Does radiation improve silicon detectors

Sensors designed for the CMS Preshower detector were irradiated with protons and neutrons to fluences equivalent up to 2x10^14 n/cm^2. The leakage current and the capacitance as well as the charge collection efficiency and the noise were measured, before and after the irradiation, for most of the detectors. We noticed, that for some detectors of a lower quality, the breakdown voltage increases after type inversion and that their leakage current, charge collection efficiency and noise are comparable to good detectors. We explain this phenomenon by two effects: a change of the distribution of the electric field and a decrease of the carriers lifetime. Defects on the p-side do much less harm after type inversion, because the maximum of the E-field is now on the n-side. Defects on the n-side still generate charge carriers, but their lifetime is much shorter and most of them recombine beforereaching by diffusion the space charge volume. The article presents the measurements of the breakdown voltage, the charge collection efficiency and the noise before and after irradiation of such sensors compared with detectors of a high initial quality.