Novel Method of Measuring Electron Positron Colliding Beam Parameters

Through the simultaneous measurement of the transverse size as a function of longitudinal position, and the longitudinal distribution of luminosity, we are able to measure the $\beta_y^\ast$ (vertical envelope function at the collision point), vertical emittance, and bunch length of colliding beams at the Cornell Electron-positron Storage Ring (CESR). This measurement is possible due to the significant ``hourglass'' effect at CESR and the excellent tracking resolution of the CLEO detector.Comment: 11 pages, 4 figures, submitted to NIM

Synchrotron X-ray Diffraction of Layering Transitions of Multilayer Nitrogen Physisorbed on Graphite

We use synchrotron x-ray diffraction for structural analysis of the behavior of multilayer nitrogen films physisorbed on graphite foam. We provide structural information and concentrations of 2D and 3D solid phases at a coverage of Θ = 8 ML (Θ / 1 ML for a %3 x %3 structure) for temperatures from below the bulk α-ß transition temperature [Tα-ß = 34 0.5 K] to above the bulk triple point [Ttp = 63 K]. Our data indicate layering begins near Tα-ß, with subsequent layering occurring as the temperature is raised; all bulk nitrogen forms disordered film layers by 48 K at Θ = 8 ML. Our results are consistent with ellipsometry studies of nitrogen on HOPG which found multilayer nitrogen on single crystal graphite to undergo a series of layering transitions above the bulk nitrogen α-ß structural transition.[U.G. Volkmann, and K. Knorr, Phys. Rev. Lett. 1991, 66, 473.] The effect of adsorption on a graphite foam substrate, which results in capillary condensation and finite size effects, is limited to a broadening and overlap of the discrete transition temperatures observed on a single crystal substrate. A phase diagram for coverages above 2 ML is proposed, summarizing this and previous work

Synchrotron x-ray-diffraction study of the structure and growth of Xe films adsorbed on the Ag(111) surface

URL:http://link.aps.org/doi/10.1103/PhysRevB.59.15464 DOI:10.1103/PhysRevB.59.15464Synchrotron x-ray scattering has been used to investigate the structure and growth of perhaps the simplest of all films: xenon physisorbed on the Ag(111) surface. High-resolution x-ray scans of the in-plane structure and lower-resolution scans (specular and nonspecular) of the out-of-plane order were performed. The Xe films were prepared under both quasiequilibrium and kinetic growth conditions, and have fewer structural defects than those investigated previously by others on graphite substrates. Under quasiequilibrium conditions, the bulk Xe-Xe spacing is reached at monolayer completion, and the monolayer and bilayer lattice constants at coexistence are inferred equal to within 0.005 Å, consistent with theoretical calculations. The Xe/vacuum interface profile for a complete monolayer and bilayer grown at quasiequilibrium is found to be sharper than for kinetically grown films. At coverages above two layers, diffraction scans along the Xe(01l) rod for quasiequilibrated films are consistent with the presence of two domains having predominantly an ABC stacking sequence and rotated 60° with respect to each other about the surface normal. Annealing of these films alters neither the population of the two domains nor the fraction of ABA stacking faults. The thickest film grown under quasiequilibrium conditions exceeds 220 Å (resolution limited). Under kinetic growth conditions, x-ray intensity oscillations at the Xe anti-Bragg position of the specular rod are observed as a function of time, indicating nearly layer-by-layer growth. Up to four complete oscillations corresponding to a film of eight layers have been observed before the intensity is damped out; the number of oscillations is found to depend on the substrate temperature, the growth rate, and the quality of the Ag(111) substrate. The specular reflectivity from kinetically grown films at nominal coverages of three and four layers has been analyzed using a Gaussian model which gives a film thickness standard deviation of 0.5 and 1.0 layers, respectively. Diffraction scans along the Xe(01l) rod of these films indicate a larger fraction of ABA stacking faults than found for thicker films. These results demonstrate the difficulty of kinetically growing Xe films thicker than two layers which have an ideal slab geometry.This work was partially supported by U.S. National Science Foundation Grant Nos. DMR-8704938, DMR-9011069, and DMR-9314235 and the U.S. Department of Energy Grant No. DE-FG02-85ER45183 of the MATRIX Participating Research Team

Ultrahigh Vacuum Chamber for Synchrotron X-ray Diffraction from Films Adsorbed on Single-crystal Surfaces

An ultrahigh vacuum chamber has been developed for structural analysis of adsorbed films and single‐crystal surfaces using synchrotron x‐ray diffraction. It is particularly well suited for investigations of physisorbed and other weakly bound films. The chamber is small enough to transport and mount directly on a standard four‐axis diffractometer and can also be used independently of the x‐ray diffractometer. A low‐current, pulse‐counting, low‐energy electron diffraction/Auger spectroscopy system with a position‐sensitive detector enables in situ characterization of the film and substrate while the sample is located at the x‐ray scattering position. A closed‐cycle He refrigerator and electron bombardment heater provide controlled substrate temperatures from 30 to 1300 K. The chamber is also equipped with an ion sputter gun, a quadrupole mass spectrometer, and a gas handling system. Details of the design and operation of the instrument are described. To demonstrate the performance of the instrument, we present some preliminary results of a study of Xe physisorbed on the Ag(111) surface

Studies of the structure and growth mode of dotriacontane films by synchrotron x-ray scattering and molecular dynamics simulations

doi: 10.1088/0953-8984/16/29/005We report on synchrotron x-ray scattering experiments and molecular dynamics simulations of the structure and growth mode of dotriacontane (n-C32H66 or C32) films adsorbed on Ag(111) and SiO2-coated Si(100) substrates. On the SiO2 surface, the x-ray measurements confirm a structural model of the solid film inferred from high-resolution ellipsometry measurements in which one or two layers of C32 adsorb with the long axis of the molecule oriented parallel to the interface followed by a monolayer in which the molecules have a perpendicular orientation. At higher C32 coverages, preferentially oriented bulk particles nucleate, consistent with a Stranski-Krastanov growth mode. On the Ag(111) surface, we again observe one or two layers of the 'parallel' film but no evidence of the perpendicular monolayer before nucleation of the preferentially oriented bulk particles. We compare the experimentally observed structures with molecular dynamics simulations of a multilayer film of the homologous C24 molecule.This work was support by US National Science Foundation under Grant Nos. DMR-9802476 and DMR-0109057. The Midwest Universities Collaborative Access Team (MUCAT) sector at the Advanced Photon Source (APS) is supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES), through Ames Laboratory under ContractNo.W-7405-Eng-82. Use of the APS was supported by the DOE BES under Contract No. W-31-109-ENG-38

Seasonal Variations of the 7Be Solar Neutrino Flux

Measuring the 7Be solar neutrino flux is crucial towards solving the solar neutrino puzzle. The Borexino experiment, and possibly the KamLAND experiment, will be capable of studying the 7Be neutrinos in the near future. We discuss (1) how the seasonal variation of the Borexino and KamLAND data can be used to measure the 7Be solar neutrino flux in a background independent way and (2) how anomalous seasonal variations might be used to discover vacuum neutrino oscillations, independent of the solar model and the measurement of the background. In particular, we find that, after three years of Borexino or KamLAND running, vacuum neutrino oscillations can be either established or excluded for almost all values of (sin^2 2 theta, Delta m^2) preferred by the Homestake, GALLEX, SAGE, and Super-Kamiokande data. We also discuss how well seasonal variations of the data can be used to measure (sin^2 2 theta, Delta m^2) in the case of vacuum oscillations.Comment: 39 pages, 13 figures, uses psfig. Now the impact of the MSW effect on vacuum oscillations taken into account. Conclusions unchanged. References adde

The microbiome of the Melitaea cinxia butterfly shows marked variation but is only little explained by the traits of the butterfly or its host plant

Understanding of the ecological factors that shape intraspecific variation of insect microbiota in natural populations is relatively poor. In Lepidopteran caterpillars, microbiota is assumed to be mainly composed of transient bacterial symbionts acquired from the host plant. We sampled Glanville fritillary (Melitaea cinxia) caterpillars from natural populations to describe their gut microbiome and to identify potential ecological factors that determine its structure. Our results demonstrate high variability of microbiota composition even among caterpillars that shared the same host plant individual and most likely the same genetic background. We observed that the caterpillars harboured microbial classes that varied among individuals and alternated between two distinct communities (one composed of mainly Enterobacteriaceae and another with more variable microbiota community). Even though the general structure of the microbiota was not attributed to the measured ecological factors, we found that phylogenetically similar microbiota showed corresponding responses to the sex and the parasitoid infection of the caterpillar and to those of the host plant's microbial and chemical composition. Our results indicate high among-individual variability in the microbiota of the M. cinxia caterpillar and contradict previous findings that the host plant is the major driver of the microbiota communities of insect herbivores.Peer reviewe

Nanoscale Observation of Alkane Delayering

Noncontact Atomic Force Microscopy and synchrotron x-ray scattering measurements on dotriacontane (n-C32H66 or C32) films adsorbed on SiO2-coated Si(100) wafers reveal a narrow temperature range near the bulk C32 melting point Tb in which a monolayer phase of C32 molecules oriented perpendicular to surface is stable. This monolayer phase undergoes a delayering transition to a three-dimensional (3D) fluid phase on heating to just above Tb and to a solid 3D phase on cooling below Tb. An equilibrium phase diagram provides a useful framework for interpreting the unusual spreading and receding of the monolayer observed in transitions to and from the respective 3D phases.Comment: 13 pages, 3 figure

Spiral surface growth without desorption

Spiral surface growth is well understood in the limit where the step motion is controlled by the local supersaturation of adatoms near the spiral ridge. In epitaxial thin-film growth, however, spirals can form in a step-flow regime where desorption of adatoms is negligible and the ridge dynamics is governed by the non-local diffusion field of adatoms on the whole surface. We investigate this limit numerically using a phase-field formulation of the Burton-Cabrera-Frank model, as well as analytically. Quantitative predictions, which differ strikingly from those of the local limit, are made for the selected step spacing as a function of the deposition flux, as well as for the dependence of the relaxation time to steady-state growth on the screw dislocation density.Comment: 9 pages, 3 figures, RevTe