Helium Gas Atmosphere Research Articles

The performance of a separated crystal X-ray monochromator during wiggler heating — Preliminary results

We report on the performance of a double crystal monochromator exposed to large heat loads from a 6 pole wiggler magnet running at 15.6 kG peak field and a 5.3 GeV electron beam. This source of synchrotron radiation can produce between 1.8 and 4.5 kW of X-ray power depending on beam conditions. Various single crystals of silicon have been tested in a nondispersive double crystal arrangement over a wide energy range along with several different cooling schemes. We report preliminary measurements of the performance of silicon crystals cooled by ambient helium gas or water cooled through conductive greases or through a 10 μm thick layer of liquid indium and gallium. A side cooled, thick crystal geometry shows good stability for photon energies from 20 to 45 keV. A thin, bottom cooled geometry shows promise at 7.4 keV. We have observed monochromator performance (rocking curve width, shift in edge, X-ray flux output vs electron beam current, harmonic rejection capabilities, etc.) under a variety of heating conditions. We summarize our suggestions for making effective use of monochromators under severe operating conditions.

Surface morphology of VLS Cd and Zn crystals grown in Helium inert gas atmosphere

Surface microtopographs were taken from VLS Cd and Zn crystals grown in a helium inert gas atmosphere and were compared with those taken from crystals grown in vacuum. During growth in vacuum, the impurity element caused the {10 1 0} and {10 1 1} fac es to become flat or striated. When these faces are flat during vacuum-growth, they contain a cavity during growth in a helium inert gas atmosphere due to the effect of the diffusion field. However, the striated faces during vacuum-growth exhibited striations also during growth in a helium gas atmosphere but without any cavity. The presence of the cavity is explained by the advancement of the “curved” steps toward the center of the face. A similar movement of the “straight” steps (striations) would not have produced any cavity.

Evaporation from molten TiC x

A new floating zone technique for keeping a molten zone composition constant has been developed. Using this new technique, the evaporation composition and the evaporation rate could be easily obtained as a function of molten TiCx composition in the range of 2800 to 3100° C. In addition, the dependence of the evaporation rate on the ambient helium gas pressure was examined in the range of 3.6 to 32 atm.

Temperature distribution in TiC crystal rod prepared by r.f. floating zone technique

The temperature distribution in a growing TiC crystal rod was calculated using a floating-zone model in which the power is supplied by r.f. heating and dissipated from the rod surface only by radiation. The calculated result agreed well with the experimental one. Therefore, it was confirmed that this model, in which a crystal is prepared in vacuum, also applies to the preparation of a TiC crystal under ambient helium gas. This model was also used to determine the best conditions for growth.

Thermal conductivity of ceramic fibrous insulators at high temperatures

The temperature and bulk density dependence of the thermal conductivity of commercial aluminosilicate fibrous insulators were studied by using the transient hot wire method. The thermal conductivity of ceramic fibrous insulators in both air and helium gas atmosphere increased with increasing bulk density. At high temperatures, however, the insulators with lower bulk density showed a higher thermal conductivity because of heat radiation. The following experimental relation between thermal conductivity λ and temperature θ was obtained for aluminosilicate fibruous insulators: λ=a exp(bθ). Relationships are given between the constants a and b and the bulk density. From the relation, the optimum bulk density of ceramic fibrous insulators can be calculated for each working temperature.

Preparation of TiC x single crystals with maximum carbon content by a floating zone technique

TiC x single crystals with a maximum carbon content ( x = 0.96–0.97) were prepared by controlling the compositions of the initial molten zone and the feed rod (a modified zone leveling method). The homogeneity of the composition was checked along the growth and radial directions. The lattice constants, densities and impurities in the crystal, starting material, feed rod and evaporation product, were measured and reported. All impurities except W, whose carbide has a low vapor pressure, were found to be refined by evaporation. The effect of the helium ambient gas pressure was examined. It was found that good quality crystals could be prepared under relatively low pressure (4–5 atm). The good quality of the crystals was confirmed by X-ray transmission topography.

Resistance and superconductivity of small lead particles

Electrical resistance measurements on films made out of small lead particles are reported. The lead particles are produced in an atmosphere of helium gas at low temperatures and simultaneously condensed onto a substrate. Immediately after condensation, the superconducting transition temperature of the particle films is lowered, corresponding to the mean particle diameter. It is very likely that in this state the particles form chains and the films possess a sponge-like structure. After annealing, the particle films behave more and more like lead films condensed in the usual way. At room temperature, the sponge-like structure of the particle films has probably completely disappeared; nevertheless, enhanced superconducting transition temperatures in comparison to bulk lead may indicate the existence of internal strain.

The influence of thermal conduction in spin-lattice relaxation experiments

Electron spin resonance and low-frequency relaxation measurements were performed on KCr alum placed in a helium gas atmosphere. The results for various gas pressures could be accurately described by a thermal conduction model that accounts for the heat conduction in the gas. It is shown that the bad heat conduction in the gas bottlenecks the spin-lattice relaxation behaviour, even at the saturation pressure.

Ignition of Laminated Composite Propellants Composed of Ammonium Perchlorate Single Crystal and Fuel-Binder Slab by Means of CO2Laser

Abstract Abstract-The ignition behavior of laminated propellants composed of ammonium perchlorate (AP) single crystal oxidizer and carboxyl terminated polybutadiene (CTPB) fuel-binder slab, has been studied by means of CO2 laser under sub-atmospheric pressure. The ignition is initiated by the reaction of carbonaceous material on AP surface with AP decomposition products and ambient helium gas has appreciably preventive influence on propellant ignition, compared with Ar and N2 gases. These phenomena might be explained by a comprehensive ignition model.

Helium blast-wave measurements of laser-heated microshell targets

Measurements of blast waves produced by laser heating of spherical glass-shell targets in an atmosphere of 20 Torr helium are described. These measurements made use of a streak camera, monitoring the self-luminosity of the strong shock, a blast probe detecting the ionization behind the shock front, and a piezoelectric pressure probe recording the pressure pulse of the shock. The time history of the blast wave is used to determine the kinetic energy of target dissembly via a simple model which is applicable in both the strong-shock and acoustic regions. These measurements show that the inferred blast-wave energy increases for larger radius–time-shock coordinates. This anomalous effect is ascribed to ’’fast’’ ion preheating of the ambient helium gas in front of the shock wave.

A simple method to transfer specimens at liquid helium temperature

A simple and reliable method to transfer irradiated specimens at liquid helium temperature without any intermediate warming is described. The transfer is performed under the helium gas atmosphere at room temperature using a small vacuum-insulated metal Dewar and consumes only a small amount of liquid helium.

Use of capillary and pinch instability for dendritic crystal growing

Contact-free melting, quenching and dendritic crystal growing is possible with a technique which uses the capillary and pinch instability of current-liquefied metal wires. The wire is melted by a condenser discharge, in a helium gas atmosphere, or in liquid helium. The liquid metal cylinder then becomes unstable because of the capillary and pinch forces and disintegrates finally into a “chain” of spherical samples. These liquid samples are subjected to very high supercoolings. Solidification (dendritic growth) occurs rapidly from a single nucleation site before the crystal touches any wall or the bottom of the apparatus. Spherical single crystals of diameters up to 0.5 mm are obtained from bcc metals (W, Mo, Ta, Nb, V, Fe) but not generally from fcc metals. Polycrystals are obtained from the latter metals, because of (hexagonal) growth stacking faults of the {111} planes. The capillary and pinch instability can be explained by a theory due to Murty. Kleen's so called “unduloid theory” is criticized.

The Cryopumping System for the Indiana University 200 WeV Cyclotron

The cryopumping system for the 200 MeV cyclotron being designed and built at Indiana University is described. The volume of the vacuum chamber is approximately 28,000 liters and the estimated net area is 3.5 × 106 cm2. Two flange mounted cryopump assemblies are used to reach a pressure in the low 10-6 Torr region. Each assembly consistg of a 20°K refrigerator which uses ambient temperature helium gas and a chevron radiation shield which uses helium gas in a closed cycle at a nominal 80°K from a cryogenerator. The pumping speed of each pump is in the region of 30,000 1/s for nitrogen at 1 × 10-5 Torr or less, plus 250,000 1/s for water vapor. Liquid nitrogen is not used for cooling the radiation shield because of explosive hazards. A cryogenic heat exchanger (part of the closed cycle) in a safe location can be used also with liquid nitrogen for cooling the 80°K helium circuit should the cryogenerator be out of use for more than its scheduled maintenance period. A method for cryopumping a beam line is also described.

A new apparatus for simultaneous differential thermal analysis and gas evolution analysis

A new apparatus is described which permits the simultaneous recording of the differential thermal analysis and gas evolution curves for a given sample from ambient to 900°. The sample is pyrolyzed in a dynamic helium gas atmosphere using a conventional DTA furnace and sample holder assembly. The evolved decomposition products are detected in the helium gas stream by means of a thermistor thermal conductivity cell. From a knowledge of the GE curve and an analysis of the decomposition products, a more accurate interpretation of the DTA curve can be made.

Triplet-Singlet Emission Spectra of Xylenes in Crystalline State at 4.2° and 77°K, and in EPA at 77°K

The triplet-singlet emission spectra of xylenes in the crystalline state at 4.2° and 77°K and in a rigid glass at 77°K have been photographed and analyzed. The samples were obtained from the National Bureau of Standards. They were kept sealed under an oxygen-free atmosphere of helium gas throughout the experiments. Large well-formed crystals were grown from the vapor on a surface cooled by liquid nitrogen, and some were grown from the melt. At 4.2°K o-xylene produced two sharp spectra similar in structure and separated by 370 cm-1 from each other. These spectra are ascribed to two distinct crystalline phases. At 77°K o-xylene did not produce a significant phosphorescence. The spectrum of m-xylene at 4.2°K consisted of a strong diffuse spectrum and a weak sharp spectrum toward shorter wavelengths. The diffuse spectrum appeared to be associated with the amorphous phase. At 77°K a broad spectrum appeared which was found to be characteristic of an aldehyde. At 4.2°K p-xylene showed a sharp spectrum beginning at 25 254 cm-1. It differs in vibrational structure from the spectrum of this compound in a rigid glass. A vibrational analysis indicated that the carrier of this spectrum is p-methylbenzaldehyde. With crystals of the p-xylene grown from the melt the genuine spectrum due to p-xylene was observed between 28 246 and 25 416 cm-1, over-lapped by the spectrum of p-methylbenzaldehyde in the longer wavelength region. These results suggest a temperature-activated sensitization which had been first observed in toluene.

Heat Transfer to Mercury in Turbulent Pipe Flow

Abstract Experimental heat-transfer results are presented for turbulent flow of nonwetting mercury in a ¾-in., 18-gage mild-steel tube with constant heat flux. The identical test heat exchanger developed for the previously reported investigation of molten lead-bismuth eutectic (10, 11, 12) was used. This present investigation also includes a series of short-duration tests for possible effects due to secondary flow, the use of argon as an alternate for a helium-gas atmosphere, and the addition of magnesium-titanium amalgam as a wetting agent. Heat-transfer tests with water preceded and followed those for mercury to establish continuing reliable performance of the test exchanger and its instrumentation. The results are correlated for the Peclet modulus range from 200 to 10,000 and are 4 to 10 per cent lower than the previously reported values for lead-bismuth and 35 to 40 per cent lower than the Martinelli-Lyon momentum theory (15, 17). No difference in heat-transfer performance was noted for upward versus downward flow or for the use of argon as a replacement for the helium atmosphere. No visible evidence of wetting occurred on the addition of the magnesium-titanium amalgam and no effect was found in the heat transfer.