THE EVAPORATION SIGNAL FROM ALPHA-PARTICLES STOPPED IN SUPERFLUID-HELIUM

Alpha particles stopped in a 3 liter volume of liquid helium at 30 mK are observed by the calorimetric detection of helium atoms evaporated from the free surface of the liquid. Quantum evaporation of the helium is produced by the rotons that are created by the alpha particle. While the energy spectrum of the 5.5 MeV alpha's from the Am-241 source has a width of less than 0.5%, the energy distribution of the observed evaporation signals extends from the low enery threshold of several keV up to a maximum of several 100 keV, depending on geometrical factors and the collection area of the calorimeter. The origin of the observed distribution may result in part from the presence of the substrate and a dependence on the direction of the track of the alpha particle. A simple model of the generation of rotons by the alpha particle will be discussed.