  We have studied magnetic excitations in the dimerized spin-Peierls phase of
CuGeO3, by high resolution inelastic neutron scattering. We measured the
well-defined spin triplet dispersive mode which is gapped throughout the whole
Brillouin zone. We also observed that this mode is separated by an unexpected
second gap of order 2 meV from a continuum of magnetic excitations extending to
higher energy. The first gap (or 'triplet gap') and its associated dispersive
mode is due to the breaking of a singlet dimer into a delocalized triplet. We
propose that the second gap (or 'solitonic gap') and the continuum correspond
to dissociation of that triplet into two unbound spin-1/2 solitons that are
separated by a dimerized region of arbitrary length.
