Measurements of total and positronium-formation cross sections for the scattering of positrons by alkali atoms

We present a review of our recent measurements of total cross sections (Q _T's) for the scattering of positrons by Na, K, and Rb, and positronium-formation cross sections (Q _Ps's) for Na and K. For our total cross section measurements, a beam transmission technique has been used. For ourQ _Ps measurements, our approach involves setting upper and lower limits onQ _Ps using a combination of (1) measuring the transmission of the positron beam with the angular discrimination of the apparatus made as poor as possible, and (2) measuring the 511 keV annihilation gamma rays in coincidence produced by the decay of para-Ps formed in the scattering cell. Comparison with theoretical calculations shows that our measuredQ _T's andQ _Ps's for Na and K agree reasonably well with a close coupling approximation (CCA) calculation which takes into account the formation of Ps in then=1 andn=2 states. In the 3–10 eV energy range, this calculation predicts a peak in theQ _T's andQ _Ps's for K which also appears in our measurements. The absence of such a peak in our measuredQ _T's andQ _Ps's (preliminary) for Na in this energy range is also consistent with the same theory. Comparisons with five-state CCA calculations ofQ _T which do not take Ps-formation into account also show good agreement with our positron-Na, K, and RbQ _T measurements for energies above 20 eV, but show dramatic departures from our measurements below 10 eV for K and Rb.     

Measurement of the fast neutron scattering and total cross sections of141Pr

The differential elastic neutron scattering cross sections of¹⁴¹Pr were measured at the incident neutron energies of 1.2, 1.7 and 1.9 MeV in the angular range between 25 and 150 degrees. At 1.7 MeV the differential inelastic neutron scattering cross sections corresponding toQ=?1122 keV, and at 1.9 MeV the ones corresponding toQ=?1122, andQ=?1295 keV were also determined. In a transmission experiment, the total cross section was measured between 0.50 and 2.42 MeV. The total and differential cross sections were calculated using the nuclear optical model. The calculated results were compared with the experimental data.     

A semi-empirical formula for total cross sections of electron scattering from diatomic molecules

A fitting formula based on the Born approximation is used to fit the total cross sections for electron scattering by diatomic molecules (CO, N₂, NO, O₂ and HCl) in the intermediate- and high-energy range. By analyzing the fitted parameters and the total cross sections, we found that the internuclear distance of the constituent atoms plays an important role in the e-diatomic molecule collision process. Thus a new semi-empirical formula has been obtained. There is no free parameter in the formula, and the dependence of the total cross sections on the internuclear distance has been reflected clearly. The total cross sections for electron scattering by CO, N₂, NO, O₂ and HCl have been calculated over an incident energy range of 10–4000 eV. The results agree well with other available experimental and calculation data.     

Differential cross sections for ionization of laser-aligned atoms by electron impact

The first experimental data are given for (e,2e) ionization from laser-aligned atoms. A linearly polarized laser excited Mg atoms to the 31P? state prior to ionization by low energy electrons. The scattered and ejected electrons were detected in coincidence and the differential cross section determined for a range of alignment angles. An asymmetric coplanar geometry was used, with one electron fixed and the other detected at different angles. The data are compared to that from the spherically symmetric 31S? state. Significant differences are found, in both magnitude and angular distribution.     

First measurements of positronium total scattering cross sections

A new type of experiment has been devised in which ortho-positronium diffusion to the metal walls of a series of parallel-sided cells is studied by measuring the annihilation lifetimes. Experimental results are subjected to an analysis in which two parameters are varied to give a best fit, one of the two parameters being the scattering cross section. The method requires a number of systematic corrections which have still to be worked out. Preliminary cross section values for helium, argon and isobutane are 0.017 πa ₀ ² , 0.09 πa ₀ ² , and 0.54 πa ₀ ² , respectively.     

Electron collisions with laser cooled and trapped metastable helium atoms: total scattering cross sections

Absolute measurements of total scattering cross sections for low energy (5-70 eV) electrons by metastable helium (2(3)S) atoms are presented. The measurements are performed using a magneto-optical trap which is loaded from a laser-cooled, bright beam of slow He(2(3)S) atoms. The data are compared with predictions from convergent close coupling and R matrix with pseudostate calculations, and we find good agreement between experiment and theory.     

Predicting total reaction cross sections for nucleon-nucleus scattering

Nucleon total reaction and neutron total cross sections to 300 MeV for 12C and 208Pb, and for 65 MeV scattering spanning the mass range, are predicted using coordinate space optical potentials formed by full folding of effective nucleon-nucleon interactions with realistic nuclear ground state densities. Good to excellent agreement is found with existing data.     

Absolute cross sections of ultra-soft x-ray radiation in electron scattering upon atoms and the soft-photons approximation

We calculated the cross sections of ultra-soft x-ray bremsstrahlung in electron scattering upon Ar, Kr and Xe. The results are consistent with the absolute values of the differential cross sections measured by Gnatchenko et al. (2009) [17] for scattering electrons with an energy of 600 eV on these atoms.     

Measurements of total cross sections for positron-K and-Rb scattering

A modification of the apparatus that we have been using to measure absolute total cross sections (Q _T′s) for positron and electron scattering by alkali metal atoms has enabled us to more easily obtainQ _T′s at low energies (<10 eV). Our positron-K and-RbQ _T measurements now extend from 1 to 98 eV and we have made the correspondingQ _T measurements for electrons in the same system using the same technique (a beam transmission technique). Our measured positron-K and-RbQ _T values show a peak in the vicinity of 6 eV and decrease substantially as the positron energy is reduced below 6 eV, in sharp contrast to the results obtained in recent 5-state close-coupling approximation calculations. At higher energies (above 10 eV), we have found that the same calculations are in quite good agreement with our measured positron-K and-RbQ _T′s. Our measurements of the correspondingQ _T′s for electrons below 10 eV do not show a peak, but rather continue to increase as the energy is decreased.     

Fast-neutron total and scattering cross sections of elemental titanium

Energy-averaged total neutron cross sections of elemental titanium are measured from ? 1.0 to 4.5 MeV with statistical accuracies of ? 1%. Differential elastic neutron-scattering angular distributions are measured from 1.5 to 4.0 MeV at incident energy intervals of ≦ 0.2 MeV. Differential cross sections for the inelastic neutron excitation of “states” in titanium at 158 ± 26, 891 ± 8, 984 ± 15, 1428 ± 39, 1541 ± 30, 1670 ± 80, 2007 ± 8, 2304 ± 22, 2424 ± 16 and 2615 ± 10 keV are measured in the incident-neutron energy range 1.5 to 4.0 MeV. Neutrons corresponding to additional states at approximately 2845 and 3010 keV were qualitatively observed. The experimental results are interpreted in terms of an energy-averaged optical-statistical and coupled-channels models including consideration of collective vibrations. The implications of these models and their use in the context of strong fluctuating structure are discussed.     

Fast-neutron total and scattering cross sections of58Ni

Neutron total cross sections of⁵⁸Ni were measured at 25 keV intervals from 0.9 to 4.5 MeV with 50–100 keV resolutions. Attention was given to self-shielding corrections to the observed total cross sections. Differential elastic- and inelastic-scattering cross sections were measured at 50 keV intervals from 1.35 to 4.0 MeV with 50–100 keV resolutions. Inelastic excitation of levels at 1.458±0.009, 2.462+-0.010, 2.791±0.015, 2.927+-0.012 and 3.059+-0.025 MeV was observed. The experimental results were interpreted in terms of optical-statistical and coupled-channels models.     

中高能电子散射全截面测量装置的设计

介绍了中高能电子散射全截面绝对测量装置的设计,采用直线透射技术.因采用了恒温控制作用室,以便较准确测量反应室的绝对气压值;同时采用静电型圆柱能量分析器,并用计数率测量模式代替通常的束流强度测量模式,从而可以大幅度减小接收立体角,抑制零度角的弹性散射贡献.通过以上几项措施,可以有效地提高测量的精度,从而可以对一些有意义的分子的中、高能电子散射全截面进行绝对测量.     

中高能电子散射全截面测量装置的设计

介绍了中高能电子散射全截面绝对测量装置的设计,采用直线透射技术.因采用了恒温控制作用室,以便较准确测量反应室的绝对气压值;同时采用静电型圆柱能量分析器,并用计数率测量模式代替通常的束流强度测量模式,从而可以大幅度减小接收立体角,抑制零度角的弹性散射贡献.通过以上几项措施,可以有效地提高测量的精度,从而可以对一些有意义的分子的中、高能电子散射全截面进行绝对测量.     

Total cross sections for electron scattering from sulfur compounds

The original additivity rule method cannot give good results for electron scattering from SO,SO2,SO2Cl2,SO2ClF,and SO2F2 molecules at low energy,because the electron-molecule scattering is simply reduced to electron-atom scattering.Considering the difference between the bound atom in a molecule and the corresponding free atom,the original additivity rule is revised.With the revised additivity rule,the total cross sections for electron scattering from these molecules are calculated over a wide energy range below 3000 eV and compared with the available experimental and theoretical data.A better agreement between them is obtained.     

Differential cross sections for electron scattering by polar molecules. Elastic scattering and vibrational excitation

By use of the effective range theory for electron scattering by polar molecules differential cross sections for elastic scattering by HCN and LiF molecules and for vibrational excitation of water molecules are calculated.     

Elastic differential cross sections of electron scattering by CF_4 at intermediate energies

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Total electron scattering cross sections for carbon dioxide at low electron energies

Absolute total electron scattering cross sections for carbon dioxide have been measured at low electron energies using a photoelectron source. The measurements have been carried out at 27 electron energies varying from 0.91–9.14 eV with an accuracy of ±3%. The cross sections obtained in the present experiment have been compared with other measurements and theoretical computations.     

Elastic differential cross sections of electron scattering by CF_4 at intermediate energies

The elastic differential cross sections(DCS)for electron scattering from CF 4 are calculated at six impact energies(in 100—700 eV)employing the independent atom model(IAM)with partial waves. The atoms are presented by a model complex optical potential which is composed of static, exchange, polarization, and absorption terms. The electron density function ρ(r) is obtained by a fitting procedue to the Dirac Hartree Fock Slater sef consistent data. Compared with available experimental data, the presen...