“Semi-Transparent” X-Ray Beam Monitor Based on Nanometric Phosphor Powder Deposited on Thin Carbon Plate

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.

I. Introduction

Third-Generation synchrotron sources provide X-ray beam sizes in nanometric size. Devices required for beam monitoring have specifications that may vary from a simple requirement of intermittent verification that the beam has not drifted outside some acceptable position limit, to the need for continuous measurements in the case where a motorized stage is required to stabilize the beam position. The X-ray beams at the third synchrotron sources are intense: after passing through a crystal monochromator, the beam (energy typically between 5 and 50 keV) will contain and will be focused to a spot size from 80 nm to 50 microns depending on the application. A permanent transmissive beam monitor is itself an optical element, and its impact on the beam quality must be tolerable. At the physical device level, pink/white beam monitoring also presents qualitatively different technical problems as compared to the monochromatic beam case, notably due to the higher beam intensity and the consequent heat load from beam absorption. The present development was foreseen for monochromatic beam at a typical energy of 14 keV. Various devices are currently in use at the ESRF for monochromatic beam intensity and position monitoring, each with their merits and drawbacks.