We describe a GaAs detector array for soft-Xray photons with energies above 250 eV designed to operate with power density levels of up to 20 W/mm2 to be used in a novel beam position monitor.

GaAs photodiode arrays have been designed for non-destructive monitoring of X-ray beam position in soft coherent beamline front ends in synchrotron light sources. A shallow p-on-n junction was employed to reduce the device photocurrent density to optimize the operation with beam power densities up to 20 W/mm2, mainly coming from hard X-rays. With this approach, the photocurrent is primarily defined by the excess carriers generated by low-energy X-ray photons absorbed near the detector surface. The p-n junction structures were grown by Molecular Beam Epitaxy and processed into 64-element linear photodiode arrays. The devices were characterized first in the visible range with a high-power Ar-ion laser and then tested in the soft and hard X-ray regions up to 10 keV at two beamlines of the National Synchrotron Light Source-II. The responsivity was measured to be 0.16 A/W at 0.7 keV and 0.05 A/W at 6 keV in agreement with modeling. At higher X-ray energies the measured responsivity was lower than predicted in the framework of the carrier diffusion model; a possible explanation is discussed.

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