Positron Annihilation with Inner-Shell Electrons in Noble Gas Atoms

Iwata, Koji; Gribakin, G. F.; Greaves, R. G.; Surko, C. M.

doi:10.1103/physrevlett.79.39

Cited by 39 publications

(80 citation statements)

References 20 publications

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“…Theories neglecting these correlations are generally called independent-particle models (IPM) because the electron wave functions are assumed not to respond to the presence of the positron. Recently, Iwata et al studied annihilations in a simpler isolated two-body positron-atom interaction by mixing noble gases with cold positrons stored in a modified Penning trap [5]. The results qualitatively matched an IPM based on a static Hartree-Fock approximation in both the low and high momentum regions of the spectra.…”

supporting

confidence: 55%

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Doppler Broadening of In-Flight Positron Annihilation Radiation due to Electron Momentum

et al. 2001

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supporting

confidence: 55%

“…However, Doppler broadening effects from the deep core electrons have not been observed using energetic or thermalized positrons. Typically with thermalized positrons annihilating in high Z materials, only the shell immediately beneath the valence bands contributes significantly to the annihilation signal [5].…”

mentioning

confidence: 99%

Doppler Broadening of In-Flight Positron Annihilation Radiation due to Electron Momentum

et al. 2001

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“…The observed spectra are Doppler broadened due to the momentum distribution of annihilating electron-positron pairs which, for the case of room-temperature positrons, is dominated by the momentum distribution of the bound electrons [23]. Thus the Doppler broadening measurements provide information about the quantum states of the annihilating electrons.…”

Section: Previous Experimentsmentioning

confidence: 98%

“…(20) and the cross section of Eqs. (23) and (24), respectively, for all available data. The values calculated from these two models correlate as well to all of the data as they do to the data for atoms and single-bonded molecules.…”

Section: The Larrichia-wilkin Modelmentioning

confidence: 99%

Positron annihilation on large molecules

et al. 2000

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Positron annihilation on molecules is known to depend sensitively on molecular structure. For example, in the case of hydrocarbon molecules, modest changes in molecular size produce orders of magnitude changes in the observed annihilation rates. Although this process has been studied for more than three decades, many open questions remain. Experimental studies are described which are designed to test specific features of the annihilation process. Two possible mechanisms of the annihilation are considered theoretically: direct annihilation of the positron with one of the molecular electrons, including possible enhancement of this process when low-lying virtual or bound positron-molecule states are present, and resonant annihilation through positron capture into vibrationally excited states of the positronmolecule complex. The dependence of annihilation rates, λ, on positron temperature, T p , is studied for the first time for molecules, and at low values of T p the dependence follows a power law, λ ∝ T −ξ , with ξ ≈ 0.5. These data are used to test the predictions of direct numerical calculations and theories of the virtual-level enhancement. Partially fluorinated hydrocarbons are studied in order to understand the rapid changes in annihilation rate produced in hydrocarbons as a result of fluorine substitution. These data are compared with the behavior expected due to direct annihilation when there is virtual or bound level enhancement. Measurements of positron annihilation on deuterated hydrocarbons are described which test the dependence of the annihilation on the nature of the molecular vibrations. The relationship of the presently available experimental data for annihilation in molecules to current theories of the annihilation process is discussed.

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“…We mention the pioneering work of: Bhatia et al (1977) for early theoretical calculations of annihilation in positron-hydrogen collisions; Brown et al (1984, for measurements of line widths and Positronium formation fractions in gases (H 2 , He) relevant to the ISM conditions; see Charlton & Humberston (2001) for a general review of positron work in gases (particularly H, H 2 , He), Puska & Nieminen (1994) for the theory of positrons in solids and on solid surfaces, and the body of work by Iwata, Surko and collaborators (Iwata et al 1996(Iwata et al , 1997b(Iwata et al , 2000 on laboratory measurements of cross sections and annihilation line widths of positrons colliding with atoms and molecules (up to and including large complex alcohol and/or aromatic molecules, some of which may exist in galactic molecular clouds). We would draw attention to the significant amount of relevant "non-astrophysical" work currently being performed by atomic and condensed matter researchers in positron physics 1 .…”

Section: Introductionmentioning

confidence: 99%

The lives and deaths of positrons in the interstellar medium

Guessoum

Jean

Gillard

2005

A&A

178

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Abstract. We reexamine in detail the various processes undergone by positrons in the interstellar medium (ISM) from their birth to their annihilation using the most recent results of positron interaction cross sections with atomic and molecular hydrogen, as well as helium. The positrons' lives are divided into two phases: the "in-flight" phase (between ≈1 MeV and tens of eV) and the thermal phase. The first phase is treated with a Monte Carlo simulation that allows us to determine the fraction of positrons that form positronium and annihilate as well as the characteristics of the annihilation emission as a function of the medium conditions. The second phase is treated with a binary reaction rate approach, with cross sections adopted from experimental measurement or theoretical calculations. An extensive search and update of the knowledge of positron processes was thus undertaken. New reaction rates and line widths have been obtained. We investigate the treatment of the complicated interactions between positrons and interstellar dust grains. Fully relevant data were not always available, but we were nonetheless able to reach satisfactory understanding of positron annihilation on grains, both qualitatively and quantitatively. All factors of the problem have been considered, including the grain size distribution and composition, the electric charge of the grains, the backscattering, positronium formation and ejection from the grain, the pick-off annihilation inside them and the partial destruction of dust in the hot regions of the ISM. New reaction rates and widths of the line resulting from the annihilation inside and outside of the grain have been obtained. The final results of our calculations (reaction rates and spectra) showed that dust is only important in the hot phase of the ISM, where it dominates all other processes. Combining the new calculations, we have constructed annihilation spectra for each phase of the ISM, considering various grain contents, as well as an overall combined spectrum for the ISM as a whole.

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Positron Annihilation with Inner-Shell Electrons in Noble Gas Atoms

Cited by 39 publications

References 20 publications

Doppler Broadening of In-Flight Positron Annihilation Radiation due to Electron Momentum

Doppler Broadening of In-Flight Positron Annihilation Radiation due to Electron Momentum

Positron annihilation on large molecules

The lives and deaths of positrons in the interstellar medium

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