Radiation damage study of Belle II silicon strip sensors with 90 MeV electron irradiation

The silicon strip sensors of the Belle II silicon vertex detector were irradiated with 90 MeV electron beams up to a 1 MeV neutron equivalent fluence of 3.1 × 1 0 13 c m − 2 , which corresponds to approximately ten years of the full-luminosity Belle II operation, including a safety factor of greater than three in the background estimation. Electrons with comparable energies are one of the major sources of beam-induced background in the silicon strip sensors during Belle II operation. We measure radiation-induced changes in sensor properties, including the full depletion voltage, sensor leakage current, noise, and charge collection. For the sensors developed for the Belle II silicon vertex detector, the sensor bulk effective doping changes polarity at a 1 MeV neutron equivalent fluence of ( 6 - 7 ) × 1 0 12 cm -2 . We also determine a damage constant of 3.7 × 1 0 − 17 A/ cm at 17 °C immediately after irradiation. Due to annealing, the damage constant decreases to about 40% of the initial value at a temperature of approximately 17 °C in the first 200 h. The temperature is then set to 20 °C between 200 and 400 h, and to 25 °C after 400 h; as a result, the decrease stabilizes at around 30% of the initial value after 1000 h. We measure sensor noise and signal charge for a sensor irradiated with the 1 MeV neutron equivalent fluence of 3.1 × 1 0 13 cm -2 . Noise increases by approximately 44% after irradiation, while charge collection does not change significantly when a sufficiently high bias voltage is applied. Even after radiation damage corresponding to a fluence of 3.1 × 10 13 cm -2 , these measurements confirm that the full depletion voltage and leakage current will remain within the acceptable range of the power supply system, and that a sufficient signal-to-noise ratio can still be maintained despite the increase in noise.