DIRECT MEASUREMENT OF THE ANDREEV REFLECTION OF A BEAM OF EXCITATIONSIN SUPERFLUID HE-3-B

We present a direct quantitative measurement of the Andreev reflection of a beam of quasiparticle excitations in the B-phase of superfluid H e-3 in the ballistic regime. The beam, generated by a black-body radia tor (which also acts as the detector), is partially Andreev reflected by a superflow gradient produced by a vibrating paddle located in fron t of the radiator hole. The retroreflective nature of the Andreev proc ess ensures that reflected quasiparticles are returned to the radiator whereas normally scattered ones are not. The Andreev reflected compon ent of the beam is detected as an increase in the quasiparticle densit y within the radiator over the value measured with no superflow.