Room Temperature Helium Research Articles

Amelioration de la conductivite superficielle du cuivre et de l'aluminium en hyperfrequences, par abaissement de temperature

The use of a high power linear accelerator leads to the research of high microwave surface conductivity in order to reduce losses by Joule effect. A simple method is to cool microwave cavities to liquid helium or liquid hydrogen temperature. After a brief survey of the theory, we give experimental results about copper and aluminium microwave resonant cavities in S band and L band. The experimental gain on surface conductivity between room temperature and liquid helium temperature is 5.6 for copper and 7.4 for aluminium at a frequency of 3160 MHz. At 1420 MHz we find 5.3 for copper and 7.8 for aluminium. This work shows the great effect of the method of preparation on microwave surface conductivity of metals at very low temperature. The surface conductivity for an electropolished surface is 1.75 times that of a machined or mechanically polished surface.

A dynamic method for determination of specific surfaces of adsorbents by continuous adsorption from a solution stream

The coefficient defining the density expansion of the dielectric function of a simple gas are expressed as thermal averages of products of two-body adiabatic polarizabilities. To allow this description a superposition form has been introduced for the three-body polarizability: this approximation leads to results which reduce to the well-known Kirkwood, dipole-induced-dipole (DID) theory only when the pair polarizabilities are replaced by their long range (dispersional) form. When non-DID effects are retained, i.e., when ab initio (e.g., Hartree-Fock) pair polarizabilities are used, significant corrections are found, but the triplet contributions are still small except at very high densities and pressures. In particular, we evaluate the second and third dielectric virial coefficients for room temperature helium, and compare our calculated values with those obtained in recent DID treatments. The marked differences found in the resulting density dependence of the Clausius-Mossotti function, and the relative insensitivity to the choice of interaction potential, are discussed in terms of short range electronic distortion.

Scattering of High-Velocity Neutral Particles. XI. Further Study of the He–He Potential

Results are presented for two studies of the scattering by room-temperature helium of beams of helium atoms with controlled energies in the approximate range 150–1500 ev. The interaction potential derived from the two sets of measurements is represented by φ(r)=5.56×10−12/r5.03 erg (0.97 A<r<1.48 A).Where there are common ranges of the interaction distance r this potential is in reasonable agreement with He–He potentials previously determined from scattering experiments. In its specified range of validity, it is somewhat lower than most of the corresponding values calculated quantum mechanically.

Susceptibility measurements of Nb between room temperature and liquid helium temperatures

Measurements have been carried out on bulk Nb between room temperature and liquid helium temperatures. The susceptibility shows an increase of 1% per 100 degrees for decreasing temperatures. No field dependence could be detected up to liquid hydrogen temperature. This is in agreement with the results obtained by other authors. In the liquid He region some anomalies appear, notwithstanding fields were used higher than the critical one. It is supposed that the sample was not completely in the normal state.

Germanium Resistance Thermometers Suitable for Low-Temperature Calorimetry

Single crystal ``bridges'' of suitably doped germanium have been encased in a strain-free manner within nonmagnetic platinum-glass capsules to provide resistance thermometers with desirable low-temperature electrical properties. The stability of the resistance of these thermometers with repeated cycling between room temperature and liquid helium temperatures and their relatively constant sensitivity (percent change in resistance per percent change in temperature) between 2 and 35°K for a wide range of resistivities are improvements over existing thermometers for this temperature range. Thermometers of this type are believed to be suitable for the most precise measurements, such as calorimetry, and are being further investigated.

Scattering of High Velocity Neutral Particles. VIII. H–He

Total collision cross sections have been measured for hydrogen atoms with energies between 700 and 2100 ev, scattered in room temperature helium. The results have been analyzed to obtain potential energy information for the interaction between a hydrogen atom and a helium atom. The potential function, which may be represented by V(r)=3.75×10−12/r3.29 ergfor r between 1.16 A and 1.71 A, gives values which agree with those calculated quantum mechanically by Mason, Ross, and Schatz. The H–He potential has been combined with a potential previously obtained for He–He to obtain a function which, it is suggested, represents the repulsive interaction between two hydrogen atoms. The derived function is in fair agreement with theoretical calculations of Hirschfelder and Linnett for the repulsive 3Σ state of the hydrogen molecule.

Further measurements on the magnetic susceptibility of germanium down to liquid helium temperatures

Synopsis New measurements are done on the susceptibility of seven samples of germanium obtained from single crystals of very pure germanium or of germanium containing small amounts of Sb (10 14 , 1016, 10 17 ). The measurements are carried out between room temperature and liquid helium temperatures. For the pure samples an increase is found of the diamagnetic susceptibility of about 5% between room and liquid hydrogen temperatures. In the liquid helium region the susceptibility is nearly constant. For the impure samples the increase of the diamagnetic susceptibility between room temperature and the liquid hydrogen temperature region is about I 1 % and 18%. For the sample with the greatest concentration of Sb, we moreover find a minimum in the susceptibility versus temperature between liquid hydrogen and liquid helium temperatures. A paramagnetic term appears at the lowest temperatures. In order to control the method measurements are also carried out on a sample of piezoelectric quartz. We find a susceptibility which is constant within 0,2% between room temperature and liquid helium temperatures.

Scattering of High-Velocity Neutral Particles. IV. He–A; A–He

Total collision cross sections have been measured for helium atoms with energies between 500 and 2100 ev, scattered in room temperature argon, and for argon atoms with energies between 600 and 2100 ev, scattered in room temperature helium. In each case, two detectors of different geometric aperature have been used, and potential energy functions have been obtained by procedures previously applied to helium scattered in helium. The average function for the potential energy between a helium atom and an argon atom derived from the measured cross sections may be represented by V(r)=9.95×10−11/r7.25ergs,for values of r between 1.64A and 2.27A. Since this potential is in good agreement with the geometric mean of the potential between two helium atoms and that between two argon atoms in the range represented by the arithmetic average of the individual interaction distances, it is suggested that the parameters of a point repelling type potential function may, in general, be combined in this manner.

Scattering of High-Velocity Neutral Particles. II. Helium-Helium

Total collision cross sections have been measured for helium atoms, with energies between 500 and 2100 ev, scattered in room temperature helium. Measurements were made with two detectors of different geometric aperture, 13.4 and 0.96 minutes. In order to obtain meaningful potential energy information from such results, it is necessary to determine effective apertures by taking into account beam shape, intensity distribution, and for the case where the beam is wider than the detector, scattering to the detector from external regions of the beam. Potential energy functions have been derived from the cross sections obtained with the two detectors. The functions agree within a factor of about 1.3 as compared with 0.17 when geometric apertures are used directly. The results from both detectors have been represented by an average function, namely, V(r)=7.55×10−12/r5.94 ergsfor values of r between 1.27A and 1.59A. The potential energy information obtained from the present experiments is in agreement with theoretical calculations of P. Rosen and H. Margenau within the uncertainty of such calculations. The information also appears to be consistent with that derived from measurements of gaseous compressibility and viscosity, for values of r between 1.90A and 2.30A.

Helium Repulsive Potential from Collision Cross Section Measurements

Total collision cross sections have been measured for fast helium atoms, having velocities corresponding to 300–1000 electron volts, scattered in room temperature helium gas. Using classical theory, attempts were made to find a repulsive intermolecular potential which could account for the experimental results. A hard sphere model or a Lennard-Jones type potential were both unsatisfactory, whereas exponential type potentials of the form V(r)=A exp (−arn) seemed promising. For the distances involved in the scattering experiments, 0.55 to 1.05A, the specific repulsive potential which predicted total collision cross sections within the experimental error was V(r)=11.3 exp (−4.63r12)×10−10erg.