UCN anomalous losses and the UCN capture cross section on material defects

Experimental data shows anomalously large ultra cold neutrons (UCN) reflection losses and that the process of UCN reflection is not completely coherent. UCN anomalous losses under reflection cannot be explained in the context of neutron optics calculations. UCN losses by means of incoherent scattering on material defects are considered and cross-section values calculated. The UCN capture cross section on material defects is enhanced by a factor of 10⁴ due to localization of UCN around defects. This phenomenon can explain anomalous losses of UCN.     

A thin film source of ultra-cold neutrons

Ultra-cold neutron (UCN) densities have been calculated for a thin film source using realistic phonon spectra of the film material (hydrogen and deuterium). The UCN's build up to a significant density inside a cryogenic container whose walls support the thin films. The UCN's density dependence on the neutron temperature,T _n, was also investigated. It was found that the density reaches a maximum value atT _n20K. A deuterium film at low temperature (5K) andT _n=30 K yields a UCN density 10^–9 ₀ where ₀ is the total flux.Work supported in part by NSF Grant # DMR-8024662     

Neutron diffraction study of polycrystalline 4He in a porous medium

The elastic (diffraction) component of the neutron scattering cross section, which carries information on the atomic structure of solid helium confined in silica aerogel, has been studied. Analysis of the crystalline structure of solid helium in a porous medium, which is determined from the existing neutron diffraction data, indicates that the superfluid phase is localized inside a hexagonal close-packed phase and is not present in a body-centered cubic crystal. It has also been revealed that the addition of the ³He isotope changes the structure of solid helium and hardly affects the formation of a superfluid phase.     

Multiquantum incoherent neutron scattering from molecular solids

General expressions are obtained for the scattering of neutrons from molecular solids with the exchange of m-quanta. Explicit expressions are obtained for the total cross section, when both the phonon and the torson modes are given by the Einstein distribution functions. The expressions can be readily utilized to understand neutron scattering from a molecular crystal.     

A liquid hydrogen source of ultra-cold neutrons

A liquid hydrogen source of ultra-cold neutrons (UCN) developed for an experimental search for the electric dipole moment of the neutron is described. The results of an investigation of the yield of UCN from gaseous, liquid, and solid hydrogen as a function of temperature are presented. The UCN counting rate obtained at the output of the 6 × 7 cm² neutron guide tube is 5 × 10⁴ n/s. This counting rate corresponds to a flux of neutrons whose velocity along the axis of the neutron guide tube is below 7 m/s. Preliminary measurements of the UCN yield from liquid and solid deuterium have been carried out.     

Scattering of ultra-cold neutrons (UCN) by superfluid helium at temperatures around 1 K

We report the first observation of the upscattering of UCN stored in a closed vessel filled with ⁴He. The observations allow the direct determination of the production rate and hence the density of the UCN stored in the helium, which are found to be in agreement with theoretical expectations. the technique offers the possibility of studying the interactions between excitations in the liquid at regions of temperature and wavelength inaccessible to presently available methods.     

Raman scattering of light by 2LO-phonons

A theory of resonance Raman scattering of light by 2LO-phonons in semiconductor crystals is presented. Wannier excitons are considered as intermediate states. Analytic expressions are obtained that take into account contributions from different chains of intermediate states. The scattering cross section is shown to be weakly dependent on the wave vector imparted to the phonon system. The theoretical model permits the calculation of the scattering cross section for the energy of the exciting radiation photon below the level of exciton resonances.     

Dissipation of ripplon flow at the surface of superfluid <Superscript>4</Superscript>He

Within the framework of quantum hydrodynamics of a superfluid helium surface, the momentum relaxation rate caused by the annihilation of two ripplons with phonon creation, inelastic phonon scattering with ripplon annihilation, and in the case of helium films one-particle ripplon scattering from the surface-level inhomogeneities introduced by the substrate roughness (new relaxation mechanism) was obtained for a ripplon gas at T?0.25 K. The contribution from the inelastic phonon scattering is negligible at these temperatures. For a film at T≤0.15 K, one-particle scattering dominates, leading to a temperature dependence of the form K∝T^5/3 for the convective thermal conductance.At higher temperatures, phonon creation with annihilation of two ripplons is the dominant mechanism, giving K∝T^?3. These results are in quantitative agreement with the available experimental data.     

Polarisationsmessungen an Elektronenstrahlen nach der Transmission durch magnetisierte Eisenschichten

Magnetized iron foils of a thickness of 500 Å were transmitted by 60 keV electrons. The spin polarization of the electron beam was investigated by Mott scattering after transmission. No polarization effect was found. Since the lowest detectable polarization degree wasP=0·003 the atomic polarization cross section must be assumed to be smaller than 0·8·10^?20cm². This cross section is the sum of the spin exchange cross section and the spin dependent part of the total scattering cross section. If the electron binding energy is neglected, the spin depentend part of the total scattering cross section can be calculated from the theory of Møller scattering. In the case of our conditions-60 keV and an aperture of 10^?3 radian-the calculated cross section is smaller than the experimental upper limit. In further experiments the electron beam was split into the energy spectrum by an electrostatic analyzer placed between the iron foil and the Mott scattering foil. In these measurements only small parts of the energy spectrum were investigated, however, even here no detectable polarization occured.     

Light-light scattering in vector electrodynamics

Expressions for scattering amplitudes with definite polarizations in the approximation ω² ? m² are found. The asymptotic γγ→γγ total cross section is evaluated for the scattering of unpolarized photons. Also the forward scattering amplitudes and the differential cross section for forward scattering are obtained.     

The interaction of Ultra-Cold Neutrons (UCN) with liquid helium and a superthermal UCN source

We discuss the interaction of Ultra-Cold and Cold Neutrons with superfluid ⁴He and show that this interaction has all the characteristics which are necessary for the achievement of extremely high densities of UCN.     

Neutrons scattering as a method for fermi surface investigations

High-T_c A15-compounds are suggested as suitable materials for which Fermi surface investigations by means of thermal neutron scattering might be possible. The structure of the differential neutron—d-electron cross section at small scattering angles is connected in a simple way with parameters defining the Fermi surface of the linear chain model. The magnitude of the cross section is estimated are compared with the phonon background.     

Spin-dependent phonon Raman scattering in CdCr2Se4

Influences of ferromagnetic ordering on the phonon Raman scattering are studied for CdCr₂Se₄ through the intensity measurements of Raman spectra between 25 and 300 K with various wavelengths of excitation light (488.0–676.4 nm). Spin-dependent enhancements of Raman cross section are observed for optical phonon lines D(168 cm^?1) and F(238 cm^?1) with excitation wavelengths of about 630 and 550 nm, respectively. This kind of phenomenon in spinel-type chalcogen chromites seems to originate in spin-dependent intermediate interactions in the excited states of specific electronic transitions with which the incident or scattered light is resonant.     

Dynamics of atom scattering from 4He nanoclusters

We present a microscopic theory and results of atom scattering calculations to determine the dispersion of surface modes (ripplons) of superfluid helium-4 nanodroplets, expanding previous work [J. Chem. Phys. 115, 10161 (2001)]. A quantum transport formalism is adapted to the many-body scattering problem, yielding both elastic and inelastic fluxes. We demonstrate that, in analogy to the dynamic structure function S(k,ω) obtained from neutron scattering, a dynamic structure function σ(k,ω) can be obtained from ³He scattering. The ³He dynamic structure function σ(k,ω) is sensitive to surface dynamics, whereas the neutron dynamic structure function S(k,ω) is dominated by bulk-like excitations, in particular by rotons. Unlike for neutron-scattering, the total inelastic cross section for atom-scattering on ⁴He nanodroplets is large which we believe makes experimental detection feasible. We also show that scattering identical particles, i.e. ⁴He atoms, does not provide information about the dispersion of surface modes. Instead, inelastically scattered ⁴He atoms preferably lose roughly half their energy.     

Comparison of small-angle p and pp elastic scattering at 52.8 GeV center-of-mass energy at the CERN intersecting storage rings

Proton-antiproton and proton-proton elastic scattering have been measured in the four-momentum transfer range 0.001|t|0.06 GeV² for center-of-mass energy 52.8 GeV at the CERN Intersecting Storage Rings (ISR). Using the known pp total cross section, a simultaneous fit to the p and pp differential cross sections yields the p total cross section; in addition, we obtain the ratio of the real-to-imaginary part of the forward nuclear-scattering amplitude and the nuclear-slope parameter for both p and pp. Our results show conclusively that the p total cross section is rising at ISR energies and lend support to conventional theories in which the difference between the p and pp total cross section vanishes at very high energy.     

Improved optical potential for low and medium energy π−-4He elastic scattering

Effects of the second-order correction in multiple scattering theory and the nucleon binding correction to the optical potential are investigated in π^?-⁴He elastic scattering and total reaction cross-sections. The second-order correction includes not only nucleon-nucleon correlation effects but also the contribution from spin (isospin) double-flip processes. It is found that the latter has an ascendency over the former. We have also shown that the nucleon binding effect is very important to reproduce the energy dependence of the total elastic cross section and the total reaction cross section, especially in the low energy and resonance energy regions.     

Impurities in an imperfect bose gas II. Quasi-particle scattering

The problem of quasi-particle scattering by a hard sphere in an imperfect Bose gas is examined in order to gain insight into the behavior of ions in He II. Wave functions and phase shifts are calculated exactly using a simple approximation for the condensate wave function. For wavelengths characteristic of temperatures down to about 0.1°K the consequent cross sections are much like those of classical sound waves. (The depression of the condensate near the sphere plays a crucial role in this result.) At longer wavelengths the total cross section approaches a constant. Such an effect in He II would lead, for temperatures less than about 0.1°K, to ion mobilities substantially smaller than one would predict using acoustic cross sections. The more complicated problem of the scattering of density fluctuations (phonons) is considered briefly, and, with the help of further approximations, it is verified that the phonon scattering exhibits the acoustic characteristics calculated for the single-particle scattering.     

Speed of sound in an optical lattice

A system of equal mixture of ⁶Li atomic Fermi gas of two hyperfine states loaded into a cubic three‐dimensional optical lattice is studied assuming a negative scattering length (BCS side of the Feshbach resonance). When the interaction is attractive, fermionic atoms can pair and form a superfluid. The dispersion of the phonon‐like mode and the speed of sound in the long‐wavelength limit are obtained by solving the Bethe‐Salpeter equations for the collective modes of the attractive Hubbard Hamiltonian.     

The total photon deuteron hadronic cross section in the energy range 0.265–4.215 GeV

The total cross section for photoproduction of hadrons on the deutron, σ_T^d, has been measured for photon energies in the range 0.265–40215 GeV. From this, using results for the photon total cross section, obtained previously with the same apparatus, the neutron total cross section has been determined in the resonance region. The resonant structure is found to be quite different from that for the proton. Thereafter the neutron cross section falls off steadily with energy, and the values obtained are consistently lower than those for the proton. Forward scattering amplitudes have been evaluated for the deuteron.     

Incoherent neutron scattering by the phonon-libron system in solid ortho-hydrogen

The incoherent scattering cross section of slow neutrons by ortho hydrogen atT=0 K has been calculated for the case where only one excitation (phonon, libron resp. a mixed mode quantum) is created. The results are compared with the experiment of Stein, Stiller and Stockmeyer and prove that only the coupling of phonons and librons can produce the measured cross section.