  In this paper we examine an alternative method for determining the neutron
multiplication factor keff in fast subcritical reactors, based on the
observable obtained by calculating the ratio between the ``fast`` component of
the neutron flux (in this case by fast we mean neutrons with kinetic energy
above 0.5 MeV) and the whole flux at all energies. The assumption behind the
present study was that the above mentioned observable would be only sensitive
to keff, provided a peripheral fuel rod is considered (i.e. far from the
neutron source in subcritical systems) and keff is sufficiently high. Indeed,
our results show that, for keff> 0.7, the ratio calculated for LFR or GFR
models turns out to show a quite similar trend well approximated by a straight
line, while a different slope is observed for a sodium reactor. In our opinion,
the results obtained show that such observable, after appropriate calibrations
on the real systems under consideration, may be used for determining keff in
all operational conditions, in particular for subcritical systems with
different types of neutron source and different values of the beam current.
