Polycrystalline CVD Diamonds for the Beam Calorimeter of the ILC

The instrumentation of the very forward region of the ILC detectors is challenging. At lowest polar angles a beam calorimeter, BeamCal, is foreseen. The main tasks of BeamCal are efficient detection of high energetic particles at lowest angles as well as monitoring of the beam collisions. A large background of electron-positron pairs generated by beamstrahlung leads to an energy deposition of 10-20 TeV per bunch crossing in BeamCal. This corresponds locally to doses up to 10 MGy per year of electromagnetic radiation. BeamCal is a compact sandwich calorimeter. Tungsten is the absorber and polycrystalline CVD diamond is under study as the sensor material to allow the operation in this harsh radiation environment. The pCVD diamond sensors under investigation are from different manufacturers fabricated usually on 4 inch wafers. We study the charge collection efficiencies of the diamond sensors as function of the applied electric field and of the absorbed dose. In our application the homogeneity and linearity of the response are of critical importance. In a first test beam period we investigated the linearity of the response of different diamond sensors up to particle fluences of 10⁷ MIP/(cm² times 10 ns). In a second test beam a high intensity beam of 10 MeV electrons was used to irradiate the diamond sensors and to investigate the behavior of the charge collection efficiency up to doses of several MGy.