Cosmic-ray and solar high energy particles penetrate the LISA experiment test masses. Consequently, an electric charge accumulates in the bodies of the masses, generating spurious Coulomb forces between the masses and the surrounding electrodes. This process increases the noise level of the experiment. We have estimated the amount of charge deposited per second on the LISA test masses by solar flares and primary cosmic-ray protons at solar minimum. The simulation has been carried out with the Fluka Monte Carlo program. A simplified geometry for the experiment has been considered. We have found a net charging rate of 37 +/- 1 e(+)/s for primary protons at solar minimum between 0.1 and 1000 GeV. The amount of charge released by a medium-strong solar flare, like that of 16 February 1984, is 10732 +/- 30e(+)/s in the energy range 0.1-10 GeV. This value increases or decreases by approximately one order of magnitude for strong (weak) solar flares. [References: 9]
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