Cosmogenic activation of germanium and its reduction for low background experiments

Production of 60Co and 68Ge from stable isotopes of germanium by nuclear active component of cosmic rays is a principal background source for the new generation 76Ge double beta decay experiments like GERDA and Majorana. The biggest contribution of cosmogenic activity is expected to be produced during transportation of either enriched material or already grown crystal. In this article the properties and feasibility of a movable iron shield are discussed. An activation reduction factor of about 10 is predicted by simulations with the SHIELD code for a simple cylindrical configuration. This is sufficient for the GERDA Phase II background requirements. The possibility of further increasing the reduction factor and physical limitations are considered. The importance of activation reduction during germanium purification and detector manufacturing is emphasized