Experimental Total Cross Research Articles

Theoretical investigation of electron collisions with sulfur monoxide in the low- and intermediate-energy range

We report a theoretical study on electron collisions with the sulfur monoxide radical. More specifically, differential, integral, and momentum-transfer cross sections are calculated and reported in the 1--500 eV energy range. Calculations are performed at the static-exchange-polarization-absorption level of approximation. A combination of the iterative Schwinger variational method and the distorted-wave approximation is used to solve the scattering equations. Our study reveals shape resonances in both the doublet and quartet spin-specific scattering channels. The occurrence of such resonances may enhance the spin-flip effects. In addition, the comparison of our calculated total absorption cross sections with existing experimental total ionization cross sections is encouraging.

Absolute and total electron-capture cross sections in slowArq+−C60collisions

We present experimental absolute and total electron-capture cross sections for ${\mathrm{Ar}}^{q+}\ensuremath{-}{\mathrm{C}}_{60}$ $(q=4,$ 6, and 8--18) collisions at $3.3q \mathrm{keV}.$ The absolute scale is based on the ${\mathrm{C}}_{60}$ vapor pressure by Abrefah et al. [Appl. Phys. Lett. 60, 1313 (1992)], which together with the results of Mathews et al. [J. Phys. Chem. 96, 3566, (1992)] are the only possible choices among the many, widely dispersed, vapor pressures reported in the literature. In order to support this statement, we calculate total electron-capture cross sections modeling ${\mathrm{C}}_{60}$ as a pointlike object and as an infinitely conducting sphere (ICS). These model results are shown to differ little, since polarization and finite-size effects are relatively unimportant for the distant collisions at which the outermost (first) electrons are transferred. We are thus able to use the semiempirical formula for absolute ion-atom cross sections by Selberg et al. [Phys. Rev. A 54, 4127 (1996)], treating ${\mathrm{C}}_{60}$ as a hypothetical atom with ionization potential ${I}_{1}=7.6 \mathrm{eV},$ and the ICS model to define lower and upper bounds for the absolute cross-section scale. We find significant oscillations in the total electron-capture cross sections as functions of q.

Positron scattering on atoms and molecules near the positronium threshold

Effects found in the total and elastic-scattering cross sections of positrons on atoms and molecules near the positronium (Ps) formation threshold are examined on the basis of R-matrix theory. In the case of atomic targets, the energy, dependence of the Ps-production cross section yields the dominant angular momentum of the outgoing Ps near threshold. The predicted effects agree semiquantitatively with experimental elastic and total cross sections. The predictions suggest that Wigner cusps in the elastic-scattering cross section should be very small for He targets and largest for Xe targets. For molecules, the variability of threshold effects in the total cross section suggests that this is caused by variable threshold effects in the nonelastic cross sections. Using three-channel R-matrix theory, the effect is illustrated for a H2 target.

Open-shell and solid state effect in resonant photoemission

Experimental total photoabsorption and partial photo-ionisation cross sections for Eu atoms and Eu metal are compared to new relativistic time-dependent local density calculations and previous many-body perturbation theory results. This comparison makes explicit for the first time the influence of open-shell and solid state effects in giant resonant absorption and resonant photo-emission.

Polarization and binding effects in K-shell ionization by 16O and 32S ions.

The theoretical model of Montenegro and Sigaud is used for the description of the experimental total K-shell ionization cross sections for 2.2-MeV/u /sup 16/O and /sup 32/S on 24 targets from /sub 21/Sc to /sub 83/Bi obtained by Seidel et al. The predictions of the model, which considers both binding and static polarization effects within the framework of adiabatic perturbation theory, are very good for the majority of the measured data in the region of validity of the model. The electron-capture process that can contribute in the region of the largest values of relative velocity xi/sub K/ is also analyzed.

Elastic and inelastic scattering of uranium ions by holmium, gold, and bismuth targets.

Angular distributions for elastic scattering of /sup 238/U ions by /sup 165/Ho, /sup 197/Au, and /sup 209/Bi at 2380 MeV have been obtained in a 2..pi.. geometry using solid state nuclear track detectors. The angular distributions of the elastic differential cross section ratios sigma/sub el/(expt.)/sigma(Ruth) are analyzed using (a) the generalized Fresnel model and (b) the parametrized S-matrix method. Quarter-point angles obtained are converted to reaction parameters such as grazing angular momenta, interaction radii, and total reaction cross sections. Values of inelastic total reaction cross sections were obtained by directly counting the three-, four-, and five-prong events along with the two-prong inelastic events as observed in the 2..pi.. geometry. Experimental total reaction cross sections are compared with optical and sharp cutoff model calculations. There is good agreement among the total reaction cross sections evaluated from elastic and inelastic data as well as with model calculations.

L-shell ionisation of medium and heavy elements by 1.2-3.0 MeV incident alpha particles

Thin targets of Yb, W, Au, Pb and Bi were bombarded by 1.2-3.0 MeV alpha particles. From the resulting L X-rays experimental Li-subshell ionisation cross section were deduced. The measured cross sections are compared with the predictions of the ECPSSR theory by expressing the data in terms of ratios of experimental to theoretical values as a function of the corrected reduced velocity xi (Li)R. Disagreements of up to 50% were revealed for the L1 subshell at xi (L1)R<or=0.3. The ECPSSR theory underestimate the L2 ionisation cross sections for all elements studied by as much as a factor of 3.3 but explains the L3 data to within 30%. In general, for heavy elements, the ECPSSR is unsuccessful in explaining the velocity dependence of the cross sections. Several factors were highlighted which may explain some of the observed disagreements. The target atomic number dependence of the experimental Li subshell and total ionisation cross sections were also investigated and the results are presented.

Application of the matrix-effective-potential formalism to electron-neon scattering at 150 - 700-eV impact energy and comparison to optical-potential calculations

We apply two methods to predict integral and differential elastic and absorption cross sections for electron-neon scattering at 150-700 eV: a matrix-effective-potential (MEP) formalism and an optical-potential formalism. For the optical potential we use the absorption potential that Green, Rio, and Ueda adjusted to give accurate absorption cross sections, and we show that the real part of the potential can be chosen so that the calculated elastic-scattering cross sections also agree with experiment. The MEP model, with no parameters adjusted to experiment, is even more successful. The agreement with available experimental elastic differential cross sections is better than 10% in many cases and is about 40% at worst; agreement with experimental total inelastic cross sections averages 3% at four energies. An interesting conclusion from the optical-potential calculations is that when the absorption potential is included in the optical potential, accurate elastic cross sections can be calculated even at high energy by using an adiabatic polarization potential.

Analysis of low-energy positron-helium total cross sections

Scattering lengths for positron-helium collisions are obtained from least-squares fits to the experimental low-energy total cross section data of Canter and co-workers (1979) and Stein and co-workers (1978). Previous conclusions regarding the quality of these experimental data based on such fits are challenged. The effect of small-angle forward elastic scattering in experiments at low energies is analysed.

A comparison of experimental and theoretical total cross sections for low-energy positron-helium scattering

The experimental values of the positron-helium total cross section at 2 eV recently measured by Kauppila et al. (1976, 1977) and Burciaga et al. (1977) are compared with a theoretical value which is believed to be a conservative lower bound on the exact result. Both experimental values are found to be significantly lower than the theoretical value, and it is suggested that the discrepancy is due to the neglect of small-angle scattering in the experiments. The angle below which scattering would need to be neglected to explain the discrepancy is calculated for each experiment, and its value is consistent with the known or probably resolution of the apparatus.

Total gamma-ray cross sections in C, Sn, W and Pt in the energy region 84-662 kev

Experimental total gamma-ray cross sections in the energy region 84-662 kev in the elements C, Sn, W and Pt are recalculated, using most of the available data with their appropriate errors, because of the inconsistencies in the existing experimental data. These values are found to be in agreement with the theoretical total gamma-ray cross sections reported by Grodstein within the range of errors. Using these values, studies are made on the scattering cross sections, and within the range of errors agreement is observed between theory and experiment. More accurate and consistent remeasurements of total gamma-ray cross sections are suggested, in order to have a better understanding of the existing discrepancies between theory and experiment in the present region of energy.

Photoelectric cross sections of gamma rays in Al, Cu, Sn, W, Au and Pb in the energy region 50-208 keV

Total photoelectric cross sections of gamma rays are determined by subtracting the theoretical scattering cross sections from the experimental total gamma-ray cross sections of Wiedenbeck in the elements Al, Cu, Sn, W, Au and Pb in the energy region 50-208 keV. These values are compared with the latest theoretical values of Rakavy and Ron, Schmickley and Pratt and the compiled data of Hubbell and Berger. Agreement is observed between the theoretical and experimental values within 5%, except in lead at 50 and 105 keV energies. A systematic measurement of total gamma-ray cross sections in lead at these energies is suggested.

Associated Production in Pion-Nucleon Collisions and Effects of Resonant States

Possibilities of explaining experimental features, angular distributions, polarization, and total cross sections in the reaction pi + p yields K0 + lambda at 910 Mev by considering N?* ( pi -N third resonance), Y* ( pi -A resonance), and K (Kresonance) are investigated. Calculations are presented in graphical and tabular forms. (L.N.N.)

Neutron-Deuteron Scattering at High Energy

The neutron-deuteron total scattering cross section at high energy has been calculated by expressing it as a sum over two-particle states over a wide range of energy. A sum rule was then applied to give the total cross section, that is, the elastic and inelastic cross sections. The calculations were performed with a Serber type exchange force for the neutron-proton force in agreement with present neutron-proton scattering data and a neutron-neutron force of arbitrary depth and exchange character. The neutron-neutron well depth was then determined for an ordinary, Serber type and pure exchange force between the two neutrons by making use of the experimental neutron-deuteron total cross section. The neutron-deuteron elastic scattering was then calculated for the three different types of neutron-neutron exchange and was found to agree best with the experimental value for the Serber type exchange (no force in odd states). The cross section for production of low energy protons was also calculated and confirmed the Serber type exchange force between the two neutrons when compared with the experimental value. Finally, a qualitative discussion of the angular and energy distribution of the low and high energy protons was given.