Investigation of a New Kind of Energetic Neutral Excitation in Superfluid Helium

We present evidence for the existence of a new kind of neutral excitation generated in superfluid helium in the presence of a ${\mathrm{Po}}^{210}$ $\alpha$-particle source..The flux of neutral excitations is negligibly small above 0.6°K and increases as the temperature is decreased. The neutral excitations travel in the liquid at low temperatures through distances greater than 1 cm without appreciable scattering or attenuation. They produce ${\mathrm{He}}_2^{+}$ ions and electrons at the free surface of the liquid, and positive ions and electrons at a suitably arranged metal plate immersed in the liquid. Experiments indicate that these neutral excitations are not photons. A possible model for the neutral is some kind of exciton. In addition, we also present evidence that, at temperatures below 1°K, electrons within superfluid helium may emerge from the liquid into the vapor above its surface when they arrive at the liquid surface trapped in vortex rings. No such behavior is observed for positive ions.