In this work, the characteristics and properties of spallation targets in ADS (Accelerator Driven System) are investigated. The objective is to obtain a high neutron flux with a hardened neutron spectrum for transuranic transmutation in a hybrid ADS-fission systems purposes. The proposed system uses a source of protons with energies between 0.3 to 1.6 GeV focusing on liquid targets such as mercury (Hg), natural lead (Pb) and a eutectic lead-bismuth alloy (LBE). The dimensions of the spallation target (radius R and length L) were modified to better understand the role of these variables in neutron yield. The neutron flux on the surface of the targets is evaluated for different energies of the incident protons and different target geometries considering the results of the simulations in MCNPX. The results show a neutron multiplicity saturation limit for incident beam energy at 1 GeV. It is verified that there are optimal dimension values for the target radius and length, respectively, at R close to 15 cm and L close to 50 cm for incident proton energies of 1 GeV. The graphical distribution of the neutron flux is also shown using the software MATLAB.