LAMBDA-POINT EXPERIMENT IN MICROGRAVITY

In October 1992 a low temperature experiment was flown on the Space Shuttle in low earth orbit, using the JPL low temperature reserarch facility. The objective of the mission was to measure the heat capacity and thermal relaxation of helium very close to the lambda point with the smearing effect of gravity removed. We describe the experiment with emphasis on the high resolution thermometry and the thermal control system. We also report preliminary results from the measurements made during the flight and compare them with related measurements performed on the ground. The sample was a sphere 3.5 cm in diameter contained within a copper calorimeter of very high thermal conductivity. The calorimeter was attached to a pair of paramagnetic salt thermometers with noise levels in the 10(-10) K range. During the mission we found that the resolution of the thermometers was degraded somewhat, due to the impact of charged particles. This effect limited the useful resolution of the measurements to almost-equal-to 2 nK from the lambda point.