  In this paper we present the experimental realization of a Nb tunnel junction
connected to a high-gap superconducting NbTiN embedding circuit. We investigate
relaxation of nonequilibrium quasiparticles in a small volume Au layer between
the Nb tunnel junction and the NbTiN circuit. We find a saturation in the
effective heat-transfer coefficient consistent with a simple theoretical model.
This saturation is determined by the thickness of the Au layer. Our findings
are important for the design of the ideal Au energy relaxation layer for
practical SIS heterodyne mixers and we suggest two geometries, one, using a
circular Au layer and, two, using a half-circular Au layer. Our work is
concluded with an outlook of our future experiments.
