Impact of low-dose electron irradiation on $n^+p$ silicon strip sensors

The response of $n^+p$ silicon strip sensors to electrons from a $^{90}$Sr 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\,$\mu $m thick float-zone and magnetic-Czochralski silicon. Their pitch was 80\,$\mu $m, and both $p$-stop and $p$-spray isolation of the $n^+$\,strips were studied. The electrons from the $^{90}$Sr 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 SiO$_2$ at the maximum was about 50\,Gy/d. % The measurements were performed over periods between 5 and 10 days. 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^\circ $C and annealing times of 18\,hours, showed that the changes can only be partially annealed. % The observations are qualitatively explained with the help of TCAD simulations in which the effects of radiation damage in SiO$_2$ have been included. The observations can be qualitatively explained by the increase of the positive oxide-charge density due to the ionization of the SiO$_2$ by the radiation from the $\beta $\,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. % Results of TCAD simulations with different oxide-charge densities and different boundary conditions at the sensor surface are presented. The relevance of the measurements for the design of $p^+n$ strip sensors is discussed