Neutron diffraction studies of NaUO3.

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.

Dose tolerance at helium and nitrogen temperatures for whole cell electron tomography

The magnetic properties of materials hosting Eu2+ (J = 7/2, 4f7) ions have attracted much attention in the science of strongly correlated electrons. In part because crystal electric field effects are impoverished for an S-state ion, as with Gd3+ intermetallics, and Eu2+ substitution in biological and optically active materials is resourceful. The magnetic structure of EuPdSn2 is not wholly resolved. Ferromagnetic and antiferromagnetic structures coexist in powder neutron diffraction patterns, and compete in the ground state. Moreover, the specific heat as a function of temperature is enigmatic and indicative of J = 5/2. We present symmetry-informed analytic magnetic structure factors for single crystal resonant X-ray Bragg diffraction using Eu atomic resonances that reveal significant potential for the technique. Europium ions use Wyckoff positions that are not centres of inversion symmetry in magnetic space groups inferred from neutron diffraction. In consequence, axial and polar Eu multipoles are compulsory components of both magnetic neutron and resonant X-ray Bragg diffraction patterns. The proposed antiferromagnetic phase of EuPdSn2 supports anapoles (magnetic polar dipoles) already observed in magnetic neutron diffraction patterns presented by Gd-doped SmAl2, and several resonant X-ray diffraction patterns.

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.

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.

Structural features responsible for lithium conductivity in Li(1+x)Ti(2-x)Al(x)(PO4)3 (x = 0, 0.2, and 0.4) samples have been investigated by Rietveld analysis of high-resolution neutron diffraction (ND) patterns. From structural analysis, variation of the Li site occupancies and atomic thermal factors have been deduced as a function of aluminum doping in the temperature range 100-500 K. Fourier map differences deduced from ND patterns revealed that Li ions occupy M1 sites and, to a lower extent, M3 sites, disposed around ternary axes. The occupation of M1 sites by Li ions is responsible for the preferential expansion of the rhombohedral R3c unit cell along the c axis with temperature. The occupation of less symmetric M3 sites decreases electrostatic repulsions among Li cations, favoring ion conductivity in Li(1+x)Ti(2-x)Al(x)(PO4)3 compounds. The variations detected on long-range lithium motions have been related to variations of the oxygen thermal factors with temperature. The information deduced by ND explains two lithium motion regimes deduced previously by (7)Li NMR and impedance spectroscopy.

Neutron diffraction and X-ray photoemission studies of the RNi 4Cu compounds (R = Ce, Pr, Nd)

We have studied the nature of magnetic ordering in the molecular magnet Mn[Fe(CN)6]2/3⋅zH2O by employing dc magnetization and neutron diffraction techniques. A ferrimagnetic coupling of Mn2+ (S = 5/2) spins and the Fe3+ (S = 1/2) spins is found for the compound. The magnetic Bragg scattering in the neutron diffraction patterns is masked by a large diffuse scattering arising due to the structural disorder in the compound. The results of Reverse Monte Carlo simulations of neutron diffraction data show that around the coordinated oxygen atoms (located at the 24e crystallographic sites with [Fe(CN)6] vacancies), there are formations of small clusters of non‐coordinated oxygen atoms.

The polarized absorption and emission spectra of single crystals of LaBr3 doped with nominally 0.15% and 0.75% 237Np have been recorded between 5000 and 25 000 cm−1 at temperatures of liquid helium and nitrogen. Fluorescence excitation spectra and both parallel and perpendicular Zeeman spectra were also recorded. Based on the number, polarization, and magnetic behavior of Stark components appearing in distinct groups, as well as on preliminary 5f-hydrogenic calculations, 14 absorption groups were identified as levels of the ground electronic configuration 5f4 of Np3+. The assumption of a pure 5f4 electronic configuration does not lead to a satisfactory interpretation of the spectrum; however, the inclusion of electrostatic configuration interaction permits a satisfactory fitting of the observed levels with an rms deviation in energy of 86.4 cm−1 using the parameters: E1=3394.3±29.1, E2=14.19±0.22, E3=317.5±1.9, ζ5f=1969.2±10.8, α=35.0±2.2, and β=−802.4±11.3 cm−1. Using 25 crystalline-field-split Stark components of the 5I4,5,6,7 levels of the ground multiplet, a first-order crystal-field perturbation calculation was performed. An rms deviation in energy of 8.0 cm−1 is obtained using the parameters: B20=−22.8±8, B40=−86.7±2.4, B60=−70.8±0.9, B66=+857.1±12.0 cm−1. The negative value obtained for B20 represents a crystal-field shielding in excess of 100% and is discussed in some detail. Parallel and perpendicular Zeeman interactions are also discussed in some detail.

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.

The visualization of macromolecular structures on voxels provides the basis for insights in dynamic events in assembly. Certain macromolecules arise as well organized frameworks. Symmetry or crystalline arrays arise through a strict geometry of subunits. Single asymmetric motifs can be superimposed on themselves taking such geometric constraints into account. Biological molecules are often composed of subunits arranged in a symmetrical manner Such conformations provide an opportunity for signal enhancement originating from the repetition of molecules in a larger configuration. Such repetitions require not only specific interactions among protein motifs, but also a built-in flexibility to accommodate conformational changes. Thus, it is advantageous to analyze critical molecular interactions with detailed protein folding information rooted in the volume density as a whole. Two-dimensional image projections can be collected in electron microscopes at temperatures of liquid nitrogen or helium. Using low-dose electron beams, viral assemblies of large lattices have been studied in vitreous ice to localize function-specific domains identified through difference imaging. Robust algorithms have been implemented based on various grid transformations to allow effective averaging of massive image data sets.

Experimental evidence has been presented on the difference in intermolecular ion-water distances obtained from X-ray and neutron diffraction methods. Simultaneous least squares fitting procedures were performed for X-ray and neutron interference terms observed for (NaCl)x(*H2O)1−x, (x = 0, 0.02, 0.05, and 0.098) and (KCl)x(*H2O)1−x, (x = 0, 0.02, 0.05, and 0.075) solutions at 25 °C, respectively. The null-water mixture was employed for neutron diffraction measurements for these solutions to eliminate structural contribution from hydrogen atoms. It has been revealed that the hydration numbers of Na+ and K+ are concentration dependent and the values for lower-concentration limit are 5 and 6, respectively. The nearest neighbor Na+⋯H2O and K+⋯H2O distances are obtained to be 2.36–2.37 and 2.75–2.82 Å, respectively. In order to examine the effect of the separate treatment of interactions between ion-oxygen and ion-hydrogen atoms in the X-ray model function, simultaneous fitting procedures were carried out for X-ray and neutron diffraction data observed for 9.8 mol% NaCl and 7.5 mol% KCl solutions by employing the individual atom model for the X-ray interference term. Obtained Na+⋯O and K+⋯O distances are ca. 0.02 Å shorter than those determined by the simultaneous fit employing the usual united model for water molecules. The nearest neighbor Cl−⋯O distance derived from the simultaneous fit by means of the individual atom model exhibits ca. 0.1 Å shorter than that obtained from the fit using the united atom model. The present Cl−⋯O distance agrees with that obtained from neutron diffraction measurements on 35Cl/37Cl isotopically substituted aqueous 5 mol% Na*Cl solutions in D2O. The simultaneous fitting analyses employing X-ray model function with the united and individual atom models of water molecule have revealed that the ion-oxygen (water) internuclear distance is significantly shorter than the average separation of electron clouds between ion and neighboring water molecule. The present results indicate that the difference in ion-water distance observed from X-ray and neutron diffraction studies mainly arises from the united atom model of X-ray diffraction data analysis assuming a spherical electron density around oxygen atom within the water molecules.