Enthalpy—pressure (H-p) diagram of He3 in the range 1.0 K ≤ T ≤ 4.17 K and 0 ≤ p ≤ 6.5 atm and inversion curve for T ≤ 4.17 K

The enthalpy-pressure (H-p) diagram of He³ was determined by experiment. All important data for the operation of a continuous He³ refrigerator working on the Joule-Thomson principle can be derived from this diagram. In particular, the achievable refrigerating capacity at preselected operating conditions may be immediately ascertained. The inversion curve of He³ for T ? 4.17 K was determined from the H-p diagram and is in fair agreement with the data for T ? 4 K already known.     

Diffusion Monte Carlo Study of the Small Mixed 3He-4He Clusters

The ground state properties of helium mixed clusters ³He_2,3,4 ⁴He₂, ³He_2,3,4 ⁴He₃, ³He_2,3,4 ⁴He₄, ³He_2,3 ⁴He₅, consisting of up to eight helium atoms are studied using variational and diffusion Monte Carlo calculations. For clusters with three and four ³He atoms released-node diffusion Monte Carlo method is used. Our calculations show that, within errorbars, clusters ³He_3,4 ⁴He₂ have the same binding energy as the cluster ³He₂ ⁴He₂, and that ³He₃ ⁴He₃ has the same binding energy as ³He₂ ⁴He₃. The clusters ³He_3,4 ⁴He₂ and ³He₃ ⁴He₃ are in states in which one or two ³He atoms are far away from the rest of the system. Other considered clusters are bound. In particular, we have shown the stability of the cluster ³He₄ ⁴He₃, which was previously considered unstable. The calculations are performed using several different interatomic potentials and the conclusions concerning stability are insensitive to the particular form of the interaction potential. We compare our results with the recent experiment and other theoretical calculations.     

Nuclear refrigeration of liquid He3

A nuclear demagnetization cryostat in which liquid He³ has been successfully cooled below 0.7 mK is described. After a general discussion on nuclear refrigeration the essential parts of the apparatus and its operation and performance are explained. The important detail of how to measure the temperature of He³ is also described. The paper is concluded with suggestions for future improvements.     

He4 film flow suppressor for a dilution still and He3 purification

The design and construction of a still for a dilution refrigerator is described. This device yielded an He³He⁴ ratio of almost 300. In addition to its use in a dilution refrigerator the still has been used for He³ purification, where it proved capable of extracting a high proportion of He³ from gas with a large He⁴ contamination.     

A new superleak to remove He for UCN experiments

The production of superleaks to remove He³ in helium for UCN experiments is described. Using one of these superleaks, He³/He⁴ ratio was found to be less than 3 × 10⁻⁹ as indicated by the UCN storage lifetime.     

Continuous adsorption refrigerator for producing temperatures below 1 K

This paper describes an evaporation refrigerator in which the pumping of the vapour above liquid He³ or He⁴ was accomplished with two alternately operating adsorption pumps. In this way temperature can be maintained constant with an accuracy to ～0.003 K for a long time; ～ 10 l helium vapour being used. The refrigeration capacity of the apparatus has been determined.The characteristics of the dilution refrigerator (namely, the rate of He³ circulation, the starting and operating temperatures) have been calculated for He³ circulation using the adsorption pumps and pumping line.This system is shown to be successful for producing very low temperatures at an He³ circulation rate of ～ 10^?5?10^?4 mole s^?1.     

An apparatus for preparing isotopically pure He4

A cryostat has been constructed for extracting pure He⁴ from He II of natural isotopic composition by means of a heat flush technique. It is shown that the method is in principle capable of yielding He⁴ which is entirely devoid of He³ isotopic impurities. A secondary heat flush, operated in conjunction with a conical heat exchanger of novel design, was used to place an experimental lower bound of 2 × 10¹⁵ on the $\text{He}{}^4\text{He}{}^3$ ratio of the product, a standard sufficient for all present or projected applications requiring isotopically pure He⁴. The apparatus produces 2 000 1 (at standard temperature and pressure) of He⁴ per thermal cycle. The design of future purifiers is discussed.     

Low-temperature heat capacity of helium films adsorbed on copper

Heat capacity measurements are described for films of pure⁴ He and a 12%He ³ mixture ofHe ³ andHe ⁴He adsorbed on copper, for 0.1<T<1.2 K.He ⁴ heat capacity isotherms show a step-type behavior as a function of coverage for the first two layers, while the mixture isotherms only show the first step. A comparison is made with recent multilayer data for helium films on Vycor published by other authors.Work supported by the National Science Foundation.     

Dynamic Surface Properties of Liquid 3He Probed by Surface State Electrons

Two-dimensional (2D) electrons floating on the liquid He surface is a powerful tool to study the dynamic surface properties of both normal and superfluid ³ He. From the mobility measurement an evidence is found for the existence of ripplons on liquid ³ He at least above 140 mK. When the 2D electrons form a Wigner solid (WS) at low temperatures, they are accompanied with surface depression under each lattice site. Low frequency mobility of the WS probes the scattering of quasiparticles from the surface depression. The damping of high frequency magnetophonon resonances of the WS is dominated by the scattering of electrons from surface excitations. A comparative measurement on both ³ He and ⁴ He shows a prominent difference in the surface properties of ³ He from ⁴ He below 70 mK.     

Anomalous cooling power of dilution refrigerators and the heat of transport of He3He4 solutions

The experimental cooling power of a dilution refrigeration employing two mixing chambers was found to exceed the theoretical cooling power by as much as 50%; with a single mixing chamber the experimental cooling power exceeded the theoretical value by 15%. This excess cooling power is explained by assuming that the He³ flow is partially diffusive. Information concerning the thermal diffusion ratio k_T and the He³ relaxation time in a He³He⁴ solution is extracted from the data.     

Rotating nuclear demagnetization refrigerator for experiments on superfluid he3

A nuclear demagnetization cryostat, capable of producing submillikelvin temperatures during rotational motion, was constructed. The apparatus has been used for NMR experiments on superfluid He³ at angular velocities up to 1.5 rad s^?1. The essential technical details and the operation of the cryostat are described.     

Properties of sintered silver powders and their application in heat exchangers at millikelvin temperatures

We present a systematic study of the properties of sintered silver powders used in heat exchangers at millikelvin temperatures. Scanning electron microscopy and surface area (BET) measurements have been performed on various powders with different degrees of sintering. Values of the Kapitza resistance with pure He³ and He³—He⁴ mixtures are given and compared with previously reported results.     

Compressional cooling of He3 to millidegree temperatures

A study of adiabatic compressional cooling of a two-phase mixture of solid and liquid He ³ has been made. Details of the experimental arrangement are given. Using a flexible elastic low temperature He ³ cell that is compressed by liquid He ⁴ and precooled by a continuously operating dilution refrigerator, cooling to below 2 m°K was obtained from starting temperatures near 20 m°K. Frictional heating is remarkably low. Calculations are presented which investigate the fraction of solid formed and which suggest rather long thermal equilibrium times in the solid He ³ nuclear spin system. Application of the method to the production of bulk polarized He ³ nuclei is discussed.This research was supported by the U.S. Atomic Energy Commission under contract AT(11-1)-34, P.A. 143.This work is based on the thesis of R. T. Johnson, submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Illinois.     

Anomalous cooling power of dilution refrigerators and the heat of transport of He---He solutions

The experimental cooling power of a dilution refrigeration employing two mixing chambers was found to exceed the theoretical cooling power by as much as 50%; with a single mixing chamber the experimental cooling power exceeded the theoretical value by 15%. This excess cooling power is explained by assuming that the He³ flow is partially diffusive. Information concerning the thermal diffusion ratio k_T and the He³ relaxation time in a He³---He⁴ solution is extracted from the data.     

Leung-Griffiths model for the thermodynamic properties of mixtures of CO2 and C2H6 near the gas-liquid critical line

Leung and Griffiths have proposed a fundamental thermodynamic equation for a binary mixture near the critical line and have successfully applied it to the mixture of He³ and He⁴ in which the critical line is a nearly linear function of composition. We have used a Leung-Griffiths type equation of state to describe the thermodynamic properties of the mixture of carbon dioxide and ethane. The critical line of this mixture is, unlike that of He³ and He⁴, a nonlinear function of composition, and the azeotropic line extends to the critical line. Comparison of the predictions of the equation to experimental data shows a good agreement for the mixture of CO₂ and C₂H₆.     

A Pomeranchuk cell which is precooled and pressurized by a He4-cycling He3 - He4 dilution refrigerator

This paper deals with a special Pomeranchuk cooling device which is used in combination with a He⁴-cycling He³-He⁴ dilution refrigerator at high pressures. The lowest temperature obtained was 1.1 mK.     

A cryostat for investigating inelastic scattering of neutrons on liquid He4 between 4.2 and 0.5 K

A cryostat is described which is designed for the research of inelastic scattering of neutrons on a large volume (3.5 l) of He⁴ between 4.2 and 0.5 K. Temperatures lower than 1.3 K are produced by means of pumping out vapour above liquid He³ (～ 330 cm³) with an adsorption pump (1 kg of coal CKT-2) located in the device itself. A minimum temperature is held during 140 h. The essential time for the cryostat start is about 10 h. The cryostat was used for the measurement of temperature dependence of the Bose-condensate density in He⁴.     

Low temperature heat exchanger for an He3He4 dilution refrigerator

The design and techniques for constructing a step-heat exchanger using sintered copper powder are reported. The results of applying it to a He³He⁴ combined dilution refrigerator are presented. The performance of the mixing chamber in a single-cycle mode is considered.     

Simultaneous measurements of heat capacity and pressure coefficient of liquid He4 and He3 - He4 mixtures

An apparatus for simultaneous measurements of heat capacities and pressure coefficients was designed and constructed for experimental studies of critical phenomena near the lambda points of liquid He⁴ and He³ - He⁴ mixtures.The pressure was measured with a capacitance pressure sensor involving a copper-beryllium thin plate (0.4 mm thickness). An oscillator circuit, including the capacitance pressure sensor, and driven by a tunnel diode was attached to the bottom of the calorimeter. The resolutions of temperature and pressure in this apparatus are ～ 2 × 10^?7 K and ～2 × 10^?6 bar (～2 × 10^?1 N m^?2) respectively.