X-ray Charge-coupled Device Research Articles

HIGH RESOLUTION X-RAY FLUORESCENCE MEASUREMENTS USING A FLAT ANALYZER CRYSTAL AND AN X-RAY CCD

High resolution X-ray fluorescence measurements were carried out using a flat analyzer crystal and an X-ray CCD (charge-coupled device). A comparison of the detector was made between the CCD and the position sensitive proportional counter (PSPC), and the advantage of the CCD in better spatial resolution and two-dimensional spatial resolution was demonstrated. Full width at half maximum (FWHM) of measured Cu Kα1,2 X-ray fluorescence spectra shows clear dependence of beam size on the sample, and the FWHM of 2.4eV can be expected with the use of an X-ray microbeam.

Preliminary experiment of fluorescent X-ray computed tomography to detect dual agents for biological study

The simultaneous observation of various information, such as blood flow, tissue metabolism and distribution of receptors, is quite important in order to understand the functional state of biomedical objects. The simultaneous detectability of contrast agents by fluorescent X-ray computed tomography (FXCT) with synchrotron radiation is examined in this study. The system consisted of a silicon (111) double-crystal monochromator, an X-ray slit system, a scanning table, a PIN diode, a highly purified germanium detector and an X-ray charge-coupled device (CCD) camera. The monochromatic X-ray beam energy was adjusted to 37.0 keV and collimated into a pencil beam of 1 x 1 mm. The fluorescent spectra of the K alpha lines for iodine and xenon were detected simultaneously. FXCT could image the distribution of both iodine and xenon agents in a phantom clearly and the contrast ratio was significantly better than that of transmission X-ray computed tomography images.

A low-mass multistage radiator for the passive cooling of X-ray charge-coupled devices in space

This paper describes the design and construction of a quasi-three-stage radiator as a means of passively cooling an X-ray charge-coupled device (CCD) which was at a distance of about 1 m from a radiating aperture having an unobscured view to cold space. The design was tested through thermal balance measurements which confirmed that a base temperature of —150°C could be achieved at the CCD. The low mass design was essential in ensuring the absence of significant resonant frequencies of vibration below 100 Hz. The system forms part of the EPIC instrument on the European Space Agency (ESA) cornerstone X-Ray Multi-Mirror (XMM) mission which was recently and successfully launched on an Ariane V rocket.

Subpixel Spatial Resolution of the X-Ray Charge-Coupled Device Based on the Charge Cloud Shape

When an X-ray photon lands into a pixel (event pixel), the primary charge is mainly collected into the event pixel. If the X-ray landing position is sufficiently close to the pixel boundary, the primary charge spills over to the adjacent pixel forming split events. We can easily understand that there are three parameters coupled together; the X-ray landing position inside the pixel, the X-ray event pattern and the primary charge cloud shape. We can determine any one of them from the other two parameters. Since we know the charge cloud shape using the multi-pitch mesh experiment, we can calculate the X-ray landing position with subpixel resolution using the event pattern. We applied our method to Ti–K X-rays for the charge-coupled device with 12 µm square pixel. Once the primary charge splits into the adjacent pixel, we can determine the X-ray landing position with subpixel resolution. Using three- or four-pixel split events, we can determine the X-ray landing position with an accuracy of less than 1 µm. For a two-pixel split event, we obtained a similar position accuracy in the split direction with no improvement in the direction perpendicular to it. We will discuss the type of charge-coupled device (CCD) which can achieve the subpixel resolution for the entire area of the CCD.

Measurement of the efficiency of gold transmission gratings in the 100 to 5000 eV photon energy range

Three x-ray spectrometers, each with a transmission grating dispersion element, are routinely used at the Z soft x-ray facility to measure the spectrum and temporal history of the absolute soft x-ray power emitted from z-pinch and hohlraum radiation sources. Our goal is to make these measurements within an accuracy of ±10%. We periodically characterize the efficiency of the gratings used in the spectrometers by using an electron-impact soft x-ray source, a monochromator, grazing-incidence mirrors, thin filters, and an x-ray charge-coupled device (CCD) detector. We measure the transmission efficiency of the gratings at many photon energies for several grating orders. For each grating, we calculate efficiency as a function of photon energy using published optical constants of gold and multiple-slit Fraunhofer diffraction theory and fit the calculation to the measurements using the physical parameters of the grating as variables. This article describes the measurement apparatus and calibration techniques, discusses the grating efficiency calculation and fitting procedure, and presents recent results.

Imaging due to Magnetic Anisotropy with Hard X-rays.

X-ray polarization-contrast images resulting from x-ray magnetic circular dichroism (XMCD) in hard x-ray region have been successfully recorded for the first time. The apparatus used consisted of an x-ray polarizer, double x-ray phase retarders, and a high-spatial-resolution x-ray charge-coupled device (CCD) detector. The sample used was a hexagonal close packed (h.c.p.) cobalt poly-crystal foil of thickness about 4 μm. The x-ray polarization-contrast image resulting from XMCD was observed at a photon energy 10 eV above the cobalt K-absorption edge (7709 eV). The observed contrast in the images was reversed by inversion of a magnetic field. Furthermore, the contrast was reversed again at a photon energy 32 eV above the cobalt K-absorption edge.

Hard x-ray spectrometers for NIF (abstract)

A National Ignition Facility (NIF) core diagnostic instrument has been designed and will be fabricated to record x-ray spectra in the 1.2–20 keV energy range. The high-energy electronic x-ray instrument has four reflection crystals with overlapping coverage of 1.2–10.9 keV and one transmission crystal covering 8.6–20 keV. The spectral resolving power varies from approximately 1000 at low energies to 315 at 20 keV. The spectrum produced by each crystal is recorded by a modified commercial dental x-ray charge coupled device (CCD) detector. The scintillators on the CCD detectors are optimized for the energy ranges. A one-channel x-ray spectrometer, using one transmission crystal covering 12–60 keV, will be fabricated for the OMEGA laser facility. The transmission crystal spectrometers are based on instruments originally designed at National Institute for Standards and Technology for the purpose of characterizing the x-ray flux from medical radiography sources. Utilizing one of those instruments and a commercial dental x-ray CCD detector, x-ray images were recorded using a single pulse from a laboratory x-ray source with a peak charging voltage of 200 kV. A resolving power of 300 was demonstrated by recording on film the Kα1 and Kα2 characteristic x-ray lines near 17 keV from a molybdenum anode. The continuum radiation from a tungsten anode was recorded in the 20–50 keV energy range. The transmission crystal spectrometer has sufficient spectral resolution and sensitivity to record the line and continuum radiation from high-Z targets irradiated by the NIF laser and the OMEGA laser.

Demonstration of X-ray linear dichroism imaging with hard X-rays

X-ray polarization-contrast images resulting from X-ray linear dichroism (XLD) in the hard X-ray region have been successfully recorded for the first time. The apparatus used consisted of an X-ray polarizer, double X-ray phase retarders and a high-spatial-resolution X-ray charge-coupled device (CCD) detector. The sample used was a hexagonal close packed (h.c.p.) cobalt single-crystal foil of thickness about 12 microm. The experiment was performed at X-ray energies of 23 and 29 eV above the cobalt K edge (7709 eV), at which the maximum linear dichroisms (approximately 3%) were observed, with their signs reversed, in the XLD spectrum measured with quadruple X-ray phase retarders. The contrasts in the images at the two X-ray energies were reversed as a result of the XLD in the sample. Furthermore, the values of the contrast in the images arising from the linear dichroism (approximately 3%) were in good agreement with those yielded by the XLD spectrum.

Lens distortion in optically coupled digital x-ray imaging.

The objectives of this research are to analyze geometrical distortions introduced by relay lenses in optically coupled digital x-ray imaging systems and to introduce an algorithm to correct such distortions. The radial and tangential errors introduced by a relay lens in digital x-ray imaging were experimentally measured, using a lens-coupled CCD (charge coupled device) prototype. An algorithm was introduced to correct these distortions. Based on an x-ray image of a standard calibration grid, the algorithm first identified the location of the optical axis, then corrected the radial and tangential distortions using polynomial transformation technique. Lens distortions were classified and both radial and tangential distortions introduced by lenses were corrected using polynomial transformation. For the specific lens-CCD prototype investigated, the mean positional error caused by the relay lens was reduced by the correction algorithm from about eight pixels (0.69 mm) to less than 1.8 pixels (0.15 mm). Our investigation also shows that the fourth order of polynomial for the correction algorithm provided the best correction result. Lens distortions should be considered in position-dependent, quantitative x-ray imaging and such distortions can be minimized in CCD x-ray imaging by appropriate algorithm, as demonstrated in this paper.

X-ray CCD response functions, front to back

For some applications, particularly in X-ray astronomy, one requires accurate knowledge of the charge-coupled device (CCD) spectral redistribution function, with errors of order 1%. We describe some recent advances in response function measurement and modeling. In particular, we show that it is now possible to isolate and separately measure the response contributions of many of the component volumes that comprise the CCD. We describe how we have deployed this capability to determine X-ray photon response functions for front-illuminated detectors, and indicate how these measurements can improve understanding of the response function of the back-illuminated CCD.

The effect of ice build-up on the soft X-ray response of a charge coupled device (CCD)

The potential for spacecraft contaminants to outgas in orbit and condense onto the cold surfaces of X-ray CCDs has been appreciated for some time. We investigate the effect of ice build-up on the low-energy response of a soft X-ray sensitive CCD. We derive curves relating the build up of ice with the partial pressure of water vapour and CCD temperature. Ice build-up predictions and ice removal strategies, based on these curves, are determined for the CCDs to be flown on the JET-X telescope. A discussion of anti-icing measures is given.

X-ray absorption near edge structure in the quantum efficiency of x-ray charge-coupled devices

We perform precise measurements of the x-ray transmission of the thin films comprising CCD gate structure, namely, phosphorus doped polysilicon, silicon dioxide, and silicon nitride. The x-ray transmission of these films shows large oscillations with small changes in energy in the vicinity of the following absorption edges: nitrogen K (400 eV), oxygen K (536 eV), silicon L and K (100 and 1840 eV, respectively). As a result, quantum efficiency of a CCD in the soft x-ray range deviates significantly from simple model predictions based on Henke et al. (1993) mass absorption coefficients. The measurements covered the range of energies from 60 to 3000 eV, using synchrotron beamlines at the Advanced Light Source (ALS; Berkeley), Physikalisch-Technische Bundesanstalt BESSY (Berlin), the Synchrotron Radiation Center (SRC; University of Wisconsin-Madison). Our model of the CCD response includes near edge x-ray absorption structure and predicts a very complicated shape of the energy dependence of the quantum efficiency around silicon and oxygen absorption edges. Experimental measurements of CCD quantum efficiency relative to a calibrated detector at the BESSY synchrotron confirmed our model predictions for both frontside and backside illuminated CCDs.

In situ calibration of a flat-field grazing incidence spectrograph with an x-ray charge-coupled device camera using a transmission grating and a laser-produced plasma for x-ray laser research

An in situ calibration experiment of a soft x-ray spectrograph for x-ray laser research has been performed. The calibrated spectrograph was a flat-field grazing incidence spectrometer with an x-ray charge-coupled device camera. The absolute sensitivity of the spectrograph was obtained for the wavelength range from 2.6 to 4.0 nm using a transmission grating spectrograph as a reference spectrograph set along the axial direction of a line-focused laser-produced plasma. The absolute sensitivity determined in this work shows nearly the same value as that deduced from the calibration experiment using synchrotron radiation.

Absolute calibration of a flat field spectrometer in the wavelength range 10–70 Å

We characterize an x-ray spectrometer in the wavelength range 10–70 Å. The instrument has been designed for the study of the soft x-ray emission from plasmas generated with intense sub-ps laser pulses. It uses a flat field reflection grating with 2400 lines per mm as dispersing element and an x-ray charged coupled device (CCD) as detector. The sensitivity of the spectrometer and CCD detector has been absolutely calibrated by means of a ns-laser generated gold plasma of well known x-ray conversion efficiency. The calibration procedure is explained in detail. Finally, we present examples of spectra emitted by a sub-ps laser plasma. From the spectra follows a resolving power of 150–350 and a low conversion efficiency into the second diffraction order of the flat field grating of only a few %. The measured frequency averaged total conversion of laser energy into x-ray energy emitted into 2π is in the range of 2.4×10−4 up to 6.2×10−3 depending on the material of the solid targets.

Measurement of the subpixel structure of AXAF CCD's

The authors present a method to measure the subpixel structure of a charge-coupled device (CCD), information necessary to accurately determine (<1% uncertainty) the absolute detection efficiency of the device. Their approach uses a thin metal film with periodically spaced holes (small, compared to the pixel size) to localize incident X-rays to a particular region of the pixel. The mesh is rotated to create a small angular misalignment between the grid holes and the CCD pixels, producing a moire effect in the data. The resultant moire pattern is compared to a CCD model, and a best fit minimization technique is used to constrain the parameters that describe the subpixel structure. This technique was developed to measure and calibrate the X-ray CCD's that will comprise one of the two focal plane instruments on-board AXAF, but it is applicable for measuring the structure of any pixelated solid state device.

Near-field spatial imaging of a Ni-like Ag 140-Å x-ray laser

We measure the two-dimensional, near-field spatial distribution of a 140-\AA{} nickel-like silver x-ray laser at the output aperture with high magnification using a curved multilayer x-ray mirror to image the output onto an x-ray charge-coupled device camera. Lasing is created by illuminating silver slab targets with a pair of 75 ps laser pulses separated by 2.2 nsec from the Vulcan laser. The two-dimensional, high-resolution, spatial image shows the x-ray laser source size and its position relative to the target surface. A dramatic change in both the position and source size are observed for the refraction compensating curved target as compared with the flat targets.

High resolution diagnostics on COMPASS-D and START

In this paper we describe three novel high resolution diagnostic systems on the Culham tokamaks. The first is a pair of high dispersion, short sampling time visible light spectrometers, employing a fast charge-coupled device (CCD) and a multianode photomultiplier as detectors, allowing unusually high bandwidth ion drift velocity measurements. Secondly, the application of an X-ray CCD to a Johann spectrometer for ion and electron core parameter characterisation is outlined. Lastly, a very high throughput, 30-channel Thomson scattering spectrometer system is described, which exploits the low f number lens design techniques, coated fibres and third-generation image intensifiers now available.

The X-Ray CCDs Developed for the Joint European X-Ray Telescope

The charge coupled devices (CCDs) developed for the Joint European X-ray Telescope (JET-X) are described in detail. A history of the development program and device performance is given. We present results from a comprehensive study to characterize the x-ray response of the flight model focal plane detectors. The goal of the program is to calibrate the efficiency, energy resolution, gain, etc. down to a precision of ∼1%. Final calibration data sets will be based on combinations of measurements and calculations. For example, the CCD quantum efficiency will be composed of discrete line measurements made at the University of Leicester test facility and calculation and synchrotron measurements from the Daresbury Synchrotron Radiation Source (SRS). The absolute normalizations will be provided by x-ray long beam pipe measurements at the Max Planck Institut für Extraterrestrische Physik (MPE) Panter test facility in Munich. Using the available data, it is shown that it is possible to calibrate the quantum efficiency, the FWHM energy resolution, and the system gain of the flight devices to better than 1%.

Mapping X-ray absorption fine structure in the quantum efficiency of an X-ray charge-coupled device

Recent measurements have shown that high-resolution silicon detectors are beginning to resolve x-ray absorption fine structure (XAFS) produced in the detectors themselves. Unless calibrated, such structure threatens to ‘pollute’ spectroscopic measurements. An experiment has begun map this structure to systematically in an x-ray charge-coupled device (CCD) over the energy range 300–2100 eV. The program is novel in that the synchrotron radiation source has to be operated with a ring current reduced by five orders of magnitude so that individual photons can be recorded by the CCD. Results of the detailed spectroscopic response around the nitrogen, oxygen and silicon K edges are presented. The measured quantum efficiency shows considerable near-edge structure which is modified by the presence of the various insulation layers used in MOS construction, reflecting the chemical rather than elemental nature of the interaction environment. It is pointed out that the measurement of XAFS in x-ray CCDs is potentially a powerful diagnostic tool with which to explore surface structures.

The X-Ray CCDs Developed for the Joint European X-Ray Telescope

The charge coupled devices (CCDs) developed for the Joint European X-ray Telescope (JET-X) are described in detail. A history of the development program and device performance is given. We present results from a comprehensive study to characterize the x-ray response of the flight model focal plane detectors. The goal of the program is to calibrate the efficiency, energy resolution, gain, etc. down to a precision of ~1%. Final calibration data sets will be based on combinations of measurements and calculations. For example, the CCD quantum efficiency will be composed of discrete line measurements made at the University of Leicester test facility and calculation and synchrotron measurements from the Daresbury Synchrotron Radiation Source (SRS). The absolute normalizations will be provided by x-ray long beam pipe measurements at the Max Planck Institut für Extraterrestrische Physik (MPE) Panter test facility in Munich. Using the available data, it is shown that it is possible to calibrate the quantum efficiency, the FWHM energy resolution, and the system gain of the flight devices to better than 1%.