A high-pressure mass spectrometric study of ion-molecule reactions in a mixture of CF4 and He

Gas phase ion-molecule reactions of positive He⁺ ions produced from helium with neutral carbon tetrafluoride have been studied. The measurements were made with a quadrupole mass spectrometer with a high-pressure ion source. The fractional abundance I_i/∑I_i of dominant ions CF₃⁺, CF₂⁺ and helium ions has been determined as a function of the (1% CF₄ and 99% He) gas mixture pressure (in the range 1.33-31.92 Pa) and of the repeller electrode potential (ranging from 3 to 10 V).     

The generation of particles to observe quantized vortex dynamics in superfluid helium

We describe a method to prepare a sample of superfluid helium-4 with hydrogen particles suspended within it. The method is to dilute hydrogen gas with helium at room temperature, and bubble the mixture through liquid helium at a temperature above the superfluid phase transition temperature, T_λ ≈ 2.17 K. The procedure yields a suspension of micron-sized particles whose total volume is about 10⁵ times smaller than the fluid volume. The fluid and suspension are then cooled to a temperature below T_λ. We show that the particles, so prepared in superfluid helium, are useful for studying superfluid flows and, in particular, the dynamics of quantized vortices. In addition, the particle-superfluid helium system is rich in not yet fully explained interactions. We review preliminary investigations that include observing the vortex lattice in rotating helium, vortex reconnection in quantized vortex turbulence, and vortex ring decay. These data illustrate the basic mechanisms of dissipation in superfluid turbulence.     

Using different drift gases to change separation factors (alpha) in ion mobility spectrometry

The use of different drift gases to alter separation factors (alpha) in ion mobility spectrometry has been demonstrated. The mobility of a series of low molecular weight compounds and three small peptides was determined in four different drift gases. The drift gases chosen were helium, argon, nitrogen, and carbon dioxide. These drift gases provide a range of polarizabilities and molecular weights. In all instances, the compounds showed the greatest mobility in helium and the lowest mobility in carbon dioxide; however the percentage change of mobility for each compound was different, effectively changing the alpha value. The alpha value changes were primarily due to differences in drift gas polarizability but were also influenced by the mass of the drift gas. In addition, gas-phase ion radii were calculated in each of the different drift gases. These radii were then plotted against drift gas polarizability producing linear plots with r2 values greater than 0.99. The intercept of these plots provides the gas-phase radius of an ion in a nonpolarizing environment, whereas the slope is indicative of the magnitude of the ion's mobility change related to polarizability. It therefore, should be possible to separate any two compounds that have different slopes with the appropriate drift gas.     

Optical and Electron Spin Resonance Studies of Destruction of Porous Structures Formed by Nitrogen–Rare Gas Nanoclusters in Bulk Superfluid Helium

We studied optical and electron spin resonance spectra during destruction of porous structures formed by nitrogen–rare gas (RG) nanoclusters in bulk superfluid helium containing high concentrations of stabilized nitrogen atoms. Samples were created by injecting products of a radio frequency discharge of nitrogen–rare gas–helium gas mixtures into bulk superfluid helium. These samples have a high energy density allowing the study of energy release in chemical processes inside of nanocluster aggregates. The rare gases used in the studies were neon, argon, and krypton. We also studied the effects of changing the relative concentrations between nitrogen and rare gas on thermoluminescence spectra during destruction of the samples. At the beginning of the destructions, \(\alpha \)-group of nitrogen atoms, Vegard–Kaplan bands of \(\hbox {N}_2\) molecules, and \(\beta \)-group of O atoms were observed. The final destruction of the samples were characterized by a series bright flashes. Spectra obtained during these flashes contain M- and \(\beta \)-bands of NO molecules, the intensities of which depend on the concentration of molecular nitrogen in the gas mixture as well as the type of rare gas present in the gas mixture.     

Helium detection in gas mixtures by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been evaluated as a tool for monitoring trace levels of helium in gas mixtures consisting mostly of hydrogen. Calibration data for helium in hydrogen was investigated at different helium concentration levels. At high concentrations of helium (>7.25%), the LIBS signal is quenched due to Penning ionization. The hydrogen alpha line (656.28 nm) was observed to broaden as the concentration of helium impurities in the hydrogen gas mixture increased. The helium line at 587.56 nm was selected as the analyte line for helium impurity detection. The effects of laser energy, the delay time between the laser pulse and data acquisition, and the gas pressure on the LIBS signal of helium were investigated to determine the optimum conditions for helium detection. The LIBS signal from the helium line at 587.56 nm shows good linear correlation with helium concentration for He concentrations below 1%. Thus, LIBS can be reliably used to detect the low levels of helium. The limit of detection for helium was found to be 78 ppm.     

氦氢混合制冷工质的爆炸极限试验

在微型低温制冷机中，为了提高制冷机的热力学效率，越来越多地使用氢氦二元混合工质。由于氦气是惰性气体，具有阻燃作用，因此，在使用时容易产生麻痹思想而引发事故。为此对不同配比的氦氢混合气进行爆炸试验，分别得到了它们的爆炸极限。结果表明：氦气对氢气虽然具有一定的阻燃作用，但当样气中氦的摩尔百分含量小于50％时，氦氢混合气的爆炸上、下限随氦的百分含量的增加而有所升高，但其爆炸区间的绝对值几乎与纯氢相近；只有当样气中氦的摩尔百分含量大于50％后，样气的爆炸下限随氢的百分含量的减少迅速递升；直至氦气和氢气摩尔比例大于10时，样气与任何比例的空气混合都不会爆炸，且较氢氮混合气更安全。     

Experimental investigation of the auxiliary gas circuit of a diffusion absorption chiller with natural and forced circulation

The auxiliary gas circuit has a significant influence on the cooling capacity and the efficiency of a diffusion absorption chiller. Nevertheless, there are only a few theoretical studies and experimental investigations concerning this topic. The reason is the difficulty to gain measurement data of the volume flow rate and the partial pressures without affecting the natural circulation of the auxiliary gas circuit negatively.This paper presents a detailed experimental investigation of the auxiliary gas circuit. The gas mixture in the circuit mainly consists of the vapour of the refrigerant (ammonia) and the inert gas (helium). A clamp-on ultrasonic flowmeter measures the volume flow rate of the gas mixture and the mole fraction of helium and ammonia continuously. The influence of an increasing volume flow on the mass transfer in the auxiliary gas circuit is shown by using a propeller to force the circulation of the gas mixture.     

Dielectric-Constant Gas-Thermometry Scale from 2.5 K to 36 K Applying 3He, 4He, and Neon in Different Temperature Ranges

New dielectric-constant gas-thermometry (DCGT) measurements were performed at PTB in the temperature range from 23 K to 36 K. For the first time, besides helium, neon was also used as a measuring gas. The measurements with helium and neon yielded thermodynamic temperature data in the range between 27 K and 36 K. A combination of these data, with former measurements in the range between 2.5 K and 26 K made using ³He and ⁴He, lead to a temperature scale which coincides with the ITS-90 within a few tenths of a millikelvin even at highest temperatures. The discussion of the uncertainty budget for neon follows the procedures already proved for helium. Furthermore, a comparison between new highly accurate ab initio calculations for the second and third density virial coefficients of helium, and the extended DCGT results are given. For the third virial coefficient of helium, the presented data allow the first experimental check of the new ab initio calculation for the range above 24 K.     

Co-doped ZnO Thin Films Fabricated by a Nanocluster-Beam Deposition System and the Influence of Flow Rate of Helium Gas on Their Properties

As a pioneer, Co-doped ZnO nanocluster-assembled thin films were fabricated by a nanocluster-beam deposition system and the influence of the flow rate of helium gas on the properties of the films was also investigated. Transmission electron microscopy indicated that Co-doped ZnO nanoclusters maintained a wurtzite structure as that of bulk materials. Also, it is found that the average size of ZnO nanoclusters decreased with the increased flow rate of helium gas. Two photoluminescence (PL) bands at 378 and 510 nm were observed. The Co-doped ZnO nanocluster-assembled thin films exhibited ferromagnetic property at room temperature. As the flow rate of helium gas increased, the corresponding saturation magnetization of the nanocluster-assembled thin films decreased from 11 to 6 μemu.     

An excimer emitter with pumping by barrier discharge in mixtures of zink diiodide vapors with inert gases

An experimental investigation is performed of the output characteristics of an excimer lamp utilizing mixtures of zinc diiodide vapors with inert gases and excited by a barrier discharge (BD) with the repetition rate of sine voltage pulses f ≤ 130 kHz. Radiation spectra of BD plasma in the range of 200–900 nm with resolution of 0.05 nm are studied, as well as the time characteristics of voltage and current and the dependence of radiation intensity on the composition of mixtures. The radiation of ZnI(B–X) excimer molecules is revealed with a maximum at λ = 602 nm, as well as of I ₂ ^* excimer molecules, lines of inert gases, and, in mixtures with xenon, of XeI* excimer molecules. The composition and pressure of gas components of mixture are optimized. It is found that the most intense radiation of ZnI* in binary mixtures is observed when helium or neon is used as buffer gas. The specific average radiation power in the visible range is 23 mW/cm².     

Experimental and simulation studies on the performance of standing wave thermoacoustic prime mover for pulse tube refrigerator

The thermoacoustic prime mover (TAPM) has gained considerable attention as a pressure wave generator to drive pulse tube refrigerator (PTR) due to no moving parts, reasonable efficiency, use of environmental friendly working fluids etc. To drive PTCs, lower frequencies (f) with larger pressure amplitudes (ΔP) are essential, which are affected by geometric and operating parameters of TAPM as well as working fluids. For driving PTRs, a twin standing wave TAPM is built and studied by using different working fluids such as helium, argon, nitrogen and their binary mixtures. Simulation results of DeltaEc are compared with experimental data wherever possible. DeltaEc predicts slightly increased resonance frequencies, but gives larger ΔP and lower temperature difference ΔT across stack. High mass number working fluid leads to lower frequency with larger ΔP, but higher ΔT. Studies indicate that the binary gas mixture of right composition with lower ΔT can be arrived at to drive TAPM of given geometry.     

Thermal evolution spectrometry of low energy helium ions injected into stainless steel and nickel targets

Low energy helium ions have been injected into 304 stainless steel, polycrystalline nickel foil and single crystal nickel targets and the gas evolution rate monitored during post-bombardment annealing. Peaks in the observed spectra have been associated with annealing of radiation damage, normal diffusion and with processes involving helium-vacancy agglomerates. Activation energies for evolution have been deduced and values in good agreement with previously published experimental and theoretical results have allowed identification of some of the processes.Entrapment probabilities have also been measured and found to be lower in stainless steel than in nickel due mainly to the inability to remove all the gas implanted into the steel without melting it. The marked fluence dependence of the entrapment probability in single crystal nickel for the higher energy (1 keV) and high fluence (10¹⁵–10¹⁶ ions cm^-2) range investigated illustrated the importance of gas atom-defect interactions in the migrational behavior of helium in this material.     

发射场氦气现场提纯技术

液氢贮箱在加注液氢前首先进行氮气置换 ,合格后再进行氦气置换 ,置换后排出的氮氦混合气混合贮存 ,混合气中He纯度为 91 6 6 % ,其余主要为氮气。结合中心现有条件 ,提出了利用高压低温冷凝法和低温吸附法相结合的方法对回收的氦气进行现场提纯 ,可以生产纯度大于 99 995 %的氦气。     

Electron attachment of oxygen in a drift chamber filled with xenon + 10% methane

The existence of O₂ contamination attenuates the pulse height and degrades its resolution in a drift chamber filled with xenon-methane (90/10) gas. The first measurement of the electron attachment coefficient due to oxygen in such a mixture is reported.     

Coupling high-pressure MALDI with ion mobility/orthogonal time-of-flight mass spectrometry

A new ion mobility/time-of-flight mass spectrometer employing a high-pressure MALDI source has been designed and tested. The prototype instrument operates at a source/drift cell pressure of 1-10 Torr helium, resulting in a mobility resolution of approximately 25. A small time-of-flight mass spectrometer (20 cm) with a mass resolution of up to 200 has been attached to the drift cell to identify (in terms of mass-to-charge ratio) the separated ions. A simple tripeptide mixture has been separated in the drift tube and mass identified as singly protonated species. The ability to separate peptide mixtures, e.g., tryptic digest of a protein, is illustrated and compared to results obtained on a high-vacuum time-of-flight instrument.     

Study of a liquid helium jet pump for circulating refrigeration systems

A basic diagram of the refrigeration system, in which liquid helium is circulated by means of a jet pump, is described. The equations have been derived to design jet devices which operate with liquid helium.In the experimental setup with a jet pump the flow rate of liquid was 5 to 10 times larger than that of compressed gas in a direct stream of the refrigerator. After pumping the pressure was equal to (0.15–0.40) × 10⁵Nm^?2. A circulating stream of liquid helium at supercritical pressure, due to the heat load corresponding to the refrigerator capacity, was heated to 0.15–0.25 K.The results of our studies permit one to determine circulation loop parameters and main geometric dimensions of the jet pump.     

The effect of hot phonons on the hole drift velocity in a p-type Si/SiGe modulation doped heterostructure

In this article the effect of hot phonons on the drift velocity of holes in the high-field transport regime of a Si/SiGe modulation doped heterostructure is presented. A theoretical model including the hot phonon production effect is implemented to compare the experimental results of hole drift velocity at high fields. At liquid helium temperature, our experimental results show that at an electric field of 1000 V/cm the hole drift velocity saturates at around v_d = 7 × 10⁵ cm/s which is in good agreement with the theoretical calculations based on the above model. The reduction of hole drift velocity at high fields is due to increasing momentum relaxation rate which is a result of the enhanced production of non-drifting longitudinal optical phonons.     

Measurements of the Nuclear Spin Relaxation Times for Small Grains of Solid Hydrogen Suspended in Liquid Helium

Measurements of the nuclear spin-lattice relaxation times (T ₁) and the nuclear spin-spin relaxation times (T ₂) are reported for suspensions of small grains of solid hydrogen in liquid helium. The measurements have been carried out for temperatures 1.8>T>4.9 K and for a nuclear Larmor frequency of 4.0 MHz. The samples were prepared by rapid condensation of hydrogen and helium gas in a precooled cell. The hydrogen pressure was maintained below the triple point to prepare a “slush” of hydrogen particles in liquid helium. The characteristic relaxation times are much shorter, typically by an order of magnitude for T ₂, than those observed and reported previously for bulk samples of solid hydrogen for the same ortho-hydrogen concentrations and applied magnetic fields. Possible origins for this difference between the relaxation in bulk and micro-geometries are discussed.     

Secondary particle identification using the relativistic rise in ionization

We report on the data analysis and techniques used to determine particle identification from the relativistic rise in ionization as detected by CRISIS, a 1 × 1 × 3 m³, 192 cell, ionization-sampling drift chamber containing an 80% argon/20% carbon dioxide gas mixture. For 100 GeV/c protons/antiprotons (pions) we obtain an ionization FWHM of 7.6% (7.9%), which compares well with the design value of 7.8%. The separation between the mean ionization values for the protons and pions was determined to be 7.9%. We also present the analysis of particle identification for secondary particles produced by interactions in the bubble chamber. A statistical method for correcting for the overlapping ionization curves from secondary protons/antiprotons and pions is discussed.     

Hyperfine Resonance of Deuterium Atoms Stabilized in Impurity-Helium Solids

We describe a method of combining the stabilization of impurities in superfluid helium and magnetic resonance techniques to study hyperfine resonance in atomic deuterium obtained by passing a D ₂ -He mixture through a radiofrequency discharge. The resulting D-D ₂ -He gas mixture penetrates a superfluid helium surface and forms a Van der Waals solid mixture containing atoms of D and He as well as D ₂ molecules. In preliminary experiments we have applied magnetic resonance (309 MHz) to the – transition to study spin-spin and spin-lattice relaxation in atomic deuterium and to provide information about the surroundings of atomic deuterium isolated in the D-D ₂ -He solid.