  The double neutron/proton ratio of nucleon emissions taken from two reaction
systems using four isotopes of the same element, namely, the neutron/proton
ratio in the neutron-rich system over that in the more symmetric system, has
the advantage of reducing systematically the influence of the Coulomb force and
the normally poor efficiencies of detecting low energy neutrons. The double
ratio thus suffers less systematic errors. Within the IBUU04 transport model
the double neutron/proton ratio is shown to have about the same sensitivity to
the density dependence of nuclear symmetry energy as the single neutron/proton
ratio in the neutron-rich system involved. The double neutron/proton ratio is
therefore more useful for further constraining the symmetry energy of
neutron-rich matter.
