Radiation hardness studies of silicon pixel detectors

At the LHC silicon vertex detectors will be exposed to hadron fluences of the order of 1015neqcm-2. In order to study the effects of radiation damage on the performances of the ATLAS Pixel Vertex Detector, several full-size detector modules were irradiated to a fluence of 1.1×1015neqcm-2 and tested in a beam at CERN. After irradiation only a modest degradation of the detector performances is observed. At the operating ATLAS bias voltage of 600 V the average signal is still 80% of the pre-irradiation value, the spatial resolution is 9.6μm and the detection efficiency is 98.2%. The LHC luminosity upgrade will increase the radiation hardness requirements by a factor of 10 and will require the development of new ultra-radiation hard vertex detectors. A detailed simulation of silicon pixel detectors irradiated to very high fluence is presented and used to study the possibility to use silicon pixel detectors at the LHC after the luminosity upgrade. The charge collection properties and the detector response were computed for different silicon materials (Standard Float Zone, Diffusion Oxygenated Float Zone, Czochralski, epitaxial silicon) and operating conditions (bias voltage, temperature). At the maximum fluence (1016neqcm-2) the signal is limited by charge trapping rather than by the thickness of the active volume. Since all the silicon materials studied so far have a similar trapping cross-section, they are all expected to collect an average signal of 2000–2500 electrons at 600 V bias voltage. A detection threshold of 1000–1200 electrons is required in order to have a 97% detection efficiency.