Talso Chui

Specific heat of liquid helium in zero gravity very near the lambda point

We report the details and revised analysis of an experiment to measure the specific heat of helium with subnanokelvin temperature resolution near the lambda point. The measurements were made at the vapor pressure spanningthe region from 22 mK below the superfluid transition to 4 μK above. The experiment was performed in Earth orbit to reduce the rounding of the transition caused by gravitationally induced pressure gradients on Earth. Specific-heat measurements were made deep in the asymptotic region to within 2 nK of the transition. No evidence of rounding was found to this resolution. The optimum value of the critical exponent describing the specific-heat singularity was found to be α = -0.0127 ′ 0.0003. This is bracketed by two recent estimates based on renormalization-group techniques, but is slightly outside the range of the error of the most recent result. The ratio of the coefficients of the leading-order singularity on the two sides of the transition is A + /A - = 1.053′0.002, which agrees well with a recent estimate. By combining the specific-heat and superfluid density exponents a test of the Josephson scaling relation can be made. Excellent agreement is found based on high-precision measurements of the superfluid density made elsewhere. These results represent the most precise tests of theoretical predictions for critical phenomena to date.

Lambda point experiment in microgravity

Abstract 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 3.5cm 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 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 ≈2 nK from the lambda point.

The fluctuation-imposed limit for temperature measurement

AbstractIn experimental sciences, random processes often place a fundamental limit on the achievable measurement resolution. A well known example is the Johnson noise voltage across a resistor. In this paper, we describe observations of the spontaneous transfer of heat in two equilibrium systems: one consisting of a thermometer linked to a reservoir, the other consisting of two thermometers connected to each other and linked to a reservoir for the purpose of temperature stabilization. In the second system, we find anti-correlations between the temperature fluctuations of the two thermometers at intermediate frequencies. We also find that the low frequency temperature noise density of the thermometers, in units of

Low temperature engineering applied to lunar in-situ resource utilization

K/\sqrt {Hz}

SQUID-based high-resolution thermometer

, is given by

Heat capacity experiment for very high resolution tests of the theory of confined materials

\sqrt {4Rk_B T^2 }