Isotropic Angular Distribution Research Articles

Production cross-section of antiprotons for the study of cosmic-ray propagation in the Galaxy

The production cross-section for antiprotons ( p ¯ ’s) in p– p collisions is presented for the study of cosmic-ray antiproton (CR- p ¯ ) propagation in the Galaxy. We propose a semi-empirical production cross-section of p ¯ , with only three free parameters, the average multiplicity, N p ¯ , the average transverse momentum, 〈 p T 〉 , and the deformation parameter, τ , which characterises the deformation from the isotropic angular distribution of p ¯ in the center of mass system (CMS). This cross-section reproduces the experimental data now available in the proton projectile energy range of 10–1000 GeV very well. We also check the applicability of the present formula for proton–nucleus collisions. In order to apply the present result for the CR- p ¯ data, we further present the energy dependence of the parameters appearing in the production cross-section of p ¯ .

Photofragment imaging study of the CH2CCH2OH radical intermediate of the OH+allene reaction

These velocity map imaging experiments characterize the photolytic generation of one of the two radical intermediates formed when OH reacts via an addition mechanism with allene. The CH2CCH2OH radical intermediate is generated photolytically from the photodissociation of 2-chloro-2-propen-1-ol at 193 nm. Detecting the Cl atoms using [2+1] resonance-enhanced multiphoton ionization evidences an isotropic angular distribution for the Cl+CH2CCH2OH photofragments, a spin-orbit branching ratio for Cl(2P1/2):Cl(2P3/2) of 0.28, and a bimodal recoil kinetic energy distribution. Conservation of momentum and energy allows us to determine from this data the internal energy distribution of the nascent CH2CCH2OH radical cofragment. To assess the possible subsequent decomposition pathways of this highly vibrationally excited radical intermediate, we include electronic structure calculations at the G3//B3LYP level of theory. They predict the isomerization and dissociation transition states en route from the initial CH2CCH2OH radical intermediate to the three most important product channels for the OH+allene reaction expected from this radical intermediate: formaldehyde+C2H3, H+acrolein, and ethene+CHO. We also calculate the intermediates and transition states en route from the other radical adduct, formed by addition of the OH to the center carbon of allene, to the ketene+CH3 product channel. We compare our results to a previous theoretical study of the O+allyl reaction conducted at the CBS-QB3 level of theory, as the two reactions include several common intermediates.

Study of thepd→pdηreaction

A study of the pd{yields}pd{eta} reaction in the energy range where the recent data from Uppsala are available is done in the two-step model of {eta} production including the final state interaction. The {eta}-d final state interaction is incorporated through the solution of the Lippmann Schwinger equation using an elastic scattering matrix element, T{sub {eta}}{sub d{yields}}{sub {eta}}{sub d}, which is required to be half off-shell. It is written in a factorized form, with an off-shell form factor multiplying an on-shell part given by an effective range expansion up to the fourth power in momentum. The parameters of this expansion have been taken from an existing recent relativistic Faddeev equation solution for the {eta}NN system corresponding to different {eta}-N scattering amplitudes. Calculations have also been done using few body equations within a finite rank approximation to generate T{sub {eta}}{sub d{yields}}{sub {eta}}{sub d}. The p-d final state interaction is included in the spirit of the Watson-Migdal prescription by multiplying the matrix element by the inverse of the Jost function. The {eta}-d interaction is found to be dominant in the region of small invariant {eta}-d mass, M{sub {eta}}{sub d}. The p-d interaction enhances the cross section in the whole region of M{sub {eta}}{submore » d}, but is larger for large M{sub {eta}}{sub d}. We find nearly isotropic angular distributions of the proton and the deuteron in the final state. All the above observations are in agreement with the data. The production mechanism for the entire range of the existing data on the pd{yields}pd{eta} reaction seems to be dominated by the two-step model of {eta} production.« less

Anomalous angular distribution of fragment ions from rare-gas diatomic molecules with intense, femtosecond, near-infrared laser pulses

Almost isotropic angular distributions are observed for fragment ions from ${\mathrm{Xe}}_{2}$ produced with intense femtosecond laser pulses ($800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, $4\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$). The results observed for ${\mathrm{Xe}}_{2}$ present a great contrast to those for ${\mathrm{I}}_{2}$ molecules used as reference molecules, which show typical anisotropic angular distributions resulting from enhanced ionization. The kinetic energy spectra of the fragment ions from ${\mathrm{Xe}}_{2}$ reveal that the internuclear distance remains near the equilibrium distance for various pulse widths from 45 to $710\phantom{\rule{0.3em}{0ex}}\mathrm{fs}$, indicating that in the rising edge of the pulses, the internuclear distance of ${\mathrm{Xe}}_{2}^{\phantom{\rule{0.4em}{0ex}}n+}$ ($n=0$, 1, 2) does not reach the favorable region for enhanced ionization even though enhancement can potentially work for ${\mathrm{Xe}}_{2}^{\phantom{\rule{0.4em}{0ex}}n+}$. The dynamic alignment of neutral ${\mathrm{Xe}}_{2}$ during the pulses is also discussed on the basis of numerical simulations.

Kα1radiation from heavy, heliumlike ions produced in relativistic collisions

Bound-state transitions in few-electron, heavy ions following radiative electron capture are studied within the framework of the density matrix theory and the multiconfiguration Dirac-Fock approach. Special attention is paid to the $K{\ensuremath{\alpha}}_{1}$ $(1{s}_{1∕2}2{p}_{3∕2}\phantom{\rule{0.2em}{0ex}}^{1,3}P_{J=1,2}\ensuremath{\rightarrow}1{s}_{1∕2}^{2}\phantom{\rule{0.2em}{0ex}}^{1}S_{J=0})$ radiative decay of heliumlike uranium ${\mathrm{U}}^{90+}$ projectiles. This decay has recently been observed at the GSI facility in Darmstadt, giving rise to a surprisingly isotropic angular distribution, which is inconsistent with previous experiments and calculations based on a ``one-particle'' model. We show that the unexpected isotropy essentially results from the mutual cancellation of the angular distributions of the $^{1}P_{1}\ensuremath{\rightarrow}^{1}S_{0}$ electric dipole and $^{3}P_{2}\ensuremath{\rightarrow}^{1}S_{0}$ magnetic quadrupole transitions, both of which contribute to the $K{\ensuremath{\alpha}}_{1}$ radiation. Detailed computations on the anisotropy of the $K{\ensuremath{\alpha}}_{1}$ radiation have been carried out for a wide range of projectile energies and are compared to available experimental data.

Photodissociation dynamics of fluorobenzene (C6H5F) at 157 and 193nm: Branching ratios and distributions of kinetic energy

Following photodissociation of fluorobenzene (C6H5F) at 193 and 157 nm, we detected the products with fragmentation-translational spectroscopy by utilizing a tunable vacuum ultraviolet beam from a synchrotron for ionization. Between two primary dissociation channels observed upon irradiation at 193 (157) nm, the HF-elimination channel C6H5F --> HF + C6H4 dominates, with a branching ratio of 0.94+/-0.02 (0.61+/-0.05) and an average release of kinetic energy of 103 (108) kJ mol(-1); the H-elimination channel C6H5F --> H + C6H4F has a branching ratio of 0.06+/-0.02 (0.39+/-0.05) and an average release of kinetic energy of 18.6 (26.8) kJ mol(-1). Photofragments H, HF, C6H4, and C6H4F produced via the one-photon process have nearly isotropic angular distributions. Both the HF-elimination and the H-elimination channels likely proceed via the ground-state electronic surface following internal conversion of C6H5F; these channels exhibit small fractions of kinetic energy release from the available energy, indicating that the molecular fragments are highly internally excited. We also determined the ionization energy of C6H4F to be 8.6+/-0.2 eV.

Anisotropy of Fibrous Tissues in Relation to the Distribution of Tensed and Buckled Fibers

Fibrous tissues are characterized by a much higher stiffness in tension than compression. This study uses microstructural modeling to analyze the material symmetry of fibrous tissues undergoing tension and compression, to better understand how material symmetry relates to the distribution of tensed and buckled fibers. The analysis is also used to determine whether the behavior predicted from a microstructural model can be identically described by phenomenological continuum models. The analysis confirms that in the case when all the fibers are in tension in the current configuration, the material symmetry of a fibrous tissue in the corresponding reference configuration is dictated by the symmetry of its fiber angular distribution in that configuration. However, if the strain field exhibits a mix of tensile and compressive principal normal strains, the fibrous tissue is represented by a material body which consists only of those fibers which are in tension; the material symmetry of this body may be deduced from the superposition of the planes of symmetry of the strain and the planes of symmetry of the angular fiber distribution. Thus the material symmetry is dictated by the symmetry of the angular distribution of only those fibers which are in tension. Examples are provided for various fiber angular distribution symmetries. In particular, it is found that a fibrous tissue with isotropic fiber angular distribution exhibits orthotropic symmetry when subjected to a mix of tensile and compressive principal normal strains, with the planes of symmetry normal to the principal directions of the strain. This anisotropy occurs even under infinitesimal strains and is distinct from the anisotropy induced from the finite rotation of fibers. It is also noted that fibrous materials are not stable under all strain states due to the inability of fibers to sustain compression along their axis; this instability can be overcome by the incorporation of a ground matrix. It is concluded that the material response predicted using a microstructural model of the fibers cannot be described exactly by phenomenological continuum models. These results are also applicable to nonbiological fiber-composite materials.

Four phases of amorphous water: Simulations versus experiment

Multiplicity of the liquid-liquid phase transitions in supercooled water, first obtained in computer simulations [Brovchenko et al., J. Chem. Phys. 118, 9473 (2003)], has got strong support from the recent experimental observation of the two phase transitions between amorphous ices [Loerting et al., Phys. Rev. Lett. 96, 025702 (2006)]. These experimental results allow assignment of the four amorphous water phases (I-IV) obtained in simulations to the three kinds of amorphous ices. Water phase I (rho approximately 0.90 gcm(3)) corresponds to the low-density amorphous ice, phase III (rho approximately 1.10 gcm(3)) to the high-density amorphous ice, and phase IV (rho approximately 1.20 gcm(3)) to the very-high-density amorphous ice. Phase II of model water with density rho approximately 1.00 gcm(3) corresponds to the normal-density water. Such assignment is confirmed by the comparison of the structural functions of the amorphous phases of model water and real water. In phases I and II the first and second coordination shells are clearly divided. Phase I consists mainly of the four coordinated tetrahedrally ordered water molecules. Phase II is enriched with molecules, which have tetrahedrally ordered four nearest neighbors and up six molecules in the first coordination shell. Majority of the molecules in phase III still have tetrahedrally ordered four nearest neighbors. Transition from phase III to phase IV is characterized by a noticeable drop of tetrahedral order, and phase IV consists mainly of molecules with highly isotropic angular distribution of the nearest neighbors. Relation between the structures of amorphous water phases, crystalline ices, and liquid water is discussed.

Site and Isotopic Effects on the Angular Anisotropy of Products in the Photodissociation of Ethene at 157 nm

We measured angular-anisotropy parameters beta(E(t)) of fragments from photolysis of ethene and four isotopic variants at 157 nm using photo-fragment translational spectroscopy and selective photoionization. The averaged beta value of products ranges from -0.17 to 0.10, depending on dissociation pathways. Angular distributions of atomic hydrogen produced from C(2)H(4) and C(2)D(4) are isotropic. For dissociation into C(2)H(2) + H(2), beta has a small negative value whereas dissociation into C(2)D(2) + D(2) has an isotropic angular distribution. The photolysis of dideuterated ethene reveals site and isotopic effects on the angular distributions of products; products H(2), HD, and D(2) from photolysis of 1,1-CH(2)CD(2) have negative, nearly zero, and positive values of beta, respectively. Molecular hydrogen from photolysis of 1,2-cis-CHDCHD has a negative beta value and the anisotropy has a trend D(2) > H(2) > HD. Photolysis of 1,2-trans-CHDCDH produced a result similar to photolysis of 1,2-cis-CHDCHD for the angular anisotropy of molecular hydrogen except slightly more isotropic. A calculation of optimized geometries of ethene in the ground electronic state and pertinent transition structures enables a qualitative interpretation of the site and isotopic effects on the angular anisotropy of products. We deduce that the photoexcited state of ethene at 157 nm has a major character (1)B(1u) that produces a transition dipolar moment parallel to the C=C bond.

SOURCES: a code for calculating (α,n), spontaneous fission, and delayed neutron sources and spectra

SOURCES is a computer code that determines neutron production rates and spectra from (alpha,n) reactions, spontaneous fission and delayed neutron emission owing to the decay of radionuclides in homogeneous media, interface problems and three-region interface problems. The code is also capable of calculating the neutron production rates due to (alpha,n) reactions induced by a monoenergetic beam of alpha particles incident on a slab of target material. The (alpha,n) spectra are calculated using an assumed isotropic angular distribution in the centre-of-mass system with a library of 107 nuclide decay alpha-particle spectra, 24 sets of measured and/or evaluated (alpha,n) cross sections and product nuclide level branching fractions, and functional alpha particle stopping cross sections for Z < 106. Spontaneous fission sources and spectra are calculated with evaluated half-life, spontaneous fission branching and Watt spectrum parameters for 44 actinides. The delayed neutron spectra are taken from an evaluated library of 105 precursors. The code outputs the magnitude and spectra of the resultant neutron sources. It also provides an analysis of the contributions to that source by each nuclide in the problem.

Photoelectron anisotropy and channel branching ratios in the detachment of solvated iodide cluster anions

Photoelectron spectra and angular distributions in 267 nm detachment of the I(-)Ar, I(-)H(2)O, I(-)CH(3)I, and I(-)CH(3)CN cluster anions are examined in comparison with bare I(-) using velocity-map photoelectron imaging. In all cases, features are observed that correlate to two channels producing either I((2)P(3/2)) or I((2)P(1/2)). In the photodetachment of I(-) and I(-)Ar, the branching ratios of the (2)P(1/2) and (2)P(3/2) channels are observed to be approximately 0.4, in both cases falling short of the statistical ratio of 0.5. For I(-)H(2)O and I(-)CH(3)I, the (2)P(1/2) to (2)P(3/2) branching ratios are greater by a factor of 1.6 compared to the bare iodide case. The relative enhancement of the (2)P(1/2) channel is attributed to dipole effects on the final-state continuum wave function in the presence of polar solvents. For I(-)CH(3)CN the (2)P(1/2) to (2)P(3/2) ratio falls again, most likely due to the proximity of the detachment threshold in the excited spin-orbit channel. The photoelectron angular distributions in the photodetachment of I(-), I(-)Ar, I(-)H(2)O, and I(-)CH(3)CN are understood within the framework of direct detachment from I(-). Hence, the corresponding anisotropy parameters are modeled using variants of the Cooper-Zare central-potential model for atomic-anion photodetachment. In contrast, I(-)CH(3)I yields nearly isotropic photoelectron angular distributions in both detachment channels. The implications of this anomalous behavior are discussed with reference to alternative mechanisms, affording the solvent molecule an active role in the electron ejection process.

Quantification and Conformational State of Lysozyme Deposition on Etafilcon, Balafilcon and Lotrafilcon Contact Lens Materials worn on a Daily Wear Basis

Electron paramagnetic resonance spectroscopy was used to monitor the orientation of muscle cross-bridges attached to actin in a low force and high stiffness state that may occur before force generation in the actomyosin cycle of interactions. 2,3-butanedione monoxime (BDM) has been shown to act as an uncompetitive inhibitor of the myosin ATPase that stabilizes a myosin.ADP.P(i) complex. Such a complex is thought to attach to actin at the beginning of the powerstroke. Addition of 25 mM BDM decreases tension by 90%, although stiffness remains high, 40–50% of control, showing that cross-bridges are attached to actin but generate little or no force. Active cross-bridge orientation was monitored via electron paramagnetic resonance spectroscopy of a maleimide spin probe rigidly attached to cys-707 (SH-1) on the myosin head. A new labeling procedure was used that showed improved specificity of labeling. In 25 mM BDM, the probes have an almost isotropic angular distribution, indicating that cross-bridges are highly disordered. We conclude that in the pre-powerstroke state stabilized by BDM, cross-bridges are attached to actin, generating little force, with a large portion of the catalytic domain of the myosin heads disordered.

Individual fundamental mode dependence of H2O vibrational excitation in the 6–8 eV resonance region by electron impact

Resonance phenomena in the bending (010), symmetric (100), and asymmetric (001) stretching vibrational modes in water (H(2)O) molecules have been investigated by measuring differential cross sections (DCSs) at fixed energy losses of 0.43, 0.46, 0.49, and 0.51 eV while sweeping the impact energies from 1.6 to 10 eV, at angles 60 degrees and 90 degrees, using a crossed-beam method. No resonancelike enhanced features have been observed in the study of the (010) mode. In the DCSs for the combined (100+001) mode, a single broad hump was observed at each angle, whose position shifts towards the higher energy side with increasing scattering angle, a feature typical of the interference profiles previously observed in a study of the (301) vibrational mode. DCSs for the (001) mode exhibited a rather isotropic angular distribution with no evidence of resonances. However, enhancement of DCSs were indeed observed for the (100) mode, at the energy loss of 0.46 eV, which is attributable to the A(1) symmetry resonance that was studied by Seng and Linder [J. Phys. B 9, 2539 (1976)], with a typical s-wave characteristic angular distribution. Theoretical analysis based on the continuum multiple-scattering approach has been carried out to provide rationale to the experimental results.

Angular anisotropy of products in the photodissociation of propene at 157 nm

The dynamics of photodissociation of propene has been investigated at 157 nm in a molecular-beam apparatus using photofragment translational spectroscopy and photoionization produced with a synchrotron. We measured angular anisotropy parameters β of several products as a function of kinetic energy. Atomic H has an isotropic angular distribution, H 2 has a small negative β, and fragments due to C–C bond rupture have a small positive β. According to these measurements we suggest that the 2 1A′(π−3p) state has a major contribution on excitation at 157 nm. Photolysis of CD 3CHCH 2 reveals site effects on the angular distributions of products.

Photoproduction of eta-mesic 3He.

The photoproduction of eta-mesic 3He has been investigated using the TAPS calorimeter at the Mainz Microtron accelerator facility MAMI. The total inclusive cross section for the reaction gamma3He-->etaX has been measured for photon energies from threshold to 820 MeV. The total and angular differential coherent eta cross sections have been extracted up to energies of 745 MeV. A resonancelike structure just above the eta production threshold with an isotropic angular distribution suggests the existence of a resonant quasibound state. This is supported by studies of a competing decay channel of such a quasibound eta-mesic nucleus into pi(0)pX. A binding energy of (-4.4+/-4.2) MeV and a width of (25.6+/-6.1) MeV is deduced for the quasibound eta-mesic state in 3He.

Lithium erosion experiments and modelling under quiescent plasma conditions in DIII-D

Lithium-sputtering erosion and transport has been measured in the outer divertor of the DIII-D tokamak. The Divertor Materials Evaluation System (DiMES) mechanism places a 2.5 cm lithium spot as a plasma-facing surface in the divertor. Plasma diagnostics and atomic lithium visible spectroscopy are used to measure the lithium erosion yield near the outer strikepoint (OSP) with swept-plasma parameters of electron temperature 5–25 eV and electron density (0.03–1.80) × 1019 m−3. Solid-phase lithium physical sputtering is measured to be less than 10% (Li/D+). The yield increases with incident energy. Physical sputtering models confirm measurements of sputtered energy and spatial distributions showing skewed angular distributions when the sample is exposed near the OSP and isotropic angular distributions when the sample is exposed to the private flux region. REDEP/WBC modelling of near-surface impurity transport agrees well with experimental measurements showing sputtered lithium neutral atoms effectively ionized about a centimetre away from the Li–DiMES probe surface. A reduction in lithium physical sputtering by a factor of 4–5 is measured when the lithium surface forms an oxide, consistent with the physical sputtering behaviour of most metal-oxides. Although of less significance than lithium atom transport, there is a modelling/data discrepancy regarding lithium ion transport with, e.g. the data showing more asymmetric ion transport than predicted.

Quantification of the spreading effect of auroral proton precipitation

A three‐dimensional Monte Carlo model has been developed to study the transverse beam spreading effect of incident energetic auroral protons during their precipitation in the Earth's upper atmosphere. Energetic protons with an isotropic angular distribution are injected at 700 km altitude. Two types of incident energy spectra, a monoenergetic and a Maxwellian distribution, are considered. Interaction of fast particles with a three‐species atmosphere (O, N2, and O2) is included through charge exchange, electron stripping, ionization, excitation, and elastic scattering collisions. A uniform geomagnetic field is assumed in the model. The spreading effect is simulated for both a fine proton beam and a proton arc of longitudinal and latitudinal extent. It is found that the main dispersion region for a fine proton beam is located in the altitude range of around 250–450 km, where the first few charge exchange collisions play a significant role. In the spreading study for a proton arc, we compare the numerical results with previous studies and give a convincing explanation by analyzing atmospheric scale heights and cross‐section data. For the purpose of the model validity check, we make a comparison of the Monte Carlo simulation with observations and the results from other models.

Combined crossed-beam studies of C(3PJ)+C2H4→C3H3+H reaction dynamics between 0.49 and 30.8 kJ mol−1

The reaction C(3PJ)+C2H4(X 1A1)→C3H3+H(2S1/2) has been studied using complementary crossed molecular beam techniques. Integral cross sections have been obtained in the range of relative translational energies ET=0.49–24.9 kJ mol−1 in experiments conducted with pulsed supersonic beams coupled with laser-induced fluorescence detection of H(2S1/2) atoms. The major reaction pathway leading to HCCCH2 (propargyl)+H has been found without any barrier, with relative integral cross sections that are proportional to (ET)−0.60±0.03 below 8 kJ mol−1. Threshold for a minor pathway, leading also to H formation, occurs around 6 kJ mol−1; the relative importance of this second pathway increases with relative translational energy. Differential cross sections have been obtained at three relative translational energies: ET=9.1, 17.2, and 30.8 kJ mol−1 in experiments conducted with continuous supersonic molecular beams coupled with universal mass spectrometric detection and time-of-flight analysis. At the lowest ET of 9.1 kJ mol−1 formation of HCCCH2 (propargyl)+H is observed to be the dominant channel with a nearly forward–backward symmetric angular distribution in the center-of-mass (cm) frame; about 35% of the total available energy is channeled into translation indicating that the propargyl radical is highly internally excited; formation of less stable C3H3 isomer(s) is minor (2%). As ET increases, formation of appreciable, increasingly larger fractions of less stable propyn-l-yl and/or cyclopropenyl isomers is also observed. These findings are consistent with the integral cross-section measurements. While formation of propargyl is thought to proceed via an osculating complex mechanism following addition of C(3PJ) to the double bond of ethylene, the dynamics of formation of the less stable isomers is going through a long-lived complex, as witnessed by an isotropic cm angular distribution. The H2 elimination channel leading to C3H2 formation has not been found to occur, which suggests that inter-system-crossing to the ground singlet C3H4 potential energy surface manifold has low probability and/or the H2-elimination process on the triplet surface is characterized by a very large exit potential barrier.

Electron kinetic energy measurements from laser irradiation of clusters

We have measured electron kinetic energy spectra from the interaction of a low-density beam of noble-gas clusters with a high-intensity femtosecond laser pulse. Electron energies were measured both through their time of flight and through measurements of electron signals as a function of retarding voltages up to 7 kV. We have observed electrons with energies up to 6 keV and a sharp peak at an arrival time consistent with the detection of photons. The electrons had a broad angular distribution peaked along the polarization axis, while the sharp peak had an isotropic angular distribution. We attribute this sharp peak to uv, xuv, or x-ray photons formed as a result of the laser-cluster interaction and were unable to observe the two-lobed electron kinetic energy distribution reported by Shao et al. [Phys. Rev. Lett. 77, 3343 (1996)] with any combination of the laser and cluster parameters used.

Classical transport of charged particles in a magnetic field

We examine the traditional transport equation for classical, charged particles diffusing in a cold, absorbing medium subject to a uniform magnetic field and in which scattering is isotropic. Steady-state solutions in plane geometry are examined in some detail; we make no expansion about an isotropic angular distribution. Restricting the motion to two dimensions captures most of the interesting features; there is some discussion of the three-dimensional case.