Ning Gao

Overview of polycapillary X-ray optics

Polycapillary optics are utilized in a wide variety of applications and are integral components in many state of the art instruments. Polycapillary optics operate by collecting X-rays and efficiently propagating them by total external reflection to form focused and parallel beams. We discuss the general parameters for designing these optics and provide specific examples on balancing the interrelations of beam flux, source size, focal spot-size, and beam divergence. The development of compact X-ray sources with characteristics tailored to match the requirements of polycapillary optics allows substantial reduction in size, weight, and power of complete X-ray systems. These compact systems have enabled the development of portable, remote, and in-line sensors for applications in industry, science and medicine. We present examples of the utility and potential of these optics for enhancing a wide variety of X-ray analyses.

Development of monolithic capillary optics for x-ray diffraction applications

One of the most attractive and potentially useful applications of capillary optics for x rays is conversion of divergent x rays from intense point sources into quasi parallel beams for diffraction applications. This is particularly important for investigation of small samples such as protein crystals. This report reviews the current status of development of monolithic tapered Kumakhov optics for protein structure analysis. Even at this early stage of development, gains of more than 30 compared to optimized systems without the optics have been achieved.

Concentration of synchrotron beams by means of monolithic polycapillary X-ray optics

Abstract Capillary optics are valuable tools for concentrating synchrotron radiation [D.J. Tiel, A.E. Stern, D.H. Bilderback and A. Lewis, Physica B 158 (1989) 314]. Single tapered capillaries are used at several facilities. However, most of these optics collect only over a small area. This can be overcome by using larger capillary structures. Polycapillary optics can deflect X-rays by larger angles than other X-ray optics that use only one or two reflections. Conventional X-ray optics that achieve similar deflections are much more energy selective than capillaries. Therefore, capillaries can achieve very short focal distances for a wide range of energies. The measurements shown here represent first tests performed with polycapillaries with large collection area. The performance with respect to transmission efficiency and spot size was evaluated for a set of four very different prototypes. It is shown that for these prototypes a significant gain may be achieved if a spot size of the order of 0.1 mm is required. Further, some characteristics of the different optics are discussed.

Confocal X-ray Fluorescence (XRF) Microscopy: A New Technique for the Nondestructive Compositional Depth Profiling of Paintings

A confocal x-ray fluorescence microscope was built at the Cornell High Energy Synchrotron Source (CHESS) to determine the composition of buried paint layers that range from 10-80 µm thick in paintings. The microscope consists of a borosilicate monocapillary optic to focus the incident beam and a borosilicate polycapillary lens to collect the fluorescent x-rays. The overlap of the two focal regions is several tens of microns in extent, and defines the active, or confocal, volume of the microscope. The capabilities of the technique were tested using acrylic paint films with distinct layers brushed onto glass slides and a twentieth century oil painting on canvas. The position and thickness of individual layers were extracted from their fluorescence profiles by fitting to a simple, semi-empirical model.

New capabilities and applications of compact source-optic combinations

Polycapillary and doubly curved crystal x-ray optics have gained broad acceptance and are now being used in a wide variety of applications. Beginning as optics integrated into research setups, they were then used to enhance the performance of existing x-ray analytical instruments and are now widely used as essential components in x-ray spectrometers and diffractometers designed to utilize their capabilities. Development of compact x-ray sources, matched to the optic input requirements have allowed large reduction in the size, power, and weight of x-ray systems which are now resulting in development of compact x-ray instruments for portable, remote, or in-line analytical tools for new applications in industry, science, or medicine.

Application of monolithic polycapillary focusing optics in MXRF

A monolithic polycapillary focusing optic, consisting of hundreds of thousands of small tapered glass capillaries, can collect a large solid angle of x-rays from a point source and guide then through the capillaries by multiple total reflections to form an intense focused beam. Such a focused beam has many applications in microbeam x-ray fluorescence (MXRF) analysis. Two monolithic polycapillary focusing optics were tested and characterized in a MXRF set- up using a microfocusing x-ray source. For the CuK(alpha ) line, the measured focal spot sizes of these optics were 105 micrometers and 43 micrometers full-width-half-maximum, respectively. When the source was operated at 16W, the average CuK(alpha ) intensities over the focal spots were measured to be 2.5 X 104 photons/s/micrometers2 and 8.9 X 104 photons/s/micrometers2, respectively. When we compared the monolithic optics to straight monocapillary optics with approximately the same output beam sizes, intensity gains of 16 and 44 were obtained. The optics were applied to the MXRF set-up to analyze trace elements in various samples and a minimum detection limit of about 2 pg was achieved for the transition elements (V, Cr, Mn, and Fe). The optics were also used to map the distributions of trace elements in various samples.

Effects of intense x-ray radiation on polycapillary fiber performance

Several applications of Kumakhov polycapillary optics require extended exposure to intense x- ray radiation. No degradation of performance has been observed when using polycapillary x- ray optics with laboratory sources. As part of an ongoing study to develop an understanding of damage mechanisms and performance limitations, borosilicate glass polycapillaries have been exposed to white beam bending magnet synchrotron radiation with peak energies of 5 and 11 keV, and focused broad band energy centered at 1.4 keV synchrotron radiation. In situ and ex situ measurements of degradation of x-ray transport efficiency have been performed at doses up to 1.8 MJ/cm2 at ambient and elevated temperatures. No decrease in transmission was observed for in situ measurement of fibers exposed to 1.4 keV photons at doses up to 1.4 MJ/cm2. Ambient temperature exposure to higher photon energies causes degradation that can be recovered by low temperature annealing. Exposure at elevated temperatures prevented any measurable damage to rigid fibers, at doses up to 800 kj/cm2.