  The response of n+p silicon strip sensors to electrons from a Sr-90 source
was measured using a multi-channel read-out system with 25 ns sampling time.
The measurements were performed over a period of several weeks, during which
the operating conditions were varied. The sensors were fabricated by Hamamatsu
Photonics K.K. on 200 micrometer thick float-zone and magnetic-Czochralski
silicon. Their pitch was 80 micrometer, and both p-stop and p-spray isolation
of the n+ strips were studied. The electrons from the Sr-90 source were
collimated to a spot with a full-width-at-half-maximum of 2 mm at the sensor
surface, and the dose rate in the SiO2 at the maximum was about 50 Gy/d.
  After only a few hours of making measurements, significant changes in charge
collection and charge sharing were observed. Annealing studies, with
temperatures up to 80{\deg}C and annealing times of 18 hours, showed that the
changes can only be partially annealed. The observations can be qualitatively
explained by the increase of the positive oxide-charge density due to the
ionization of the SiO2 by the radiation from the {\ss} source. TCAD simulations
of the electric field in the sensor for different oxide-charge densities and
different boundary conditions at the sensor surface support this explanation.
  The relevance of the measurements for the design of p+n strip sensors is
discussed.
