Full-field X-ray Fluorescence Imaging Research Articles

The study of X-ray full-field imaging method for low atomic number elements based on Compton scattering

Full-field X-ray fluorescence imaging cannot image low atomic number (Z < 12) materials because the characteristic X-ray fluorescence produced by low atomic number (Z < 12) materials is severely attenuated in air. In this paper, we propose a method to combine Compton scattering with full-field X-ray fluorescence imaging, which can realize the imaging of both metals (high atomic number) and organic materials (low atomic number), and compensate the defect that the conventional FF-XRFI technique cannot image materials with low atomic number such as organic materials. The imaging experiments were performed using two materials, zirconium metal and polyethylene, and the imaging results were optimized using a density filtering algorithm, and the results indicate that it is possible to use this method to image samples of high and low atomic number with resolution.

A new full-field XRF imaging station at Synchrotron Light Research Institute.

A full-field X-ray fluorescence imaging (FXI) station was recently developed at beamline BL8 of Synchrotron Light Research Institute (SLRI), Thailand. An unfocused, synchrotron X-ray beam from the bending magnet with a size of 2 mm (vertical) × 13 mm (horizontal) and photon energy of 10 keV was employed in the FXI experiments. A sample stage was tilted by 7.5° to enlarge the vertical beam size. X-ray fluorescence images were recorded by an energy-dispersive, 256 × 256 array, pn-type charge coupled device detector equipped with a polycapillary optics, providing a full-frame image size of 12.3 mm × 12.3 mm. The incident photon flux per pixel was 3 × 104 photons s-1 (100 mA)-1 and the experimental spatial resolution was 68 µm. Image processing was carried out offline using an in-house MATLAB program capable of elemental selection and inhomogeneity intensity correction. Elemental detection limits of FXI were found to decrease with increasing atomic number, i.e. 0.3 to 0.03 wt% for Z = 19 (K) to 30 (Zn). Compared with the BL6b microbeam imaging (µXI) station at SLRI with higher photon flux per pixel, 3 × 1010 photons s-1 (100 mA)-1, a tenfold sample area can be obtained and 13 times higher peak-to-background (PKB) ratio at Zn Kα measured with the same experimental time (8 h). Simultaneous measurement of FXI is more time-efficient against the long overhead times of µXI scanning over large pixel numbers, >65000. To demonstrate potential applications of the new FXI station, various types of samples were examined: dendritic limestone, ancient bronze and dried fish. Analyzed elemental images enabled us to identify areas rich in Mn on the limestone, Sn and Cu separation in the bronze, and Zn nutrition in the dried fish eye.

Full-field x-ray fluorescence imaging using a Fresnel zone plate coded aperture

We present an approach to full-field x-ray fluorescence imaging that uses a Fresnel zone plate as a coded aperture positioned between the object and detector. For this particular coded aperture, the incoherent image formation is identical to the coherent propagation image (inline hologram) of an equivalent absorption object. In the first experiment, we show that prior knowledge of the coded aperture structure and position is no longer required. Fast and robust reconstruction algorithms well known from coherent imaging can be applied. We demonstrate the approach using x-ray fluorescence excited by a collimated Cu- K α beam at an ordinary laboratory x-ray source. Images are recorded with a field of view of 1 m m 2 and resolution of 35 µm. The simple and efficient imaging scheme is well suited for further dissemination of x-ray fluorescence imaging and translation to a broader user community, which could take advantage of full-field imaging with chemical contrast. More generally, the scheme also enables full-field imaging of other incoherent radiation processes for which lenses are either not available at all or not with sufficient numerical apertures. Examples include Compton imaging, imaging of inelastic neutron scattering, or γ -ray imaging in nuclear medicine.

Characterization of micro pore optics for full-field X-ray fluorescence imaging

Elemental mapping images can be achieved through step scanning imaging using pinhole optics or micro pore optics (MPO), or alternatively by full-field X-ray fluorescence imaging (FF-XRF). X-ray optics for FF-XRF can be manufactured with different micro-channel geometries such as square, hexagonal or circular channels. Each optic geometry creates different imaging artefacts. Square-channel MPOs generate a high intensity central spot due to two reflections via orthogonal channel walls inside a single channel, which is the desirable part for image formation, and two perpendicular lines forming a cross due to reflections in one plane only. Thus, we have studied the performance of a square-channel MPO in an FF-XRF imaging system. The setup consists of a commercially available MPO provided by Photonis and a Timepix3 readout chip with a silicon detector. Imaging of fluorescence from small metal particles has been used to obtain the point spread function (PSF) characteristics. The transmission through MPO channels and variation of the critical reflection angle are characterized by measurements of fluorescence from copper and titanium metal fragments. Since the critical angle of reflection is energy dependent, the cross-arm artefacts will affect the resolution differently for different fluorescence energies. It is possible to identify metal fragments due to the form of the PSF function. The PSF function can be further characterized using a Fourier transform to suppress diffuse background signals in the image.

Application of Factorisation Methods to Analysis of Elemental Distribution Maps Acquired with a Full-Field XRF Imaging Spectrometer

The goal of the work was to investigate the possible application of factor analysis methods for processing X-ray Fluorescence (XRF) data acquired with a full-field XRF spectrometer employing a position-sensitive and energy-dispersive Gas Electron Multiplier (GEM) detector, which provides only limited energy resolution at a level of 18% Full Width at Half Maximum (FWHM) at 5.9 keV. In this article, we present the design and performance of the full-field imaging spectrometer and the results of case studies performed using the developed instrument. The XRF imaging data collected for two historical paintings are presented along with the procedures applied to data calibration and analysis. The maps of elemental distributions were built using three different analysis methods: Region of Interest (ROI), Non-Negative Matrix Factorisation (NMF), and Principal Component Analysis (PCA). The results obtained for these paintings show that the factor analysis methods NMF and PCA provide significant enhancement of selectivity of the elemental analysis in case of limited energy resolution of the spectrometer.

Characterization of a large-area full-field X-ray fluorescence imaging system based on a commercial full-frame CMOS camera

A new large-area and low-cost full-field X-ray fluorescence imaging system was developed for simultaneous mapping of several chemical elements. It consists of a commercially available full-frame visible-light CMOS digital camera coupled to a tungsten pinhole-collimator. The X-ray fluorescence is produced by irradiating the sample with an external X-ray tube. The system magnification can be continuously adjusted from 0.55 to 1.43 by changing the position of the pinhole in the collimator. To allow X-ray sensitivity, the glass window of the CMOS sensor was removed and the front glass cover of the camera was replaced by an X-ray window. The x-ray spectra and 2D X-ray fluorescence images of different elements are obtained by processing a series of images acquired in a spectroscopy mode using the CMOS camera. The camera is able to resolve fluorescence lines up to 22 keV, with an energy resolution of 213 eV at 6.4 keV. The experimental spatial resolution is 105 μm when using a 90 μm pinhole at 1.43× magnification. The capabilities of the system in performing the 2D resolved X-ray fluorescence imaging were investigated by testing samples of different materials and geometries. The obtained X-ray fluorescence spectra and images show that the system is able to accurately identify the presence of several different elements and visualize their spatial distributions.

Full-field X-ray fluorescence imaging with a straight polycapillary X-ray collimator

Due to the availability of X-ray imaging detectors, full-field X-ray fluorescence (FF-XRF) imaging technique has become achievable, which provides an alternative to scanning X-ray fluorescence imaging with a micro-focus X-ray beamline. In this paper, we present a setup based on straight capillary optics and an energy-dispersive hybrid pixel detector, which can perform simultaneous mapping of several chemical elements. The photon transmission efficiency and spatial resolution are compared between two X-ray collimation setups: one using pinhole optics and one using straight polycapillary optics. There is a tradeoff between the spatial resolution and transmission efficiency when considering X-ray optics. When optimizing the spatial resolution, using straight capillary optics achieved a higher intensity gain when comparing with the pinhole setup. Characterization of the polycapillary imaging setup is performed through analyzing various samples in order to investigate the spatial frequency response and the energy sensitivity. This developed setup is capable of FF-XRF imaging in characteristic energies below 20 keV, while for higher energies the spatial resolution is affected by photon transmission through the collimator. This work shows the potential of the FF-XRF instrument in the monitoring of toxic metal distributions in environmental mapping measurements.

Modelling of vignetting effects in full-field X-ray fluorescence imaging system based on pinhole optics

The full field X-ray fluorescence (FF-XRF) imaging is a technique capable of analysing the spatial distribution of elements in a sample. An alternative technique is the scanning macro-XRF method. Both techniques are powerful tools for non-destructive analysis of artworks mainly historical paintings. Technically, FF-XRF imaging is usually realised by coupling of a pinhole camera and a position sensitive and energy dispersive X-ray detector. The main advantages of the pinhole camera are its simplicity and infinite depth of field, however it introduces vignetting. For a visible light pinhole camera one distinguishes natural vignetting following the cos4 law and mechanical vignetting due to finite thickness of the camera. For an X-ray pinhole camera one has to take into account an additional component of vignetting due to absorption of X-rays in the air or other gas unless the setup is kept in vacuum. The goal of this study is to work out an analytical model of vignetting effects for a FF-XRF apparatus using an X-ray pinhole camera. Complete quantitative characterisation of the vignetting effects is a key step towards proper correction of the vignetting artefacts in recorded images. The elaborated analytical model has been applied to the images acquired with the FF-XRF apparatus being developed and an example of vignetting correction is presented.

On the way to full-field X-ray fluorescence spectroscopy imaging with coded apertures

The new reconstruction algorithms for near-field coded aperture imaging show a good performance even in cases where the established method by convolution fails, thus providing a basis for a new approach to full-field X-ray fluorescence imaging.

Expanding a polarized synchrotron beam for full-field x-ray fluorescence imaging.

Full-field x-ray fluorescence (XRF) imaging is an efficient technique for investigating element composition of a sample and the corresponding spatial distribution. Eliminating scattering x-rays is important for visualizing diluted/trace elements clearly. However, using the linear polarization of synchrotron radiation to remove scattering in full-field XRF imaging has not been feasible for many years because a synchrotron beam is inherently narrow in the direction perpendicular to the polarization and a large imaging area and a low scattering background cannot be simultaneously achieved. In this study, the trade-off was solved by expanding a synchrotron beam in the direction perpendicular to the polarization using an asymmetric-cut Si crystal. Large areas of samples were illuminated. In addition, a collimator plate, which only transmitted scattering x-rays that spread in the polarization direction, was used for imaging. Therefore, the detected scattering intensity was low. The present full-field XRF imaging scheme with a size-expanded polarized synchrotron beam is well suited for visualizing diluted/trace elements. It could be extended to x-ray absorption edge fine structure imaging for analyzing the chemical state of diluted/trace elements in inhomogeneous samples.

X-ray standing wave technique with spatial resolution: In-plane characterization of surfaces and interfaces by full-field x-ray fluorescence imaging

The present paper describes a powerful extension of the x-ray standing-wave technique that combines it with the capability of full-field x-ray fluorescence imaging, so that the impurity depth profiles at all different parts of a nanolayer material can be measured in parallel. The imaging capability improves the robustness and reliability of the x-ray standing-wave technique in investigating inhomogeneous two-dimensional materials and soft interfaces. It can also visualize the three-dimensional element-specific structure at a buried interface with a depth resolution at the nanometer level.

Scanning and Full Field X-Ray Fluorescence Imaging with Laboratory X-ray Source

Scanning and Full Field X-Ray Fluorescence Imaging with Laboratory X-ray Source

CCD camera as feasible large-area-size x-ray detector for x-ray fluorescence spectroscopy and imaging

As X-ray fluorescence radiation isotropically spreads from the sample, one of the most important requirements for spectrometers for many years has been a large solid angle. Charge-coupled device (CCD) cameras are quite promising options because they have a fairly large area size, usually larger than 150 mm2. The present work has examined the feasibility of a commercially available camera with an ordinary CCD chip (1024 × 1024 pixels, the size of one pixel is 13 μm × 13 μm, designed for visible light) as an X-ray fluorescence detector. As X-ray photons create charges in the CCD chip, reading very quickly the amount is the key for this method. It is very simple if the charges always go into one pixel. As the charges quite often spread to several pixels, and sometimes can be lost, it is important to recover the information by filtering out the unsuccessful events. For this, a simple, versatile, and reliable scheme has been proposed. It has been demonstrated that the energy resolution of the present camera is 150 eV at Mn Kα, and also that its overall achievement in seeing minor elements is almost compatible with conventional X-ray fluorescence detectors. When the CCD camera is combined with a micro-pinhole collimator, full field X-ray fluorescence imaging with a spatial resolution of 20 μm becomes possible. Further feasibility in practical X-ray fluorescence analysis is discussed.

Elemental mapping in a contemporary miniature by full-field X-ray fluorescence imaging with gaseous detector vs. scanning X-ray fluorescence imaging with polycapillary optics

Energy dispersive X-ray imaging can be used in several research fields and industrial applications. Elemental mapping through energy dispersive X-ray imaging technique has become a promising method to obtain positional distribution of specific elements in a non-destructive way. To obtain the elemental distribution of a sample it is necessary to use instruments capable of providing a precise positioning together with a good energy resolution. Polycapillary beams together with silicon drift chamber detectors are used in several commercial systems and are considered state-of-the-art spectrometers, however they are usually very costly. A new concept of large energy dispersive X-ray imaging systems based on gaseous radiation detectors emerged in the last years enabling a promising 2D elemental detection at a very reduced price. The main goal of this work is to analyze a contemporary Indian miniature with both X-ray fluorescence imaging systems, the one based on a gaseous detector 2D-THCOBRA and the state-of-the-art spectrometer M4 Tornado, from Bruker. The performance of both systems is compared and evaluated in the context of the sample's analysis.

Time-Lapse Observation of Electrolysis of Copper Sulfate with a Full-Field X-ray Fluorescence Imaging Microscope

The time-lapse observation of the electrodeposition of copper in copper sulfate solution was performed by imaging X-ray fluorescence from the copper deposition. The X-ray fluorescence was directly imaged with a full-field Wolter mirror microscope, which was constructed at the Photon Factory. Controlling the electric current in the solution from 0 to 71.7 µA, the deposition of copper on a Pt cathode was directly observed by imaging its X-ray fluorescence. One exposure time for obtaining an X-ray fluorescence image was 80 s. Then, it was 17 min later from the beginning of the electrolysis when the X-ray fluorescence image of the electrodeposition is observed for the first time. At this exposure time, the detection limit of the mass of copper was estimated to be 0.60 pg/image, which was calculated using test samples of 1.00×10-3–1.00 mol/l copper sulfate solutions.

Application of a charge-coupled device photon-counting technique to three-dimensional element analysis of a plant seed (alfalfa) using a full-field x-ray fluorescence imaging microscope

A full-field x-ray fluorescence imaging microscope using a Wolter mirror was constructed at Photon Factory BL3C2. White x rays from a bending magnet were used to excite x-ray fluorescence and to enhance the x-ray fluorescence intensity. A photon-counting method using a charge-coupled device was applied to obtain an x-ray fluorescence spectrum at the image plane. The spatial distributions of some specific atoms such as Fe and Zn were obtained from photon-counting calculations. An energy resolution of 220 eV at the Fe Kalpha line was obtained from the x-ray fluorescence spectrum by the photon-counting method. The newly developed three-dimensional element mappings of the specific atoms were accomplished by the photon-counting method and a reconstruction technique using computed tomography.

Full-field x-ray fluorescence imaging microscope with a Wolter mirror

Full-field x-ray fluorescence (XRF) and transmission x-ray microscopic images were obtained with a Wolter-type mirror (10× magnification). A synchrotron radiation white beam (4–20 keV) from a bending magnet beamline at the Photon Factory was used to obtain XRF images and a conventional laboratory x-ray source (8.04 keV) was used to obtain transmission x-ray images. The effects of the coherent and the incoherent scattering x rays on the contrast of an XRF image were estimated. The scattering angle between the incidence x ray and the optical axis of the XRF microscope should be 90° to obtain the highest contrast image when the incidence x ray is horizontally polarized. Observation of small metallic inclusions in the synthesized diamond showed that the contrast of the XRF image was better than that of the transmission x-ray image.