Positive-ion mobility in dilute solutions of 4He in 3He

Abstract

The mobility of positive ions in essentially pure 3He and in solutions with 100, 300, 1000, 3000, and 10,000 ppm of 4He has been studied at pressures of 60 Torr, 10, 20, and 25.6 bar. For temperatures below about 100 mK the pureliquid mobility data are consistent with a ln (T 0/T) behavior except at the very lowest temperatures, where field-dependent effects may cause departures; both phenomena are in agreement with recent calculations of Bowley based on the Josephson-Lekner formalism. In the more concentrated solutions an increase in the ion radius (corresponding to a decrease in the mobility) is observed starting below ~200mK; this is interpreted as a polarization-potentialinduced phase separation resulting in a liquid 4He-rich “halo” about the solid ion core. At still lower temperatures a vanishing 4He content causes the solution data to rather abruptly rejoin the pure-liquid data. At still higher 4He concentrations additional very low-mobility species are observed which are likely ions with large metastable halos produced by a large 4He concentration at the field-emission tip where the ions are created.