Summary of the fifth international workshop on slow positron beam techniques for solids and surfaces

Weiss, Alexander; Ottewitte, E.H.; Augenstein, Bruno; Denison, A. B.; Ghosh, V.J.; Jacobsen, F. M.; Krištiaková, K.; Lynn, K.G.; Nieminen, R.M.; Ritley, K. A.; Simpson, P. J.

doi:10.1016/0969-806x(94)90228-3

Cited by 3 publications

(2 citation statements)

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“…Guessoum, Ramaty and Lingenfelter (1991) added spectral considerations to the positron-dust process but kept a simplified picture of the dust and showed that unless the abundance of dust in the annihilation regions is increased many times compared to "normal" quantities, grains change the spectrum of the line mostly in the hot phase. Astrophysical knowledge of grains has since increased considerably (see the reviews by Draine 2003, Li 2004, and Krugel 2003 and although a huge body of work exists on positron interactions with solids and solid surfaces (Nieminen & Oliva 1980;Huttunen et al 1990;Dannefaer et al 1996;Dupasquier & Mills 1995;Puska & Nieminen 1994;Weiss et al 1994;Suzuki et al 2003;Mills et al 1989;Mokrushin et al 2003;Pi et al 2003;Weber & Lynn 2000), experiments dealing with positrons and dust like materials remain scarce. We here summarize our best understanding of the relevant information we have collected on dust grains in our context.…”

Section: Grains and Positron Processesmentioning

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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Section: Grains and Positron Processesmentioning

confidence: 99%

The lives and deaths of positrons in the interstellar medium

Guessoum

Jean

Gillard

2005

A&A

178

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“…Tsytovich and Wharton [7] suggested trapping positrons in an MM from a LINAC source. Later on, Boehmer [8] employed cyclotron heating to trap moderated positrons from a radioactive source and to heat the trapped positrons to relativistic energies. The most successful experimental approach, to obtaining positron plasmas, however, has turned out to be the scattering from a buffer gas into a Penning trap.…”

Section: Introductionmentioning

confidence: 99%

Quantitative and qualitative analyses of the mKdV equation and modeling nonlinear waves in plasma

et al. 2023

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In this paper, nonlinear electrostatic structures on the ion time scale in plasma consisting of two populations of electrons (cold and hot), positrons, and warm adiabatic ions are investigated. The multiple scale method is used to derive the modified Korteweg–de Vries equation (mKdVE). The Jacobi elliptic function expansion method (JEFEM) is employed to find some exact analytical solutions such as periodic, solitonic, and shock solutions. It is shown that the variation in the plasma parameters of interest, for our model, allows the existence of solitary and periodic structures and no shocks. It is also shown that the most important plasma parameters for the plasma model under consideration are positron concentration, α, and the percentage of cold and hot electrons, represented by the parameters μ and ν, respectively. Additionally, the qualitative behavior of the mKdVE is studied using dynamical system theory. The topological structure of the solution is discussed in the phase plane. In this work, the phase plane analysis, which is restricted to the discrete values of the parameter, is extended to the continuous range of the parameter using a bifurcation diagram. Bifurcation diagrams are drawn to forecast the behavior of the solution for specifically chosen essential plasma parameters. The analytical solution and the qualitative behavior of the solution presented in this paper are shown to be compatible with each other. The results presented here are general and can be gainfully employed to study a variety of nonlinear waves in space, laboratory plasmas, and astrophysical plasmas.

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In situ small angle X‐ray scattering study on structural evolution of crosslinked polytetrafluoroethylene during deformation

Tian

Yang

et al. 2013

J of Applied Polymer Sci

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The structural evolution of virgin and crosslinked polytetrafluoroethylene (PTFE) during stretching was studied by in situ synchrotron small-angle X-ray scattering (SAXS). Both yield and tensile stress of crosslinked PTFE increased with increasing crosslinking density. During stretching, for virgin PTFE, amorphous chains gradually turned to tensile direction at early stage, perpendicularly arranged lamellar stacks appeared at high strains (>140%). While for crosslinked PTFE, lamellar structure was observed even at lower strains; with increasing irradiation dose, the lamellar structure became obvious and the long period decreased. Four-point SAXS patterns were observed only in 3000kGy-dosed PTFE during deformation, which indicated that an alternately tilted lamella arrangement called herringbone structure was formed. Radiation dose induces crosslinked networks formed, which can carry part of local stress during deformation, resulting in the increase of yield and tensile stress. Crosslinking density is an important factor on structural evolution. In addition, a deformation mechanism of different crosslinked PTFE is proposed.

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Summary of the fifth international workshop on slow positron beam techniques for solids and surfaces

Cited by 3 publications

References 0 publications

The lives and deaths of positrons in the interstellar medium

The lives and deaths of positrons in the interstellar medium

Quantitative and qualitative analyses of the mKdV equation and modeling nonlinear waves in plasma

In situ small angle X‐ray scattering study on structural evolution of crosslinked polytetrafluoroethylene during deformation

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