Emission of Band-Gap-Energy Positrons from Surfaces of LiF, NaF, and Other Ionic Crystals

Mills, A. P.; Crane, W. S.

doi:10.1103/physrevlett.53.2165

Cited by 47 publications

(9 citation statements)

References 17 publications

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“…Unfortunately, contrary to Ref. 9, it now does not appear that ionic crystals would make very good moderators for muons, 16 although solid Ne could possibly give a useful slow-muon yield. On the other hand, the large yield of reemitted positrons and their long diffusion length suggests that the rare gases will be excellent slow-positron moderators.…”

Section: Armentioning

confidence: 63%

Positron Dynamics in Rare-Gas Solids

1986

Phys. Rev. Lett.

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Using a beam of slow positrons in ultrahigh vacuum, we estimate the positron mean free path and energy-loss rate and measure the inelastic thresholds due to exciton, electron-hole pair, and positronium formation in solid rare-gas targets. The measurements are used to explain the large emission energies of positrons from the wide-band-gap solids in terms of a hot-positron model. The large diffusion length of the hot positrons in the solid rare gases makes them very efficient moderators for producing slow positrons. PACS numbers: 71.60. + Z, 78.70.Bj, 79.20.Hx There have been many studies of positrons in ionic crystals. 1 Measurements have revealed the positronium binding energies, wave function, effective mass, and formation mechanisms. Powdered insulators provided the first sources of positronium in vacuum 2,3 and the first practical efficient moderator for producing slow positrons. 4 Recently there have been studies of the positronium formation mechanism at the surface of ice, 5 of positronium emission from quartz, 6 and of positron reemission by various ionic solids. 7 "" 9 Unfortunately, this large body of work has not led to a precise picture of how a low-energy positron interacts with an insulator. The present study attempts to rectify this situation by careful measurements on the ideal wide-band-gap insulators, the rare-gas solids.When positrons are implanted into an ionic solid at kiloelectronvolt energies, one observes the reemission of positrons with a spread of kinetic energies comparable to the band-gap energy. In Ref. 9 this reemission was interpreted in terms of a modified version of the positronium breakup mechanism originally proposed by Canter et ai 4 In this model the reemitted positrons result from positronium formed in the bulk which diffuses to the surface and breaks up, emitting the positron via an Auger-iike process. We now find that positrons are copiously emitted with several electronvolts of kinetic energy from solid Ne, Ar, Kr, and Xe. However, because of the simplicity of these materials we have also been able to examine the emission process in detail by measuring the mean free path and inelastic thresholds for few-electronvolt positrons. The measurements are not consistent with positronium breakup and suggest that the emitted positrons are simply those that have not thermalized and happen to scatter back to the surface. Our new results also show that the rare-gas solids are a new class of very efficient slow-positron moderators.We propose to explain the observed positron reemission as follows. In any material, energetic positrons lose energy rapidly by making inelastic collisions involving electronic transitions. However, in an insulator there can be no more of these events once a positron has less energy than is needed to make an electron-hole pair, an exciton, or positronium. The positron continues to lose energy by creating phonons, but since the maximum phonon energy (E m&x ) is small, the diffusion length of the hot positron is large. Positrons reaching the surface before their...

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Section: Armentioning

confidence: 63%

Positron Dynamics in Rare-Gas Solids

1986

Phys. Rev. Lett.

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“…Incident beams of positrons have been used to study Ps diffusion in crystalline and amorphous ice (Eldrup et al y 1983(Eldrup et al y , 1984(Eldrup et al y , 1985, in a variety of ionic solids (Mills and Crane, 1984), and on oriented crystals of quartz , all of which are summarized in Table VII. The diffusion coefficients are all small (on the order of 0.1 cmVsec or less).…”

Section: Psmentioning

confidence: 99%

“…b Sferlazzo, Berko, and Canter, 1987. c Dupasquier, 1984 Sferlazzo, Berko, and Canter, 1985. e Paulin, 1968, 1972. f Kakimoto, Hyodo, and Fujiwara, 1985. g Mills and Crane, 1984. h Boev and Arefiev, 1985.…”

Section: Psmentioning

confidence: 99%

Interaction of positron beams with surfaces, thin films, and interfaces

1988

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Recent advances in the study of solid surfaces and thin films using variable-energy positron beams are reviewed. In the first part the authors discuss the process of positron moderation and technical aspects of positron beam production and application. The second part is (roughly) organized in sections that apply to increasing time scales appropriate to the positron-solid interaction. These are (a) first encounter and scattering effects, (b) energy loss and stopping profiles,

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“…The following processes to form Ps at the surface have been reported in the literature: (a) implanted positrons can diffuse to the surface, capture an electron and leave as Ps, 2 (b) implanted positrons can get trapped into a surface state that can be thermally activated into Ps emission, 3 and (c) Ps can be formed in the bulk of the material and can diffuse back to the surface where it is emitted. [4][5][6] Time-Of-Flight (TOF) spectroscopy is the most accurate method to measure the energy distribution of emitted orthopositronium (oPs) in a direct way. Sferlazzo 3,6 used this method to study oPs emission from Al, MgO and ␣-SiO 2 surfaces.…”

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confidence: 99%

Diffusion length of positrons and positronium investigated using a positron beam with longitudinal geometry

et al. 2004

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Positronium emission from single crystalline Al2O3, MgO and vitreous a-SiO2 surfaces was studied as a function of the positron implantation energy E by means of Doppler broadening spectroscopy and Compton-to-peak ratio analysis. When the Ge-detector is in-line with the positron beam, the emission of para-positronium yields a red-shifted fly-away peak with intensity I-pPs(e). An analysis of I-pPs(e) versus E for Al2O3 and MgO where no Ps is formed in the bulk (f(Ps)=0) results in positron diffusion lengths L+(Al2O3)=(18+/-1) nm and L+(MgO)=(14+/-1) nm, and efficiencies for the emission of Ps by picking up of a surface electron of f(pu)(Al2O3)=(0.28+/-0.2) and f(pu)(MgO)=(0.24+/-0.2). For a-SiO2 the bulk Ps fraction is f(Ps)(a-SiO2)=(0.72+/-0.01), f(pu)(a-SiO2)=(0.12+/-0.01) and the diffusion lengths of positrons, para-positronium and ortho-positronium are L+(SiO2)=(8+/-2)nm, L-pPs(SiO2)=(14.5+/-2) nm and L-oPs(SiO2)=(11+/-2)=nm. Depending on the specimen-detector geometry the emission of Ps at low implantation energy may cause either an increase or a decrease of the width of the annihilation line shape at low implantation energies

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Emission of Band-Gap-Energy Positrons from Surfaces of LiF, NaF, and Other Ionic Crystals

Cited by 47 publications

References 17 publications

Positron Dynamics in Rare-Gas Solids

Positron Dynamics in Rare-Gas Solids

Interaction of positron beams with surfaces, thin films, and interfaces

Diffusion length of positrons and positronium investigated using a positron beam with longitudinal geometry

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