Temperature dependence of the charge collection efficiency in heavily irradiated silicon detectors

Two silicon diode detectors, Al/n+/n/p+/Al, were exposed to fluences of 1.19×1014 and 2.23×1015 equivalent 1 MeV neutrons/cm2, respectively. After this exposure the detectors were stored at room temperatures for 2 yr (1.19×1014) and six months (2.23×1015). During this time they were thermally cycled around 4.2 K and room temperature a number of times in order to make measurements. The charge collection efficiency is measured to be (at 77 K) 100% for the less severely irradiated detector and 50% for the detector exposed to high levels of radiation. The same results apply to operation at 4.2 K, while no recovery is observed at 195 K. By examining the signal response of the irradiated detectors to α particles, it is shown that some of the radiation damage after reverse annealing is in the form of electron and hole traps, which are either weakly, or not at all, temperature dependent.