Positronium formation in positron scattering from hydrogen and helium atoms: the distorted-wave approximation

Mandal, P.; Guha, S.; Sil, N. C.

doi:10.1088/0022-3700/12/17/019

Cited by 88 publications

(27 citation statements)

References 31 publications

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“…However, in all cases except for the results of [59] and [29], the theoretical ratios should be considered overestimates (by around 20% or so) since their Q Ps refer to n=1 or n=1,2 only. Even so, bar for those of Mandal et al [16], these ratios are lower than experiment. Once again, a good agreement is found between experiment and the coupledpseudostate calculation of [29] for dQPs dΩ (n=1)/Q Ps (all n) who allowed for n=1,2 formation explicitly and n>2 through the 1 n 3 scaling [58].…”

Section: Psmentioning

confidence: 69%

“…This shows an initial de- [16]; (◻) DWA [16]; (⋯⋯⋯) close-coupling [19]; (-⋅ -) eikonal approximation [21]; (------) DWA [25]; (--) second order DWA [26]; (△) 27-state coupled-pseudostate approach [29,58]; thick line (near the threshold), Kohn variational method [59]. Ar: (△) truncated coupled-static [24].…”

Section: Psmentioning

confidence: 95%

“…Included for He are the results of calculations performed within various approximations. Of these, the first order Born approximation (FBA) and distorted-wave approach (DWA) of Mandal et al [16] both predict a shape similar to experiment; the DWA also agrees in magnitude. However other DWA results [25,26], although also similar in shape, are considerably lower especially in the peak region.…”

Section: Psmentioning

confidence: 95%

“…However other DWA results [25,26], although also similar in shape, are considerably lower especially in the peak region. The main differences between the various DWA calculations is in the form of the target wavefunction used and the fact that exchange was not included explicitly in work of Mandal et al [16] who multiplied both the differential and integrated cross-sections obtained by a factor of two. The results of an eikonal approximation [21] have a magnitude close to experiment at the lowest energy considered but which decreases at a somewhat faster rate.…”

Section: Psmentioning

confidence: 99%

“…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.…”

mentioning

confidence: 99%

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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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Section: Psmentioning

confidence: 69%