Cryogenic temperature performance of heavily irradiated silicon detectors

The charge collection efficiency (CCE) of silicon detectors, previously irradiated with high neutron fluences, has been measured at 4.2, 77 and 195 K. The CCE recovery measured after 1.2×1014 n/cm2 is 100% at a bias voltage of 50 V. For 2×1015 n/cm2 the most probable signal collected for minimum ionising particles is 13 000 electrons, corresponding to 50% CCE, at a bias voltage of 250 V. Negligible difference has been observed between 77 and 4.2 K operation, while no recovery was measurable at 195 K. The timing of the signal was measured to be better than 5 ns. The samples were irradiated and stored at room temperature and cooled only when operated. Reproducible results were obtained after several weeks and several thermal cycles.