Fragmentation of Positronium in Collision with He Atoms

Armitage, S.; Leslie, D.; Garner, A J; Laricchia, G.

doi:10.1103/physrevlett.89.173402

Cited by 61 publications

(61 citation statements)

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“…Although we note that recent experimental and theoretical results [28] do not support the findings of [27], the interpretation of Shah et al [27] might be compatible with the findings by Sarkadi [29], who investigated the fragmentation of positronium (Ps) in Ps-He collision also with the CTMC method and compared the results with the experimental data of Armitage et al [30]. While good agreement has been found with the shape of the measured longitudinal energy distribution of the positron, he has predicted that the maximum of the electron peak should be shifted to lower energies due to the dynamical polarization of the target.…”

Section: 223202 (2005) P H Y S I C a L R E V I E W L E T T E R S contrasting

confidence: 29%

Energy-Sharing Asymmetries in Ionization by Positron Impact

2005

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The triply differential cross section of molecular hydrogen for ionization by 50 eV positrons has been determined, for the first time, for both the ejected electron in coincidence with the remnant ion and for the scattered projectile. Asymmetries in the energy sharing between the two light particles in the final state are observed, with the electron spectrum being shifted to significantly lower (and the scattered positron to correspondingly higher) energies than expected. A similar shape is observed in the case of the ejected electron spectrum from a helium target at the same excess energy. DOI: 10.1103/PhysRevLett.95.223202 PACS numbers: 34.85.+x The correlated dynamics of few interacting particles is a fundamental physics problem that may be exemplified through the process of ionization. Despite the tremendous progress in its theoretical description during the past decade or so (e.g., [1,2]), concomitant experimental investigations remain essential in assessing the accuracy of the various approaches and in guiding further developments. In this respect, particularly sensitive are studies in which there are two or more light particles (e.g., an electron and a positron) in the final state and which yield cross sections which are differential in the energy and/or angular distribution of the ejected electron(s) and/or scattered projectile. The most stringent among these is the triply differential cross section (TDCS) in which all the kinematic parameters are determined. A significant body of data has been gained using the (e; 2e) method (e.g., [3]) and, more recently, the COLTRIM technique (e.g., [4]), which has been applied to electron, photon, proton, and ion impact. Differential studies with positrons, mainly confined to doubly differential investigations (e.g., [5][6][7]), remain scarce but are desirable both intrinsically and for comparison with equivelocity electrons or protons to probe the role of the projectile charge or mass on the collision dynamics (e.g., [8,9]). In this Letter, we report experimental TDCS results for positron impact ionization of simple molecular and atomic targets that reveal major discrepancies with current quantum-mechanical treatments and should thus provide new insights into the understanding of three-body correlated dynamics.Over the past decade, sophisticated distorted wave calculations have been developed based on the 3-Coulombwave final-state wave function (3C) of Brauner et al. [10] which approximates the strictly inseparable many-body system in terms of pairs of interacting particles. At lower energies, the use of the eikonal approximation for the initial state has been found to improve agreement with experiments (e.g., [11]). While such methods have been successful with a variety of projectiles and over a wide energy range, nonperturbative approaches remain superior at lower energies. Particularly noteworthy in this respect are the exterior complex scaling (ECS) method (e.g., [2]), which yielded the first accurate TDCS for e ÿ -H ionization for the case of equal energy-s...

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Section: 223202 (2005) P H Y S I C a L R E V I E W L E T T E R S contrasting

confidence: 29%

Energy-Sharing Asymmetries in Ionization by Positron Impact

2005

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“…However, in some cases where this was not found to be so, Q + T and/or Q Ps T were varied in order to achieve pressure-independence and to estimate the associated uncertainty on dQPs dΩ (≤ 10% in all cases, except for CO 2 at 139 eV where ≤ 18% applies). An additional systematic uncertainty arises from that in R d , estimated to be +8% and -(20-30)% [38]. At each energy, the absolute differential Ps formation cross-section has been computed as the weighted mean of the results across the pressure range if within errors, else the mean and standard error are reported.…”

Section: Psmentioning

confidence: 99%

“…(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].…”

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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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“…With the advent of mono energetic energy tunable positronium (Ps) beams [ 20,21 ] , attention is also being focused both experimentally [22][23][24] and theoretically [25][26][27][28] on the breakup process of the projectile Ps atom .…”

Section: Introductionmentioning

confidence: 99%

Breakup of positronium in a collision with aLi+ion

Roy

Sinha

2009

Phys. Rev. A

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ABSTRACT:Fragmentation of ground state ortho Positronium ( Ps) in collision with Li ion is noted in the e energy spectrum in contrast to the sharp ELP peak for a heavy projectile. Two salient features are noted in the present study: i) the shift of the e DDCS peak (summed over + e angles) towards higher ejection energy with respect to half the residual energy of the system, ii) comparison of the e & + e energy spectra reflect a strong e -+ e asymmetry with respect to the ratio. Both these features could be attributed to the post collisional two center effect on the e due to its parent nucleus ( + e ) and the screened target ion . Two different wave functions of the Li ion are chosen in order to test the sensitivity of the present results with respect to the choice of the wave function.

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Fragmentation of Positronium in Collision with He Atoms

Cited by 61 publications

References 27 publications

Energy-Sharing Asymmetries in Ionization by Positron Impact

Energy-Sharing Asymmetries in Ionization by Positron Impact

Absolute Differential Positronium-Formation Cross Sections

Breakup of positronium in a collision with aLi+ion

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