GEANT4 validation in the calculation of absorbed dose in air due to gamma radiation emitted from the soil

Geant, is a monte Carlo code system conceived essentially for optimization of detectors in particle physics field. It found some usage in other high energy branches such as medical physics. GEANT4 toolkit system, written in C++ oriented object programation language, is the recent version of Geant code. This powerful simulation tool is believed to be a viable alternative to other codes based on Monte Carlo methods in the calculation of photon transport. We have developed a new optimized geometry to simulate the soil in a Monte Carlo calculation of absorbed dose in air due to gamma radiation from sources distributed in the ground. This geometry was successfully implemented in Geant4 System. The method relative to the implementation is described. Simulation tests were conducted to compare the results issued from different approches were the soil is modelled either by a simple shape like a cylinder or by our new geometry. This work seems to be important to validate Geant4 in the calculation of absorbed dose in air due to radioactive sources distributed in the soil and to confirm the usage of GEANT4 toolkit system in such field of nuclear physics