Limits on spurious contributions to integrated crosssections for photoexcitation and de-excitation of isomers

The large values of integrated cross sections for the excitation and de-excitation of nuclear isomers in (γ, γ′) reactions provide strong encouragement for the feasibility of an optically pumped gamma-ray laser. For this reason, sources of possible spurious contamination of the measurements were carefully considered. This paper reviews an analysis of possible sources as well as experimental limits on contamination of the isomeric yields. The question of spurious contributions from (n, γ) or (n, n′) reactions was examined by estimating the level of thermal, epithermal and fast neutron fluxes based on possible source material in the accelerator environments. Such possibilities were severely reduced by the range of photon energies employed in the studies of 1.5-6 MeV. The expected fluxes were below levels necessary to produce significant isomeric yields in this energy range. Next, experiments were conducted in accordance with standard neutron activation-foil techniques to directly measure any fluxes of neutrons in the accelerator environments. Measurements for fast neutron fluxes were completely negative under even the most likely conditions with a 6 MeV medical linac. Measurable fluxes of thermal and epithermal neutrons were obtained. However, in typical cases the amount of isomeric activation due to “slow” neutrons was 1% of the total activation and 6% in the worst case based on measured fluxes and known values of cross sections. This revised version was published online in August 2006 with corrections to the Cover Date.     

A neutron reflectometry study of polystyrene network interfaces

We have used neutron reflectometry to measure interfacial widths between two polystyrene films, where either one or both films are crosslinked. The observed interfacial width between two networks is larger than the size expected for “dangling ends”, which suggests motion of heterogeneous regions of the networks. In the case when one of the networks is replaced by a linear polymer, the interfacial profile can be asymmetric with a diffusion “front” of linear polymer penetrating the network to a length scale of up to 200 ?. In the case of a more densely crosslinked network and a high molecular weight linear polymer the interface is symmetric implying negligible penetration. Received: 4 December 1996 / Revised: 13 October 1997 / Accepted: 23 December 1997     

Evidence for a K<Superscript>π</Superscript> = 1/2<Superscript>+</Superscript> isomer in neutron-rich <Superscript>185</Superscript>Ta

In this paper we present for the first time the measured delayed neutron (DN) yields and time spectra from high-energy protons interacting with thick ^natPb targets. The 1GeV protons from the accelerator impinged on targets of different thicknesses producing a huge number of spallation-fission products, some of which can be DN precursors. After the beam is switched off, the DNs were detected with optimized ³He counter. The production yields of light DN precursors as ¹⁷N and “usual” fission products as ⁸⁷Br and ⁸⁸Br, which dominate the total DN activity, are obtained both for thin and thick targets. These new data are of great interest for the new generation high-power spallation targets based on liquid-metal technologies. Our findings also should help to constrain the physics models within the simulations codes.     

Detection of submicron-sized raft-like domains in membranes by small-angle neutron scattering

Using coarse grained models of heterogeneous vesicles we demonstrate the potential for small-angle neutron scattering (SANS) to detect and distinguish between two different categories of lateral segregation: 1) unilamellar vesicles (ULV) containing a single domain and 2) the formation of several small domains or “clusters” (~10 nm in radius) on a ULV. Exploiting the unique sensitivity of neutron scattering to differences between hydrogen and deuterium, we show that the liquid ordered (lo) DPPC-rich phase can be selectively labeled using chain deuterated dipalymitoyl phosphatidylcholine (dDPPC), which greatly facilitates the use of SANS to detect membrane domains. SANS experiments are then performed in order to detect and characterize, on nanometer length scales, lateral heterogeneities, or so-called “rafts”, in ~30 nm radius low polydispersity ULV made up of ternary mixtures of phospholipids and cholesterol. For 1:1:1 DOPC:DPPC:cholesterol (DDC) ULV we find evidence for the formation of lateral heterogeneities on cooling below 30 °C. These heterogeneities do not appear when DOPC is replaced by SOPC. Fits to the experimental data using coarse grained models show that, at room temperature, DDC ULV each exhibit approximately 30 domains with average radii of ~10 nm.     

Phonon density of states of tetracyanoethylene from coherent inelastic neutron scattering at Dhruva reactor

Inelastic neutron scattering experiments to determine phonon density of states of coherent scattering samples of polycrystalline complex solids are generally intensity-limited and therefore are feasible only at high flux facilities. Phonon density of states of the monoclinic phase of tetracyanoethylene at 300 K, obtained using the medium resolution triple axis spectrometer at the new Indian medium flux reactor Dhruva are reported here. The raw data is converted to the “neutron weighted” phonon density of states by applying suitable corrections. Comparison made with results from a theoretical calculation based on a semirigid molecule model of lattice dynamics is fair. Results from Dhruva are also consistent with that obtained (to be published) at the high flux pulsed neutron source (ISIS) of the Rutherford Appleton Laboratory in United Kingdom.     

Neutron laue diffraction in a weakly deformed crystal at the Bragg angles close to π/2

An essential magnification of an external force acting on a diffracting neutron for the Bragg angles θ_B close to the right one is observed. Any external action (caused by either crystal deformation or external force affected the neutron) results in a bend of the so called “Kato trajectories” inside the crystal and, for the case of a finite crystal, gives considerable variation of the intensities of both diffracted neutron beams (direct and reflected). It is shown that the magnification factor is proportional to tan² (θ_b) and can reach (10²−10³) for Bragg angles surfficiently close to 90°. The text was submitted by the authors in English.     

Modelling the structure and ionic conduction of amorphous (AgI)x(AgPO3)1−x with the RMC method

For the first time neutron diffraction, X-ray diffraction and EXAFS data have been combined simultaneously using the RMC method to model the fast-ion conducting glass, (AgI)_x(AgPO₃)_1−x. This material is of considerable technological and scientific interest due to its high ionic conductivity at ambient temperature. We present some details of the RMC technique and highlight some of the structural information obtained from our models. The origin of the “first sharp diffraction peak” in the neutron diffraction data is explained, about which there has been considerable speculation. Diffusion pathways for ionic conduction are observed. A simple analysis of available free volume shows that a percolation transition in the ionic conductivity occurs between x=0.2 and 0.3, in agreement with a prediction based on conductivity measurements. This study highlights the considerable power that these developments of the RMC method have for the structural modelling of complex amorphous materials. Paper presented at the 1st Euroconference on Solid State Ionics in Zajkynthos, Greece, 11–18 Sept. 1994     

Progress in direct-drive inertial confinement fusion research at the laboratory for laser energetics

Direct-drive inertial confinement fusion (ICF) is expected to demonstrate high gain on the National Ignition Facility (NIF) in the next decade and is a leading candidate for inertial fusion energy production. The demonstration of high areal densities in hydrodynamically scaled cryogenic DT or D₂ implosions with neutron yields that are a significant fraction of the “clean” 1-D predictions will validate the ignition-equivalent direct-drive target performance on the OMEGA laser at the Laboratory for Laser Energetics (LLE). This paper highlights the recent experimental and theoretical progress leading toward achieving this validation in the next few years. The NIF will initially be configured for X-ray drive and with no beams placed at the target equator to provide a symmetric irradiation of a direct-drive capsule. LLE is developing the “polar-direct-drive” (PDD) approach that repoints beams toward the target equator. Initial 2-D simulations have shown ignition. A unique “Saturn-like” plastic ring around the equator refracts the laser light incident near the equator toward the target, improving the drive uniformity. LLE is currently constructing the multibeam, 2.6-kJ/beam, petawatt laser system OMEGA EP. Integrated fast-ignition experiments, combining the OMEGA EP and OMEGA Laser Systems, will begin in FY08.     

Double passage of electromagnetic waves through magnetized plasma: approximation of independent normal waves

Polarization properties of electromagnetic waves, double-passed through magnetized plasma, are studied. Analyses are performed in the case of non-interacting normal modes, propagating in homogeneous and weakly inhomogeneous plasmas, and for three kinds of reflectors: metallic plane, 2D corner retro-reflector (2D-CR), and cubic corner retro-reflector (CCR). It is shown that an electromagnetic wave, reflected from a metallic plane and from a CCR, contains only “velocity-preserving” channels, whose phases are doubled in comparison with those of a single-passage propagation. At the same time, an electromagnetic wave reflected from a 2D-CR is shown to contain both “velocity-preserving” and “velocity-converting” channels, the latter converting the fast wave into the slow one and vice-versa. One characteristic feature of “velocity-converting” channels is that they reproduce the initial polarization state near the source, which might be of practical interest for plasma interferometry. In the case of circularly polarized modes, “velocity-preserving” channels completely disappear, and only “velocity-converting” channels are to be found.     

Optical and Thermal Simulator for Laser-assisted Magnetic Recording

“Laser-assisted magnetic recording”, in which a recording media is heated by a laser beam while writing data, is attracting attention as a technology that enables a recording density of 1 Tb/in.². There exists another technology for media in which the recording layer is constructed with many small projections that enable high magnetic coercivity. This is called “patterned media”. For developing hard disk drives using these methods, we developed a simulator that analyzes the optical intensity distribution from the optical head for laser-assisted recording and the temperature profile on the patterned media. The simulator calculates the optical model using the finite-difference time-domain (FDTD) method. The thermal analysis of the three-dimensional model allows fast calculations using the alternating direction implicit (ADI) method. The heat source distribution data for thermal analysis is calculated in order to use the results of optical analysis. The optical and thermal analyses of the laser-assisted recording model were investigated with the simulator.     

A discrete nonetheless remarkable brick in de Sitter: The “massless minimally coupled field”

Over the last ten years interest in the physics of de Sitter space—time has been growing very fast. Besides the supposed existence of a “de Sitterian period” in inflation theories, the observational evidence of an acceleration of the universe expansion (interpreted as a positive cosmological constant or a “dark energy” or some form of “quintessence”) has triggered a lot of attention in the physics community. A specific de Sitterian field called “massless minimally coupled field” (mmc) plays a fundamental role in inflation models and in the construction of the de Sitterian gravitational field. A covariant quantization of the mmc field, à la Krein—Gupta—Bleuler was proposed in Class. Quantum. Grav. 17, 1415 (2000). In this talk, we will review this construction and explain the relevance of such a field in the construction of a massless spin-2 field in de Sitter space—time.     

Zone mirror for image formation with neutrons

We propose a concave blazed reflection zone mirror as an achromatic neutronoptical device for high-resolution image formation with ultracold neutrons. The possibility of application to a neutron microscope is discussed. Work supported by the German “Bundesministerium für Forschung und Technologie”.     

Atomic structure of Ge quantum dots on the Si(001) surface

In situ morphological investigation of the “105” faceted Ge islands on the Si(001) surface (hut clusters) have been carried out using an ultra high vacuum instrument integrating a high resolution scanning tunnelling microscope and a molecular beam epitaxy vessel. Both species of hut clusters-pyramids and wedges-were found to have the same structure of the “105” facets which was visualized. Structures of vertexes of the pyramidal clusters and ridges of the wedge-shaped clusters were revealed as well and found to be different. This allowed us to propose a crystallographic model of the “105” facets as well as models of the atomic structure of both species of the hut clusters. An inference is made that transitions between the cluster shapes are impossible.     

洁净核能源技术和D-Li强中子源的应用设想

在讨论国际科学界普遍关注的加速器驱动核动力装置技术,及其在未来洁净核能源开发和利用核嬗变方法处理核废料等方面的应用意义和可行性的同时,从我国的实际出发,提出了立足国内,利用中能（４０ＭｅＶ）、强流（２×１２５ｍＡ）质子加速器作Ｄ－Ｌｉ超强中子源,开展零功率加速器驱动核裂变装置技术的原理性研究和核嬗变、聚变材料、抗核加固及核物理研究,以及生产放射性同位素的初步设想. The accelerator driven nuclear power technology, as one of the most important scientific projects in the world, is evaluated. Some of its applications on transmuting nuclear waste, producing fission energy and doing scientific researches are also discussed. A proposal of using high intensity D Li fast neutron source as driven source of a zero power accelerator driven fission facility is outlined for the principle research and multi application purposes based on the present scientific...     

Detecting hidden spatial and spatio-temporal structures in glasses and complex physical systems by multiresolution network clustering

We elaborate on a general method that we recently introduced for characterizing the “natural” structures in complex physical systems via multi-scale network analysis. The method is based on “community detection” wherein interacting particles are partitioned into an “ideal gas” of optimally decoupled groups of particles. Specifically, we construct a set of network representations (“replicas”) of the physical system based on interatomic potentials and apply a multiscale clustering (“multiresolution community detection”) analysis using information-based correlations among the replicas. Replicas may i) be different representations of an identical static system, ii) embody dynamics by considering replicas to be time separated snapshots of the system (with a tunable time separation), or iii) encode general correlations when different replicas correspond to different representations of the entire history of the system as it evolves in space-time. Inputs for our method are the inter-particle potentials or experimentally measured two (or higher order) particle correlations. We apply our method to computer simulations of a binary Kob-Andersen Lennard-Jones system in a mixture ratio of A₈₀B₂₀ , a ternary model system with components “A”, “B”, and “C” in ratios of A₈₈B₇C₅ (as in Al₈₈Y₇Fe₅ , and to atomic coordinates in a Zr₈₀Pt₂₀ system as gleaned by reverse Monte Carlo analysis of experimentally determined structure factors. We identify the dominant structures (disjoint or overlapping) and general length scales by analyzing extrema of the information theory measures. We speculate on possible links between i) physical transitions or crossovers and ii) changes in structures found by this method as well as phase transitions associated with the computational complexity of the community detection problem. We also briefly consider continuum approaches and discuss rigidity and the shear penetration depth in amorphous systems; this latter length scale increases as the system becomes progressively rigid.     

Ultra low momentum neutron catalyzed nuclear reactions on metallic hydride surfaces

Ultra low momentum neutron catalyzed nuclear reactions in metallic hydride system surfaces are discussed. Weak interaction catalysis initially occurs when neutrons (along with neutrinos) are produced from the protons that capture “heavy” electrons. Surface electron masses are shifted upwards by localized condensed matter electromagnetic fields. Condensed matter quantum electrodynamic processes may also shift the densities of final states, allowing an appreciable production of extremely low momentum neutrons, which are thereby efficiently absorbed by nearby nuclei. No Coulomb barriers exist for the weak interaction neutron production or other resulting catalytic processes. PACS 24.60.-k, 23.20.Nx     

Reconstruction of the Fermi surface in the pseudogap state of cuprates

Reconstruction of the Fermi surface of high-temperature superconducting cuprates in the pseudogap state is analyzed within a nearly exactly solvable model of the pseudogap state, induced by short-range order fluctuations of the antiferromagnetic (AFM), spin-density wave (SDW), or a similar charge-density wave (CDW) order parameter, competing with the superconductivity. We explicitly demonstrate the evolution from “Fermi arcs” (on the “large” Fermi surface) observed in the ARPES experiments at relatively high temperatures (when both the amplitude and phase of the density waves fluctuate randomly) towards the formation of typical “small” electron and hole “pockets,” which are apparently observed in the de Haas-van Alphen and Hall resistance oscillation experiments at low temperatures (when only the phase of the density waves fluctuate and the correlation length of the short-range order is large enough). A qualitative criterion for the quantum oscillations in high magnetic fields to be observable in the pseudogap state is formulated in terms of the cyclotron frequency, the correlation length of fluctuations, and the Fermi velocity. The text was submitted by the authors in English.     

Axion emission from a magnetized neutron gas

By using the polarization density matrix for a neutron in a magnetic field, the axion luminosity of magnetic neutron stars that is associated with the flip of the anomalous magnetic moment of degenerate nonrelativistic neutrons is calculated. It is shown that, at values of the magnetic-field induction in the region B ≳ 10¹⁸ G, this mechanism of axion emission is dominant in “young” neutron stars of temperature about a few tens of MeV units. At B ∼ 10¹⁷ G, it is one of the basic mechanisms. The Fermi energy of a degenerate neutron gas in a magnetic field is found, and it is shown that there is no such mechanism of axion emission in the degenerate case.     

Hysteresis Phenomenon in Deterministic Traffic Flows

We study phase transitions of a system of particles on the one-dimensional integer lattice moving with constant acceleration, with a collision law respecting slower particles. This simple deterministic “particle-hopping” traffic flow model being a straightforward generalization to the well known Nagel–Schreckenberg model covers also a more recent slow-to-start model as a special case. The model has two distinct ergodic (unmixed) phases with two critical values. When traffic density is below the lowest critical value, the steady state of the model corresponds to the “free-flowing” (or “gaseous”) phase. When the density exceeds the second critical value the model produces large, persistent, well-defined traffic jams, which correspond to the “jammed” (or “liquid”) phase. Between the two critical values each of these phases may take place, which can be interpreted as an “overcooled gas” phase when a small perturbation can change drastically gas into liquid. Mathematical analysis is accomplished in part by the exact derivation of the life-time of individual traffic jams for a given configuration of particles. This research has been partially supported by Russian Foundation for Fundamental Research and French Ministry of Education grants.     

Perspectives on Current Issues Is “Anthropic Selection” Science?

I argue that there are strong reasons for resisting as a principle of science the concept of “anthropic selection.” This concept asserts that the existence of “observers” in a universe can be used as a condition that selects physical laws and constants necessary for intelligent life from different laws or physical constants prevailing in a vast number of other universes, to thereby explain why the properties of our universe are conducive to intelligent life. My reasons for limiting “anthropic selection” to the realm of speculation rather than permitting it to creep into mainstream science include our inability to estimate the probabilities of emergence of “observers” in a universe, the lack of testability through direct observation of the assumed high variability of the constants of nature, the lack of a clear definition of an “observer,” and the arbitrariness in how and to what questions anthropic selection is applied.