An electrostatic brightness-enhanced timed positron beam for atomic collision experiments

Kövér, Á.; Williams, AE; Murtagh, D. J.; Fayer, S. E.; Laricchia, G.

doi:10.1088/0957-0233/25/7/075013

Cited by 8 publications

(17 citation statements)

References 39 publications

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“…The total cross sections have been measured in the energy range (10-300) eV. The lower limit is just below the first ionization threshold E i ¼ 12.62 eV and slightly above the Ps formation threshold, E Ps ¼ 5.82 eV.The equipment used for this experiment has been described previously [29]. Briefly, positrons from a 22 Na source are moderated by a stack of three annealed tungsten meshes (20 μm wire and 70% transmission) [30].…”

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“…The equipment used for this experiment has been described previously [29]. Briefly, positrons from a 22 Na source are moderated by a stack of three annealed tungsten meshes (20 μm wire and 70% transmission) [30].…”

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confidence: 99%

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confidence: 99%

“…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. This is an annealed W(100) foil (thickness ≃50 nm) with a measured remoderation efficiency of 0.1 [29]. The remoderator (rm) is floated at a potential (V rm ) to accelerate the positrons to the required beam energy (E þ ¼ eV rm þ jϕj), where ϕ ¼ −2.7 AE 0.1 eV is the positron work function for the current remoderator [29], and e is the elementary charge.…”

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

et al. 2016

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“…The beam constructed within a national programme at the Australian National University in Canberra is dedicated mainly to atomic physics [44]. Currently operating positron beams, among others, in Helsinki [45], Halle [46], London [47], Delft [48], and Orleans [49] rarely go beyond national contexts and specialized applications, with the exception of the NEPOMUC facility, where scientists of European countries have access within the EU great facilities NI3 programme.…”

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Toward a European Network of Positron Laboratories

et al. 2015

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Positron defectoscopyPositron annihilation is one of the few really nondestructive techniques in material science, giving unique information on types and concentrations of defects, even at a single parts per million (ppm) level, see, for example, [1]. Applications of positron annihilation techniques span from semiconductors [2,3] and metals [4] to polymers [5] and porous media [6]. In each case, answers given by positron methods are unique and complementary to other techniques.Two most common techniques, the analysis of Doppler broadening (DB) of the 511-keV annihilation line and the positron lifetime measurements, are to some extent complementary. The broadening brings information on momenta of electrons that positrons annihilate with. The main contribution to the annihilation comes from valence electrons, that is, possessing a few electron-volts energy. In the relativistic transformation, these energies translate to broadening of a few kiloelectron-volts, i.e. similar to the intrinsic resolution of gamma detectors used in experiments. Therefore, the information on relative contributions from valence and core (i.e., from defect sites) electrons is not straightforward. Somewhat phenomenological line-broadening parameters (central area S parameter, W lateral area, V 3 annihilation) are used, and some normalization is needed to make comparison between different experiments and theories. Evaluation of DB requires not only information of the overlap between a positron and electrons in the material but also a detailed

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Moderation and diffusion of positrons in tungsten meshes and foils

Williams

Murtagh

Fayer

et al. 2015

Journal of Applied Physics

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The efficiency of tungsten meshes and thin foils for moderation of fast positrons from 22Na has been investigated in transmission geometry and a fair agreement has been found with previous experimental results where directly comparable. For foils, the dependence on material thickness is found to be similar to the prediction of the Vehanen-Mäkinen diffusion model; however, the magnitude is 5–10 times lower. A broad consensus is observed between experiment and the results of a three-dimensional model developed in this work. For a given thickness, meshes are found to be generally better than foils by around a factor of 10 with a maximum efficiency (∼10−3) comparable to that achieved with thin single crystal foils, in accord with previous measurements and the results of the present model.

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An electrostatic brightness-enhanced timed positron beam for atomic collision experiments

Cited by 8 publications

References 39 publications

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

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

Toward a European Network of Positron Laboratories

Moderation and diffusion of positrons in tungsten meshes and foils

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