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Synchrotron sources provide increasingly smaller X-ray beams (sub-micrometric size) and experiments are of course, more sensitive to X-ray beam movement owing to thermal effect or machine vibration. Sensors used to evaluate in-situ the position and shape of the beam are a powerful diagnostic device to compensate the variation of the beam by feedback correction. Powder of nanometric size, 100 nm diameter, of europium-doped gadolinium oxide has been prepared and deposited by spin-coating on quartz substrate for evaluation and finally on a thin vitreous carbon plate for ldquosemi-transparentrdquo X-ray beam monitor applications and detector alignment purposes. The 400 nm thick layer allows to measure the beam position with only 8% X-ray absorption at 14 keV. 92% of the X-ray beam is still available to investigate a diffraction pattern or a tomography of a sample. Powder size, annealing temperature, rotation speed and thickness of the layers are key parameters to optimize the optical quality and light yield of the thin phosphor screen. A low-cost 2D detector has been set up at the European Synchrotron Radiation Facility (ESRF). The system, based on lens coupling and a digital CCD camera allows a sub-micrometer precision measurement as well as beam profile information.