Study of evolution of active volume in irradiated silicon detectors

Light pulses emitted by a red (670 nm) Light Emitting Diode (LED) have been used to study the charge collection properties of non-irradiated and irradiated n-type silicon detectors. The advantages of red LED light pulses, compared to low-range alpha particles, are the availability of an external trigger and a very shallow distribution of the created e–h pairs (<10 μm). These features, combined with the use of a fast current amplifier and a 2.5 Gs/s sampling oscilloscope, allow the study of the evolution of the electric field in irradiated detectors. Evidence of a sensitive region on both sides of the detector has been observed. The model of the diode that depletes from the n+ junction side after conductivity-type inversion is discussed and the electric field distribution in the inverted detector is presented. A first evaluation of the strength of the electric field in the undepleted bulk of the detector is proposed.