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 the ALiBaVa read-out system. The measurements were performed over a period of several weeks, during which several operating conditions were varied. The sensors were fabricated by Hamamatsu on 200\,$\mu $m thick float-zone and magnetic-Czochralski silicon. Their pitch is 80\,$\mu $m, and both $p$-stop and $p$-spray isolation of the $p^+$ 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 0.6 mGy/s. The dose in the SiO$_2$ at the end of the measurements was about 500\,Gy. Significant changes in the charge collection and charge sharing were observed as function of $^{90}$Sr irradiation dose. Annealing studies, with temperatures up to $80^\circ $C and annealing times of 18\,hours, show that the changes can only be partially annealed. The observations are qualitatively explained with the help of TCAD simulations in which the effects of radiation damage in SiO$_2$ have been included. The relevance of the measurements for the design and use of $p^+n$ strip sensors in different radiation environments is discussed.