Design of a Proton-induced Quasimonochromatic Micro-XRF Setup for Wet Biological Samples

We designed and constructed a proton-induced quasimonochromatic micro-XRF system for wet biological samples. A horizontal beam of MeV protons delivered by a 1.6 MV tandem accelerator was bent downward by 45° using a dipole magnet. A copper (Cu) foil target was placed perpendicular to the incident proton beam. The thickness of the foil was 30 μm, which is slightly larger the proton range. This Cu foil was used also as a vacuum window for the X-rays. The proton-induced quasimonochromatic X-rays (QMXRs) emitted to the atmosphere through the foil were focused by a polycapillary X-ray half lens. The monochromaticity and intensity of the QMXR microbeam as a function of the incident proton energy were investigated. The QMXR microbeam impinged on the horizontal sample surface with an incident angle of 45°, and emitted fluorescent X-rays from the sample were collected by a Si-PIN X-ray detector positioned at 90° with respect to the incident microbeam. The size of the X-ray beam was 250 μm × 350 μm. To demonstrate the capability of the micro-XRF setup for in-vivo measurement of trace elements in biological samples, we measured cobalt (Co) accumulated in leaves of duckweed floating on a Co solution surface. By using standard samples which simulated the duckweed leaf, the Co concentration in the leaf in the order of 100 ppm could be quantitatively determined.