1997 年 32 巻 6 号 p. 257-262
The investigation of the cooling process in the vicinity of λ-transition in liquid helium is of crucial importance for cryogenic engineering. In spite of the 2nd order transition, when the temperature crosses downward over the λ-point, a He I-He II interface appears. In this study, the dynamic behavior of a He I-He II phase boundary was experimentally investigated. When He I under the condition of saturated vapor pressure is suddenly cooled from an initial state at a temperature slightly higher than Tλ by vapor evacuation, an interface appears and propagates downward from the liquid helium-free surface in a cryostat. We investigated the interface behavior by measuring the temperature with a superconductive thin-film temperature sensor. It was confirmed that the He I-He II interface propagates downward at a speed of several centimeters per second in direct proportion to the cooling rate and that the interface speed decreases as the distance from the free surface increases. A preliminary theoretical approach for the phenomena is also discussed briefly. It was confirmed that a modified Stefan problem can be a fundamental model for the construction of the theory.