Study of the evolution ot the active volume in irradiated silicon detectors

Red (670 nm) LED light was used to study the charge collection properties of non-irradiated and irradiated n-type silicon detectors. The advantages of red LED, compared to low-range alpha particles, are the availability of an external trigger, and a very shallow distribution of the created electron-hole pairs (< 10 µm). These features, combined with the use of a fast current amplifier and a 2.5 G s/s sampling oscilloscope, allow the electric field evolution in irradiated detectors to be studied. Evidence of a sensitive region on both sides of the detector was observed. The model of the diode d epletion volume from the n+ junction side after conduction-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.