[en] The expansion of prospection activity for oil and natural gas in the Rio de Janeiro State brought an increasing use of neutron area monitors by this sector and consequently an increase in demand for calibration of such instruments. There is a need to ensure the traceability and reliability in calibration conditions, which often are lost due to the transport, operation and manipulation, in addition to some defects. The Laboratory of Neutrons (LN), integrating the National Laboratory for Metrology of Ionizing Radiation (LNMRI/IRD), is developing a portable system for testing of area monitors, which may be carried out by the users of the instruments in places where they are used. The first phase of this project consists in the simulation of the geometry of the system, using computational code, based on method of Monte Carlo, MCNP5. In this simulation, the rates of ambient dose equivalent were defined to sensitize area monitors. The modeling of the source of AmBe 20.35 GBq (550 mCi) was also made in a block of polyethylene to define the thickness of the system. The definition of the geometry of the system and discussions for the use of this system in the field are also presented additionally. (author)