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

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.