Showing papers on "Proton published in 2003"

Induced magnetic ordering by proton irradiation in graphite.

Abstract: We provide evidence that proton irradiation of energy 2.25 MeV on highly oriented pyrolytic graphite samples triggers ferro- or ferrimagnetism. Measurements performed with a superconducting quantum interferometer device and magnetic force microscopy reveal that the magnetic ordering is stable at room temperature.

Voltage-Gated Proton Channels and Other Proton Transfer Pathways

Abstract: Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H+ channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H+ch...

Statistical analysis of relativistic electron energies for cyclotron resonance with EMIC waves observed on CRRES

Abstract: Electromagnetic ion cyclotron (EMIC) waves which propagate at frequencies below the proton gyrofrequency can undergo cyclotron resonant interactions with relativistic electrons in the outer radiation belt and cause pitch-angle scattering and electron loss to the atmosphere. Typical storm-time wave amplitudes of 1-10 nT cause strong diffusion scattering which may lead to significant relativistic electron loss at energies above the minimum energy for resonance, E-min. A statistical analysis of over 800 EMIC wave events observed on the CRRES spacecraft is performed to establish whether scattering can occur at geophysically interesting energies (less than or equal to2 MeV). While E-min is well above 2 MeV for the majority of these events, it can fall below 2 MeV in localized regions of high plasma density and/or low magnetic field (f(pe)/f(ce,eq) > 10) for wave frequencies just below the hydrogen or helium ion gyrofrequencies. These lower energy scattering events, which are mainly associated with resonant L-mode waves, are found within the magnetic local time range 1300 4.5. The average wave spectral intensity of these events (4-5 nT(2)/Hz) is sufficient to cause strong diffusion scattering. The spatial confinement of these events, together with the limited set of these waves that resonate with less than or equal to2 MeV electrons, suggest that these electrons are only subject to strong scattering over a small fraction of their drift orbit. Consequently, drift-averaged scattering lifetimes are expected to lie in the range of several hours to a day. EMIC wave scattering should therefore significantly affect relativistic electron dynamics during a storm. The waves that resonate with the similar toMeV electrons are produced by low-energy (similar tokeV) ring current protons, which are expected to be injected into the inner magnetosphere during enhanced convection events.

Gas-Phase Infrared Spectrum of the Protonated Water Dimer

Abstract: The protonated water dimer is a prototypical system for the study of proton transfer in aqueous solution. We report infrared photodissociation spectra of cooled H + (H 2 O) 2 [and D + (D 2 O 2 ] ions, measured between 620 and 1900 wave numbers (cm −1 ). The experiment directly probes the shared proton region of the potential energy surface and reveals three strong bands below 1600 cm −1 and one at 1740 cm −1 (for H 5 O 2 + ). From a comparison to multidimensional quantum calculations, the three lower energy bands were assigned to stretching and bending fundamentals involving the O⋯H + ⋯O moiety, and the highest energy band was assigned to a terminal water bend. These results highlight the importance of intermode coupling in shared proton systems.

Proton Transport through Water-Filled Carbon Nanotubes

Abstract: Proton transfer along 1D chains of water molecules inside carbon nanotubes is studied by simulations. Ab initio molecular dynamics and an empirical valence bond model yield similar structures and time scales. The proton mobility along 1D water chains exceeds that in bulk water by a factor of 40, but is reduced if orientational defects are present. Excess protons interact with hydrogen-bonding defects through long-range electrostatics, resulting in coupled motion of protons and defects.

CHIANTI—An Atomic Database for Emission Lines. VI. Proton Rates and Other Improvements

Abstract: The CHIANTI atomic database contains atomic energy levels, wavelengths, radiative transition probabilities, and electron excitation data for a large number of ions of astrophysical interest. Version 4 has been released, and proton excitation data are now included, principally for ground configuration levels that are close in energy. The fitting procedure for excitation data, both electrons and protons, has been extended to allow nine-point spline fits in addition to the previous five-point spline fits. This allows higher quality fits to data from close-coupling calculations where resonances can lead to significant structure in the Maxwellian-averaged collision strengths. The effects of photoexcitation and stimulated emission by a blackbody radiation field in a spherical geometry on the level balance equations of the CHIANTI ions can now be studied following modifications to the CHIANTI software. With the addition of H I, He I, and N I, the first neutral species have been added to CHIANTI. Many updates to existing ion data sets are described, while several new ions have been added to the database, including Ar IV, Fe VI, and Ni XXI. The two-photon continuum is now included in the spectral synthesis routines, and a new code for calculating the relativistic free-free continuum has been added. The treatment of the free-bound continuum has also been updated.

Probing the threshold to H atom transfer along a hydrogen-bonded ammonia wire.

Abstract: We characterized the entrance channel, reaction threshold, and mechanism of an excited-state H atom transfer reaction along a unidirectionally hydrogen-bonded "wire" -O-H...NH3...NH3...NH3...N. Excitation of supersonically cooled 7-hydroxyquinoline.(NH3)3 to its vibrationless S1 state produces no reaction, whereas excitation of ammonia-wire vibrations induces H atom transfer with a reaction threshold approximately 200 wave numbers. Further translocation steps along the wire produce the S1 state 7-ketoquinoline.(NH3)3 tautomer. Ab initio calculations show that proton and electron movement along the wire are closely coupled. The rate-controlling S1 state barriers arise from crossings of a pipi* with a Rydberg-type pisigma* state.

High energy proton acceleration in interaction of short laser pulse with dense plasma target

Abstract: The generation of high energy protons from the interaction of a short laser pulse with a dense plasma, accompanied by a preformed low density plasma, has been studied by particle-in-cell simulations. The proton acceleration toward the laser direction in the preformed plasma is characterized by a time-dependent model and the peak proton energy is given. The effect of electron recirculation on the rear side sheath acceleration is discussed and it is found that the peak proton energy increases in inverse proportion to the target thickness. These results shed light on the peak proton energy dependence on laser intensity, laser pulse length, and target thickness. Finally the optimal parameters of the laser pulse for large ion peak energy and conversion efficiency are discussed.

Scaling behavior at high p T and the p / π ratio

Abstract: We first show that the pions produced at high ${p}_{T}$ in heavy-ion collisions over a wide range of high energies exhibit a scaling behavior when the distributions are plotted in terms of a scaling variable. We then use the recombination model to calculate the scaling quark distribution just before hadronization. From the quark distribution, it is then possible to calculate the proton distribution at high ${p}_{T},$ also in the framework of the recombination model. The resultant $p/\ensuremath{\pi}$ ratio exceeds one in the $\mathrm{i}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{e}\ensuremath{-}{p}_{T}$ region where data exist, but the scaling result for the proton distribution is not reliable unless ${p}_{T}$ is high enough to be insensitive to the scale-breaking mass effects.

Two-Photon Exchange and Elastic Electron-Proton Scattering

Abstract: Two-photon exchange contributions to elastic electron-proton scattering cross sections are evaluated in a simple hadronic model including the finite size of the proton. The corrections are found to be small, but with a strong angular dependence at fixed Q{sup 2}. This is significant for the Rosenbluth technique for determining the ratio of electric and magnetic form factors of the proton, and partly reconciles the apparent discrepancy with the results of the polarization transfer technique.

Kinetics of proton transport in water

Abstract: The excess proton mobility in water has attracted scientific attention for more than a century. Detailed theoretical concepts and models are also presently in strong focus in efforts toward understanding this ubiquitous phenomenon. In the present report, we discuss a theoretical framework for rationalizing the excess proton mobility, based on computer simulations, theory of proton transfer (PT) in condensed media, and analysis of classical proton conductivity experiments over broad temperature ranges. The mechanistic options involved are (i) classical hydrodynamic motion of the hydronium ion (H3O+), (ii) proton transfer from hydronium to a neighboring water molecule, and (iii) structural diffusion of the Zundel complex (H5O2+), the processes all controlled by orientational fluctuations or hydrogen bond breaking in neighboring hydration shells. Spontaneous conversion of excess proton states between Zundel and hydrated hydronium states and between hydrated and bare hydronium states are the crucial parts of ...

Systematic study of deformed nuclei at the drip lines and beyond

Abstract: An improved prescription for choosing a transformed harmonic-oscillator (THO) basis for use in configuration-space Hartree-Fock-Bogoliubov (HFB) calculations is presented. The new $\text{HFB}+\text{THO}$ framework that follows accurately reproduces the results of coordinate-space HFB calculations for spherical nuclei, including those that are weakly bound. Furthermore, it is fully automated, facilitating its use in systematic investigations of large sets of nuclei throughout the periodic table. As a first application, we have carried out calculations using the Skyrme force SLy4 and volume pairing, with exact particle-number projection following application of the Lipkin-Nogami prescription. Calculations were performed for all even-even nuclei from the proton drip line to the neutron drip line having proton numbers $Z=2,4,\dots{},108$ and neutron numbers $N=2,4,\dots{},188$. We focus on nuclei near the neutron drip line and find that there exist numerous particle-bound even-even nuclei (i.e., nuclei with negative Fermi energies) that have at the same time negative two-neutron separation energies. This phenomenon, which was earlier noted for light nuclei, is attributed to bound shape isomers beyond the drip line.

Water alignment and proton conduction inside carbon nanotubes.

Abstract: First-principles molecular dynamics simulations have been carried out to investigate the structure, electronic properties, and proton conductivity of water confined inside single-walled carbon nanotubes. The simulations predict the formation of a strongly connected one-dimensional hydrogen-bonded water wire resulting in a net electric dipole moment directed along the nanotube axis. An excess proton injected into the water wire is found to be significantly stabilized, relative to the gas phase, due to the high polarizability of the carbon nanotube.

Simulation of DNA damage after proton irradiation.

Abstract: Friedland, W., Jacob, P., Bernhardt, P., Paretzke, H. G. and Dingfelder, M. Simulation of DNA Damage after Proton Irradiation. Radiat. Res. 159, 401–410 (2003). The biophysical radiation track simulation model PARTRAC was improved by implementing new interaction cross sections for protons in water. Computer-simulated tracks of energy deposition events from protons and their secondary electrons were superimposed on a higher-order DNA target model describing the spatial coordinates of the whole genome inside a human cell. Induction of DNA double-strand breaks was simulated for proton irradiation with LET values between 1.6 and 70 keV/μm and various reference radiation qualities. The yield of DSBs after proton irradiation was found to rise continuously with increasing LET up to about 20 DSBs per Gbp and Gy, corresponding to an RBE up to 2.2. About half of this increase resulted from a higher yield of DSB clusters associated with small fragments below 10 kbp. Exclusion of experimentally unresolved mu...

Coronal shocks and solar energetic proton events

Abstract: From 1996 July through 2001 June, less than half (43/98) of all favorably located (from solar western hemisphere sources) metric type II radio bursts were associated with solar energetic proton (SEP) events observed at Earth. When western hemisphere metric type IIs were accompanied by decametric-hectometric (DH; 1-14 MHz) type II emission (observed by Wind/WAVES) during this period, their association with ~20 MeV SEP events (with peak fluxes ≥10-3 protons cm-2 s-1 sr-1 MeV-1) was 90% (26/29), versus only 25% (17/69) for metric IIs without a DH counterpart. Overall, 82% (63%) of all SEP events with visible disk origins were associated with metric (DH) type II bursts, with the percentage associations increasing with SEP event size to 88% (96%) for ~20 MeV SEP events with peak intensities of ≥10-1 protons cm-2 s-1 sr-1 MeV-1. Our results are consistent with the following possibilities (which are not mutually exclusive): (1) large ~20 MeV SEP events result from strong shocks that are capable of persisting well beyond ~3 R☉ (the nominal 14 MHz plasma level); (2) shock acceleration is most efficient above ~3 R☉; and (3) shocks that survive beyond ~3 R☉ are more likely to have broad longitudinal extents, enabling less well connected shocks to intercept open field lines connecting to Earth.

Proton number fluctuation as a signal of the QCD critical end-point

Abstract: We argue that the event-by-event fluctuation of the proton number is a meaningful and promising observable for the purpose of detecting the QCD critical end point in heavy-ion collision experiments. The long range fluctuation of the order parameter induces a characteristic correlation between protons which can be measured. The proton fluctuation also manifests itself as anomalous enhancement of charge fluctuations near the end point, which might be already seen in existing data.

Temperature-dependent hydrogen-bond geometry in liquid water.

Abstract: We have determined the hydrogen-bond geometry in liquid water from 0 to 80 °C by combining measurements of the proton magnetic shielding tensor with ab initio density functional calculations. The resulting moments of the distributions of hydrogen-bond length and angle are direct measures of thermal disorder in the hydrogen-bond network.These moments, and the distribution functions that can be reconstructed from them, impose quantitative constraints on structural models of liquid water.

A coherent interpretation of the form factors of the nucleon in terms of a pion cloud and constituent quarks

Abstract: The recent measurements of the electric form factor of the neutron suggest that its shape may be interpreted as a smooth broad distribution with a bump at $Q^2 \approx$ 0.3 (GeV/c)2 superimposed. As a consequence the corresponding charge distribution in the Breit frame shows a negative charge extending as far out as 2 fm. It is natural to identify this charge with the pion cloud. This realisation is then used to reanalyse all old and new data of the electric and magnetic from factors of the proton and the neutron by a phenomenological fit and by a fit based on the constituent quark model. It is shown that it is possible to fit all form factors coherently with both ansaetzen and that they all show the signal of the pion cloud.

HSAB principle applied to the time evolution of chemical reactions.

Abstract: Time evolution of various reactivity parameters such as electronegativity, hardness, and polarizability associated with a collision process between a proton and an X- atom/ion (X = He, Li+, Be2+, B3+, C4+) in its ground (1S) and excited(1P,1D,1F) electronic states as well as various complexions of a two-state ensemble is studied using time-dependent and excited-state density functional theory. This collision process may be considered to be a model mimicking the actual chemical reaction between an X-atom/ion and a proton to give rise to an XH+ molecule. A favorable dynamical process is characterized by maximum hardness and minimum polarizability values according to the dynamical variants of the principles of maximum hardness and minimum polarizability. An electronic excitation or an increase in the excited-state contribution in a two-state ensemble makes the system softer and more polarizable, and the proton, being a hard acid, gradually prefers less to interact with X as has been discerned through the dro...

Proton acceleration from high-intensity laser interactions with thin foil targets.

Abstract: Measurements of energetic proton production resulting from the interaction of high-intensity laser pulses with foil targets are described. Through the use of layered foil targets and heating of the target material we are able to distinguish three distinct populations of protons. One high energy population is associated with a proton source near the front surface of the target and is observed to be emitted with a characteristic ring structure. A source of typically lower energy, lower divergence protons originates from the rear surface of the target. Finally, a qualitatively separate source of even lower energy protons and ions is observed with a large divergence. Acceleration mechanisms for these separate sources are discussed.

A microscopic estimate of the nuclear matter compressibility and symmetry energy in relativistic mean-field models

Abstract: The relativistic mean-field plus random phase and quasiparticle random phase approximation calculations, based on effective Lagrangians with density-dependent meson-nucleon vertex functions, are employed in a microscopic analysis of the nuclear matter compressibility and symmetry energy. We compute the isoscalar monopole response of 90Zr, 116Sn, 144Sm, the isoscalar monopole and isovector dipoles response of 208Pb, and also the differences between the neutron and proton radii for 208Pb and several Sn isotopes. The comparison of the calculated excitation energies with the experimental data on the giant monopole resonances restricts the nuclear matter compression modulus of structure models based on the relativistic mean-field approximation to Knm 250-270 MeV. The isovector giant dipole resonance in 208Pb and the available data on differences between the neutron and proton radii limit the range of the nuclear matter symmetry energy at saturation (volume asymmetry) of these effective interactions to 32 MeV < a4 < 36 MeV.

Viewing the proton through 'color' filters

Abstract: While the form factors and parton distributions provide separately the shape of the proton in coordinate and momentum spaces, a more powerful imaging of the proton structure can be obtained through phase-space distributions. Here we introduce the Wigner-type quark and gluon distributions which depict a full-3D proton at every fixed light-cone momentum, like what is seen through momentum ("color") filters. After appropriate phase-space reductions, the Wigner distributions are related to the generalized parton distributions (GPDs) and transverse-momentum dependent parton distributions, which are measurable in high-energy experiments. The new interpretation of GPDs provides a classical way to visualize the orbital motion of the quarks, which is known to be the key to the spin and magnetic moment of the proton.

Discovery of Interacting Molecular Gas toward the TeV Gamma-Ray Peak of the SNR G 347.3–0.5

Abstract: Supernova remnants (= SNR) are suggested to be sites of cosmic-ray acceleration. In particular, it has been an issue of keen interest whether cosmic ray protons are being accelerated in a SNR which emits TeV γ-rays. A crucial observational test for this is to find dense molecular gas towards the SNR, because such molecular gas can best verify the existence of cosmic-ray protons via pion decay to γ-rays. Here, we show that new high-resolution mm-wave observations of interstellar CO molecule have revealed molecular gas at 1kpc distance interacting with the TeV γ-ray SNR G 347.3−0.5, and that a molecular cloud of ∼ 200 solar masses is clearly associated with the TeV γ-ray peak, providing strong evidence for proton acceleration. We have estimated the total energy of accelerated protons to be ∼ 1048erg, which corresponds to an acceleration efficiency of ∼ 0.001, posing an observational constraint on the proton acceleration.

Calculated Masses of Heaviest Nuclei

Abstract: Masses of heaviest nuclei are calculated within a macroscopic-microscopic approach. Even-even, odd-A, and odd-odd nuclides are considered. A large region of nuclei with proton and neutron numbers of Z=82–128 and N=126–192, respectively, is analyzed. The results are compared with those of other macroscopic-microscopic and of recent microscopic Hartree-Fock-BCS calculations. Alpha-decay energies are also given.

Coupling of light-induced electron transfer to proton uptake in photosynthesis

Abstract: Light energy is transformed into chemical energy in photosynthesis by coupling a light-induced electron transfer to proton uptake. The resulting proton gradient drives ATP synthesis. In this study, we monitored the light-induced reactions in a 100-kDa photosynthetic protein from 30 ns to 35 s by FTIR difference spectroscopy. The results provide detailed mechanistic insights into the electron and proton transfer reactions of the QA to QB transition: reduction of QA in picoseconds induces protonation of histidines, probably of His126 and His128 in the H subunit at the entrance of the proton uptake channel, and of Asp210 in the L subunit inside the channel at 12 μs and 150 μs. This seems to be a prerequisite for the reduction of QB, mainly at 150 μs. QA− is reoxidized at 1.1 ms, and a proton is transferred from Asp210 to Glu212 in the L subunit, the proton donor to QB−. Notably, our data indicate that QB is not reduced directly by QA− but presumably through an intermediary electron donor.