**4.2. Radiation event generator**

To numerically generate the particles with the spectral, spatial and angular distributions mimicking all the characteristics of the natural background, as introduced and defined in section 2, we use the G4 General Particle Source (GPS) [31] which is part of the Geant4 distribution. The module allows the user to define all the source parameters, in particular the energy of the emitted particles from a given energy distribution defined in a separate input file.

**Figure 6.** Differential fluxes of atmospheric muons given by the QARM (top) [22-23] and PARMA (bottom) [33-34] models at the location of the Altitude SEE Test European Platform (ASTEP, latitude North 44° 38' 02'', longitude East 5° 54' 26'', altitude 2555 m, see www.astep.eu). The atmospheric proton spectrum calculated with the PARMA model is also plotted (bottom).

For atmospheric particles, the energy distributions of neutrons, protons, pions and muons reaching the ground level are available in the literature or on the web as functions of latitude, longitude and altitude. They are obtained from direct measurements and/or from Monte Carlo simulations. For the neutron flux, we use the experimental atmospheric spectrum presented in Figure 1, which is actually the reference curve (for high-energy neutrons above 1 MeV) for the JEDEC Standard JESD89A [32]. For the other atmospheric particles (mainly muons and protons), we adopted the QinetiQ Atmospheric Radiation Model (QARM) [22-23] and the PARMA model [33-34], which are specifically developed for prediction of the radiation in the atmosphere for a given location and date. Figure 6 shows the differential fluxes of atmospheric muons (resp. protons) given by these two models (resp. by PARMA).

Another important issue in Monte-Carlo simulation is the strong zenith angular dependence of atmospheric showers. To make our Geant4 GPS primary particle sources more realistic, we introduce in simulations the angular dependence of the primary flux intensity in the form I(θ) ~ cosn (θ) where θ is the zenith angle and n a parameter fixed to n=3.5 for neutrons [35], and for muons n=2 [36].

For the simulation of alpha-particle emitters present in the IC materials, we directly generate in the code the random positions and emission directions with uniform probability densities for each daughter element of the considered decay chain (uranium or thorium).
