HEAT CAPACITY AND THERMAL RELAXATION OF BULK HELIUM __ _/VERY NEAR THE LAMBDA POINTJ. A. Lipa*, D. R. Swanson*, J. A. Nissen*, and T. C. P. Chui +*Department of Physics, Stanford University, Stanford, CA.+Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA.ABSTRACTIn October 1992 a low temperature experiment was flown on the Space Shuttle in low Earth orbit.The objective of the mission was to measure the heat capacity and thermal conductivity of helium veryclose to the lambda point with the smearing effect of gravity removed. We report preliminary results fromthe experiment, and compare them with related measurements performed on the ground. The samplewas a sphere of helium 3.5 cm in diameter contained within a copper calorimeter of very high thermalconductivity. The calorimeter was attached to a pair of high resolution paramagnetic salt thermometerswith noise levels in the 10-10/( range and suspended from a high stability thermal isolation system.During the mission we found that the resolution of the thermometers was degraded somewhat due to theimpact of charged particles. This effect limited the useful resolution of the measurements to about twonanokelvins from the lambda point. The results reported here are limited to about ten nanokelvins fromthe transition.INTRODUCTIONSince the first high resolution measurements of the heat capacity singularity at the lambda point,this transition has become the most important testing ground for theories of second order phasetransitions. The transition is very sharp because of the strain-free nature of the fluid and its relatively lowcompressibility. With sample heights of the order of millimeters, values of t as small as 10-7 can bereached on Earth before intrinsic distortion is encountered. Here t = I1-T/T_,I is a dimensionlesstemperature parameter, and TX is the transition temperature. The ultimate limit in ground basedmeasurements is usually encountered when finite size effects 1 become appreciable, generally near t=5x10 "8. In space, the lambda transition may be sharp to t ~ 10"12 or so2, in optimal conditions. Since theearly 70's the aim of most high resolution experiments in this region has been to test the renormalizationgroup (RG) formalism developed by Wilson3 to predict the singular thermodynamic behavior observed inJoint "L+I" Science Review for USML-1 and USMP-1with the Microgravity Measurement Group, September22-24, 1993, Huntsville, Alabama, USA.5