We report on a newly developed scanning positron microbeam based on threefold moderation of positrons provided by the high intensity positron source NEPOMUC. For brightness enhancement a remoderation unit with a 100 nm thin Ni(100) foil and 9.6% efficiency is applied to reduce the area of the beam spot by a factor of 60. In this way, defect spectroscopy is enabled with a lateral resolution of 33 μm over a large scanning range of 19×19 mm 2 . Moreover, 2D defect imaging using Doppler broadening spectroscopy (DBS) is demonstrated to be performed within exceptional short measurement times of less than two minutes for an area of 1×1 mm 2 (100×100 μm 2 ) with a resolution of 250 μm (50 μm). We studied the defect structure in laser beam welds of the high-strength age-hardened Al alloy (AlCu 6 Mn, EN AW-2219 T87) by applying (coincident) DBS with unprecedented spatial resolution. The visualization of the defect distribution revealed a sharp transition between the raw material and the welded zone as well as a very small heat affected zone. Vacancy-like defects and Cu rich precipitates are detected in the as-received material and, to a lesser extent, in the transition zone of the weld. Most notably, in the center of the weld vacancies without forming Cu-vacancy complexes, and the dissolution of the Cu atoms in the crystal lattice, i.e. formation of a supersaturated solution, could be clearly identified.

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The neutron-induced positron source Munich (NEPOMUC) provides a monochromatic low-energy positron beam with an intensity of > 1 × 10 9 e + s −1 and a full width at half maximum (FWHM) diameter of about 10 mm. To create a small beam focus or sharp positron pulses of 100 ps FWHM the beam brightness needs to be enhanced by remoderation. This is achieved by focusing the primary beam magnetically onto a tungsten single crystal W(100) in reflection geometry. Afterwards the beam exhibits an intensity of > 5 × 10 7 e + s −1 and a diameter of < 2 mm FWHM. To further optimize the beam quality of NEPOMUC we redesigned the remoderation unit. The new setup allows a precise positioning of the remoderator crystal within the focus of the magnetic lens. Additionally, a replacement of the crystal within several minutes and without breaking the beamline vacuum is possible that offers the opportunity for systematic tests of different remoderator materials.