Background predictions for the Xenon100 experiment at the Gran Sasso underground laboratory

The Xenon10 experiment has achieved the world's most stringent limits on spin-independent WIMP-nucleon cross sections after six months of stable underground operation. A substantial increase in sensitivity requires a larger sensitive mass coupled to a strong decrease in background. Xenon100, which is currently under construction, will operate a total mass of 150 kg of liquid Xenon, viewed by 250 PMTs, in the XENON10 shield starting in spring 2008. One of the new features is an active liquid Xenon layer around the target volume, which, along with a strong selection of detector and shield materials, and the removal of electrical feed-throughs and the cooling system from the vicinity of the detector, will allow a reduction in the expected background by about two orders of magnitude. Here we present background predictions from both gamma- and neutron-induced events, based on Geant4 Monte Carlo simulations of the detailed XENON100 detector geometry