
4. Results of the first test experiments Fig. 3. The range of amplification of the improved image plate detector. The sensitivity is given in arbitrary scale units of the control electronic.
Fig. 4. The dynamic range of X-ray dose that is accessible with the image plate detector. Open symbols indicate the experimental data after a correction for the natural background radiation which was estimated to be 4 nGy.
For the first test experiments the new detector has been used in combination with a large volume high-pressure cell (Paris Edinburgh cell) and monochromatic X-ray radiation of 50 keV energy. With this monochromatic setup it is possible to record powder diffraction pattern under pressures up to 10 GPa and temperatures of up to 1500 K within less than a minute including the exposure time. However, the sample is surrounded by material, which partly acts as pressure transmitting medium. Thus high background and even diffraction cones can be caused through diffraction of X-rays by the medium, as exemplary shown in Fig. 5. This background level can be reduced by collimation optics in formerly used energy dispersive setup, which utilizes a white X-ray beam and a single point energy dispersive Ge detector. However, the energy dispersive setup has also several disadvantages, which make the quantitative analysis of diffraction pattern by full structural refinement (Rietveld refinement) almost impossible. First, because of unknown absorption corrections for the white
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Fig. 5. Diffraction pattern of BN powder sample taken with the new X-ray detector in combination with a large volume high-pressure cell at a pressure of 2 GPa and a temperature of 600 K. Several diffraction cones are caused by graphite, which surrounds the sample in the large volume cell and is used as a pressure transmitting medium. The exposure time was 30 s and a pixel size of 80 lm was used for the readout of the image plate. The readout took additionally 12 s.
radiation no reliable intensity information can be extracted. Secondly, due to the use of a single point detector only the reflections of a few powder grains are recorded. Therefore, no corrections for preferred orientations in the sample can be made and only poor statistical information of the average intensities can be obtained. Thus the only possibility to obtain data suitable for structure refinement is to use the monochromatic setup and to overcome the problem of high background. Because the background decreases with the reciprocal of the square of the sample to detector distance, it is advantageous to increase this distance and to use a detector with large X-ray sensitive area. In the case where the same range of diffraction angles is detected and the distance is adapted to the area of the detector the background level decreases with the reciprocal of the image area. Thus the background level is already about one order lower in the case of the image plate detector presented here compared to CCD-based X-ray detectors. In addition the background can be almost completely removed by displacing the press relative to the incident beam, recording the diffraction pattern of the material, which surrounds the sample as shown exemplary in Fig. 6 and subtracting this pattern as shown in Fig. 7. As will be presented in more detail in a future publication careful analysis of the background
subtracted diffraction patterns revealed excellent quality of the data. Even in the case of the light material BN it was possible to perform a Rietveld refinement resulting in an error of only 1%. It should be noted, that in this case the total time to collect the patterns was less than 2 min and the majority of this time was used for the exposure, which can be significantly shortened in the case of the study of heavier materials. This time is about two orders of magnitudes shorter compared to previous studies [12]. However, even compared to another commercial image plate detector, which can be alternatively used at the ID30 beamline, the new detector seems to have about one order of magnitude higher sensitivity, which shortens additionally the exposure time. From a general point of view the possibility of the new detector to record rapidly 2D images of radiation scattered even by light elements under conditions of simultaneous extreme pressure and temperature opens a new field of investigations. In particular, it now becomes possible to perform in situ studies of synthesis of superhard materials of which cubic boron nitride and diamond are examples. But also the study of the kinetics of phase transitions under high pressure or single-crystal work under high pressure will become possible with the new detector.
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Fig. 6. Diffraction pattern of the material surrounding the sample taken with the new X-ray detector in combination with a large volume high-pressure cell. Compared to Fig. 5 the cell was displaced slightly during the exposure. Therefore, only diffraction cones of the surrounding material, which is mainly composed of graphite, were recorded.
which influence the performance of the detector are discussed. First promising results of powder diffraction experiments using BN as a sample in a large volume high pressure cell at the ID30 beamline of the ESRF at high pressure and temperature are reported. A Rietveld refinement of the obtained data down to an error of 1% has been possible. Compared to previously reported experiments the total time needed for the experiment has been reduced by about two orders of magnitude to about 2 min. Acknowledgements Fig. 7. Circularly integrated 1-d diffraction pattern of the BN sample and the pressure medium as shown in Fig. 5 (a), of the pressure medium surrounding the sample as shown in Fig. 6 (b) and of the final data taken by subtracting pattern b from pattern a (c).
5. Summary A new image plate X-ray detector, which combines large image area, high dynamic range and high resolution with high-readout speed, is presented. The high-readout speed of 9 s for an image of 250 mm]305 mm enables a high duty cycle in the use of the X-ray radiation, which is of special importance at the 3rd generation synchrotrons. The advantages of the detector compared to CCDbased X-ray detectors and physical processes,
The authors like to acknowledge the support of the University of Erlangen, Institut fu¨r Werkstoffwissenschaften VI and thank Prof. Ross Angel and Dr. Larry Finger for valuable discussions.
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