Using lobster-eye optics in hard-x-ray imaging systems
By their nature, x-rays are difficult to focus refractively. One way to manipulate x-rays is to reflect them from smooth metal surfaces at small, grazing angles of incidence. Because of the very-small critical grazing angles (of less than 3.6 arc-minutes for 60keV x-rays), and relatively large inner capillary diameters, current x-ray reflection optics are large (on the meter scale), expensive, hard to align, and have small fields of view.1,2 A more advanced form of x-ray focusing optics, known as lobster-eye (LE) optics,3–7 may provide a better alternative.
Current x-ray LE optic systems are based on slumped leadglass microchannel plates (MCPs).6,7 However, the spectral range of operation of existing MCP-based LE optics is limited to x-rays with energies of less than 4keV (or λ greater than 310pm). Harder x-rays with smaller critical grazing angles cannot be focused efficiently by glass MCPs because their walls are inaccessible for polishing and metallization. This energy range constraint of the MCP-based LE optics limits its application to vacuum chambers or open-space astronomical applications with infinite distance to the objects (such as stars).
At Physical Optics Corporation (POC), we have built new LE structures from flat, thin, ideally-polished metal ribs that form long, hollow, metal microchannels to focus hard x-rays with energies greater than 40keV (or λ < 21pm) from objects at finite distances. The LE x-ray focusing optics use the internal reflection of x-rays from the gold-plated metal-mirror walls of the microchannels. This approach offers full control of the surface quality of the elements, because both sides of the flat ribs can be polished to the accuracy needed for hard-x-ray optics.
To optimize the performance of the LE x-ray focusing optics, we derived an imaging equation covering the transversal, angular, and longitudinal magnifications of the LE. We showed that it is possible to minimize the defocusing of the image by relying on the basic properties of the LE optics, which reduce the blurring effect of photons from outside the imaged area. We can also tailor the specular reflectivity of the LE metal surface as a function of the incident grazing angle and the energy of the x-rays, to increase the control of such blurring. The efficiency of the LE optics in collecting/focusing x-rays can also be increased several times by using metals such as iridium or osmium, which have large-acceptance incident-grazing angles, on a LE element surface.
POC has used these considerations to develop and assemble LE optics from flat metal ribs polished on both sides. These optics have several advantages critical for hard-x-ray focusing systems: a choice of initial materials, including various metal sheets, silicon wafers, and glass; low weight; the ability to get high-quality surface polishing across the whole surface of the square LE cells, including corners; a high fill factor (greater than 80%); cost-effectiveness; and compatibility with existing fabrication procedures. These advantages are important for practical application of the LE optics for high-sensitivity, high-resolution x-ray imaging in applications outside astronomy.
We have used the new LE hard-x-ray optics to develop the idea of using staring (non-scanning) Compton backscattering to image hidden objects.8 In contrast to existing scanning pencilbeam systems, our LE x-ray imaging system's optics simultaneously acquire backscattering photons from an entire scene irradiated by a wide-open cone beam from an x-ray generator. For the first time, we used hard x-rays (in the 40 to 60keV range, or wavelengths between 40 and 20pm) to produce Compton-backscattered staring images of obscured metal and organic objects. The images were produced in real time with high sensitivity after the rays had penetrated through steel walls.
The hard x-ray LE lens we have developed looks a promising basis for a compact, real-time x-ray backscattering inspection system for objects at finite standoff distances. Our results show that we can optimize the LE structure and the reflecting properties of its elements to enhance the penetration of the inspection system to 1.9mm of steel at standoff distances of between five and 50m, with adequate rescaling of the LE lens and x-ray source.
Our next step will be to make optimized LE flat metal elements with the surface quality and coating necessary for hard x-ray optics. We will then assemble them into a scaled-up, large field-of-view, high-resolution, low-weight hard-x-ray lens. We'll combine this lens with x-ray generators and an x-ray camera to create compact field prototypes. Finally, these will be tested for use in various applications including: non-destructive evaluation; industrial, security, and other surveillance; cargo inspection; bomb detection; and medical imaging.
