HEAT-CAPACITY AND THERMAL RELAXATION OF BULK HELIUM VERY NEAR THE LAMBDA POINT

In October 1992 a low temperature experiment was flown on the Space Sh uttle in a low earth orbit. The objective of the mission was to measur e the heat capacity and thermal conductivity of helium very close to t he lambda point with the smearing effect of gravity removed. We report preliminary results from the experiment, and compare them with relate d measurements performed on the ground. The sample was a sphere of hel ium 3.5 cm in diameter contained within a copper calorimeter of very h igh thermal conductivity. The calorimeter was attached to a pair of hi gh-resolution paramagnetic salt thermometers with noise levels in the 10(-10) K range and suspended from a high-stability thermal isolation system. During the mission we found that the resolution of the thermom eters was degraded somewhat due to the impact of charged particles. Th is effect limited the useful resolution of the measurements to about t wo nanokelvins from the lambda point. The results reported here are li mited to about ten nanokelvins from the transition.