We investigate the topological properties of the chaotic invariant set asso
ciated with the dynamics of scattering systems with three or more degrees o
f freedom. We show that the separation of one degree of freedom from the re
st in the asymptotic regime, a common property in a large class of scatteri
ng models, defines a gate which is a dynamical object with phase space sepa
rating invariant manifolds. The manifolds form an invariant set causing sin
gularities in the scattering process. The codimension one property of the m
anifolds ensures that the fractal structure of the invariant set can be stu
died by scattering functions defined over simple one-dimensional families o
f initial conditions as usually done in two-degree-of-freedom scattering pr
oblems. It is found that the fractal dimension of the invariant set is not
due to the gates but to interior hyperbolic periodic orbits.