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The use of Coincidence Doppler-Broadened (CDB) positron annihilation spectra for surface analysis requires ensuring that a significant fraction of the annihilation signal originates from positrons annihilating at the surface. We present measurements of CDB spectra obtained using a high-purity germanium (HPGe) detector in coincidence with a sodium iodide (NaI) detector for a series of incident positron beam energies ranging from 2 eV to 20 keV on multilayer graphene (6–8 layers) on copper. The CDB data were analyzed by fitting the Doppler spectra using a linear combination of spectra derived from measured data obtained at the incident beam energies mentioned together with a Gaussian function representing the spectrum associated with para-positronium decay. These fits were used to determine the fraction of incident positrons that annihilate at the surface and in the bulk. We compare the results obtained from fitting the full Doppler-broadened spectra with those derived from a VEPFIT analysis of the S parameters as a function of beam energy, using the same set of Doppler spectra. Both approaches indicate that most annihilation events occur at the surface for beam energies below 5 keV. However, the results from full spectrum fitting suggest a significantly larger bulk annihilation fraction than VEPFIT in the 100 eV to 5 keV range, highlighting the importance of accounting for bulk contributions when using CDB for surface analysis at beam energies above 100 eV.
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Energy distribution and adiabatic guiding of a solid-neon-moderated positron beam
Slow positrons are generated from a 22Na source and cone-shaped solid neon moderator and extracted as a magnetically guided beam. Measurements are presented for the mean parallel and perpendicular energies and the radial distribution of the beam particles. Over a distance of 7 m, where the magnetic field B varies from 0.005 to 0.12 T, the beam transport is found to be adiabatic for mean energies up to 50 eV. Non-adiabatic effects, evidenced by an increase in energy in motion perpendicular to B, are observed at larger transport energies. The implications of these observations for buffer-gas positron traps and other positron-transport beamlines are discussed.
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- Award ID(s):
- 1702230
- PAR ID:
- 10196730
- Date Published:
- Journal Name:
- Journal of Physics B: Atomic, Molecular and Optical Physics
- Volume:
- 53
- ISSN:
- 0953-4075
- Page Range / eLocation ID:
- 085701
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1088/1361-6455/ab7642
