  We measured the Josephson radiation emitted by an InSb semiconductor nanowire
junction utilizing photon assisted quasiparticle tunneling in an AC-coupled
superconducting tunnel junction. We quantify the action of the local microwave
environment by evaluating the frequency dependence of the inelastic Cooper-pair
tunneling of the nanowire junction and find the zero frequency impedance
$Z(0)=492\,\Omega$ with a cutoff frequency of $f_0=33.1\,$GHz. We extract a
circuit coupling efficiency of $\eta\approx 0.1$ and a detector quantum
efficiency approaching unity in the high frequency limit. In addition to the
Josephson radiation, we identify a shot-noise contribution with a Fano factor
$F\approx1$, consistently with the presence of single electron states in the
nanowire channel.
