Blackbody source and detector of ballistic quasiparticles inB3: Emission angle from a wire moving at supercritical velocity

Using a new quasiparticle detector and source consisting of a blackbody radiator, we have made the first measurements of the divergence of a beam of quasiparticles produced by a vibrating-wire resonator moving at supercritical velocities in superfluid ${}_{}{}^3{}_{}{}^{}\mathit{B}$ at the zero-temperature limit. At velocities just above the critical velocity of Δ/3${\mathit{p}}_{\mathit{F}}$ we see a very narrow beam directed along the direction of motion of the wire. As the maximum wire velocity increases the beam spreads to saturate at a half-angle divergence of approximately 55° at a maximum velocity of order Δ/${\mathit{p}}_{\mathit{F}}$.