Charging of isolated proof masses in satellite experiments such as LISA

We report on a Monte Carlo simulation of electrostatic charging of the LISA proof masses by cosmic-ray protons and alpha particles, developed using the Geant4 toolkit. A positive charging rate of 58+/-17 +e/s (proton charges per second) was obtained with the minimum Geant4 energy threshold for the production of secondary particles by electromagnetic processes. This charging rate does not seem to depend strongly on the tracking of low-energy secondary electrons, and is some 5 times larger than that found in previous simulations. The difference is only partly explained by the slightly larger proof mass considered in this study. This figure is used to place new limits on the required discharge time of the LISA test masses.