Changes of the particle detection properties of irradiated silicon microstrip sensors after room temperature annealing

Abstract

The electrical properties of hadron irradiated silicon detectors change over several years after irradiation. This annealing process has a strong dependence on temperature and it can be accelerated or decelerated by lowering or elevating the temperature at which the sensors are kept. This is exploited to investigate the long term behaviours of irradiated silicon detectors that are, or will be, installed in the experiment at the current and upgraded LHC at CERN. Elevated temperatures (up to 80°C) are used to accelerate the effect of annealing to study the expected changes of the sensor performances over several years of room temperature equivalent time. Low temperatures are applied to the sensors also when not operated to suppress undesired effects of annealing. The acceleration factors with respect to nominal room temperature (RT = 20°C) have been established monitoring the changes of the capacitance-voltage characteristics (CV) with time at various temperatures. In the experiments, the maximum high temperature envisaged out of operation cannot exceed much 20°C. It is important to measure the changes of the relevant parameters (charge collection reverse current, noise) at this temperature to verify the annealing behaviours in realistic conditions for planning the operation scenario (i.e. bias voltage and temperature during and outside operation) of the silicon sensors. We show here the study of room temperature annealing of the charge collection, reverse current and noise of silicon microstrip detectors after two doses of hadron irradiation (2 and 10 × 1015 neq cm−2) . These doses are chosen to represent the expected levels in the future upgrade of the LHC at CERN (High Luminosity LHC, HL-LHC) for the microstrip and pixel layers. The measurements show that a suitable choice of annealing time at 20°C can partially recover the degraded charge collection and reduce the reverse current after a given dose of hadron irradiation.