Positronium formation and diffusion in crystalline and amorphous ice using a variable-energy positron beam

Eldrup, Morten Mostgaard; Vehanen, A.; Schultz, Peter J.; Lynn, Kelvin G.

doi:10.1103/physrevb.32.7048

Cited by 84 publications

(41 citation statements)

References 44 publications

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“…Most of the positrons that diffuse inside the grain will reach the surface, the diffusion length being greater than the diameter of most of the grains, but upon reaching the surface the positron will be captured in a "surface state" and form Ps there or be ejected, if the "positron work function" of the material is negative, which is the case for our dust grains (Rosenberg et al 1987). A fraction of the Ps atoms formed at or near the surface will also be ejected if the "Positronium work function" for the material is negative, which is also the case for our dust (Hodges & Stott 1973;Nieminem & Oliva 1980;Eldrup et al 1985;Rice-Evans & Rao 1988;Puska & Nieminem 1994).…”

Section: Grains and Positron Processesmentioning

confidence: 91%

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: 91%

The lives and deaths of positrons in the interstellar medium

Guessoum

Jean

Gillard

2005

A&A

178

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“…Very low values of L a (a fewÅ at room temperature (RT)) were in fact reported in [12] for some amorphous polymers. In [13] a large difference was found between the positronium diffusion coefficient in semicrystalline ice (about 0.2 cm 2 /s) and in amorphous ice (roughly 10 −3 cm 2 /s). Trapping of positronium in defects in the amorphous ice was assumed as the reason of the large difference.…”

Section: Examples Of Morphology Influence On Positron Annihilation Chmentioning

confidence: 98%

Influence of Polymer Crystallinity and Morphology on Positron Annihilation Characteristics

Dębowska

2006

Acta Phys. Pol. A

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Some most representative results for semicrystalline polymers by positron annihilation spectroscopy method, showing the influence of crystallinity and morphology of samples on positron annihilation characteristics, are reminded. The latest finding of the author's experiment on modified polyamide 6 is presented. The latter reveals that the interfacial zone between a lamella and an amorphous region may influence the ortho-positronium characteristics. The role of chain defects is stressed.

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“…where R 1 , R 0 , P 0 , P 1 are the R and P values corresponding to 100% and 0% o-Ps formation respectively [19]. Information about the e + and Ps pick-off annihilations were obtained by PALS measurements carried out on the capped sample.…”

Section: Methodsmentioning

confidence: 99%

Study of Positronium formation in nano-channelled silicon as a function of sample temperature

Mariazzi

DiNoto

Ravelli

et al. 2013

J. Phys.: Conf. Ser.

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Abstract. Oxidized nanochannel in silicon have been demonstrated to be suitable for positronium (Ps) formation and cooling also at low sample temperature. To investigate the Ps yield and to clarify the Ps formation mechanism we studied, by Positron Annihilation Spectroscopy (PAS), nanochanneled Si p-type samples in the 150-430 K temperature range. Ps yield was found to be constant in the 150-300 K temperature range, then it increases up to ~50% of its value from 350-400 K. This effect is associated to a decrease of the fraction of positrons annihilating in Si and in the SiO 2 layer on the nanochannels surface. This finding is compatible with the thermal decrease of the positive charge distribution at the Si/SiO 2 interface limiting e + reaching the SiO 2 layer and to a charge rearrangement at the SiO 2 surfaces.

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Positronium formation and diffusion in crystalline and amorphous ice using a variable-energy positron beam

Cited by 84 publications

References 44 publications

The lives and deaths of positrons in the interstellar medium

The lives and deaths of positrons in the interstellar medium

Influence of Polymer Crystallinity and Morphology on Positron Annihilation Characteristics

Study of Positronium formation in nano-channelled silicon as a function of sample temperature

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