Cryogenic Si detectors for ultra radiation hardness in SLHC environment

摘要

Radiation hardness up to 10~(16)n_(eq)/cm~2 is required in the future HEP experiments for most inner detectors. However, 10~(16)n_(eq)/cm~2 fluence is well beyond the radiation tolerance of even the most advanced semiconductor detectors fabricated by commonly adopted technologies: the carrier trapping will limit the charge collection depth to an effective range of 20-30 μm regardless of depletion depth. Significant improvement of the radiation hardness of silicon sensors has been taken place within RD39. Fortunately the cryogenic tool we have been using provides us a convenient way to solve the detector charge collection efficiency (CCE) problem at SLHC radiation level (10~(16)n_(eq)/cm~2). There are two key approaches in our efforts: (1) use of the charge/current injection to manipulate the detector internal electric field in such a way that it can be depleted at a modest bias voltage at cryogenic temperature range (≤ 230 K); and (2) freezing out of the trapping centers that affects the CCE at cryogenic temperatures lower than that of the LN_2 temperature.rnIn our first approach, we have developed the advanced radiation hard detectors using charge or current injection, the current injected diodes (CID). In a CID, the electric field is controlled by injected current, which is limited by the space charge, yielding a nearly uniform electric field in the detector, independent of the radiation fluence. In our second approach, we have developed models ofrnradiation-induced trapping levels and the physics of their freezing out at cryogenic temperatures. In this approach, we intend to study the trapping effect at temperatures below LN_2 temperature. A freeze-out of trapping can certainly help in the development of ultra-radiation hard Si detectors for SLHC. A detector CCE measurement system using ultra-fast picosecond laser with a He cryostat has been built at CERN. This system can be used to find out the practical cryogenic temperature range that can be used to freeze out the radiation-induced trapping levels, and it is ready for measurements on extremely heavily irradiated silicon detectors. Initial data from this system will be presented.