The pnCCD Color X-Ray Camera (CXC)
Simultaneous X-ray Fluorescence and Diffraction
|Physical pixel size||48 µm × 48 µm × 450 µm|
|Number of physical pixels||264 × 264 (69,696) full frame area, plus frame store area|
|Active area||12.7 mm × 12.7 mm (161 mm2) full frame area|
|Frame rate||up to 1,000 Hz (264 × 264 pixel)|
|Pixel readout rate||up to 70 MegaPixels/s|
|Windowing mode||24 × 264 pixels (smallest window)|
|Readout noise (rms)||ENC typ. 3e- / pixel at 100Hz, 200Hz |
ENC typ. 4e- / pixel at 400Hz
ENC typ. 8e- / pixel at 1000Hz
|Energy resolution||145 eV for Mn-Kα|
|Sub-pixel spatial resolution||Δx < 15 µm (rms) for 1 keV X-rays|
|Charge handling capacity||up to 400,000 signal electrons per pixel|
|Radiation hardness||up to 1014 photons/cm2 at 10 keV|
The CXC system is equipped with a 264 × 264 pixel pnCDD with a physical pixel size of 48 µm × 48 µm and an imaging area of 161 mm2. The on-chip electronics in combination with a fully column-parallel readout enables low noise operation at about 3 e-/pixel (ENC), and frame rates of up to 1000 Hz. Due to the advanced detector design, the internal potential distribution of the pnCCD can be adjusted by the operation voltages depending on the requirements of the individual experiments. Thus, excellent energy resolution (145 eV for Mn-Kα) can be achieved, as well as high charge handling capacity (up to 400000 e- per pixel), and anti-blooming modes for handling extremely high amounts of signal charges.
Excellent elemental information between 200 eV and 30 keV with an energy resolution of 145 eV (FWHM) at 5.9 keV (Mn-Kα) and 83 eV at 277 eV (C-K).
Ultra-fast readout and excellent signal-to-noise ratio
Standard full frame rate of 100, 200, 400, 1,000 fps, up to 8,000 fps using windowing and binning. Excellent signal-to-noise ratio even at high readout speeds with typical 3e-at 100 fps up to 8e- at 1,000 fps.
High dynamic range
The CXC is capable to provide a HCHC mode (High Charge Handling Capacity, up to 400,000 e- per pixel), as well as an anti-blooming mode for handling extremely high amounts of signal charges. The impact of the HCHC mode is depicted below:
Inimitable single photon sensitivity (left image) and high intensity imaging (right image) with 400,000e- per pixel (corresponding to 10³ photons with an energy of 1 keV) at the same time.
Depending on the experimental requirements, some of our customers are interested in a spatial resolution beyond the CXC pixel size of 48 µm. This subpixel resolution can be realized with two approaches, which can also be combined.
The first approach, the so-called "optical subpixel resolution", reaches this goal by equipping the CXC with magnifying policapillary optics or by using pinhole optics. A spatial resolution of approx. 8 µm was demonstrated with a CXC equipped with a magnifying 6:1 polycapilary optics [B. De Samber et al. (2019)]. Compared to a polycapillary optics, a pinhole optics provides the capability for "zooming", i.E. to vary the magnification factor according to the experimental needs, however at the expense of lower detector count rates. For a detailed evaluation of both methods of optical subpixel resolution with the CXC please refer to B. De Samber et al. (2019).
The second approach is the so-called "computational subpixel resolution". This method is based on dedicated data post-processing. A spatial resolution better than 3 µm at 1.3keV was demonstrated with this method by S. Ihle et al. (2017), please refer to this publication for a detailed discussion. As the native CXC frms6 raw data format provides the full raw data information, it enables the user to apply such a method e.g. as an optional data post-processing "on demand" on any native CXC raw data set which meets the requirements regarding the count rate and photon statistics.
The pnCCDs have an outstanding heritage in diverse fields of science. For example, two projects based on the pnCCDs which provided excellent data and a multitude of highly rated publications in journals with high impact factor like nature, are the EPIC camera and the CAMP instrument.
The pnCCDs are the core element of the EPIC camera on board of the European X-ray observatory XMM-Newton [Soltau (1996), Strüder (2001)], which was launched in 1999 and is still operative. The excellent quality of the EPIC data led to several thousand publications.
A ground based application of the pnCCD is the CAMP instrument which was installed at the LCLS (Linac Coherent Light Source) at SLAC for several years (see e.g. [Strüder (2010), Chapman (2011), Seibert (2011), Loh (2012), Rudek (2012), Johansson (2012)]) and was afterwards installed at FLASH at DESY.
The pnCCD Principle
The pnCCDs are back illuminated, three-phase CCDs on a fully depleted silicon substrate. Their operation is based on the principle of sideward depletion and on transfer registers formed by pn-junctions. The outstanding characteristics include a homogeneous and thin photon entrance window leading to high quantum efficiency values between 100 eV and 20 keV and an excellent radiation hardness as well as high charge handling capacity.
With an X-ray source at a high incident angle, the CXC system enables imaging of samples with large topographic features with minimal shadowing. Thus, in places like the eyes on the statue shown below, the traces left behind by ancient paint can be identified. The iron based pigments show up as traces in this area of the statue, along with a lead based pigment in the middle. A flexible imaging system based on the CXC is capable of measuring many different types of samples, from flat, polished specimens, to rough samples with little to no sample preparation [I. Reiche (2013)].
Therefore the FF-µXRF technique satisfies the requirement of artists and archeologists for imaging and analysis of cultural heritage objects. It allows analysts to quickly identify critical areas and hidden structures, with the advantage that the imaging technology is extremely similar to other cameras commonly used for optical, UV-VIS and NIR imaging. The final result is always an image that contains spatial and spectroscopic information.
As a matter of course, due to the capability of the CXC to record energy dispersive images with the full spectroscopic information stored for every pixel of the images, the generation of various element-specific 3D sinograms from one µCT data set is possible. Combining the CXC with a magnifying polycapillary enhances the spatial resolution beyond the physical pixel size of the pnCCD [De Samber (2019)].