Effects of trapped charge on the signal formation and detection efficiency for subsequent pulses in irradiated silicon sensors

Envisaged high energy physics experiments like the Future Circular Collider require unprecedented radiation hardness of the detectors, as well as short readout time due to high luminosity and occupancy. Silicon has proven to be extremely radiation hard, clear and fast signals can be recorded even at fluences close to $1\times 10^{16}~n_{eq}/\mathrm{cm}^{2}$. The signal formation in silicon strip sensors, irradiated and annealed until the phenomenon of charge multiplication occurred, was studied. ATLAS12EC R0 mini sensors were tested by means of Edge TCT measurements at temperatures around $-20^\circ C$. It was observed that the flow of generated charge changes the signal pulse in time, especially in charge multiplication regime. Moreover, it was observed that the detection of subsequent signals separated even several microseconds is altered by the charge trapped during the first pulses.The effects of trapped charge on the electrical configuration of a sensor is well known as a pumping effect in larger band-gap materials like diamond, but is often neglected for silicon at this relatively high temperature.The investigation of the effect created by trapped charges in silicon sensors using subsequent pulses allows to gain information on important parameters such as de-trapping times. Furthermore, it shows a severe impact on the sensor performance in a pile-up scenario. The irradiation fluence and hence the effective doping concentration, the temperature and the amount of initially created charge have a large impact on this phenomenon.The presented measurements help to characterize this phenomenon and particular attention was paid at the application point of view.