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.     

An improved He3-He4 mixing chamber for single-cycle experiments.

A new type of mixing chamber is described with which any dilution refrigerator, capable of reaching 30 mK continuously, can now attain temperatures below 3 mK when operating in the single-cycle mode.In a preliminary experiment this mixing chamber used in a dilution refrigerator with an additional superfluid He⁴ circulation of 2 × 10^?3 moles s^?1 reached a temperature of 4 mK continuously.     

Measurements of Torsional Oscillations and Thermal Conductivity in Solid 4He

Polycrystalline samples of hcp ⁴He of molar volume 19.5?cm³ with small amount of ³He impurities were grown in an annular container by the blocked-capillary method. Three concentrations of ³He, x ₃, were studied: isotopically purified ⁴He with the estimated x ₃≤10^?10, ‘well-grade’ helium with x ₃～3×10^?7 and a specially prepared mixture with x ₃=2.5×10^?6. The torsional oscillator response and thermal conductivity were investigated before and after annealing. The temperature and width of the torsional anomaly increase with increasing x ₃. Annealing resulted in an increased phonon mean free path but often in little change in the torsional oscillator response. While the magnitude of the torsional anomaly and phonon mean free path can be very different in different samples, no correlation was found between them; this implies that these two properties are controlled by different types of crystal defects. It seems plausible that the mean free path of thermal phonos at ～200?mK is controlled by vibrating dislocations while the magnitude of the frequency shift of torsional oscillations is governed by static defects such as pinned dislocations and grain boundaries.     

The Annealing Process in Solid 4He

We have used a torsional oscillator with square cross section and a resonance frequency of 185 Hz to confirm the nonclassical rotational inertia (NCRI) discovered by Kim and Chan (Nature 427:225, 2004; Science 305:1941, 2004). We have also found a strong correlation between the NCRI signal and a high dissipation Q ^?1 of 4×10^?6 of the oscillation above the transition temperature. Here, we present preliminary results of the annealing process in ⁴He at a pressure of 26 bar. When holding the temperature constant above 1 K we have observed a immediate rise in the period and a slow decay of the dissipation. The equilibrium value of Q ^?1 decreases with increasing temperature.     

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.     

Using anisotropic heat flux sensors in aerodynamic experiments

We present the results of comparative measurements of the heat flux to a flat plate in a supersonic flow at a Mach number of M = 6, which were performed using the two following anisotropic heat sensors with different thicknesses of sensor elements: (i) Atomic Layer Thermo Pile (ALTP, Fortech GmbH, Germany) with a thickness of ～0.5 × 10^?6 m and (ii) gradient heat flux sensor (GHFS, St. Petersburg State Polytechnic University, Russia) with a thickness of ～2 × 10^?4 m. The ALTP sensor can be used for directly measuring heat fluxes in processes with a characteristic time above 10^?6 s. A method for mathematically processing the GHFS response signal is proposed that allows heat flux oscillations to be revealed in gasdynamic process with a characteristic time on the order of 10^?4 s.     

Superfluidity of solid4He

Using a procedure suggested by Leggett, an upper bound to the superfluid fraction in ground state solid ⁴ He slightly above the melting density is obtained numerically. The value obtained is 0.3±0.1. To judge the usefulness of this upper bound, we examine the conditions under which a symmetrized product of single-particle functions times a Jastrow function exhibits ODLRO, a necessary and sufficient condition for superfluid flow. It is found that ifU _ij (U _ij=? φ _i (x)φ _j (x) dx, and φ _i (x) is a single-particle wave function centered on the pointi) satisfies σ′_i U _ij>x, wherex varies from unity for long rangeU _ij (i.e.,U _ij decreases slowly enough asR _i?R_j increases) to a value of 12/7 for nearest-neighbor overlap only in the hcp lattice, then there is ODLRO, but not otherwise. Therefore, if the accepted single-particle functions are the true ones, then there is no ODLRO in solid ⁴ He, since the overlap is too small. We have explored the possibility of adding a flat tail, of magnitude λ′(VN)^?1/2 to the accepted single-particle functions. It is shown that if λ → 1 [λ ² =(λ′) ² +2(vNV ^?1)^1/2, andv=(? φ_i(x)dx)²], the system wave function becomes a pure Jastrow function, whereas if λ²?1??2×10 ^?1 , we have in effect the case where λ′=0; furthermore, there is ODLRO if λ ² ?1～?2×10 ^?1 . It is also concluded that the superfluid fraction upper bound of 0.3±0.1 obtained here as well as one suggested by Leggett are not very useful. We have not attempted to establish if there is some value of λ satisfying the above inequality such that the ground-state energy is lower than the value it takes for λ′=0.     

The detection sensitivity of heavy impurities in Si using 280 keV He2+ and C2+ back-scattering

Background limitations, especially those involving pulse pile-up, on the sensitivity to detection of heavy elements near the surface of Si have been investigated with back-scattered 280 keV He²⁺ and C²⁺. Using gated electronics, pulse pile-up was reduced by as much as a factor of 1/60, which permitted a detection sensitivity, i.e. equal signal and background, of approximately 2×10¹¹ Tl atoms/cm² for He²⁺ back-scattering, while C²⁺ back-scattering data showed a detection sensitivity of about 2×10¹⁰ Tl atoms/cm². Back-scattering from Si implanted with 20 keV Tl⁺ to doses of 1×10¹³ and 1×10¹² ions/cm² was used for experimental confirmation of the He²⁺ and C²⁺ back-scattering sensitivities.     

Helium-4 isotopic purification without a superleak

We report the successful operation of a He⁴ isotopic purification cryostat which depends for its operation solely on the phenomenon of superfluid heat flush. The product He³ content was determined as being less than 3 × 10^?11.     

Adsorption of 4He N and 4He N 3He Clusters on Cesium

The ground-state properties of helium clusters ⁴He _N and ⁴He _N ³He, for N≤ 40, adsorbed on the surface of cesium are studied using variational and diffusion Monte Carlo calculations. Binding properties are determined using two different Cs–He interaction potentials. For the smallest clusters, self-binding on a Cs surface is stronger than in two or three dimensions. For N > 10 self-binding in three dimensions is stronger than on Cs for both types of Cs–He interaction potential considered. The obtained binding energies and structure are compared to results of recent density functional calculations. The emergence of edge states of ³He atom, localized along the contact line of ⁴He cluster with a cesium surface, is studied. First indication that ³He atom prefers to be close to the contact line appears already for the ⁴He₃ ³He cluster.     

16O contamination in 4He analysis beams

The ⁴He⁺ beam from a Van de Graaff accelerator is often accompanied by an ¹⁶O⁺ beam of the same energy. If, after acceleration and before magnetic analysis, one electron is stripped from the oxygen ion to form ¹⁶O²⁺, these ions will not be separated from ⁴He⁺ during magnetic analysis. The ¹⁶O²⁺ fraction in the ⁴He⁺ beam was measured by analyzing back-scattering spectra for a thin Au film and the O to He ion charge ratio of 2:1 was confirmed by electrostatic deflection. Ion source conditions and the pressure in the beam lines strongly affect the ¹⁶O²⁺ fraction and the dependence of the O²⁺/O⁺ ratio on pressure is found to be in approximate agreement with predicted values. A formula for estimating the ¹⁶O²⁺ beam intensity, based on charge exchange data and measurement of the primary ¹⁶O⁺ beam intensity, is N[O²⁺] = 13 P(torr) l(cm) N[O⁺], where P is the total gas pressure, l the length of the beam line between the accelerator and switching magnet and N[O²⁺] and N[O⁺] are the beam intensities of the respective species. The ¹⁶O²⁺ contamination of ⁴He⁺ beams is easily removed by electrostatic separation.     

Two-point calibration of standard capsule platinum resistance thermometers for the range 13.81 K to 273.15 K

A method of calibrating a standard capsule platinum resistance thermometer (prt) over the range 13.81 K to 273.15 K is described. Measurements of the resistance of the prt are needed at only two fixed points, the boiling point of He⁴ (4.2 K) and the ice-point (273.15 K), both of which are easy to realize. For a prt with a residual resistance ratio (R_{4.2 K}/R_{273.15 K}) of less than 4 × 10^?4, the method provides a calibration on ipts 68 with an uncertainty of 20 mK over the entire range 13.81 K to 273.15 K. For prts with residual resistance ratios between 4 × 10^?4 and 7 × 10^?4, the calibration uncertainty is 75 mK from 13.81 K to 40 K and 20 mK from 40 K to 273.15 K.     

Properties of TiNxfilms reactively sputtered in an argon-nitrogen atmosphere

The dependence of the resistivity and temperature coefficient of resistivity (TCR) of TiN_x films on nitrogen pressure is described. The partial nitrogen pressure was varied from 10^?5 Torr to 2×10^?4 Torr. The maximum value of the resistivity (216 μΩ cm) and the lowest negative value of TCR (?33ppmK^?1) were obtained in the nitrogen pressure range (2?4)×10^?5 Torr. The minimum value of the resistivity (44 μΩ cm) and the highest positive value of the TCR (1160 ppm K^-1 were obtained in the nitrogen pressure range (4?10)×10^?5 Torr. The influence of aging temperature up to 573 K on the resistance changes are shown. X-ray diffraction analysis indicated the presence of oriented or non-oriented TiN in these films.     

Vapour pressure thermometry in high magnetic fields using a Pirani gauge

Vapour pressure thermometry has the advantage of not being affected by magnetic fields. Therefore, a study was carried out of thermometry below 1 K using a Pirani pressure gauge in the vapour of He³. Its sensitivity and error in temperature measurements above 0.55 K are dV/dT ≥ K^?1 and 0.4% respectively.Additionally, a semiconductor pressure sensor in the vapour of He⁴ was examined for the vapour pressure thermometry.     

Simultaneous Absorptance and Thermal-Diffusivity Determination of Optical Components with Laser Calorimetry Technique

The laser calorimetry (LCA) technique is used to determine simultaneously the absorptances and thermal diffusivities of optical components. An accurate temperature model, in which both the finite thermal conductivity and the finite sample size are taken into account, is employed to fit the experimental temperature data measured with an LCA apparatus for a precise determination of the absorptance and thermal diffusivity via a multiparameter fitting procedure. The uniqueness issue of the multiparameter fitting is discussed in detail. Experimentally, highly reflective (HR) samples prepared with electron-beam evaporation on different substrates (BK7, fused silica, and Ge) are measured with LCA. For the HR-coated sample on a fused silica substrate, the absorptance is determined to be 15.4?ppm, which is close to the value of 17.6?ppm, determined with a simplified temperature model recommended in the international standard ISO11551. The thermal diffusivity is simultaneously determined via multiparameter fitting to be approximately 6.63?×?10^?7?m² · s^?1 with a corresponding square variance of 4.8?×?10^?4. The fitted thermal diffusivity is in reasonably good agreement with the literature value (7.5?×?10^?7?m² · s ^?1). Good agreement is also obtained for samples with BK7 and Ge substrates.     

NMR Studies of 3He Impurities in 4He in the Proposed Supersolid Phase

Preliminary results are reported for measurements of the NMR relaxation times of very dilute ³He in samples of solid ⁴He at low temperature, 0.25 K <T< 1.3 K. The results were obtained for carefully prepared samples with different ³He concentrations. The measurements of the spin-spin relaxation time, T ₂, show several interesting features. A temperature independent plateau attributed to the exchange motional narrowing is observed down to the lowest temperature studied, and the observed variation of T ₂ with ³He concentration favors the nonlinear theory suggested by Landesman. The best fit to the data is given by T ₂ ∝ x ₃ ^?1.89±0.1 rather than x ₃ ^?1 . No evidence of an exchange-phonon bottleneck for the spin-lattice relaxation is seen down to 25 mK. The vacancy activation energy is determined to be 13.5±0.3 K for a sample with x ₃=5×10^?4 and molar volume 20.9 cm³.     

Nickel diffusion in samarium sulfide

The diffusion of nickel atoms in samarium monosulfide (SmS) has been studied for the first time. Using the sequential layer removal technique, it was found that the coefficient of diffusion of the ⁶³Ni radioactive isotope at a temperature of T = 1050°C amounts to D ～ 1.8 × 10^?10 cm²/s in SmS single crystals and to 5.3 × 10^?9 and 1.2 × 10^?10 cm²/s for the fast and slow diffusion components, respectively, in polycrystalline SmS. The process of nickel diffusion in thin polycrystalline SmS films was studied using an X-ray diffraction technique. The coefficient of diffusion at T = 400°C in thin-film samples according to these data is ～10^?13 cm²/s.     

A combined 3He-4He dilution refrigerator

A refrigerator capable of operating both in evaporation and dilution modes is described. The refrigerator provides preliminary cooling of investigated samples down to a starting temperature of about 0.3 K and permits the location of samples directly in the mixing chamber as well as their replacement at any stage of operation.The replacement of samples lasts about 20 min. The cooling time for a sample from room temperature down to 15 mK is about 3 h. The minimal temperature is 10 mK, and the ³He circulation rate is about 5 × 10^?4mols^?1.     

Precise temperature control by a manostat utilizing a sheet of a thin film material

A new type of manostat has been developed, in which a thin-film material is utilized. It is possible to control the liquid He vapour pressure within 0.1 Torr (1.3 × 10 Nm^?2) in the range between 1 atm (1.01 × 10⁵Nm^?2) and 4 Torr (5.3 × 10²Nm^?2) by use of a thin polyethylene film sheet.