- Email: [email protected]
Detectors for X-ray Coherent Diffractive Imaging
ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 604 (2009) 130–132
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Detectors for X-ray Coherent Diffractive Imaging Christopher J. Hall a,b,, Robert A. Lewis a,b a b
Monash Centre for Synchrotron Science, Monash University, Wellington Road, Clayton 3800, Australia The Australian Research Council Centre of Excellence in Coherent X-ray Science, Australia
a r t i c l e in f o
a b s t r a c t
Available online 25 January 2009
This paper examines both current and future requirements for detectors in Coherent X-ray Diffractive Imaging. A set of criteria for a suitable detector for a specific form of CXDI, Fresnel Coherent Diffractive Imaging, is derived. Observations about the challenges to soft X-ray detector science from this new imaging technique are discussed. & 2009 Elsevier B.V. All rights reserved.
Keywords: Coherent X-ray imaging Soft X-ray detectors
1. Introduction Scientific gains made by the ability to image objects at scales of tens of nanometres are driving the evolution of several new short wavelength microscopy techniques. In the life sciences, significant progress is being made through these technologies in understanding cell and inter-cellular processes. Similar progress is being enabled by the techniques in nanomaterials research. The advent of X-ray microscopes, which use synchrotron sources, has meant image resolutions below 10 nm are now within reach even with wet samples. Furthermore, the use of the coherence from these sources has given rise to a novel form of lens-less X-ray microscopy: Coherent X-ray Diffractive Imaging (CXDI). This relies on an ability to reconstruct images of an object from measurements of its diffracted intensity. CXDI works if the coherence of the illumination is larger than the object being imaged and some aspects of the incident light wave field are known a-priori. The potential of the technique was highlighted in 1999 when Miao et al. [1] published a paper in Nature showing a reconstructed image of a non-periodic object at X-ray wavelengths with a resolution of around 75 nm. Since then images of biological samples with resolutions down to 30 nm [2] and tomographic reconstructions of hard materials down to 8 nm [3] have been published. CXDI can be thought of as a way of imaging nanoscale structures using a method similar to crystallography [4]. The Fourier transform of the wavefield from the object is captured by measuring the X-ray diffraction pattern. This is mathematically inverted to recover the wavefield as it exits the object. However, a major difference from crystallography is that structures being imaged in CXDI are not repetitive in space, so the diffraction signal is both continuous and very weak. This fact determines a need for
Corresponding author at: Monash Centre for Synchrotron Science, Monash University, Wellington Road, Clayton 3800, Australia. E-mail address: [email protected] (C.J. Hall).
0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.026
synchrotrons or other bright sources of incident X-ray illumination. The well-known phase problem is also to be overcome in CXDI. Intensity measurements by the detector of the Fourier space images do not provide phase information, which would allow a simple inverse transform to be made accurately. However, the way CXDI is performed does allow a solution. If the object is spatially isolated so that sampling in Fourier space meets the Nyquist criteria, a detection of intensity provides an ability to constrain the data in both domains. The idea of a ‘support’ around the object, which is known to be featureless, permits an iterative solution to be made [5]. There has been a lot of activity in the CXDI area over the last few years. In Australia the Centre of Excellence (CoE) in Coherent X-ray Science has been working with a particular form of CXDI known as Fresnel CXDI (FCDI) [6]. FCDI uses a non-planar incident wave field on the object, which has some advantages in accuracy and speed of convergence. CXDI has the potential to rival electron microscopy in spatial resolution, but without the inconvenience of requiring a vacuum around the sample. However, CXDI is a technically challenging technique requiring all the source, optics and detectors to be developed if its full potential is to be realised. Our focus at the Monash Centre for Synchrotron Science is in a specific part of the FCDI facility being constructed by the CoE: the X-ray detector. Commercially available direct detection CCDs are used for the majority of CXDI experiments at the moment. These versatile soft X-ray detectors have allowed the field to progress to the point where the technique is feasible as a user facility. However, the limitations CCDs impose on the technique need to be addressed before CXDI can really fulfil its potential. The work of our CoE is aimed at life science issues. For biological samples the X-ray scattering cross-sections for the photon energies at which CXDI is used are relatively small. The diffraction patterns are consequently weak so sensitivity is a prime consideration for a detector. Back-illuminated silicon CCD detectors score well in this respect. They have a quantum efficiency close to 100% and are able to detect single photons of 3 keV. Arguably, a more challenging
ARTICLE IN PRESS C.J. Hall, R.A. Lewis / Nuclear Instruments and Methods in Physics Research A 604 (2009) 130–132
131
Table 1 Criterion for selecting a soft X-ray detector technology for CXDI. Criterion
Essential
Desirable
Explanation
Dynamic range
104
107
Pixelation
2048 2048
44096 44096
Pixel size Energy range (keV)
o100 mm 0.5–3
o50 mm 0.5–3
Detective quantum efficiency (%) Rate capability
10
50
410 /pixel/s
4106/pixel/s
Frame rate (Hz) Vacuum compatibility (bar)
1 10
1 10
Dead time fraction (%)
10
1
Parallax error Gaps in FOV
o1 pixel at 201 Acceptable
None None
The detector’s single frame saturation level divided by the RMS dark image noise for the same exposure time. In an additive noise model the dark image noise is the sum of the variances due to read noise (sread) and dark noise (sdark). The number of evenly spaced pixels in the two dimensions of the image over the field of view (FOV). Assuming square pixels. This is the size on the active area of the detector. The range over which 410% Detective Quantum Efficiency (DQE) is achieved. DQE describes the degradation of signal-to-noise ratio (SNR) as a photon field is detected. It is defined as the ratio of the squares of the SNR in the incident photon field to the SNR in the detected image. This level of DQE to be at least maintained across the energy range given in criterion 4. Maximum rate at which the pixel retains an intensity measurement linearity of 1%. Measured after dead time correction in the case of a counting detector. The maximum rate at which full fields can be read out from the detector. The vacuum which can be maintained with the detector operational in the sample chamber. For a counting detector, the fraction of time in which the detector is insensitive to incoming photons. Measured at the maximum rate. The attenuation length for the lowest energy photons in criterion 4, projected. In any tiled detector array there are bound to be discontinuities at the boundary of the tiles. These may lead to gaps in the sampling of the incident intensity.
4
4
6
This was the result of a ‘trade study’ criterion preparation exercise.
parameter to tackle is capturing the large dynamic range inherent in a CXDI image. In FCDI images the central part of the detected pattern is a magnified far-field image (hologram) of the object. This region is required as input to the reconstruction algorithm and therefore must be recorded accurately. The holographic region is typically 6 orders of magnitude brighter than the diffracted intensity out at higher angles. X-ray CCD imagers are unable to record the whole range of intensity in a single capture. The noise level to full-well capacity puts a limit on this parameter. In order to overcome the limit, hundreds of individual frames are summed in order to capture the full data set. Clearly this is an inefficiency that can be reduced. For each capture the CCD must be clocked out inevitably adding readout noise. With a synchrotron as the X-ray source, the read-out time and exposure time for an individual frame are similar so the inefficiency is around 50%. A detector that would allow longer exposures whilst capturing the full range of intensity would improve the efficiency by many fold. Two driving specifications of dynamic range and sensitivity lead to consideration of a photon counting detector. Typical rates from the brightest part of the FCDI image are o106 photons mm 2 s 1, making some form of the soft X-ray hybrid pixel detector very attractive. One of the many technical challenges to a successful FCDI is the requirement that the registration of the wavefront of the illuminating field incident and the object stays very stable during the exposure. Aiming for a spatial resolution of below 10 nm means the stability must be ideally less than 1 nm over the time of exposure. Our FCDI optical system will achieve close to this, but the technology required is expensive. A detector that can be rapidly read out might prove an alternative means to satisfy this demand. If sufficient signal circuitry can be provided locally to a pixel, it is possible to add a time stamp to data from each pixel. Photon arrival times may be correlated with movement sensors in the object stage. A post-collection correction could then be made to compensate for drift and vibration in the relative position of the beam and object. When surveying detector technologies that might be applicable to CXDI, the following basic parameters were considered: the detector needs to efficiently detect low-energy X-rays, it needs to be position sensitive and it needs to provide a sufficient dynamic
range for the technique. Quantifying these demands involved close consultation with the groups who are developing the optics and reconstruction algorithms. Demands from the image reconstruction process are key to specifying the acceptable non-ideal parameters from the detector. The results of this consultation are summarised in Table 1. As is often the case, different degrees of compromise were accepted by the individuals who helped compile this list, which resulted in a binary categorisation of parameters. Each criterion had a specification, which was considered essential for any successful FCDI detector. Any detector or technology that did not meet all of the ‘essential’ parameters would not be considered for next-generation FCDI instrumentation. Realising that there may be scientific gains in pushing some of these specifications further, a second category of ‘desirable’ specifications was also tabulated. It was these parameters that give a weighted score to help rank the solutions in our final analysis. The order in which these parameters appear in the table gives the priority to the criteria for weighting. Although terms used in the table might be familiar to those in the field, some explanation of the criterion is given to clarify what is meant in each case.
2. Hybrid pixel detectors for FCDI The idea of building an X-ray detector technology that taps into the extensive industrial capabilities for silicon-integrated circuit fabrication has been around for decades. The Hybrid Pixel Detector (HPD) consists of a matrix of circuits above which a suitable detector material is connected, usually through flip-chip or bump bonding [7]. For small research groups, realising anything other than a proof of concept detector has taken a long time. CERN has successfully spawned an organization to exploit its investment in this area. The MediPix collaboration has demonstrated a wide range of demands that such a device has to offer.1 However, they are now commercially available systems in the market. Dectris Pilatus is the first and there are others set to 1
http://medipix.web.cern.ch/MEDIPIX/
ARTICLE IN PRESS 132
C.J. Hall, R.A. Lewis / Nuclear Instruments and Methods in Physics Research A 604 (2009) 130–132
follow this lead.2 With academic access to some of the silicon fabrication processes, it is now within the reach of research groups to design and build detectors tailored specifically for a particular task. We have recently embarked on a design study to investigate using HPD technology for FCDI. Our group is building infrastructure for HPD development. We believe that with appropriate adaptations we could design an HPD specifically for the shorter term requirements of FCDI. 3. Conclusion New X-ray imaging techniques that come under the general term Coherent X-ray Diffractive Imaging will impose significant demands on the detector. Although directly illuminated CCDs are currently being successfully used, the limitations they impose on dynamic range and readout speed will need to be addressed as the techniques mature. We have drawn up a list of specifications that will suit one particular type of CXDI; Fresnel Coherent Diffractive
2
http://pilatus.web.psi.ch/pilatus.htm
imaging. This list will be used in the future to assess technologies and individual instruments as to their suitability.
Acknowledgements The authors would like to acknowledge the support of the Australian Research Council Centre of Excellence in Coherent X-ray Science. References [1] J.W. Miao, et al., Nature 400 (6742) (1999) 342. [2] J.W. Miao, et al., Proceedings of the National Academy of Sciences of the United States of America 100 (1) (2003) 110. [3] I.K. Robinson, et al., Physical Review Letters 8719 (19) (2001). [4] S. Marchesini, et al., Physical Review B 68 (14) (2003). [5] J.R. Fienup, Applied Optics 21 (1982) 2758. [6] G.J. Williams, et al., Physical Review Letters 97 (2) (2006). [7] G. Riley, Bump, dip, flip: single chip, Surface Mount International, 1987.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Detectors for X-ray Coherent Diffractive Imaging Christopher J. Hall a,b,, Robert A. Lewis a,b a b
Monash Centre for Synchrotron Science, Monash University, Wellington Road, Clayton 3800, Australia The Australian Research Council Centre of Excellence in Coherent X-ray Science, Australia
a r t i c l e in f o
a b s t r a c t
Available online 25 January 2009
This paper examines both current and future requirements for detectors in Coherent X-ray Diffractive Imaging. A set of criteria for a suitable detector for a specific form of CXDI, Fresnel Coherent Diffractive Imaging, is derived. Observations about the challenges to soft X-ray detector science from this new imaging technique are discussed. & 2009 Elsevier B.V. All rights reserved.
Keywords: Coherent X-ray imaging Soft X-ray detectors
1. Introduction Scientific gains made by the ability to image objects at scales of tens of nanometres are driving the evolution of several new short wavelength microscopy techniques. In the life sciences, significant progress is being made through these technologies in understanding cell and inter-cellular processes. Similar progress is being enabled by the techniques in nanomaterials research. The advent of X-ray microscopes, which use synchrotron sources, has meant image resolutions below 10 nm are now within reach even with wet samples. Furthermore, the use of the coherence from these sources has given rise to a novel form of lens-less X-ray microscopy: Coherent X-ray Diffractive Imaging (CXDI). This relies on an ability to reconstruct images of an object from measurements of its diffracted intensity. CXDI works if the coherence of the illumination is larger than the object being imaged and some aspects of the incident light wave field are known a-priori. The potential of the technique was highlighted in 1999 when Miao et al. [1] published a paper in Nature showing a reconstructed image of a non-periodic object at X-ray wavelengths with a resolution of around 75 nm. Since then images of biological samples with resolutions down to 30 nm [2] and tomographic reconstructions of hard materials down to 8 nm [3] have been published. CXDI can be thought of as a way of imaging nanoscale structures using a method similar to crystallography [4]. The Fourier transform of the wavefield from the object is captured by measuring the X-ray diffraction pattern. This is mathematically inverted to recover the wavefield as it exits the object. However, a major difference from crystallography is that structures being imaged in CXDI are not repetitive in space, so the diffraction signal is both continuous and very weak. This fact determines a need for
Corresponding author at: Monash Centre for Synchrotron Science, Monash University, Wellington Road, Clayton 3800, Australia. E-mail address: [email protected] (C.J. Hall).
0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.01.026
synchrotrons or other bright sources of incident X-ray illumination. The well-known phase problem is also to be overcome in CXDI. Intensity measurements by the detector of the Fourier space images do not provide phase information, which would allow a simple inverse transform to be made accurately. However, the way CXDI is performed does allow a solution. If the object is spatially isolated so that sampling in Fourier space meets the Nyquist criteria, a detection of intensity provides an ability to constrain the data in both domains. The idea of a ‘support’ around the object, which is known to be featureless, permits an iterative solution to be made [5]. There has been a lot of activity in the CXDI area over the last few years. In Australia the Centre of Excellence (CoE) in Coherent X-ray Science has been working with a particular form of CXDI known as Fresnel CXDI (FCDI) [6]. FCDI uses a non-planar incident wave field on the object, which has some advantages in accuracy and speed of convergence. CXDI has the potential to rival electron microscopy in spatial resolution, but without the inconvenience of requiring a vacuum around the sample. However, CXDI is a technically challenging technique requiring all the source, optics and detectors to be developed if its full potential is to be realised. Our focus at the Monash Centre for Synchrotron Science is in a specific part of the FCDI facility being constructed by the CoE: the X-ray detector. Commercially available direct detection CCDs are used for the majority of CXDI experiments at the moment. These versatile soft X-ray detectors have allowed the field to progress to the point where the technique is feasible as a user facility. However, the limitations CCDs impose on the technique need to be addressed before CXDI can really fulfil its potential. The work of our CoE is aimed at life science issues. For biological samples the X-ray scattering cross-sections for the photon energies at which CXDI is used are relatively small. The diffraction patterns are consequently weak so sensitivity is a prime consideration for a detector. Back-illuminated silicon CCD detectors score well in this respect. They have a quantum efficiency close to 100% and are able to detect single photons of 3 keV. Arguably, a more challenging
ARTICLE IN PRESS C.J. Hall, R.A. Lewis / Nuclear Instruments and Methods in Physics Research A 604 (2009) 130–132
131
Table 1 Criterion for selecting a soft X-ray detector technology for CXDI. Criterion
Essential
Desirable
Explanation
Dynamic range
104
107
Pixelation
2048 2048
44096 44096
Pixel size Energy range (keV)
o100 mm 0.5–3
o50 mm 0.5–3
Detective quantum efficiency (%) Rate capability
10
50
410 /pixel/s
4106/pixel/s
Frame rate (Hz) Vacuum compatibility (bar)
1 10
1 10
Dead time fraction (%)
10
1
Parallax error Gaps in FOV
o1 pixel at 201 Acceptable
None None
The detector’s single frame saturation level divided by the RMS dark image noise for the same exposure time. In an additive noise model the dark image noise is the sum of the variances due to read noise (sread) and dark noise (sdark). The number of evenly spaced pixels in the two dimensions of the image over the field of view (FOV). Assuming square pixels. This is the size on the active area of the detector. The range over which 410% Detective Quantum Efficiency (DQE) is achieved. DQE describes the degradation of signal-to-noise ratio (SNR) as a photon field is detected. It is defined as the ratio of the squares of the SNR in the incident photon field to the SNR in the detected image. This level of DQE to be at least maintained across the energy range given in criterion 4. Maximum rate at which the pixel retains an intensity measurement linearity of 1%. Measured after dead time correction in the case of a counting detector. The maximum rate at which full fields can be read out from the detector. The vacuum which can be maintained with the detector operational in the sample chamber. For a counting detector, the fraction of time in which the detector is insensitive to incoming photons. Measured at the maximum rate. The attenuation length for the lowest energy photons in criterion 4, projected. In any tiled detector array there are bound to be discontinuities at the boundary of the tiles. These may lead to gaps in the sampling of the incident intensity.
4
4
6
This was the result of a ‘trade study’ criterion preparation exercise.
parameter to tackle is capturing the large dynamic range inherent in a CXDI image. In FCDI images the central part of the detected pattern is a magnified far-field image (hologram) of the object. This region is required as input to the reconstruction algorithm and therefore must be recorded accurately. The holographic region is typically 6 orders of magnitude brighter than the diffracted intensity out at higher angles. X-ray CCD imagers are unable to record the whole range of intensity in a single capture. The noise level to full-well capacity puts a limit on this parameter. In order to overcome the limit, hundreds of individual frames are summed in order to capture the full data set. Clearly this is an inefficiency that can be reduced. For each capture the CCD must be clocked out inevitably adding readout noise. With a synchrotron as the X-ray source, the read-out time and exposure time for an individual frame are similar so the inefficiency is around 50%. A detector that would allow longer exposures whilst capturing the full range of intensity would improve the efficiency by many fold. Two driving specifications of dynamic range and sensitivity lead to consideration of a photon counting detector. Typical rates from the brightest part of the FCDI image are o106 photons mm 2 s 1, making some form of the soft X-ray hybrid pixel detector very attractive. One of the many technical challenges to a successful FCDI is the requirement that the registration of the wavefront of the illuminating field incident and the object stays very stable during the exposure. Aiming for a spatial resolution of below 10 nm means the stability must be ideally less than 1 nm over the time of exposure. Our FCDI optical system will achieve close to this, but the technology required is expensive. A detector that can be rapidly read out might prove an alternative means to satisfy this demand. If sufficient signal circuitry can be provided locally to a pixel, it is possible to add a time stamp to data from each pixel. Photon arrival times may be correlated with movement sensors in the object stage. A post-collection correction could then be made to compensate for drift and vibration in the relative position of the beam and object. When surveying detector technologies that might be applicable to CXDI, the following basic parameters were considered: the detector needs to efficiently detect low-energy X-rays, it needs to be position sensitive and it needs to provide a sufficient dynamic
range for the technique. Quantifying these demands involved close consultation with the groups who are developing the optics and reconstruction algorithms. Demands from the image reconstruction process are key to specifying the acceptable non-ideal parameters from the detector. The results of this consultation are summarised in Table 1. As is often the case, different degrees of compromise were accepted by the individuals who helped compile this list, which resulted in a binary categorisation of parameters. Each criterion had a specification, which was considered essential for any successful FCDI detector. Any detector or technology that did not meet all of the ‘essential’ parameters would not be considered for next-generation FCDI instrumentation. Realising that there may be scientific gains in pushing some of these specifications further, a second category of ‘desirable’ specifications was also tabulated. It was these parameters that give a weighted score to help rank the solutions in our final analysis. The order in which these parameters appear in the table gives the priority to the criteria for weighting. Although terms used in the table might be familiar to those in the field, some explanation of the criterion is given to clarify what is meant in each case.
2. Hybrid pixel detectors for FCDI The idea of building an X-ray detector technology that taps into the extensive industrial capabilities for silicon-integrated circuit fabrication has been around for decades. The Hybrid Pixel Detector (HPD) consists of a matrix of circuits above which a suitable detector material is connected, usually through flip-chip or bump bonding [7]. For small research groups, realising anything other than a proof of concept detector has taken a long time. CERN has successfully spawned an organization to exploit its investment in this area. The MediPix collaboration has demonstrated a wide range of demands that such a device has to offer.1 However, they are now commercially available systems in the market. Dectris Pilatus is the first and there are others set to 1
http://medipix.web.cern.ch/MEDIPIX/
ARTICLE IN PRESS 132
C.J. Hall, R.A. Lewis / Nuclear Instruments and Methods in Physics Research A 604 (2009) 130–132
follow this lead.2 With academic access to some of the silicon fabrication processes, it is now within the reach of research groups to design and build detectors tailored specifically for a particular task. We have recently embarked on a design study to investigate using HPD technology for FCDI. Our group is building infrastructure for HPD development. We believe that with appropriate adaptations we could design an HPD specifically for the shorter term requirements of FCDI. 3. Conclusion New X-ray imaging techniques that come under the general term Coherent X-ray Diffractive Imaging will impose significant demands on the detector. Although directly illuminated CCDs are currently being successfully used, the limitations they impose on dynamic range and readout speed will need to be addressed as the techniques mature. We have drawn up a list of specifications that will suit one particular type of CXDI; Fresnel Coherent Diffractive
2
http://pilatus.web.psi.ch/pilatus.htm
imaging. This list will be used in the future to assess technologies and individual instruments as to their suitability.
Acknowledgements The authors would like to acknowledge the support of the Australian Research Council Centre of Excellence in Coherent X-ray Science. References [1] J.W. Miao, et al., Nature 400 (6742) (1999) 342. [2] J.W. Miao, et al., Proceedings of the National Academy of Sciences of the United States of America 100 (1) (2003) 110. [3] I.K. Robinson, et al., Physical Review Letters 8719 (19) (2001). [4] S. Marchesini, et al., Physical Review B 68 (14) (2003). [5] J.R. Fienup, Applied Optics 21 (1982) 2758. [6] G.J. Williams, et al., Physical Review Letters 97 (2) (2006). [7] G. Riley, Bump, dip, flip: single chip, Surface Mount International, 1987.