Berlin 2024 – wissenschaftliches Programm

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KFM: Fachverband Kristalline Festkörper und deren Mikrostruktur

KFM 9: KFM Poster Session

KFM 9.1: Poster

Montag, 18. März 2024, 18:00–20:00, Poster E

Modelling backscattered positron capture at the FRM II coincidence Doppler broadening spectrometer — Danny R Russell, Francesco Guatieri, •Leon Chryssos, and Christoph P Hugenschmidt — FRM II - Technische Universität München, München, DE

The coincidence Doppler broadening spectrometer (CDBS) at the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) provides state-of-the-art, depth dependent detection of defects and chemical composition at the annihilation site. A monoenergetic positron micro-beam (50 µm FWHM) is guided onto a sample where positrons annihilate with electrons. The Doppler broadening of the characteristic 511 keV annihilation peak is measured by observing both emitted photons simultaneously.

The measurement quality depends on the size and energy of the beam. However, up to ∼ 40% of the incident positrons are backscattered at the sample surface. The annihilation events occurring when these backscattered positrons return to the sample or annihilate in experimental hardware contribute unwanted signal to the measured spectrum. We present simulations and hardware design for an upgrade to the CDBS which will allow backscattered positron capture. We use an in-house particle tracking code to design a positron dump that will capture backscattered positrons at a negatively biased electrode and will be shielded from detector lines of sight. This will remove the unwanted signal from the detected spectrum and improve the quality of CDBS data.

Keywords: Positron; Beam; Backscatter; Spectrometry