It is not possible to produce gamma sources that emit all the energies needed to calculate accurately the response of a TAS detector. For this reason, it is better to perform a Montecarlo simulation of the response. For this simulation to be reliable, the interactions of all the particles emitted in the decay (γ, e−/e+, Auger e, x rays, etc.) have to be modeled accurately, and the geometry and materials in the way of these particles have to be well reproduced. Also, the light production of the scintillator has to be included. The way to perform this simulation is explained in detail in paper by D. Cano-Ott et al. GEANT3 and GEANT4 are well suited for these kind of simulations.