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DEVELOPMENT OF A SCANNING POSITRON MICROSCOPE FOR DEFECT ANALYSIS IN MATERIALS SCIENCE

Objective


The beam intensity has been evaluated in separate measurements with a specially manufactured radioactive source of 1 mm diameter and a channeltron as a detector.

With a freshly conditioned moderator a very reproducible situation is obtained, lasting several weeks in a vacuum of 10{-9} mbar. This corresponds to an overall yield of more than 5000 e+ per second and mCi.

In order to measure the spatial resolution at the remoderator position, the remoderator was replaced by a gold grid (360 um spacing, 30 um bars), which was positioned at the image plane of the single-pole lens. A line width of less than 15 um FWHM is determined from a line scan and the total spot size is less than 20 um.

With a measured remoderation efficiency of 23% the phase-space density of the reemitted positrons from this pulsed positron microsource will exceed the one of the first moderator by a factor of about 3 x 10{4}. Up to now, the typical gain in phase-space density of a single remoderation stage was only about 20. This means that in the scanning positron microscope only one remoderation stage will replace three conventional remoderation stages, reducing the required primary source strength by a factor of 25. This progress is due to the well balanced transport system in the beam, the outstanding properties of the single pole lens and the application of time bunching, which contributes with a factor of more than 50 to the total gain in the phase-space density.

The image from the electron beam, which passes the same optical column as the positron beam, exhibits the surface morphology and is also a quick reference for selecting regions of interest for positron studies, for perfect focussing and for alignment of the optical column. A present, the achievable spatial resolution of the positron image has not yet been evaluated experimentally.

Therefore the subsequent estimates are based on the evidence from test measurements on the actual performance of the components (eg primary beam, remoderator, optical column, axial magnetic field of the probe forming lenses, etc). With the remoderator cooled to liquid nitrogen temperature and the final 58Co positron source of 1 mm diameter, this will result in the following predicted spatial resolutions: 0.5 um FWHM at 1 keV and 0.2 um FWHM at 10 keV positron energy.

The actual remoderation and bunching efficiencies have been determined to 23% and 70%, respectively. For a primary source of 1 Ci 58Co, this corresponds to about 5.8 x 10{5} positrons per second at the specimen and an event rate of about 2 x 10{4} per second for a BaF2 scintillator.

The presented scanning positron microscope with micrometer resolution and with a pulsed structure is the first operational system of its kind. This system is designed and built not only for demonstration of the principle but also for general applications under normal laboratory conditions.

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Coordinator

Universität der Bundeswehr München
Address
Werner-heisenberg-weg 39
85577 Neubiberg
Germany

Participants (1)

Università degli Studi di Trento
Italy
Address
Via Mesiano
38050 Povo Trento