Within the experimental program on deeply bound pionic states at GSI  the most recent beamtime was devoted to a systematic study of the (d, 3He) reaction on Sn isotopes, which was considered to be promising for two reasons: i) The existence of 3s1/2-neutrons in one of the last bound orbitals dramatically enhances the population of the pionic 1s state, being quasi-substitutional at the recoil-free condition. ii) The long chain of stable isotopes of Sn allows to observe for the first time the isotope effect in deeply bound pionic states and thus to seperately deduce the isoscalar and the isovector part of the pion-nucleus potential. The isovector strength parameter b ⋆ 1 is of fundamental interest due to its direct relation to the pion decay constant and chiral order parameter fπ. d 2 σ/d ΩdE [ µb/sr/MeV
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