Total and ionization cross-sections of N<Subscript>2</Subscript> and CO by positron impact: Theoretical investigations

In this paper we report our calculations on several important total cross-sections (TCSs) of positron impact on isoelectronic N₂ and CO molecules, treated in the complex spherical potential formalism. Basically the total (complete) cross-section Q _T consists of elastic and inelastic contributions. Our total inelastic cross-section (Q _inel) contains ionization and electronic excitations together with positronium formation. Our goal here is to bifurcate Q _inel further to deduce total ionization cross-section, using the ‘complex scattering potential–ionization contribution’ (CSP-ic) method of electron–atom/molecule scattering. The present range of positron energy is 15–2000 eV. All the resulting cross-sections are in a good general accord with the existing data. This work highlights the importance of various scattering channels in e⁺-N₂ and e⁺-CO interactions at intermediate and high energies.     

Elastic and inelastic scattering of positrons from hydrogenlike ions

Summary The total elastic cross-sections of the collisions of positrons with various hydrogenlike targets (with nuclear chargeZ=1, 2, 3, 4, 5 and 11) are determined at 13 values of the incident energy lying below 6.8 e V(the positronium formation threshold of e⁺-H scattering). The inelastic scattering of positrons by the same targets is investigated for the first time at energies above the positronium formation thresholds. Both scattering processes are treated using a coupled-static formalism and an iterative Green's function partial wave expansion technique. The resulting cross-sections demonstrate the stability of this technique and emphasize the importance of exploring the aspects of positron-ion collisions more fundamentally on both theoretical and experimental levels.     

Influence of vertex coulomb effects on peripheral partial amplitudes for the consecutive transfer of two protons in A(X,Y)B peripheral nuclear reactions induced by light loosely bound (exotic) nuclei at low energies

The amplitude for the consecutive transfer of two protons in A(X, Y)B peripheral nuclear reactions induced by loosely bound light (exotic) nuclei and described by a nonrelativistic square Feynman diagram in which the first transferred proton is loosely bound while the second one in tightly bound is considered. It is shown that the inclusion of three-ray Coulomb vertex effects in the square diagram leads to the appearance of an additional “Coulomb” singularity in the variable cos θ (here, θ is the c.m. scattering angle), this singularity being closer to the physical domain, −1 ≤ cos θ ≤ 1, than the well-known “triangle” singularities corresponding to the amplitude of the square diagram in which the internal line is contracted. The asymptotic behavior of the partial-wave amplitudes for l ≫ 1 that are generated by the aforementioned singularities is found explicitly. A comparative analysis of the resulting partial-wave amplitudes for l ≫ 1 is performed for specific peripheral nuclear reactions induced by ⁸B and ¹²N ions at various energies.     

Collisions of fast electrons with positronium atoms

Summary Elastic and inelastic collisions of fast non-relativistic electrons with positronium atoms have been studies in the Born-Ochkur approximation. It has been shown that exchange plays an important role for transitions between states with the same parity as in this case the direct scattering amplitude vanishes. Numerical results for the total and ortho-para conversion cross-sections for the 1s→1s, 1s→2s and 1s→2p transitions have been presented for projectile energies varying from 0.1 to 10 keV. According to charge symmetry, presented results apply also to positron projectiles.     

Parton densities and dipole cross-sections at small x in large nuclei

Unintegrated gluon densities in nuclei, dipole-nucleus cross-sections and quark densities are numerically investigated in the high-color limit, with the scattering on a heavy nucleus exactly described by the sum of fan diagrams of BFKL pomerons. The initial condition for the evolution in rapidity is quickly forgotten, and the gluon density presents a “supersaturation” pattern, as previous studies indicated. Both dipole-nucleus cross-sections and quark densities present the expected saturation features. Identifying the position in transverse momentum l of the maximum of the gluon distribution with the saturation momentum , at large rapidities all distributions depend on only one variable, or . Received: 4 April 2001 / Revised version: 24 April 2001 / Published online: 8 June 2001     

np → ηd near threshold and the s -wave ηN amplitude

We calculate the process np → ηd near threshold using a separable potential model of the coupled ηN - πN - ππN subystems, and a relativistic three-body calculation for the ηd scattering amplitude. The ππN channels are represented by an effective σN channel, and we compare the case where the σ and π masses are related by m _σ = 2m _π and no width is considered, to another where the mass and width of the σ -meson are taken from ππ scattering data. The np → ηd cross-section can be well described up to about 60MeV by models where the real part of the ηN scattering length lies between 0.4≤Re(a _ηN)≤0.6 fm which allows us to determine the s -wave ηN scattering amplitude for -60≤E≤60 MeV.     

Rotational excitations in muonic atom scattering by molecular hydrogen

A computational scheme based on the infinite order sudden approximation (IOS) is proposed for treating molecular and screening effects in muonic atom scattering at low energies. With this scheme the differential (dσ/dΩ(ɛj← 0|θ) and the total σ_tot(ɛ) cross-sections of muonic atom scattering on molecules of hydrogen isotopes are calculated at energies ɛ_rot=ℏω_I⩽ɛ⩽ɛ_vib= ℏω_03BD;. The IOS permits a quantitative analysis of the dependence of the differential cross-sections on the scattering angle and on the rotational excitations, particularly the rotational rainbow effects. An attractive feature of this approach is also to include the energy and the angular dependencies in the “input” cross-sections describing the muonic atom scattering on each bare nucleus of the molecule. The calculated total cross-sections ω_tot(ɛ) are in agreement with the data obtained earlier with the pseudopotential approach. This revised version was published online in August 2006 with corrections to the Cover Date.     

Zeros of the helicity amplitudes of Σ−-p and Λ-p elastic scattering

The forces of interaction as reflected in the cosθ plane analytic structures of the helicity amplitudes of Σ⁻-p and Λ-p scattering are optimally exploited using the conformal mapping technique of Cutkosky and coworkers. A suitable parametrization of these amplitudes in the mapped variable is then chosen so that it can see the zeros better. These zeros are then located by making a good fit to the differential scattering cross-section curves. The effect of these zeros upon the cross-sections is discussed. Values for the phase shifts and coupling parameters between channels of fixedJ values are also computed.     

Positrons — an alternative probe to electron scattering

Interaction of positrons with atoms and molecules differs from electron interaction due to the difference in polarity of the charge. This makes positrons an alternative tool to study atomic and molecular structure. Recent measurements of the total cross-sections for positron scattering at low energies on He, Ar, H₂, N₂, C₆H₆, C₆H₁₂, C₆H₇N carried out at Trento University [Karwasz et al., Acta Phys. Pol. 127, 666 (2005)] are discussed and compared to electron scattering results. All measured total cross-sections exhibit an increase with decreasing positron energy in the limit of zero energy; H₂, N₂, Ar, show regions of constant cross-section which are a few eV-wide, characteristic of scattering on a hard-sphere potential. Helium shows two resonant structures much below the positronium formation threshold. They may be attributed to virtual positronium formation. In conclusion, positron scattering is complementary to electron scattering. The total cross-sections do not show Ramsuaer minima but constant values, and new resonances appear.     

Coupled-static calculations of the partial and total cross sections of positron-lithium inelastic scattering

The inelastic scattering of positrons by lithium atoms has been investigated in details using a combination of the frozen-core and coupled-static approximations. The ground state of the target has been described by Walters' wave functions. The partial cross sections corresponding to 8 values of the total angular momentuml (0l7) are determined for 21 values of the incident energy lying between 0·1 and 1000 eV. Our results illustrate the stability of the iterative numerical technique employed. They demonstrate the role played by each partial wave in the total cross sections and emphasize the argument that the effect of positronium formation on the total collisional cross sections diminishes when the incident energy is larger than 30 eV.     

Vibrationally elastice − — H2O scattering at intermediate energies

Vibrationally elastic total cross-sections ofe ⁻—H₂O scattering are calculated at intermediate energiesE _i=10–300 eV. The interaction potentials are treated in spherical models. The dipole rotational excitation, which is significant but not dominant above 10 eV, is treated incoherently. Effects of electronic excitation-ionization, significant above 30 eV or so, are considered through a complex optical potential. A dynamically distorted charge-density is employed to calculate the imaginary part of the complex potential. Comparisons are made with recent theoretical and experimental data. The mutual agreement is better in total cross-sections than in differential cross-sections.     

Empirical relation and establishment of shell effects in (<Emphasis Type="Italic">n</Emphasis>, 2<Emphasis Type="Italic">n</Emphasis>) reaction cross-sections at 14 MeV

The experimental data for (n, 2n) reaction cross-sections around 14 MeV neutron energy have been collected from the literature and analysed for the isotopes having 1 ≤ Z ≤ 82. The empirical relations for the reaction cross-sections have been obtained, which show fairly good fits with the experimental values. The shell effects have been established at magic nucleon numbers for (n, 2n) reaction cross-sections around 14 MeV neutron energy. The odd-even effects have also been observed as the cross-sections for odd-mass nuclei are higher than their neighbouring even-even nuclei.      

Alpha particle scattering in the rigid projectile approximation

We study elastic α-particle scattering offp,α-particle and¹²C targets at 17.9 GeV/c incident momentum in the rigid projectile approximation of the Glauber model. Differential and total cross-sections are computed and compared with the data. Reasonable agreement with the observed differential cross-sections is found for small momentum transfers but short-range dynamical correlations in the target will probably have to be taken into account to get better agreement at larger momentum transfers, particularly in the case of α-¹²C scattering.     

Effects of Ps polarization potentials on Ps total cross-sections of the collisions of positrons with alkali metal atoms and alkaline-earth positive ions

Summary The previous form of the coupled-static approximation applied to the inelastic scattering of positrons by alkali metal atoms is improved by switching on the polarization potentials of the positroniums. The effect of these potentials on the total elastic and positronium formation cross-sections of three different scattering problems (namely the collisions of positrons with lithium, sodium and potassium atoms) is discussed in details. Particularly, we devote our interest to the comparison between the resultant positronium formation cross-sections and those of the inelastic collisions of positrons with berylium, magnesium and calcium positive ions.     

Multiphoton electron detachment from H−: a model study

Summary N-photon cross-sections for the detachment of an electron from the hydrogen anion H⁻ are evaluated by perturbation theory for 1≤N≤4 in terms of a one-electron model for the anion founded on the Fermi pseudopotential. Our results complement recent findings obtained in terms of a zero-range plane-wave approach to multiphoton detachment rates. The authors of this paper have agreed to not receive the proofs for correction.     

New results for the production of Λ and Σhyperons in antineutrino scattering from protons

We obtain total and differential cross-sections for the strangeness changing charged current reactions ˉ + p → Λ + L ⁺ and ˉ + p → Σ⁰ + L ⁺ , where L is a charged lepton, either an electron, muon or tau. We do this by making use of the standard dipole form factors normally used and of the new form factors recently obtained from recoil proton measurements in electron-proton electromagnetic scattering. We also obtain the contributions of the individual form factors to the total and differential cross-sections for both sets of form factors. We find that the differential and total cross-sections for Λ production change only slightly between the two sets of form factors but that the differential and total cross-sections change substantially for Σ⁰ production. We discuss the possibility of distinguishing between the two cases for the experiments planned by the MINERν A Collaboration.     

Elastic scattering ofα-particles from20Ne

A relatively simple procedure using nuclear interaction calculated microscopically from two-nucleon potential employing equivalence of resonating group method and generator coordinate method has been used to calculate the differential cross-sections (DCS) forα +²⁰Ne elastic scattering atE _lab=18.0, 20.2, 21.9, 23.0 and 27.2 MeV. The absorption effects due to the opening of the non-elastic channels are taken into account approximately by the sharp cut-off of lower partial waves. The anomalous large oscillations of the DCS at backward angles atE _lab=18.0 and 27.2 MeV are reproduced. The calculated results are in fair agreement with the experimental data.     

Investigation of the very heavy cosmic abundances at source estimated from balloon-borne passive-detector results over alice springs

Summary A detailed study on the relative abundances of very heavy elements of cosmic rays with charges 10≤Z≤28 in the interstellar medium has been estimated from the results obtained from the balloon-borne passive-detector experiments flown for 32 hours from Alice Springs. The recovered CR-39 (HCB) detectors were etched for 96 hours in 7.5N NaOH solution at 80°C. About 2126 elliptic etch pit cones were scanned from the 250 cm² area of the track detector and the chemical abundances of elements with charges 10≤Z≤28 above 2 GeV/n have been estimated. We have adopted the charge-changing cross-sections calculated from the abrasion/ablation model after Norbury and duly fitted in the slab model to account for the loss of intensity due to nuclear fragmentation in the upper atmosphere. The predicted source composition is comparable with the derived result of Lezniak and Webber. Our result on (Sc−Cr)/Fe flux ratio at 6.52 GeV/n energy is in accord with the results derived from the leaky-box model.     

Band structure and dominating electron-scattering mechanisms in Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis>−<Emphasis Type="Italic">y</Emphasis></Subscript>Mn<Subscript>x</Subscript>Fe<Subscript>y</Subscript>Se

Magnetic and kinetic properties as well as transmission and absorption spectra of Hg _1−x−y Mn _x Fe _ySe (0.09 ≤ x ≤ 0.099 and 0.001 ≤ y ≤ 0.01) crystals are investigated at H ≈ 0.5–6 kOe in the temperature range T = 77–300 K. The band parameters are determined on the basis of experimental data. It is found that in the crystals under study at T ≈ 300 K, electron scattering by polar optical phonons dominates, direct optical band-to-band transitions occur, and replacement of a part of Mn atoms by Fe for x + y = 0.1 results in an increase in E_g ^op with Fe content. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 35–39, March, 2007.     

Feshbach resonances in positron scattering from sodium

S-, P- and D-wave Feshbach resonances in positron-sodium scattering have been investigated by using the momentum space coupled-channels optical method. The target continuum and positronium (Ps) formation channels are included via an optical potential. Feshbach resonances below the target excitation and Ps (n = 2) formation thresholds are predicted and the effects of channel-coupling scheme, target continuum and Ps formation channels on the resonance energy and resonance width are discussed. We have also found the Wigner cusp structures at the inelastic channel-opening thresholds in positron-sodium scattering cross sections.