The design of an X-ray microprobe at the SRS Daresbury (UK)

Synchrotron radiation proves to be an excellent source for photon induced X-ray fluorescence (XRF). The radiation is white over a broad wavelength range, highly linearly polarized, confined into a small solid angle and has a high intensity. The polarization facilitates a strong reduction of the background in XRF element analysis. The high intensity and large cross section yield a good applicability for trace element analysis. A doubly bent Si(111) crystal combined with a diaphragm as a design for a photon microprobe was numerically examined to focus synchrotron radiation. Beam size, flux and spectral distribution at the focus have been calculated. The study revealed a beamsize of 50 × 50 μm ² containing enough flux to obtain a detection limit of 10 ^-7 weight fraction {g}/{g} in a measuring time of about 5 s per pixel. In a first try out with a focusing crystal with radius of R = 600 mm a beam intensity of 10 ¹¹ photons in a spot of about 300 μm has been achieved.