CONDENSATION OF SUPERSATURATED He4 VAPOR IN A CLOUD CHAMBER

The critical supercooling, ΔTc, required to produce visible condensation in pressure-limited adiabatic expansions of presumably ion-free saturated He4 vapor has been measured in a small glass cloud chamber. The transient gas temperatures were measured during expansions by using a carbon resistance thermometer in a Wheatstone bridge. An oscilloscope was used in place of a galvanometer, and its trace was photographed during expansions. Low-amplitude temperature oscillations in the gas, which might not normally have been detected, were frequently observed in the early stages of this work. These oscillations either appeared spontaneously ("Taconis Resonances"), or could be shock-excited by an expansion. These oscillations were subsequently eliminated. Condensation thresholds were then measured using starting temperatures from 4.2 to 1.7 °K. ΔTc dropped from about 50 to 60 mdeg above the λ point to less than 20 mdeg below the λ point. The critical supersaturations required to produce condensation were thus always less than 105%. The supersaturations were calculated without making the usual, but highly implausible, assumption that the expanding gas is ideal. Assuming that the condensation nuclei are embryonic droplets arising accidentally from density fluctuations in the supersaturated vapor, the critical droplet radius is found to be about 10−6 cm above 2.5 °K, although the theoretical treatment here is not rigorous.