Performance of p-bulk microstrip sensors under 60Co γγ irradiation at rates expected at the HL-LHC

We are developing p-bulk microstrip sensors for the high luminosity upgrade of the LHC accelerator, HL-LHC. The stability of FZ (float zone) wafers available to Hamamatsu Photonics was examined by irradiating them at rates expected at the HL-LHC. They show degradation in the operational voltage at low dose but recover after the dose is accumulated. The instability is dependent on the bias voltage and dose rate, and also on the irradiation history. We have characterized the instability and attributed the cause to the charge concentration at the electrode edge. The strip isolation, which is degraded while in irradiation, is shown not to induce any practical problem for the operation.