Polycapillary optics can control x rays over a broad range of angles and energies and have been used as focusing collectors for x-ray astronomy, to produce large area collimated beams for wafer analysis, and to provide small focused beams for protein crystallography with low power x-ray sources. They are also being developed for a number of medical applications, including the removal of Comptonscattering with the resultant improvement in contrast and resolution in mammography, the production of monochromatic parallel beams for high contrast imaging in a clinical setting, and the detection and localization of radioactive tracers in prostate cancer. Other exciting applications are extensions of measurements normally performed at synchrotrons into laboratory or clinical settings because of the increased efficiency of source utilization.
Polycapillary optics are arrays of hollow glass tubes used to collect, focus, and redirect x-ray and neutron beams. A typical polycapillary fiber is shown in Figure 1. X rays striking the interior of these hollow channels at grazing incidence are guided along the channel by total external reflection in a process similar to the way fiber optics guide light. The reflection of x rays, which are reflected down the length of the capillary, is governed by the critical angle, which is approximately 1.5 mrad or 0.1°at 20 keV and is inversely proportional to photon energy. X rays are transmitted down in hollow glass tubes with high efficiency so long as the incidence angles are kept smaller than the critical angle. Focusing or collecting effects come from the overlap of the beams from thousands of capillary channels, rather than from the action within a single tube. As for single bore capillaries, x rays can be transmitted down a curved hollow tube as long as the tube is small enough and bent gently enough to keep the angles of incidence less than the critical angle for total reflection, #c.
As shown Figure 2, the angle of incidence for the ray near one edge increases with tube diameter. The requirement that the incident angles remain less than the critical angle necessitates the use of tiny tubes. Polycapillary fibers have tube diameters that are much smaller than the fiber diameter, while still maintaining high open area. Typical channel sizes are between 2 and 12 mm. Thousands of such fibers are strung through lithographically produced metal grids to produce a multifiber lens. Alternatively, a larger diameter polycapillary fiber can be shaped into a one-piece, monolithic, optic, as shown in figure 3.
Read more about the physics of polycapillary optics.
Articles on Polycapillary Optics:
- Synchrotron Focusing and Radiation Damage<