Development of polycapillary x-ray optics for scatter rejection

Many medical imaging and industrial applications for x rays require large area optics with good scatter rejection. Preliminary scatter rejection and contrast measurements show that a prototype long borosilicate optic increases the contrast by a factor of 1.7 by decreasing the scatter transmission nearly a factor of 10 at 20 keV. Since borosilicate optics have higher scatter transmissions at high energies, the optics have to be fairly long to give good scatter rejection at high energies. However, long optics are complex to manufacture and have increased defect rates. Lead glass would allow the optic to be much shorter and still give good contrast enhancement, because of the superior absorption of leaded glass. In order to investigate the feasibility of using leaded glass polycapillary optics for these applications, measurements and simulations have been performed on the behavior of leaded glass polycapillary fibers in the 9-80 keV energy range. These fibers have an outer diameter of about 0.5 mm, channel diameter of 12 mum and fractional open area from 40% to 60%. The transmission efficiencies of these fibers of different types and lengths were measured as a function of source location and x-ray energy. The measurements were analyzed using a geometrical optics simulation program, which included the effects of roughness, waviness, bending and glass inclusions. Despite low transmission at low energies, leaded glass polycapillary optics with a length of 30-60 mm seem promising for many high energy (>20 keV) x-ray applications. Type B fibers with a 60 mm length have a transmission efficiency of 50% in the 35-40 keV energy range, which is 83% of fiber's open area, and very low scatter transmission of 0.18% at 80 keV. The same fiber with a shorter length of 30 mm has a transmission which is higher at lower energies, and more than 52% in the 20-40 keV energy range.