A Liquid Helium Distributing Refrigerator for Materials Testing Machines

Interstitial halogen centers in X-irradiated CsBr

Measurements of reflectivity, photoluminescence and cathodoluminescence for various electron beam voltages were performed on p-type single crystals at liquid nitrogen, hydrogen and helium temperatures. By an application of the microreversibility principle and consideration of the reflectivity data, the nature and the energies of the transitions were determined. In addition, we identify the lines due to the annihilation of exciton-neutral defect complexes, in ground or excited state. When the luminescence is modulated by a CO2 laser (hνIR ~ 130 meV) effects are observed at liquid helium temperature. A quenching effect is observed for the free-to-bound and pair recombinations. Quenching of the luminescence due to the annihilation of exciton isoelectronic oxygen complexes is a direct corroboration of the binding-in-two-step theory.

We have investigated the performance of a few silicon photomultipliers to readout inorganic scintillators operated at cryogenic temperatures. Such detectors, that would need to operate close to liquid helium (LHe) temperatures and in a magnetic field of a few Tesla, have been proposed by our group as a way to build an active polarized target to be used for neutrino physics, for the search of axions and CP violation, as suggested by some theoretical papers. In the present work we investigate in some details the performance of a few different silicon photomultipliers produced by Hamamatsu and Advansid down to liquid helium temperature, to verify the temperature range of usability of these devices and their performance at cryogenic temperatures.

The photoluminescence (PL), optical reflectance, optical transmittance and piezoelectric photothermal spectra were successfully observed between liquid helium and room temperatures for high quality p-type P-doped ZnTe substrates. In the PL spectrum of the ZnTe substrate (1 × 1018 cm—3), three distinct kinds of peaks which were due to the radiative recombination of the exciton bound to a neutral acceptor (I1) and free to a neutral acceptor (FA) with longitudinal optical phonon replicas were present at liquid helium temperature. The bandgap energy was estimated to be 2.394 eV by adding the reflection peak (2.381 eV) to the exciton binding energy. Furthermore, the activation energy of the phosphor acceptor impurity was estimated to be 65 meV. On the other hand, in the higher carrier concentration substrate (8 × 1018 cm—3), the PL intensity was much smaller than in that of the lower carrier concentration substrate. This means that radiative recombination processes decrease with increasing carrier concentrations.

A beryllium-walled cryostat is described which allows collection of nearly a full sphere of accurate three-dimensional X-ray diffractometer data at liquid-nitrogen (LN2) and liquid-helium (LHe) temperatures. Cryogen consumption is 0.3–0.4 liters of LHe per hour at 30°K and 0.1–0.2 liters of LN2 per hour at 78°K. The specimen is mounted on a copper block, the temperature of which is controlled to within 0.1°. Measurements with a powder sample of TbVO4 show that at 33°K the specimen temperature is within 2° of the temperature of the copper block. The cryostat has been used for the collection of several low-temperature data sets. Some special modifications in data-collection and data-processing software are described.

Increased present day use of cryogenics necessitates an understanding of the behavior of small leaks at liquid helium temperatures. Little is to be found in the literature except that related to super-fluid effects in the range 1.4 to 2.9 K. We report the results of an investigation on the behavior of two small leaks (leak rates in the vicinity of 2×10−7 atm cc air/s at room temperature) at 4.2 K in contact with gaseous as well as liquid helium. In both cases, a significant increase in the leak rate was found when the leaks were in contact with liquid helium at 4.2 K. The observed behavior is explained using a model based on the changes in the physical properties of helium with temperature.

The emission spectra from NaI(T1) excited by X-Rays at the temperature of liquid hydrogen showed the peaks at approximately 616, 546, 437, 379, 332, 299 and 285 m µ. The 332 m µ band is predominant at only the temperatures of liquid hydrogen and liquid helium, and is thought to arise from the T1 dimers, consisting of a pair of T1 + ions substitutionally displacing two alkali ions in the nearest neighbour (<110> direction). T1 dimer becomes an effective luminescent emission center at very low temperature. The emission spectra from CsI(T1) excited by X-rays at the temperature of liquid helium showed a superposition of 595, 537 and 474 m µ bands. The luminescence being weaker at lower temperature than at the room temperature, it shows no band in the ultraviolet region. In the following, temperature dependence of emission spectra from NaI(T1) and CsI(T1) excited by X-rays are reported and some discussion concerning with also some experiments carried out by other authors are presented.

An explanation is proposed for the maxima in the dielectric constant and density observed in liquid helium slightly above the superfluid transition temperature. It is shown that they may be caused by a phase transition that results in spontaneous polarization of the helium. This kind of transition is possible in the liquid because when an atom is polarized its energy levels are lowered, so that the polarization is energetically favorable, especially for light atoms. In addition, the development of a dipole moment in an atom leads to the appearance of an additional electric field and, therefore, to an increase in the energy. The possible spontaneous polarization of the system of atoms is determined by the competition among these factors. In accordance with this concept, as the temperature of liquid helium is lowered there is an initial phase transition into a polarized normal phase, followed by one from this nonzero polarization state into the superfluid state. The transition of the normal fluid into the polarized phase is described in terms of a phenomenological theory of phase transitions. The transition can be of first or second order.

Using improved processing conditions, we have succeeded in increasing the transport critical current density (Jc) of powder-in-tube fabricated silver sheathed Bi-Pb-Sr-Ca-Cu-O superconducting tapes. Jc values at zero field now exceed 1.6×105 A/cm2 (Ic∼164 A) at 4.2 K and 1.3×105 A/cm2 (Ic∼133 A) at 20.2 K for short tape samples. At 77 K the same samples exhibit values greater than 3.0×104 A/cm2 (Ic∼32 A). They continue to carry 6.5×104 A/cm2 at 4.2 K and 4.8×104 A/cm2 at 20.2 K with applied magnetic fields as high as 20 T. Long lengths of tape were also fabricated and measured by winding the composites into small spiral or pancake coils. The small pancake coils carried 4.8×104 A/cm2 at 4.2 K and 5.2×103 A/cm2 at 77 K, zero field. More recently, 1 m lengths have been measured with Jc’s as high as 1.0×104 A/cm2 at 77 K, zero field. Detailed transport measurements at liquid helium and hydrogen temperatures with applied magnetic fields up to 20 T are reported for short and long lengths of monocore tapes.

A cryostat enabling thermal conductivity measurements of specimens deformed plastically at liquid helium temperatures

23 - THE CHANGE OF RESISTANCE OF GOLD CRYSTALS AT VERY LOW TEMPERATURES IN A MAGNETIC FIELD AND SUPRA-CONDUCTIVITY: CRITICAL REMARKS REGARDING THE RECENT WORK OF MEISSNER AND SCHEFFERS

Magnetic susceptibility measurements were carried out on octacoordinated uranium(IV) complexes, such as tetrakis(acetylacetonato)uranium (IV), tetrakis(trifluoroacetylacetonato)uranium (IV), tetrakis(benzoylacetonato)uranium(IV), tetrakis(dibenzoylmethanato)uranium(IV), tetrakis(thenoyltrifluoroacetonato)uranium(IV), tetrakis(cupferronato)uranium(IV), and uranium(IV)tetrachloride, in the temperature range from room temperature to the temperature of liquid nitrogen or liquid helium. The Curie-Weiss law holds above the temperature of liquid nitrogen, with a magnetic moment of 3.5–2.92 BM. For the latter two complexes, a temperature-independent paramagnetism was observed in the low temperature range; the other complexes also seemed to show the same trend. We tried to interpret the magnetic behavior on the basis of the crystal-field model of the square antiprism structure or the dodecahedron structure of the complexes.

Amendment to: Electron beam damage to organic specimens at liquid helium temperature

Far-i.r. spectra of napthalene, anthracene, biphenyl, diphenyl ether, diphenylamine, diphenyl sulfoxide, and benzophenone at liquid helium temperature

EPR and microwave absorption of alkali metal-doped fullerene superconductors