Moderation and diffusion of positrons in tungsten meshes and foils

Williams, AE; Murtagh, D. J.; Fayer, S. E.; Andersen, Stine Linding; Chevallier, J.; Kövér, Á.; Reeth, P. Van; Humberston, J W; Laricchia, G.

doi:10.1063/1.4930033

Cited by 10 publications

(12 citation statements)

References 25 publications

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“…The electrostatic positron beam at UCL [26], previously used to carry out measurements of σ T for e þ þ ðHe; KrÞ [27] and e þ þ H 2 O [14] with an energy-independent discrimination θ ≃ ð1-2°Þ, has been used to produce an electron beam with only slight modifications. Briefly, the secondary electrons, ejected from a tungsten mesh moderator [28] upon the impact of fast β þ particles emitted from 22 Na source, are accelerated to 2 keV by applying a negative bias to the moderator and focused to a ∼1 mm radius spot onto a (W-mesh) remoderator. A negative potential, V Rm , applied to the remoderator allows the beam energy to be varied according to (E − ¼ ejV Rm j þ ϕ), where ϕ has been experimentally determined to be between 1.2-1.7 eV.…”

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confidence: 99%

Angle-Resolved Electron Scattering from H2O near 0°

et al. 2019

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An electron beam, characterized by a high-angular discrimination (≃0.7°), has been used to measure the total (elastic plus inelastic) cross section of H 2 O in the energy range 3-100 eV. Broad coincidence is found with recent experiments, including a pronounced shoulder in the 6-12 eV region. However, at energies ≲6 eV, the present cross sections are ≃30% higher. Furthermore, forward scattering has been probed in the angular range 0°-3.5°and measures of the average (rotationally and vibrationally summed) differential elastic cross sections for incident energies ≤12 eV are obtained at a scattering angle ≃1 ∘ . The measurements, which provide the first test of theoretical predictions in an angular region experimentally unexplored until now, are found to be within 1 standard deviation of corresponding ab initio R-matrix calculations.

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confidence: 99%

Angle-Resolved Electron Scattering from H2O near 0°

et al. 2019

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“…Briefly, positrons from a 22 Na source are moderated by a stack of three annealed tungsten meshes (20 μm wire and 70% transmission) [30]. A set of primary lenses transports the beam at 3 keV from the moderator and focuses it to a small beam spot (R ∼ 1 mm) at the remoderator.…”

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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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“…The contribution of mirrored positrons (white area), in the ejection layer on the moderator surface opposite to the source, is enhanced compared to the case where no CT is used (black area). This mimics the so-called reflection "geometry" which is known to have a higher slow positron yield [61].…”

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confidence: 70%

High Efficiency Cyclotron Trap Assisted Positron Moderator

et al. 2018

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We report the realisation of a cyclotron trap assisted positron tungsten moderator for the conversion of positrons with a broad keV-few MeV energy spectrum to a mono-energetic eV beam with an efficiency of 1.8(2) % defined as the ratio of the slow positrons divided by the β + activity of the radioactive source. This is an improvement of almost two orders of magnitude compared to the state of the art of tungsten moderators. The simulation validated with this measurement suggests that using an optimised setup even higher efficiencies are achievable.

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Moderation and diffusion of positrons in tungsten meshes and foils

Cited by 10 publications

References 25 publications

Angle-Resolved Electron Scattering from H2O near 0°

Angle-Resolved Electron Scattering from H2O near 0°

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

High Efficiency Cyclotron Trap Assisted Positron Moderator

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