Differential Ps-formation and impact-ionization cross sections for positron scattering on Ar and Kr atoms

Falke, T; Brandt, T.; Kühl, O; Raith, Wilhelm; Weber, Marc H.

doi:10.1088/0953-4075/30/14/015

Cited by 18 publications

(12 citation statements)

References 6 publications

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“…Unfortunately, because of the much weaker beam intensities available, very little differential ionization data is available for positron impact. Singly differential data has been reported by Falke et al [5] while Schmitt et al [6] and Kövér and Laricchia and coworkers [7,8] have performed several doubly differential electron emission studies. In our laboratory we have measured doubly differential energy loss yields for scattered positrons [9,10].…”

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confidence: 89%

Differential Electron Emission for Single and Multiple Ionization of Argon by 500 eV Positrons

2006

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Triply differential electron emission cross sections are measured for single ionization of argon by 500 eV positrons. Data are presented for coincidences between projectiles scattered into angles of 3 degrees and electrons with emission energies less than 10 eV that are observed between 45 and 135 degrees along the beam direction. For interpretation, these are compared to cosine squared representations of the binary and recoil lobes which are convoluted over experimental parameters. Singly differential electron emission data for double and triple ionization by positrons are also presented.

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confidence: 89%

Differential Electron Emission for Single and Multiple Ionization of Argon by 500 eV Positrons

2006

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“…However, whilst theoretical predictions for the differential Ps formation cross-section ( dQPs dΩ ) are available for atomic [8][9][10][11][12][13][14][15][16] and molecular [17,18] hydrogen, the noble gases [9,16,[19][20][21][22][23][24][25][26][27][28][29] and the alkali metals [30][31][32][33][34], experimental data remain scarce. Indeed, available measurements are confined to H 2 [35], Ar and Kr [36,37] and, due to unknown positron and Ps detection efficiencies (ε In this communication, we present the first absolute experimental determinations of dQPs dΩ for He, Ar, H 2 and CO 2 . The values are extracted from measurements of the production efficiency of the Ps beam at UCL and thus correspond to a small angle around 0…”

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confidence: 99%

Absolute Differential Positronium-Formation Cross Sections

et al. 2015

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The first absolute experimental determinations of the differential cross-sections for the formation of ground-state positronium are presented for He, Ar, H2 and CO2 near 0 ○ . Results are compared with available theories. The ratio of the differential and integrated cross-sections for the targets exposes the higher propensity for forward-emission of positronium formed from He and H2.PACS numbers: 36.10. Dr, 34.80.Bm, 34.80.Uv The formation of positronium (Ps, the bound state of an electron and a positron) is an important channel in the scattering of positrons from atoms and molecules e.g. [1][2][3], accounting for up to 50% of the total cross-section, with experimental and theoretical investigations of its integrated formation cross-sections available for a wide range of atoms and simple molecules, e.g. [3][4][5]. Recent experimental studies also include its formation in an excited state [6] or accompanied by ionic excitation [7]. However, whilst theoretical predictions for the differential Ps formation cross-section ( dQPs dΩ ) are available for atomic [8][9][10][11][12][13][14][15][16] and molecular [17,18] hydrogen, the noble gases [9,16,[19][20][21][22][23][24][25][26][27][28][29] Details of the experimental arrangement employed at UCL for producing a beam of Ps atoms, together with a review of recent advances, may be found in [41]. In brief, the Ps beam is produced by charge-exchange of positrons (e + ) with a target gas (A), i.e. e + + A → Ps + A + , and detected downstream by a channel-electronmultiplier (CEM or CEMA) in coincidence with one or more γ-ray detectors (e.g. CsI or NaI) where it has been found to be composed predominantly of groundstate atoms [42,43].Depending on the relative spin orientation of its constituents, ground-state Ps may be formed in an ortho-( 3 S 1 ) or para-( 1 S 0 ) state. The two are characterized by lifetimes differing by three orders of magnitude (142 ns and 125 ps, respectively) and different annihilation modes (dominantly 3-γ and 2-γ, respectively). Only ortho-Ps reaches the detection region.In order to determine dQPs dΩ (a measure of the probability that Ps is emitted within a solid angle dΩ = 2πsinθdθ), we have measured the number of Ps atoms (ε where τ Ps is the lifetime of ortho-Ps, t its flight-time to the detector and ǫ Ps the 'true' Ps beam production efficiency. In Eq.(1), ǫ m Ps may be seen to depend on the (energy-dependent) ratio of the positron to positronium detection efficiencies R d also determined by our group [38][39][40].By studying the variation of ǫ Ps with gas pressure, optimum beam operating conditions may be determined for a given target and Ps energy [39,40,44,45]. An example is shown in Figure 1 for production of 20 eV Ps from CO 2 . Here ǫ Ps may be seen to increase and then decrease with increasing pressure. This variation may be expressed as:

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“…The curves have been shifted vertically for display purposes. Data are for 120 eV positron impact and are taken from Falke et al (1997). Right; fractions for double and triple ionization by 750 eV e + and e À impact (solid symbols with and without error bars) as a function of energy loss compared to fractions for photon impact (solid and dashed lines).…”

Section: Positron Impact Techniques and Datamentioning

confidence: 99%

“…6 shows results from two other types of differential ionization studies. The left portions shows relative intensities measured as a function of observation angle for various ionization channels of argon, e.g., single and double target ionization, positronium formation (electron capture), and transfer ionization (Falke et al, 1997). The curves have been displaced vertically for display purposes but note that the single electron processes, namely single target ionization and Positronium (Ps) formation, demonstrate similar angular distributions while the two-electron processes, e.g., double and transfer ionization, have nearly identical angular distributions.…”

Section: Positron Impact Techniques and Datamentioning

confidence: 99%

Ionization studies using positron and electron beams

DuBois

2007

Radiation Physics and Chemistry

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Differential Ps-formation and impact-ionization cross sections for positron scattering on Ar and Kr atoms

Cited by 18 publications

References 6 publications

Differential Electron Emission for Single and Multiple Ionization of Argon by 500 eV Positrons

Differential Electron Emission for Single and Multiple Ionization of Argon by 500 eV Positrons

Absolute Differential Positronium-Formation Cross Sections

Ionization studies using positron and electron beams

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