VERTICAL PHOTON-BEAM POSITION MEASUREMENT AT BENDING MAGNETS USING LATERAL DIODES

Commercial position sensitive diodes which were designed for measurements with visible light have been used to measure beam position in a white-light beamline, exposing them to the hard-x-ray part of the dipole spectrum. In preliminary studies it was found that the quantum efficiency of the lateral diodes is comparable to that of Si XUV diodes. At 1.5 keV a quantum efficiency of about 100 electrons per photon was measured in the central position. No long-term drifts of the efficiency have been observed. The position sensitivity of the diodes is almost independent of photon energy, but is at least a factor of 2 higher than in the visible range. Hence in a white-light beam, a photon energy window was defined by a water-cooled Al-absorber foil of 200 μm thickness. Behind the foil two lateral diodes were arranged at 3.5 (15 mm length) and 7 m (30 mm length) behind the source. The horizontal position of the diodes was staggered so that the first diode does not occlude the beam to the second one. With this system, the vertical source position in a bending magnet can be detected with 10 μm accuracy while the emission angle can be detected down to values of 2 μrad. © 1995 American Institute of Physics.