Particle detection by evaporation from superfluid helium

We report the first experiments in which 5-MeV alpha particles are detected via evaporation from a bath of superfluid helium. The \ensuremath{\alpha} excites phonons and rotons in the liquid helium, and these excitations are sufficiently energetic to evaporate helium atoms when they reach the free surface of the liquid. The approximate overall efficiency of this process has been determined, and we compare this with expectations. We have also been able to detect evaporation induced by a flux of \ensuremath{\gamma}'s from a $^{137}\mathrm{Cs}$ source.