Magnetic field-free measurements of the total cross section for positrons scattering from helium and krypton

Fayer, S. E.; Loreti, A.; Andersen, Stine Linding; Kövér, Á.; Laricchia, G.

doi:10.1088/0953-4075/49/7/075202

Cited by 11 publications

(18 citation statements)

References 52 publications

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“…In this respect, beams that employ magnetic fields are more likely to transport FSPs from the interaction region to the detector, with a dependence of σ T upon the magnetic field strength persisting even for weak fields (e.g., ≃4-9 Gauss [11,12]). This type of systematic effect can be reduced by using electrostatic beams which enable both the interaction and detection regions to be field free, thus, facilitating the attainment of a smaller (energy-independent) angular acceptance [25][26][27].For targets with a low polarizability, α (e.g., ≃1.38 a:u: for He), FSPs effects are small (e.g., [25]). However, in the case of H 2 O (α ≃ 9.8 a:u: and permanent dipole moment, d ≃ 1.85 D), they are expected to result in large errors [16,19], e.g., ≃100% of the measured σ T at 10 eV [28].…”

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confidence: 99%

“…For targets with a low polarizability, α (e.g., ≃1.38 a:u: for He), FSPs effects are small (e.g., [25]). However, in the case of H 2 O (α ≃ 9.8 a:u: and permanent dipole moment, d ≃ 1.85 D), they are expected to result in large errors [16,19], e.g., ≃100% of the measured σ T at 10 eV [28].…”

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confidence: 99%

“…The positrons are then transported around a 90°bend through a cylindrical mirror analyzer before reaching the interaction region. Here, an aluminium cylindrical cell is situated of length ¼ 53 mm, inner radius ¼ 25.4 mm, and aperture radius ¼ 0.5 mm [25]. A position sensitive detector (PSD) terminates the flight path.…”

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High-Resolution Measurements of e++H2O Total Cross Section

et al. 2016

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Using a purely electrostatic positron beam, the total cross section of positrons scattering from H 2 O has been measured for the first time with a high angular discrimination (≃1°) against forward scattered projectiles. Results are presented in the energy range (10-300) eV. Significant deviations from previous measurements are found which are, if ascribed entirely to the angular acceptances of various experimental systems, in quantitative accord with ab initio theoretical predictions of the differential elastic scattering cross section. DOI: 10.1103/PhysRevLett.117.253401 While the apparent imbalance between matter and antimatter in the Universe remains a major puzzle in science [1,2], much progress in the understanding of the interactions between the two has been achieved through studies of controlled collisions of positrons ðe þ Þ and positronium (Ps, the short-lived atom made of an electron and a positron) with atoms and molecules [3][4][5][6][7].At low energies, the static and polarization interactions tend to cancel for positrons reducing their scattering probability in comparison with electrons. However, polarization often enhances direct ionization by positrons, so that they can be more penetrating and more ionizing than electrons, a result of potential import in analyses of astrophysical (e.g., [8]) and atmospheric events (e.g., [9]) as well as in positron-track simulations for dosimetry in positron emission tomography (e.g., [10]). In turn, these studies contribute to the motivation for investigating the interaction of e þ with water which accounts for about 60% of the human body and which is the most abundant greenhouse gas in the atmosphere.Measurements of positron-water total cross section (σ T ) were first carried out 30 years ago [11,12]. Since then, only a few new results have been added, experimentally [13][14][15] and theoretically [16,17], without a satisfactory agreement emerging among them. The integral (σ el ) and differential (dσ el =dΩ) elastic (el) scattering cross sections for e þ þ H 2 O have also been measured recently [17], complementing theoretical determinations [18,19].Because of the long-range forces involved in the scattering of charged projectiles from a polar molecule such as H 2 O, one of the major difficulties in measuring σ T (even in the case of electrons, e.g., [20][21][22][23]) lies in discriminating against the considerable flux of small forward-angle scattered particles (FSPs) (e.g., [16,19]). The largest error associated with FSPs arises from elastic scattering and rovibrational inelastic processes which cannot be easily distinguished from the incident flux via energy loss discrimination since this is smaller than (or comparable to) typical beam energy resolutions (e.g., the first vibrational excitation from the ground state J ¼ 0 is ≃1595 cm −1 [24]). Detection of FSPs leads to a systematic underestimate of the beam attenuation and, thus, the measured total cross sections. In this respect, beams that employ magnetic fields are more likely to transport FSPs from the...

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confidence: 99%

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High-Resolution Measurements of e++H2O Total Cross Section

et al. 2016

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“…This can possibly explain why the TCS by Nagumo are higher than other data measured in presence of magnetic field in the scattering region. However the latest experiment by Fayer et al [50] with another magnetic field-free system does not confirm the results of Nagumo et al [48,49].…”

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confidence: 81%

A rigid sphere approach to positron elastic scattering by noble gases, molecular hydrogen, nitrogen and methane

Fedus

2016

Eur. Phys. J. D

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Abstract.A simple potential model of a rigid sphere combined with an adiabatic dipole polarization (∼r −4 ) is tested for positron-atom and positron-molecule elastic collisions. The numerical model, which is based on the analytical solution of radial Schrödinger equation for r −4 potential, depends solely upon the average dipole polarizability of the target and one adjustable parameter -the radius of a hard core. The validity of model is assessed by an extensive comparative study against numerous experimental cross-sections and theoretical phase-shifts of angular momentum partial waves for positrons scattered elastically by He, Ne, Ar, Kr, Xe, H2, N2 and CH4. In particular it is shown that this very simple approach can be used to model positron elastic collisions with targets characterized by moderate dipole polarizabilities (Ar, Kr, H2, N2) in good agreement with experiments for impact energies covering almost entire range from the positronium formation threshold down to the zero energy.

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