Measurements Of He Research Articles

Sound velocity measurement of nuclear-ordered U2D2 solid 3He along the melting curve

The sound velocity of a single-domain 3He crystal was measured in the nuclear-ordered low-field phase and the paramagnetic phase along the melting curve, using 10.98 MHz longitudinal sound. The temperature dependence of the sound velocity along the melting curve was explained by a nuclear spin contribution and the molar volume change along the melting curve. By comparing the measured velocity with thermodynamic quantities, we extracted the Grüneizen constant for the exchange energy. The anisotropy of the velocity in the ordered phase was investigated for three samples and was found to be smaller than 2×10 −5 in Δ v/ v. The attenuation coefficient of the sound was much smaller than 0.2 cm −1 .

Fluctuation effects in spin diffusion measurements in liquid 3He

We have measured spin diffusion coefficients of liquid 3He at a frequency of 920 kHz by pulsed NMR. By analyzing our data in the framework of the Leggett–Rice theory we got a spin diffusion coefficient and a Leggett–Rice parameter α= λωτ simultaneously at each temperature. On approaching the superfluid transition the spin diffusion coefficients showed a deviation from predictions of the Fermi liquid theory. The deviation at low pressure was larger than that at high pressure. This anomaly may be due to the effects of fluctuations of superfluidity which were recently observed in the viscosity measurement of liquid 3He.

Stopping cross-section measurements of 4He in TiN 1.1O 0.27

The stopping cross-section for 4He projectiles in TiN x compounds has been measured using the backscattering method. A multi-compound marker layer deposited between the test film and the substrate was used to obtain the stopping cross-section at several energies with one energy of the incident beam. Two RBS spectra at definite tilt angles of the sample are taken for each beam energy. The assistance of computer codes to synthesize RBS spectra is very useful to obtain the pertinent information from the displacements of the peaks of the marker layers. Stopping cross-section values are obtained with an estimated uncertainty of about 6%.

A Preliminary Heat Capacity Measurement of 3He in Aerogel near its Superfluid Transition

We report simultaneous heat capacity and torsional oscillator measurement of 3 He in aerogel near the superfluid transition. The heat capacity has a peak at the temperature T c where the torsional oscillator shows the onset of superfluid decoupling. The coincidence of these signatures suggests that 3 He in aerogel does undergo a true thermodynamic transition.

Review of Big Bang Nucleosynthesis and Primordial Abundances

Review of Big Bang Nucleosynthesis and Primordial Abundances

Variational density matrix approach to the temperature-dependent elementary excitation spectrum of two-dimensional liquid4He

Using the variational density matrix method, we obtain a temperature-dependent elementary excitation spectrum for two-dimensional liquid4He. For more precise results, we use a Jastrow-Feenberg-type trial wave function and include the contribution of elementary diagrams within the hypernetted chain approximation. The behavior of the excitation spectrum as a function of the temperature and density in two dimensions is similar to that of the bulk system, but has a smaller roton minimum. The roton minimum of the excitation spectrum decreases with increasing temperature and increases with increasing density at low densities but decreases at large densities. The results agree well with Monte Carlo calculations and are closer than pevious theories to experimental measurements of4He film adsorbed on substrates.

Effect of gravity on the thermal conductivity measurement of4He nearT λ

Precision measurements near the superfluid (λ) transition of4He is limited by a gravity induced inhomogeneity inT λ. Critical Dynamics in Microgravity (DYNAMX, a NASA preproject) will study the thermal conductivity in the nonlinear region in Microgravity. Unlike the study of static properties, the thermal conductivity can be derived from the measurement of the spatial temperature profile which is essentially alocal measurement of the temperature gradient. We present a method which, in principle, fully removes the apparent impact of gravity from the measurement. The inevitably finite spatial resolution of the probe and temperature resolution of the thermometer, however, make microgravity essential to study the region of interest.

Determination of the thermodynamic phases of4He adsorbed on H2 plated graphite

Thermodynamic measurements have been used to investigate the surface phases of monolayer4He adsorbed on graphite plated with 1.45, 2.15, and 3.15 layers of H2. In each case the4He preferentially adsorbed on the lower completed layer. In this paper we report the measurements of4He on the 1.45 layer H2 graphite plating. For this plating the4He goes into a condensed phase below 0.87 K. The high temperature-low density film entropy has been calculated.

Calorimetric and nuclear byproduct measurements in electrochemical confinement of deuterium in palladium

Abstract We present the results of a new experiment with our multicell set-up implemented with mass spectrometric measurements of4He and a highly improved neutron detector. The excess heat measured is comparable with the results of other laboratories, but no neutrons were found and the tritium excess was lower than expected from the power excess. 4He has been measured in the electrolysis gases and a tentative correlation of 4He with excess power is presented and discussed.

3He film flow on a round rim beaker

The superfluid properties of thin (100–150 nm) of3He were investigated by measuring the rate at which a beaker of liquid3He emptied itself through the adsorbed film, with the film thickness δ decreasing as the level dropped. A beaker rim with a semicircular cross-section was used to provide a well defined geometry and to avoid the effects of small scratches that may have affected earlier experiments. The film thicknesses were determined by Atkins' oscillaton measurements of4He films on the same surface. The superfluid transition temperature in the filmTcF was suppressed below the bulk valueTcB, and was close to being described by 2δ/ξ(TcF) = π, as expected for A-phase. The critical current density was more than an order of magnitude smaller than expected for pair-breaking. When a4He monolayer was adsorbed on the substrate, there was no suppresson ofTcF.

Viscous measurement of superfluid 3He in high magnetic field

Viscosity of superfluid 3He was investigated in a high magnetic field of 12 Tesla with a vibrating wire viscometer. A sharp decrease was observed at A 1 and A 2 transitions.

Effects of 4He coverage for the boundary condition of superfluid 3He.

We present measurements of 4He coating effects on the boundary wall of superfluid 3He in confined geometries for four different pore sizes. Using the fourth sound technique, the systematic study has been carried out for both the superfluid density and the Q-value of the resonance signal.

Determination of Lithium in 13 Geochemical Reference Materials by Thermal‐Neutron Capture and Helium‐3 Mass Spectrometry

Thirteen geochemical reference standards have been analysed for lithium by a technique originally developed for assay of lithium in biological materials. The technique consists of thermal neutron irradiation, then measurement of 3He from decay of 3H produced via the reaction 6Li(n, α)3H. The results are compared with literature values.

Superfluid transition of 4He confined in porous glasses

Recent results of superfluid density and heat capacity measurements of 4He confined in porous Vycor and aerogel glasses of very high porosity are reviewed. These measurements support the interpretation that the superfluid transition of 4He confined in Vycor belongs to the same universality class as that of bulk 4He. The superfluid density of 4He in aerogel glass can be described by a simpel power law to a reduced temperature of 10 -4 K; however, the resultant critical exponent and heat capacity behavior are found to be distinctly different from that of bulk.

Comment on heat-capacity and sticking measurements of 4He adsorbed on evaporated Ag films: Bose statistics in two dimentions.

A Comment on the Letter by T. W. Kenny and P. L. Richards, Phys. Rev. Lett. 64, 2386 (1990).

NMR measurements of 4He in thin 3He4He mixture films

Evolution of 3 He from a 2-d Fermi liquid to bulk-like liquid on thin 3 He 4 He mixture films shows stepped structure in magnetization and NMR relaxation times as a function of 3 He coverage d 3 . Spin-echo measurements in a weakly polarizing field over the temperature range 30 < T < 250 mK for 0.01 < d 3 < 4 layers are compared at three 4 He coverages d 4 = 1.77, 2.14, 2.91 atomic layers.

Determination of 3He with a tandetron accelerator at 0.8 MV

A measurement of 3He in natural gas was carried out using a 1 MV tandetron accelerator at 0.8 MV terminal voltage and an rf ion source. To eliminate interfering backgrounds of H 3, HD and 3H, helium ions in charge state 2+ were selected. The 3He/ 4He ratio in the helium gas was measured to be 1.4 × 10 −7. The background originating from 4He 2+ ions through charge changing processes in the residual gas of the accelerator and scattering in the switching magnet was removed completely with an Al absorber in front of the detector. The sensitivity for determining the 3He/ 4He ratio was better than 10 −10.

Helium in soil gas: a method of mapping groundwater circulation systems in fractured plutonic rock

The Carnmenellis granite is one of seven Variscan plutons exposed in SW England which intrude a folded and thrusted succession of metasedimentary and metavolcanic rocks of Devonian and Carboniferous age. The granite contains higher than average radionuclide concentrations which give rise to enhanced heat flow and fracture-controlled, low enthalpy hydrothermal circulation. Measurements of 4He in soil gas have been made at 91 sites over the pluton and the surrounding host rocks, in order to assess variation of 4He emanation in relation to the distribution of radionuclides and the groundwater circulation system. The pattern of variation obtained shows little correlation with the results of an airborne total gamma-ray activity survey, but agrees well with the distribution of 4He in spring waters. Because the occurrence of zones of high 4He concentration in these spring waters shows a significant correlation with local variations of heat flow within the pluton, it is believed that the distribution of upwelling limbs of groundwater circulation cells controls the groundwater 4He flux. The spatial association of high values of 4He in soil gas with high values in spring waters thus suggests that fracture-controlled groundwater circulation systems can be mapped using the soil gas method, even in the absence of springs. However, the variation of 4He concentrations in soil gas is more complex than that given by spring waters. This is due to variation in soil moisture content affecting the pore space concentration of 4He. Nevertheless, resolution of this effect is possible by statistical analysis of the 4He distribution.

Analysis of ultratrace lithium and boron by neutron activation and mass-spectrometric measurement of 3He and 4He

A new technique for analysis of lithium and boron at ultratrace concentrations (<10 −8g g −1) is described. The method consists of mass-spectrometric assay of 3He from decay of tritium produced by thermal-neutron reaction on 6Li, and 4He produced by thermal-neutron reaction on 10B. Two neutron-irradiation facilities were used: the McMaster reactor, which is 235U-enriched and light-water moderated; and a graphite-moderated thermal column attached to the 235U-enriched, heavy-water-moderated core at the National Bureau of Standards (NBS) reactor. In the McMaster irradiations, fast neutrons (>0.2 MeV) induce the reactions 14N(n, 3H) 12C, 12C(n, α) 9Be, 16O(n, α) 13C, and 14N(n, α) 11B. These reactions become serious sources of error in samples such as human blood which have very low concentrations of lithium and boron, and high concentrations of nitrogen, carbon and oxygen. In the NBS thermal column, fast-neutron reactions are virtually absent, and only corrections for thermal-neutron capture by deuterium, and thermal-neutron (n, α) reactions on oxygen, sulfur, chlorine, potassium, and calcium need to be taken into account. Results are presented for various actual samples including human blood and its components, and some standards biological reference materials, to provide a realistic base for other workers to judge the reliability of the method.

Ternary fission involving4He emission in the16O (144 MeV)+232Th and12C (108 MeV)+197Au reactions

The results of coincidence measurements of4He emission with fission fragments in reactions of12C(108 MeV) ions with a197Au target and of16O(144 MeV) ions with a232Th target are presented. On the basis of a Monte Carlo kinematic simulation of nuclear reactions the experimental energy spectra and velocity distributions of alpha particles have been analyzed. A conclusion has been drawn that the main source of4 He emission is evaporation from the fissioning compound nucleus. Substantial part of alpha particles was emitted from fully accelerated fission fragments. Some of4He nuclei with an average energy of about 16 MeV (in the CM system) emitted mainly perpendicular to the fission axis were identified as being similar long-range alpha particles produced in ternary fission of heavy nuclei at a low excitation energy. The emission multiplicities of these particles are considerably higher than those observed at a low excitation energy. The experimental results are compared with the statistical model predictions.