Abstract
The fraction of positronium (Ps) emitted from a surface of a germanium single crystal at high temperature is usually assumed to approach unity at zero positron implantation energy. In the experiment, however, the determination of the absolute Ps fraction is not straight forward since recording a reference spectrum with $100\,\%$ Ps formation remains demanding. We use GEANT4-simulated detector responses to $2\gamma$ and $3\gamma$ radiation sources mimicking positron and Ps annihilation inside the (coincidence) Doppler-broadening spectrometer at NEPOMUC, FRM II, in order to derive a reliable value for the Ps fraction reemitted from a Ge(100) target heated close to its melting point. Analysis of the measured spectra by fitting the simulated spectra shows an absolute value of $(72\,\pm{4})\,\%$ maximum Ps formation, contradicting the $100\,\%$ assumption.
Highlights
Positronium (Ps) is a unique matter-antimatter hydrogenlike bound state of a positron and an electron
We use GEANT4-simulated detector responses to 2γ and 3γ radiation sources mimicking positron and Ps annihilation inside the Doppler-broadening spectrometer at Neutron-induced Positron Source Munich (NEPOMUC), FRM II, in order to derive a reliable value for the Ps fraction reemitted from a Ge(100) target heated close to its melting point
The analysis of energy-resolved γ spectra, as, for example, obtained with high-purity germanium (HPGe) detectors, will be briefly reviewed using a spectrum recorded for a Ge target with the Doppler-broadening spectrometer (CDBS) in Munich [35] at 15 keV positron implantation energy
Summary
Positronium (Ps) is a unique matter-antimatter hydrogenlike bound state of a positron and an electron. By referring to an experiment performed by Mills [19], it is often assumed that positron implantation with various energies and extrapolating to zero energy yields 100% Ps formation in germanium at high temperature, while other loss channels such as positron annihilation from a surface state can be neglected. Such a spectrum showing 100% Ps formation, is demanding to be obtained experimentally and cannot be verified. By including the relevant parts of the spectrometer, we simulated the γ spectra for 2γ and 3γ annihilation in order to reliably estimate the absolute fraction of thermal and nonthermal Ps emitted from a Ge(100) surface
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