In a hybrid slow positron beam, extraction of the positrons from the magnetic field (∼1 T for the PSI beam) to field-free space is a necessary operation. A theoretical and experimental demonstration is given which shows that the following proposed beam extraction method works as predicted: magnetic transport of the slow positrons down to field strengths of ∼100 G and passage through an aperture grid of ∼10 cm diameter in a field termination shield followed by some brightness enhancement stages. The simulation and measurement of the magnetic-field distributions along the beam axis show a steep drop down of the field from ∼100 G to a few gauss within 1 cm of shield thickness and a quasiuniform spreading of the transverse field strength across the grid opening. Measurement of transmission and divergency (transverse energy) of the beam exiting the extraction aperture confirmed theoretical estimations and ray tracing calculations for the aperture design used to be of the order of 75% and 20 eV, respectively. These data as a function of field strength and beam energy are used for optimization of the final extraction aperture design (≳85% transmission) to be used in the PSI high intensity beam facility.

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Featuresof positronsannihilated in benzene were investigated by the threegamma coincidences technique, together with lifetime spectroscopy and a magnetic quenching experiment. A high three-quantum yield was found; the result reveals the presence of two positron bound states: a 'relaxed' positronium system and a more complex one.

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