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Extremely radiation hard sensors are needed in particle physics experimentsto instrument the region near the beam pipe. Examples are beam halo and beamloss monitors at the Large Hadron Collider, FLASH or XFEL. Currently artificialdiamond sensors are widely used. In this paper single crystal sapphire sensorsare considered as a promising alternative. Industrially grown sapphire wafersare available in large sizes, are of low cost and, like diamond sensors, can beoperated without cooling. Here we present results of an irradiation study donewith sapphire sensors in a high intensity low energy electron beam. Then, amultichannel direction-sensitive sapphire detector stack is described. Itcomprises 8 sapphire plates of 1 cm^2 size and 525 micro m thickness,metallized on both sides, and apposed to form a stack. Each second metal layeris supplied with a bias voltage, and the layers in between are connected tocharge-sensitive preamplifiers. The performance of the detector was studied ina 5 GeV electron beam. The charge collection efficiency measured as a functionof the bias voltage rises with the voltage, reaching about 10 % at 950 V. Thesignal size obtained from electrons crossing the stack at this voltage is about22000 e, where e is the unit charge.  The signal size is measured as a function of the hit position, showingvariations of up to 20 % in the direction perpendicular to the beam and to theelectric field. The measurement of the signal size as a function of thecoordinate parallel to the electric field confirms the prediction that mainlyelectrons contribute to the signal. Also evidence for the presence of apolarisation field was observed.

Investigation of a direction sensitive sapphire detector stack at the 5 GeV electron beam at DESY-II