For transmutation systems based on externally driven sub-critical assemblies with a fast neutron spectrum, there is an incentive to expose the actinides directly to the source neutrons, since these neutrons have higher energies than the fission neutrons. To evaluate the transmutation effectiveness of such systems, a parameter study based on the PHOENIX system, i.e. a sodium-cooled system with a minor actinide (MA) oxide fuelled target was performed. An interesting result is that the high-energy source neutrons give rise to a 20-25% increase in the fission-to-capture ratio of the important (fissionable) nuclides. Moreover, the performance of such a system can be further improved by substituting the oxide fuel by metal fuel and by reducing the volume fraction of steel in the target. Replacing the liquid sodium coolant by liquid lead has only a small effect on the fission-to-capture ratio, however, for a given proton current, the neutron production in the target increases. 17 refs., 5 tabs., 2 figs.