Direct observation of the Andreev reflection of a beam of excitations in superfluidB3

We describe the first direct measurement of the Andreev reflection of a beam of quasiparticle excitations in the B phase of superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ in the ballistic regime (T/${\mathit{T}}_{\mathit{c}}$≤0.2). The beam is generated by a blackbody radiator. The changing effective excitation gap which gives rise to the Andreev reflection is provided by a superflow gradient along the direction of the beam induced by a moving paddle. The component of the beam reflected by Andreev processes is observed via the retroreflective nature of the process which returns the reflected excitations to the blackbody and thereby increases its temperature over the value observed with no superflow.