A Fast Pulsed Source of Ballistic R− Rotons in Superfluid 4He

Superfluid4He is unique in having well-defined excitations (R−rotons) with momentum oppositely directed to their velocity. If a beam of R−rotons can be produced, it could be unambiguously detected by quantum evaporation because the atoms will emerge in the opposite quadrant to that for atoms evaporated by R+rotons and phonons. Previous work shows that a heated metal film which is immersed in superfluid4He only creates phonons and R+rotons. A sponge-like heater does appear to produce R−rotons but, because it has a long time constant, it cannot be used in time of flight studies. We have developed a source that produces fast pulses of R−rotons suitable for time of flight measurements. The method uses interactions between R+rotons to create R−rotons, so a transient high density of R+rotons in a small confined volume is needed. The source appears to operate as we expect from a model of the evolution of the R+and R−roton populations.