X-ray tomography is a workhorse tool of non-destructive imaging. It is used to probe three-dimensional structures across
a wide range of length scales for objects that offer good absorption contrast to x-rays. In recent years extremely high
resolution imaging (on the order of tens of nanometres) has become possible due to technological advances in x-ray
optics. At the same time the requirement for strong absorption contrast has been relaxed thanks to the advent of new
experimental and algorithmic techniques in phase imaging. Advances in both resolution and phase imaging can be
combined to image biological samples at the sub-cellular level. I will report on recent advances in our work including
improvements to the current approaches in extracting phase information at high resolution from measurements of the
diffracted intensity from a sample. I will also discuss our current experimental status.
We have developed a quality function that quantifies how well features are reconstructed in 3D phase contrast tomographic reconstructions for pure phase samples. We consider image formation using the free space propagation method for each projected data-set. A partially coherent radiation field originating from an extended source is also incorporated. This means the model is suitable for a laboratory based micro focus x-ray source. The quality function is useful for optimizing the tomographic reconstruction for given feature sizes in an object. We then use the quality function to develop filters for improving the reconstruction quality for high spatial frequency objects.
X-ray lithographic conditions for high aspect ratio SU-8 resist structures are characterized for potential application in x-ray optics and bioMEMS. The effects of the main process parameters such as exposure dose, post exposure bake, development time and the packing density of the microfabricated features on the development depth and increase in feature size at the top portion of the resist (as compared to that in mask) were investigated. As test samples, we fabricated 1mm high, densely-packed SU-8 structures comprising of 30μm square pillars with spacings of 36μm, 12μm and 6μm. Dissolution rates are found to be longer for densely packed structures than predicted by simple physical models based on isolated structures. We examine the effect on dissolution rates of the density of features in our structures. We also optimised our process with respect to the parameters described above using the Taguchi method. We find that optimisation of the development time with exposure dose and post-exposure bake time can reduce the dimensional error to ~ 3% for certain densely-packed structures.
It is well-known that the loss of phase information at detection means that a diffraction pattern may be consistent with a multitude of physically different structures. This paper shows that it is possible to perform unique structural determination in the absence of a-priori information using x-ray fields with phase curvature. We argue that significant phase curvature is already available using modern x-ray optics and we demonstrate an algorithm that allows the phase to be recovered uniquely and reliably.
The Lobster-ISS instrument is an X-ray all sky monitor proposed as an attached payload on the zenith platform exposed payload facility of the European Space Agency (ESA) Columbus module of the International Space Station (ISS). The basic instrument consists of six microchannel plate X-ray telescopes, collectively providing wide-angle (22.5 x 162 sq.degree) astronomical X-ray imaging in the 0.1 - 3.5 keV energy band. In this paper we describe computer modeling software underway at the University of Melbourne to provide an accurate simulation of the operation of the Lobster-ISS in its low Earth orbit environment. We exhibit some preliminary exposure maps and examples of the X-ray images that the instrument should produce given our simulation of its operation.
The manipulation of x-rays by phase structures is becoming more common through devices such as compound refractive lenses, blazed zone-plates and other structures. A spiral phase modulation structure can be used to condition an x-ray beam to produce an x-ray vortex. An x-ray beam in this form can be used as the first step towards a self-collimating beam. Also it can be used as a controllable pathological feature in studies of x-ray phase retrieval.
We describe the microfabrication of a spiral phase modulation structure by excimer laser ablation. A multi-step fabrication using 15 separate chrome-on-quartz mask patterns is used to create a 16 step spiral staircase structure approximating the desired spiral ramp. The results of simulations and initial experimental results are presented.
We describe the design of Lobster-ISS, an X-ray imaging all-sky monitor (ASM) to be flown as an attached payload on the International Space Station. Lobster-ISS is the subject of an ESA Phase-A study which will begin in December 2001. With an instantaneous field of view 162 x 22.5 degrees, Lobster-ISS will map almost the complete sky every 90 minute ISS orbit, generating a confusion-limited catalogue of ~250,000 sources every 2 months. Lobster-ISS will use focusing microchannel plate optics and imaging gas proportional micro-well detectors; work is currently underway to improve the MCP optics and to develop proportional counter windows with enhanced transmission and negligible rates of gas leakage, thus improving instrument throughput and reducing mass. Lobster-ISS provides an order of magnitude improvement in the sensitivity of X-ray ASMs, and will, for the first time, provide continuous monitoring of the sky in the soft X-ray region (0.1-3.5 keV). Lobster-ISS provides long term monitoring of all classes of variable X-ray source, and an essential alert facility, with rapid detection of transient X-ray sources such as Gamma-Ray Burst afterglows being relayed to contemporary pointed X-ray observatories. The mission, with a nominal lifetime of 3 years, is scheduled for launch on the Shuttle c.2009.
Lobster-eye optics are an exciting advance in the field of x-ray astronomy, specifically as focusing optics for wide field of view telescopes. In its simplest form the optic is a square packed array of square-channels. Typical channel dimensions are width 10 - 30 mm, length 300 - 1000 mm, and wall thickness of ~2 - 5 mm. These dimensions raise the question of whether such devices can be made in the deep x-ray LIGA regime. Following recent success in fabricating a low aspect ratio Lobster-eye structure, we discuss some of the parameters for, and production issues involved in making, a useful high aspect ratio Lobster-eye prototype. We report on our initial attempts to produce high aspect ratio Lobster-eye optics using the LIGA process with a Graphite substrate.
The prospect of making a lobster-eye telescope is drawing closer with recent developments in the manufacture of microchannel-plate optics. This would lead to an x-ray all-sky monitor with vastly improved sensitivity and resolution over existing and other planned instruments. We consider a new approach, using deep etch x-ray lithography, to making a lobster-eye lens that offers certain advantages even over microchannel-plate technology.
Lobster-eye optics have attracted much attention and effort in recent years due to their unique x-ray focusing capabilities. While many advances have been made in the manufacture and analysis of these optics, their characterization and the determination of their metrology remains constrained by the shortcomings of current techniques. We present a faster, better and cheaper method for the determination of many of the metrological parameters of lobster-eye optics. Optical images of the entrance and exit surfaces of an optic are taken. Applying our technique to these images allows measurement of all the geometrical properties that previously have been found to be the major contributors to focusing defects. In addition, the number of free parameters required in fitting a simulated to a measured x-ray image can be greatly reduced. We present results for the characterization of an existing lobster-eye optic and the improved modeling thereby obtained which are in very good agreement with experimental x-ray focusing data.
We describe our design for a mini-Schmidt all-sky monitor. By using standard micro-machining techniques we are able to build a module that is smaller, lighter and has a greater open area than previous prototypes. In addition, we retain the benefit of high quality metal-coated flat glass reflecting surfaces.
The concept of the lobster eye optics was proposed in the nineteen seventies. It has gained widespread interest in x- ray astronomy for its potential for constructing compact and focusing x-ray all sky monitors with unprecedented sensitivities. The majority of the efforts of developing a practical implementation of this optics has been devoted toward slumping square-pore micro-channel plates. While the advantages of the slumped micro-channel plates are obvious in that they can achieve potentially arc-second angular resolutions, the smoothness requirements for reflecting x- rays are hard to meet by micro-channel plates. It is not clear how the interior of the micro-channel plate pores can be polished to the desired smoothness. In this paper we propose the feasibility of a more straightforward approach of implementing the lobster eye optics with flat glass mirrors assembled in a standard Kirkpatrick-Baez configuration. We demonstrate with both simulations and laboratory test results that this implementation is both practical and meets al the requirements of an x-ray all sky monitor.
Lobster-eye optics have been proposed as an exciting development in the field of x-ray all-sky monitors. However, to date their potential has mainly been analyzed in the context of an all-sky monitor for a small satellite mission. We examine the wide range of parameters available for lobster-eye optics with different configurations. The sensitivity of the various schemes is calculated. We have also examined the current state of the art in actual lobster-eye optics. We present our experimental results and discuss realistic targets for manufacture. The impact of these targets on the calculated sensitivities is also described.
Square-channel capillary, or 'Lobster-eye' arrays have been shown to be the optimum geometry for array optics. This configuration leads to a novel class of conditioning devices for x-ray and neutron beams. We present the first result of the focusing of neutrons with a lead glass square-channel array. This array, designed for soft x-rays, performs comparably with neutrons. Finally, we describe a novel method for the fabrication of glass square channel arrays.
Conditioning neutron and X-ray beams is best achieved with glancing-incidence reflective optics. Square micro-channel arrays offer an increasingly practical geometry for this implementation. We present results for focussing neutrons with two such arrays, one with channel size of 32 micrometer, which places us truly in the microscopic regime. These two arrays, designed for soft X-rays, perform comparably with neutrons.
We present a conceptual design for a new x-ray all sky monitor (ASM). Compared with previous ASMs, its salient features are: (1) it has a focusing capability that increases the signal to background ratio by a factor of 3; (2) it has a broad-band width: 200 eV to 15 keV; (3) it has a large x-ray collection area: approximately 102 cm2; (4) it has a duty cycle of nearly 100%, and (5) it can measure the position of a new source with an accuracy of a few minutes of arc. These features combined open up an opportunity for discovering new phenomena as well as monitoring existing phenomena with unprecedented coverage and sensitivity.
We report investigations of the x-ray focusing of a square channel capillary array. We use x rays with an energy of about 1.5 keV from a laser produced plasma. We find the focal structure to be consistent with theoretical expectations. The images were recorded using x-ray film and, to within the precision with which we were able to analyze the results, the data is consistent with an array with negligible channel tilt and a surface roughness of 1.5 nm rms. This is the best performance yet reported for lobster-eye x-ray optics.
We set forth a conceptual design for an x-ray all-sky monitor based on lobster-eye wide-field telescopes. This instrument, suitable for a small satellite, would monitor the flux of objects as faint as 2 multiplied by 10-12 erg cm-2 s-1 (0.5 - 2.4 keV) on a daily basis with a signal-to-noise of 5. Sources would be located to 1 - 2 arc- minutes. Detailed simulations show that crosstalk from the cruciform lobster images would not significantly compromise performance. At this sensitivity limit, we could monitor not just x- ray binaries but fainter classes of x-ray sources. Hundreds of active galactic nuclei, coronal sources, and cataclysmic variables could be tracked on a daily basis. Large numbers of fast transients should be visible, including gamma-ray bursts and the soft x-ray breakout of nearby type II supernovae.
X-ray focusing using square channel capillary arrays is reviewed. We review our theoretical understanding of these devices and go on to examine their potential in the context of x-ray astronomy as an approach to the construction of a lobster-eye telescope. We show that a reasonably small device has the potential, in principle, to improve the sensitivity of wide field of view x-ray telescopes by an order of magnitude. We go on to briefly review our experimental work and indicate that these devices are getting close to realizing their theoretical potential.
We present results of the experimental tests of various arrays of reflectors acting as X-ray optical elements. Planar reflectors were aligned with a common center of curvature to create a 1D focusing array. The experiments were carried out using X-ray radiation in the spectral range 3.5 keV to 7 keV produced at beamline X-27C of the National Synchrotron Light Source at the Brookhaven National Laboratory. The initial beam had width of 90 mm, a focal distribution had a width of 8 mm and a maximum intensity gain of 7.3, as measured with a 0.4 mm diameter pinhole. An X-ray beamsplitter consisting of arrays of reflectors has also been designed and tested. The separation of X-ray beams is 40 mrad and intensity of each of two beams is about 0.32 of the original X-ray beam.
X-ray focusing using square channel capillary arrays is reviewed. We present some experimental results obtained using a variety of array configurations and we deduce that channel misalignment and surface roughness are the prime factors limiting the performance of these devices. We present results obtained using a stacked array of commercial precision bore square tubing and deduce that the reflectivity from these unetched surfaces is superior to that from etched micro-channel plate blanks. The improved surface quality implied by this reflectivity result is confirmed using atomic force microscopy. We also present results of a new drawing technique that we have developed.