Monte Carlo simulation of nuclear resonance fluorescence setup at IFUSP: MCNPX and GEANT simulations results (I) - López-Pino, N; Corrales, Y; Aguilar-Mena, J; Padilla Cabal, F; Bernal-Castillo, J L; Maidana, Nora Lia; Martins, Marcos Nogueira; Vanin, Vito Roberto; Guevara, Maria Victoria Manso (Reunião de Trabalho sobre Física Nuclear no Brasil, 2008)