Measurements and analysis of radiation effects in polycapillary x-ray optics

Abstract

Polycapillary x-ray optics are arrays of large numbers of small hollow glass tubes which deflect x rays by successive total external reflection. These optics have growing numbers of applications in areas ranging from medical imaging to microanalysis. An accelerated radiation effects study has been performed to understand the performance limitation of these optics for medium to high intensity radiation applications, to study x-radiation damage mechanisms, and to investigate possible ways to mitigate the radiation effects on x-ray transmission efficiency. Exposures have been done in white beam bending magnet radiation with peak energies at 5 and 11 keV and focused broad band radiation centered at 1.4 keV. In situ and ex situ measurements of loss of x-ray transport efficiency have been executed at doses up to 1.8 MJ/cm2. Thin polycapillary fibers displayed noticeable bending and experienced substantial degradation of x-ray transmission. Thicker polycapillary fibers showed a linear but much slower transmission loss as a function of total dose. Annealing effectively restored the low energy (∼8 keV) transmission efficiency of the fibers. Exposure of these fibers at slightly elevated temperatures prevented any measurable loss in the low energy transmission efficiency. A variety of analytical techniques has been used to understand these results. No significant change was observed in the chemical composition of the capillary surface. Profile measurements and high energy transmission efficiency spectra, along with computer simulation studies, suggest that radiation induced bending is the primary cause of transmission efficiency degradation of the fibers.