Monte Carlo studies and optimization for the calibration system of the Gerda experiment

The GERmanium Detector Array, Gerda, searches for neutrinoless double β decay in 76Ge using bare high-purity germanium detectors submerged in liquid argon. For the calibration of these detectors γ emitting sources have to be lowered from their parking position on the top of the cryostat over more than 5 m down to the germanium crystals. With the help of Monte Carlo simulations, the relevant parameters of the calibration system were determined. It was found that three 228Th sources with an activity of 20 kBq each at two different vertical positions will be necessary to reach sufficient statistics in all detectors in less than 4 h of calibration time. These sources will contribute to the background of the experiment with a total of ( 1.07 ± 0.04 ( stat ) − 0.19 + 0.13 ( sys ) ) × 10 − 4 cts / ( keV kg yr ) ) when shielded from below with 6 cm of tantalum in the parking position.